Figuring out Breaker Sizes For a 48V Off-Grid System

Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

Greetings,

I am upgrading a 48V off-grid system in Haiti (equipment in my signature); I have posted in a couple other forums and received some good feedback. I also got some great advice from an esteemed "old friend". I will be travelling by the end of he month to do the upgrade, and would definitely appreciate if you could help reviewing and filling the gaps on my configuration.

Array 6 X 315W panels (1890W) in a 3S2P configuration - Array VMP 111 Volts - PV Array Current 17.04 Amps - ISC Array 17.86 Amps - Max Output current based on this configuration 76 Amps

Combiner Breakers: 2 MNPV15 – Breakers may not be necessary for 2 parallel strings, they will be handy for testing and troubleshooting.(Panel fuse is 15 A)

PV Output Disconnect Breaker = 100 Amps (76 Amps CC Max output * 1.25 NEC derating = 95 Amps) Cable 4AWG THHN

PV Input Disconnect Breaker = 50 Amps ?? - This breaker will make it easier to service the controller.

  17.86 Isc-array *1.25 NEC derating =22.32 Amps

8 AWG RHW PV cable(33ft) will run in conduit from the combiner to a PV+bus added to the MNDC enclosure - a 4 AWG THHN cable will go from PV+bus to breaker to Classic P+

A 50 Amps breaker was chosen because of the ampacity of the 8 AWG PV cable, and since smaller breaker rating can be used on a heavier wire, the rated ampacity for the # 4 AWG wire was ignored. Thoughts?

I have another question reading inverter breaker, but I am saving it for later.

I appreciate you took the time to read this post.

Harry


48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

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Comments

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #2

    Harry,

    just a few points.......breaker derating....MNPV breakers are not 100% duty cycle and need to be derated for that and the NEC requires a derating on the breaker so the minimum breaker size on that is 1.25 for not 100% duty cycle plus 1.25 derating in general so the derating for the p.v. panel breakers is 1.25 times 1.25 or 1.56 times maximum current in that wire. So 15 amps is correct for that panel string. You are correct having a breaker on each string will make trouble shooting easier. Strings of 2 panels will be marginally not enough voltage for some situations so strings of three is approiate.

    At the controller , using MNDC breakers......these are rated 100% duty cycle so derating factor is 1.25. So 100 amp is correct, With a MPPT controller you will need to shut off solar input before shutting off the battery to controller breaker so I would use a p.v. input breaker. If you use a MNDC breaker here the required derating is 1.25 but if you use a MNPV breaker the derating is 1.56 times the maximum array current. I would use a 40 amp MNDC breaker on the PV input, this will protect the wire but will also protect the controller.

    4 gauge THHN is rated 100 amps in the NEC so you are not Ignoring the NEC rating. You cannot use the 90 degree rating when connecting that wire to a 75 degree terminal, you need to use the rated ampacities at 75 degrees.

    Im going to base the inverter breaker on experience since you have not identified the inverter.....The Cotek SD-2500-148 has a maximum draw of 90 amps so that breaker would be 125 amps for a 100% duty cycle breaker like the MNDC125

    Little note on the NEC..(National Electrical Code).......definition of 100 % duty cycle........any device that is capable of running more than 2 hours is a "continuous duty device" and does need a 100% duty cycle circuit protection device. This can be accomplished with a 100% duty cycle breaker rated at 125% of maximum circuit current or a non 100% duty cycle breaker derated at the 1.25 X 1.25 = 1.56 derating.....example of this...a household water rated at 19.8 amps at 240 volts will require 10 gauge (30 amp) wire and a 25 amp breaker. In a typical household only the water heater is a 100% duty cycle device...it can run more than 2 hours from startup cold to hot...unless you have electric furnace......typical heaters will not run more than 2.0 hours continuous.

    So your design is good....you have done the footwork! Good job!

    david

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

    Thank you David for providing so much info that would be useful in the future as well.

    Regarding the MidNite breakers, I was under the impression all of them could be used at 100% rated current when mounted in a MidNite enclosure. In that case should we only take the wire derating in consideration and just multiply the array current by 1.25?

    You have mentioned breakers have terminals rated for use with 75°C. I did notice the only rating for THHN insulated cable is calculated at 90 degrees on NEC310.15(B)(16) table, and there is no value for 75 degrees. I am just pointing it out since other novices like me could be confused.

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    My Cotek inverter is the SD-2500-148 as you have guessed , I have added it to my signature. I do have a few questions regarding inverter breaker and inline fuse, but I have saved them for later jut to not be overwhelming. Your explanations are already answering some of those questions.

    Thanks again,

    Harry


     


    48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

  • VicVic Solar Expert Posts: 3,136 ✭✭✭✭

    IMO,   MidNite MNEPV AND MNEDC Breakers ARE rated for 100% Continuous currents  --  on the MNEPV (DIN Rail breakers)  look at the tag on the side of the breaker.   It shows the Handle Rating,   AND the Trip Current.   The tag shows that the MBEPV breakers ARE rated for 100% continuous current,   IMO.

    FWIW,   Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭

    Vic,

    I got that information directly from boB at MidNite....MNDC breakers (Carlingswich D , E , and F series) are rated continuous duty.....MNPV Breakers (Circuit Breaker International) are not rated continuous duty........I'm really sure on this.....

    On MidNite website describing the MNEPV breaker it does say that MNEPV breakers can safely disconnect at full rated current repeatedly....without damage....It does not say that they can be used continuously at full rated current

    No AC breakers for your home panel are rated 100 % continuous duty, not even Square D QO line or Cutler-Hammer CH line, the two best plug on buss breakers available.

    In industry it's a different picture....Carlingswich Switch is used widely in industry....

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • halfcrazyhalfcrazy Solar Expert Posts: 720 ✭✭✭

    Midnites DC and AC breakers are all 100% duty cycle, I just talked to TecnoDave but here is the data sheet on the MNEPV series


    http://www.midnitesolar.com/pdfs/MNEPV_Series_Circuit_Breakers_DataSheet.pdf

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #7

    Vic,

    I just spoke with Ryan at MidNite and he said that the MNEPV breakers are 100% duty cycle, he is MidNite's applications engineer and the authority on this. SO IM WRONG......I FESS UP! I definitely goofed it up, I did get that information from somewhere.....Downloaded the CBI data sheet and it confirms this....ALSO ...these are magnetic/hydraulic breakers like the MDEDC breakers from Carlingswitch.

    My apologies to the forum! I don't want to provide inaccurate data.

    I will wear a dunce cap all week!.....hey it's Friday...been a long week, I'll wear the dunce cap next week!

    david

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

    Hi David,

    You’ve been so helpful in these forums, no apologies needed here, and definitely no dunce cap. Actually, guys like you deserve a medal for wanting to help. You were the first reply to my post, and you did at 12:30 AM, your time with multiple paragraphs.

    Thans again,

    Harry


    48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin

    Yes, Midnite and Outback--Their breakers (or specific breaker lines) are rated at 100% continuous current (as I understand). Not standard, from what I had seen over the years with "typical" breakers.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭

    I did learn much from this forum as well as MidNite forum . I had been off grid for a few years battling Chinese controllers that could not be configured for high capacity battries that I prefer. No user adjustable settings, absolutely no manufacturers support. Even with high level background in electrical/electronics industry, I could not get my Chinese MPPT controllers to top off my batteries, they just would not supply a high enough voltage to get past 80% S.O.C. no matter what I did, After a few years just looking around here I bought a classic 150 from wind-sun, Things work great now. I can program any voltage that is needed to top off my batteries. It was learning through the expereince of other users that made the difference, that and a manufacturer who actually supports their product. I sold off my Cheepo Chinese controllers and only buy well respected product. I'm a bit biased, I've had very few problems with my MidNite gear and when that problem arises MidNite is there with full support. I broke off the blue terminal cover and a peice landed in the lower cooling fan of the classic making an annoying whine. MidNite not only replaced the broken terminal cover but supplied a new fan as well. I would not consider a user caused damage to be warrantable but MidNite tech support insisted and that classic has been restored with new fan and terminal covers. I now have two Classic's and two Kids, at least 6 big baby boxes MNDCQUAD, MNDC10 all MN breakers. As well as some homemade boxes full of MidNite breakers. It was users here and MidNite forum that guided me to a system that has not had a day without power for at least 5 years now.I have inverters from other manufacturers but only well respected ones....MagnaSine...Exeltech...Cotek. I have burned up many in my service truck including some made by well respected American manufacturers using power tools in a mobile work shop....Econoline E-350 with multiple power sources....dual alternators 130 amp at 14.4 volt and 160 amp at 28.8 , 4kw 120 volt gen set Onan Emerald 4.0 these forums as well as a few others that I frequent are a true treasure trove of real time user expereince, not manufacturers dreams of what their systems will do....some are so wildly optomystic that it could not possibly do that. Just read an advertisement from Power Jack about their 12 volt 12,000 watt inverter.....laughable.....but people still buy that crap then want someone to make it do that. Real user expereince is worth much more than all the krap coming out of the Madison Avenue ad men. Anything to sell the krap, get it out the door......$$$$

    I am still wearing the dunce cap....I try to get it right.....I have never had a system failure in what I do commercially, my designs all incorporate a safety factor, maybe too much safety overbuild......naw!

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • DaangeroussDanDaangeroussDan Registered Users Posts: 23 ✭✭

    I only use one time fast acting fuses coming out of my treefort solar arrays into the charge controller and from the controller to the battery bank. Breakers are too slow and must get hot prior to tripping, fuses are instant and stand a better chance of saving gear. These fuse boxes come with a disconnect (pullout) so shutting the system down is a snap, as I am always working on it and adding new elements and ideas in my quest for energy independence.

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #12

    Proper solar breakers are not thermal..........they are magnetic/hydraulic They do not consider temperature

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • littleharbor2littleharbor2 Solar Expert Posts: 1,654 ✭✭✭✭
    edited April 2019 #13

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 540 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

    Very refreshing story David! A lot of us could relate, we often try to get “ affordable ” equipment to later realize we could have saved ourselves some money by getting the right and dependable equipment, usually more expensive, in the first place. How often one buys an affordable piece of equipment to find out, it can’t really do what it was bought for, or craps out prematurely etc...Then they go out and buy another one a bit more expensive to get the same result. Finally they get wise, and buy the equipment they should have purchased in the first place - Guilty as charge.

    There could be plenty reason why one might not purchase the “ right” equipment, and settle for something that may be less dependable. Recently I purchased an eight hundred dollars inverter knowing I should have spent double that money to get a more suitable and robust unit. The problem is I must bring an inverter with me during my next trip, and the more suitable inverter would have put me over the allowable baggage weight. Therefore I decided to get “second best” which I could use as a backup after I get a chance to ship the heavier unit and install it when I get a chance to go back.

    I know I am preaching to the choir here, more expensive doesn’t necessarily mean better. When it comes to RE equipment some reputable manufacturer’s name will always be the first on the list, and for good reason, they have a good track record in this industry.

    Hats off to Midnite Solar! They have done a remarkable job when it comes to Balance of System, and their product list is hors pair. As a bonus,our gracious host carries the majority of their equipments.

    Btw David, if you insist on wearing that silly dunce cap, although “ I forbade you ”, you must post a picture. 🤣

    harry

    


    48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭

    Their latest 12,000 watt 12 volt inverter looks like they sawed the bottom off the earlier 12kw 12 volt inverter anyone need a boat anchor!

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • VicVic Solar Expert Posts: 3,136 ✭✭✭✭
    edited April 2019 #16

    Hi Ryan,

    Thank you for being there  --  at MidNite  --   AND here !

    Thanks for the Link to the CBI MNEPV breakers.  It looks like it has been updated a bit since the last time we saw it.

    And,   Dave,   would not take this too seriously.   We all have made tiny,   little misstatements.    Don't worry about it !

    Good Luck,   Harry with your Haiti project.      Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭

    Harry,

    Not to worry, some newby stole it from me. LOL

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #18

    Vic,

    i missed your comment earlier, thanks I try but sometimes data gets mixed up

    CBI did revise the breaker a few years ago. They used to have different clamping screws, the new ones are much better, I can buy the driver for them, Klein makes it.

    david

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

    Thank you all for your contribution. Now let's talk about the inverter

    The inverter is the 2.5K Inverter Cotek SD-2500-148 and will be installed as followed:

    • Pos battery post - inline fuse near battery - MNDCxx breaker in MidNite MNDC enclosure - inverter - approximate cable length 5 ft
    • Neg battery post - 500A, 50 MV DC Shunt - inverter.
    • Cable will start in free air than in a conduit connected to the DC enclosure. From the DC enclosure the cable will be in a conduit until they get near the inverter's posts.

    What would be the biggest and safest combination choice for inline fuse, breaker and cable (MTW /THW Cobra-Flex)? I was looking at the following 3 breakers MNEDC90, MNEDC100 and MNDC125, and I can always create an adapter to make either one of them fit in the MNDC enclosure. I assume the ampacity of the breaker inside the DC enclosure will determine the size of the other elements.

    I know the breaker/fuse primary goal is to protect the wire, and we still we have to keep the inverter in mind.

    The Cotek manual recommends a 90 amp inline fuse and 4 AWG cable < 6ft. It also mentioned bigger cable can be used for longer distance to avoid reducing the inverter performance. I have attached a screen-print of the specs that can be found in the manual.

    I have posted the same question in other forums, and I not sure I got a definite answer.

    I would prefer to use bigger cable than 4AWG , if possible. The inveter's post can take cable lugs of size 2/0 at least. As a matter of fact, 2 compression lugs of 2/0 came inside the inverter box.

    I also looked at the NEC chart , and I am not sure if my wiring will be considered as in free air or in conduit.

    Thank you kindly,

    harry


    48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #20

    4 AWG is way too small 2/0 is more in line with American inverter standards. The voltage drop is more critical at lower voltages and larger gauge wire is used to compensate. The inverter is fused internally at 90 amps. The correct breaker would be the MNDC125RT . I would not use the Bussman circuit breaker at 48 volt system, maximum charging voltage 64 volts or so.......pushing the limits of the Bussman too far. One point that is often missed with inverter installations is that inverters switch the power input quite rapidly setting up reactive relationship with the large capacitors in the inverter and the inductance of the wiring. One thing that will help with this issue is to keep the inverter positive and negative wires close, I tape them togather every 3 inches or so. Seems to help. Others have noted this. I have in my shop a new Cotek SD1500-124HW I am installing, and will be ordering a SD-2500-148HW for my next install for the same person at another location. I have manuals on everything I service. I use a Cotek SK-1500-124 for my nite time inverter. It has been in use 6 years now. Never had an issue with it. Accidently trying to start a Skil HD-77M Skillsaw shut it down but the inverter started back up with the power switch. That big a Skilsaw is quite an overload for that inverter. I smoked a larger ProSine doing that...

    on the conduit......in the NEC conduit is considered "sheathing" protecting the wiring if it runs less than 5 feet or so and does not terminate in a box at both ends. Wiring as such is considered "free air" .....unusual but in an older house often we can't get in the wall so we mount a switch box on the surface, run conduit up to just above the ceiling and use romex inside the "sheathing" In the code you cannot put romex in conduit but in this case it is "sheathing" same principle here for your battery cable.

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭

    Since there are internal fuses, 15A × 6 for 90A, a higher rating for the external OCP would be of no benefit, since it's purpose is to protect the conductors, it should be rated according to the smallest gauge used within said circuit. Since the current carrying capacity of the conductors proposed is far greater than the required maximum for the inverter, my inclination would be to size the OCP close to the 90A internal protection say 100-125A. This will minimize the potential short circuit energy dissapation in the event of a short circuit, or arc flash, that a higher value would allow.

    Regarding the free air conduit question, the rating would be the lowest rate, the in conduit value. Since the OCP would limit the current to far less than the conductors carrying capacity this should be of no concern.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #22

    Circuit breaker sizing takes into account of the NEC requirement that the circuit be able to carry 1.25 time the maximum current that the device consumes. In this case Cotek does not supply data on the current drawn. They only specify that the internal fusing is 6 X 15 amp fuses. (Quoting installation manual " these fuses are for reverse polarity protection") These fuses are automotive type ATO fuses clamped in holder and tightend down with screws and difficult to access. These fuses blow much quicker than a standard 90 amp line fuse. The external breaker is to provide convient access to turn off inverter power if needed. It is not a good idea to have to remove inverter cabling with power on to replace the in built fuses. The external breaker is to protect the wiring and provide the ability to depower the inverter if need be. The fact that a device has internal fusing does not waive the requirement for circuit protection. The code sets do not consider anything within the device. That is the job of the NRTL be it UL, ETL or any other NRTL. (Nationaly Recognized Testing Laboratory)

    with the assumption that the maximum rated current is 90 amps then 90 X 1.25 safety factor the correct size breaker would be 112.5 amps. The next standard size is 125 amps. The correct breaker would be the MNDC125 RT.

    On the issue of the temperature the "free air rating" would be used here as I stated in my previous post last paragraph. Please reread that paragraph for the definition of "protective sheathing" in the code.......it's quite clear that for this kind of installation "sheathing" is approiate since the conduit cannot be terminated to the inverter or the batteries.

    David

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • Sunny-HTISunny-HTI Registered Users Posts: 20 ✭✭

    Thank you David and mcgivor, very interesting read. Based on your feedback and other input I have received before, I think it's safe to connect the inverter to a MNEDC125RT breaker installed in the DC enclosure.

    What would be the optimal inverter cable size I can safely use? NEC310.15(B)(16) table calls for 115 A (# 2 AWG) and 130 A (#1 AWG) ; cable insulation is THW.

    I know some of you don't necessarily use an additional fuse near the battery, but since the battery is the source of power, and the primary role of a fuse is to protect the wire I would prefer to add a fuse the closest to the source possible. The size of that fuse will depend on the cable size chosen to accommodate the 125 Amp MNEDC125RT.

    A Marine Rated Battery Fuse (MRBF) is the closest you can get to a battery since it's mounted right on the post, however there have always been the dreaded question " Are they suitable for a 48V system with a max voltage rating of 58Vdc" ? There are a few posts on the subject in this forum, and BB makes some good points as always.Here is one of those posts: https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

    Absorption voltage for my battery bank is 58.8 V and EQ is 64.80, but I have read fuses operate from current, not voltage. I am very inclined to use the MRBF type as the inline fuse.

    For the record, as a do-it-yourselfer, I am solely responsible for what I do, and not what I read and have been told. I can only be grateful for all the advices received in the forums.

    Oh! don't forget we are still trying to determine cable and OCPD sizes :)

    Best Regards,

    harry


    48V Off-grid System - 1890W panel array (6 x 315) in 3S2P configuration, MNPV3 Combiner, MNDC Enclosure, Whiz Bang JR, Classic 150 CC, 2.5K Inverter Cotek SD-2500-148, 4 x 12v 204 AH FLA Trojan (SPRE 12 225)...

  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin

    Voltage ratings--You can twist them into almost anything you want... A 60 Volt (being equalized) battery and a 58 volt fuse--Put a dead short on the battery wiring, it will no longer be 60 volts, but probably 1/2 that or less. So--Fuse OK for application (?).

    I once asked a UL Engineer (that was testing our equipment for UL Registration) about voltage ratings... I forget if it was the 220 Volt rated fuse/breaker on a 240 (and really up to 264 VAC high line for utility power), or if it was a 48 volt breaker on a "48 volt volt" battery bus (that can go to 60 volts+) was appropriate or not for circuit protection. He just looked at me and then continued with his work.

    In engineering, there are some "magic number" rules of thumb... Sqrt(2), 2, 10...

    Starting with 10, anything that is 10x "bigger" (or >10x) than something else, the something else can be ignored (i.e.., a 10 amp load and 100 amp load, don't worry about the 10 amp load effect on the system).

    2 -- Anything that differs by 2x or less, they are "the same" and both have equal weight/effects that need to be worried about.

    Srt(2) -- When dealing with power and energy... Besides the usual P=I*V equations for power/heating/etc... There is P=I^2*R and P=V^2/R ... So, this is a variation of the 2x rule... 1.414x higher voltage or current (with constant R) squared is 2x more heat and needs to be addressed in primary engineering design tasks.

    Another example, for typical structural designs, the concrete/wood/etc. elements are typically 10x stronger than needed--And this is because there are typically flaws in materials that need to be "swamped" by the sheer bulk of the rest of the material to prevent failures.

    The point at which arcs are sustained is roughly 12 volts (does differ a little bit with specific metals, copper, tin, tungsten, etc.). So, anything under 12 volts, arcing is not an issue--And over 12 volts, you need to take arc failures into account (switches, fuses, breakers, broken connections, etc.). And that DC current sustains arcs much better than AC current (hence the different AC vs DC voltage and current ratings for switches, fuses, breakers).

    There are specific physical issues which make this "different" than "that". And there are lots of rules of thumbs that also help us sort the wheat from the chaff. Following code is a good way to focus on your questions/designs and assume that "the rest" has been properly addressed by others (the folks that made the copper wire to spec, made the raw insulation to spec, the folks that molded the insulation onto the copper wire bought the correct materials, followed defined processes to mate the two, and did proper testing to insure that it meets the requirements when shipped to the customer/end installer (the UL/NRTL independent auditors)).

    And with working with fuses/breaker/etc... They are rated to "open" based on current... However, their ability to safely "interrupt" current flow is dependent on voltage/current/polarity/orientation/and frequency (AC/DC/etc.). The device must be able to "manage" the energy in the arc that is created with the opening of the circuit. An improperly specified fuse/breaker is worse than useless, it can a source of fire:

    So, using a 58 volt rated fuse on a 48 volt battery bank (that can range from 42 volts to 62+ VDC), I would not lose sleep over it.

    As always, check the specifications:

    http://www1.cooperbussmann.com/pdf/85bc5172-fdf2-45df-a501-8cf9d8e13efc.pdf

    And install them correctly (you need an insulating washer under the nut to make current pass through the fuse, and not bypass via the stud).

    If you are very concerned because you have a large AH battery bank with lots of heavy cabling (remember, cabling has resistance, and large AWG cables have very low resistance, so the maximum short circuit current will be higher--And another reason to not "thow copper" at your wiring--The copper resistance can limit current and allow you to use a less expensive over current device) (the MBR fuse is rated for different AIC/AIR maximum currents from 10,000 amps @ 14 volts to 2,000 amps @ 58 volts), then you should look other fuses (NAWS, our host, sells Class "T" fuses and holders):

    https://www.littelfuse.com/~/media/electrical/datasheets/fuses/industrial-and-ul-fuses/littelfuse_fuse_jlln_jlls_datasheet.pdf

    Interrupting Ratings

    AC: 200 kA rms symmetrical

    DC: 50kA (1 - 30A) 20kA (35 - 1200A)

    https://www.solar-electric.com/search/?q=300+amp+fuse

    And look at breakers too... Good quality breakers are not much more expensive than high power fuse+holder+spare fuse--Plus breakers can also be used as on on/off switch for servicing your equipment too and just turning off when not being used.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin

    I should add--My suggestions on a 58 volt rated fuse for a 48 volt battery bank are my own. They do not represent the opinions/suggestions of our host NAWS. And you are the person that will make the final design choice(s) and do the installation. Not me.

    -Bill "me" B.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #26

    Harry,

    EDITED to clarify

    I have advised you to use 2/0 cable for this inverter in this application. Cotek is hinting by supplying 2/0 terminals. Based on what American manufacturers (MagnaSine and Outback) recommend for their inverters drawing this much power you should be using 2/0 . The code is really written based on utility power, not based on lower voltages seen in solar systems. The code does not say that you cannot use larger wire there, rather it says the minimum size of the wire is based on the amperage of the circuit. The code (NEC) considers electronic gear to be "black boxes" , does not care what is in the "black box" just what goes in and comes out. Within the "black box" is under the authority of the testing agency the UL , ETL, or any other NRTL (Nationaly Recogined Testing Agency) .

    I did a calculation based on the wire size recommended by Outback and MagnaSine and found the value of .018-.019 amps ............Edit, I meant to say watts, not amps, End Edit.....per circular mil of copper. I posted a chart of what the biggest inverter manufacturers use for comparison. I just looked around for it and can't find it. It was probably on MidNite forum.......I use the same online name at every website that I frequent.....I am a MidNite fanatic and you will see many posts there. I can be found at other forums as well, not just solar! If I can't find a defenitive answer I look at the biggest/best manufacturers are doing. All these companies have engineering staff to do these calculations. I am assuming that their engineers did the engineering analysis and "borrow" their expertise without asking, I analyze what the proven manufacturers have done to find the answer for what the other manufacturers have not stated. That is where I get the cable size 2/0. Cotek is not clear on the recommended cable size, only the minimum size acceptable to the inspectors.

    Please feel free to call any of the sales staff at NAWS they share their expereince freely. On the issue of the fuse at the battery.........Some fuse styles will not protect at too high a voltage....they will arc current over the blown part of the fuse and not provide a safe shutdown. Marine fusing as supplied by West Marine (Bussman) does not meet the somewhat higher voltages encountered in 48 volt inverters. Modern Marine use is 12/24 volts some older commercial fishing boats were 36 volts.......that has pretty much disappeared here but it can still be found......NAWS does sell inverter fuses and fuse blocks. One big fail mode is way more expensive than a big fuse/fuse block, cheap insurance!

    david

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin

    David,

    A 2/0 cable for a 70 amp continuous load? Sounds like way overkill (unless we are talking about a very long cable run):

    • 2,500 Watts * 1/0.85 AC inverter typical eff * 1/42.0 battery cutoff = 70 Amps maximum continuous current
    • 70 amps * 1.25 NEC derating = 87.5 amps suggested branch wiring and breaker/fuse rating

    If using 90C rated insulation, 4 AWG is certainly reasonable. And 2 AWG would not be a bad choice either for lower temperature rated insulation...

    The rule of thumb of .018-.019 amps per circular mil seems to be more the fusing current of a copper conductor rather than the estimated current capacity:

    https://www.ihiconnectors.com/AWG%20wire%20sizes.htm

    4 AWG ~ 41735 cmils

    • 0.018 amps per cmil * 41,735 cmil = 751 Amps

    https://www.powerstream.com/wire-fusing-currents.htm

    For 4 AWG, the fusing current for copper wire is (very roughly) 945 amps.

    I am a bit confused.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,513 ✭✭✭✭✭

    > And install them correctly (you need an insulating washer under the nut to make current pass through the fuse, and not bypass via the stud).


    Not always. The Blue Seas MRBF

    https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A

    • Isolated stud design uses standard M8 hardware and permits stacking of terminals


    has an insulated stud on a pedestal that needs no fiber washer. They do not supply a washer, or state that one must be used.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • TecnodaveTecnodave Registered Users Posts: 437 ✭✭✭✭
    edited April 2019 #29

    BB,

    I will find that calculation and repost it here, pretty sure it's on MidNite forum and I was discussing the issue with Classic Crazy.

    It is not based on the code, what MagnaSine and Outback are recommending for the MS4024 and the VFX3524<minor edit

    It is based on the wattage of the unit not the amperage

    OK I see the confusion.......I did write amperage, not wattage. My bad......well I cannot copy from there to here

    midnite forum post name : "Help for configuring a 48 volt system in Haiti. Reply #10

    And I did make a huge mistake in that post, Larry and I were comparing the manufacturers recommendation for the cable size Larry and I both have 24 volt systems and I have a Cotek SK-1500-124 inverter as my nite inverter,

    somehow I forgot that Harry's system is 48 volt.......I was more focused on communicating with Larry at the time so indeed 2/0 at 48 volts is too large and not needed

    It is interesting to note that Cotek does include 2/0 terminals with their inverters, all of my installs came with 2/0. I have not yet ordered the SD-2500-148HW that I will be installing next for the same customer that I am installing the SD-1500-124HW for

    I think that I will defer to your judgement on the wire size. Most of what I do is tiny house super energy efficient. My thought is that families need 48 volt systems to adequately power their needs, especially if they are accustomed to utility power. Single users can be taught about energy efficiency..........families cannot!

    David

    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin
    edited April 2019 #30

    Thank you Mike... I had never seen the the fiber washer until I looked up the MRBF rated fuses and looked at the glossy (link in previous post). I missed the insulated stud--And I am relieved that it is on the BlueSea mounts (at least). I was afraid that a major fact was missing in my recommending those fuse blocks.

    David,

    The 2,500 Watt inverter Harry will be using is a 48 volt device... So, it does draw much less current than a 12 (or 24 volt) version (which would necessitate larger DC Conductors). [Very sorry to be repetitive David, I see you did go back and find this is a 48 volt device. I missed that sentence in your above post. -BB]

    I am very happy to see the dramatic increase in low wattage high (DC input) volt inverters out there... It makes it easier to have smaller AC inverters to power most of the smaller loads in a cabin (LED lighting, computer, cell phone chargers, etc.) and only power on the high wattage inverters during the day (running shop, pumping water to cistern, running washing machine, etc.).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,331 admin

    David,

    Do not feel like you have to drop back on wire sizing calculations... I am certainly skipping over a whole bunch of code stuff (conduit fill, ambient temperatures, wet locations, etc.).

    I try to error on the conservative side a bit... And avoid the page of dense type going through the NEC manual and their tables to specify the "exact" wire type/gauge/etc. for the application (it has been decades since I had to do that to any minor degree).

    I usually "oversize" the cabling (voltage drop, 1.25 NEC continuous derating), which covers "over" of of the details that the NEC defines for "optimum" or minimum requirements--And may or may not apply in these applications.

    And mixing/missing information in posts/threads... Join the club. I am the poster boy (man, person, mxn, ?) for that. 😜

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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