Cable size and fuses

I finally bought all the equipment (almost): 6 Kyocera 130 (junction box); 1 MX60; 1 Exceltech 1100 (24 volts); and 4 GC2 batteries (6volts, 216 amps).

I need some advice regarding cable size, fuses and anything else I may need to put everything together.

Distance from panels to MX60: 45 feet
Distance from MX60 to batteries: 6 feet
Distance from batteries to inverter: 6 feet
Distance from inverter to small sub-panel (circuit breakers): 50 feet

Is it better to connect the Kyoceras in series or series-parallel?

I would REALLY APPRECIATE any and every advice.

Comments

  • westbranchwestbranch Solar Expert Posts: 4,858 ✭✭✭✭
    Re: Cable size and fuses

    as a starting point have you looked at the 'sticky' at the top of the 'General' section to assess your starting point regarding line size in a 24 v setup?

    Cheers
    Eric.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Eric:

    Yes, I did. But - forgive my ignorance - I got no specific answers.
    Maybe I am not as smart as I thought.

    Thanks anyway.
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    Do you have an NEC (National Electric Code book)? If not, it is probably worth getting one or photo copying a few of the tables out of it as there are several issues when deciding minimum wiring sizes and types...

    First, if you use the Voltage Calculator in the sticky mentioned, you can determine the "optimal" size of copper wiring (AWG) you would want for your installation... Also, you need to read the manual(s) for any equipment you are connecting

    For example:

    6x Kyocera 130w panels. (Kyocera KC130TM???)
    Max Power Voltage - Vmp 17.6 Vdc
    Open Circuit Voltage - Voc 21.9 Vdc
    Voltage Temp Coeff - Vtoc -0.0821 V/°C
    STC Rating - Pmp 130 Wstcdc
    Max Power Current - Imp 7.39 Adc
    PTC Rating 115.2 Wptc

    Assuming 14F-95F operational range:
    Voc=24.75 volts (at 14F)
    Vmp=13.9055 (at 95F)

    24 volt battery bank (assume about 31 volts max required for equalization)

    Assume Current Model MX 60:

    140 volts maximum input (running)
    150 volts never to exceed
    2 volts or more over max battery bank requirement (33 volts minimum)

    Need at least 3 Kyocera panels in series (41.7 volts minimum)
    Max of 6 Kyocera panels in series (148.75 volts maximum--any more and will fry MX 60))

    Given the above specs, you should probably pick 3 series x 2 strings in parallel--but I will also show 6 x 1 string for reasons below (if you are in a warm climate, 6x1 may be OK).

    Now, need to figure out wire gauge. 45 feet x 2 (+/- wire runs), 7.39 amps per string:

    3x2 panels => 2x7.39amps=14.78 amps -- from default settings on spread sheet, 90 feet and 41.7 volts for panels. 8 awg wire gives a 2.2% (about 1 volt) drop.

    For an example, now lets try the 6x1 panel setup (very close to being too high of voltage for your MX 60 controller--will depend on your local weather conditions... 7.39 amps, 83.4 volts: Now you can run 12 AWG wire and get 2.8% drop (about 2.4 volts)--by using much less copper wire.

    Now, you have to go to the NEC and find the correct wire (buried, conduit, conduit fill ratio, insulation type/rating, UV exposure (or not), etc.). I don't have an NEC book--so I can't help you here. (remember, that you need to multiply the current by 1.25 as a safety margin when looking for the appropriate size/rated wire for NEC).

    Also, you will have to look up the documentation for your Kyocera Panels and find out the series fuse rating--Normally the fuse is not required for a single or 2 parallel strings. But for more strings in parallel, you will have to protect your panels (in case they short circuit) with a fuse/breaker in your combiner box.

    Generally, the MX 60 manual should have a good explaination of how to connect all of this together--plus the www.outbackpower.com website has example wiring diagrams for many different situations.

    I am sure that I probably missed something and somebody here will correct me--but this is basically what you have to do for every leg of your system.

    And remember the fuses/breakers are to protect the wiring--not the devices themselves (but read the manuals to make sure). You have to size the wiring/fuses/breakers for 1.25 the maximum expected load (usually at low battery voltage).

    However, you can design it for your needs... For example you may have a 3kW inverter but you will only have to power 1.5 kW of load--so you don't have to wire for 3kW (breaker/wire) if you are not using that load (but remember to divide by the inverter efficiency, multiple by 1.25 NEC factor, and use I=P/V where V=battery bank low voltage--~21 volts in your setup--to calculate maximum current--and watch for starting currents on well pumps and such).

    I have to go right now--hope this helps.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses

    Amilkar,

    In addition to buying/borrowing a copy of the NEC, I recommend you download John Wiles' "Photovoltaic Power Systems and the 2005 National Electrical Code: Suggested Practices". His document focuses on the requirements of NEC Section 690, which deals specifically with PV equipment.

    Appendix E, which can be downloaded separately, walks through the calculations for a variety of system configurations.
    Appendix I, "Selecting Overcurrent Devices and Conductors in PV Systems", will be especially useful.

    See: http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/PVnecSugPract.html

    Wiring the six KC-130 modules in series ("72 V") is generally not advisable. The MX60's operational input voltage limit is 140 VDC. The STC Voc of six KC-130's in series is 21.9 V x 6 = 131.4 V. Accordingly, the MX60 would suspend operation and disconnect the array if the ambient temperature fell below ~10 C / 50 F. If the ambient temperature fell to ~-10 C / 14 F, the MX60 could be damaged and your warranty put at risk.

    I therefore assume you plan to configure your array as a "36 V, 3 x 2" system, with three modules wire in a series string (36 V nominal), and two parallel series strings. If so, you'll need to determine if you will use one circuit breaker or two between the array output and the controller input.

    More later,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Bill; Crewzer:

    THANKS!

    Your help is greatly appreciated.

    Nevertheless, I have to ask: I think both of you prefer the "two x 36 volts configuration". Any particular reason? Is 2 x 36 "better" than just one string of 72 nominal volts?

    Second: The system will be installed in Puerto Rico (In fact, the panels are ALREADY INSTALLED, but obviously not connected; my house is built completely of concrete).
    Temperatures in my town (Aguada) in Puerto Rico never fell below 75 or 80 degrees.

    Third: I know I will need breakers or fuses, but I do not know which one is better nor what fuse or breaker to use (30 amps? 45 amps?)

    Well, thanks for your patience, and please, again, forgive my poor english.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses
    Temperatures in my town (Aguada) in Puerto Rico never fell below 75 or 80 degrees.
    Even first thing in the morning?
    Nevertheless, I have to ask: I think both of you prefer the "two x 36 volts configuration". Any particular reason? Is 2 x 36 "better" than just one string of 72 nominal volts?
    Two reasons: One is related to the ambient temperature and high Voc issue discussed above. The second reason is that the MX60's operating efficiency drops as the input voltage increases. With the array generating ~1,000 W to your "24 V" system, I'd estimate the "high voltage" efficiency loss to be ~1-1/2%.

    But, you may be able to make up part of this loss by running one "large" wire pair between the array and the controller instead of two "small" pairs.

    More on conductors and circuit breakers later.

    Jim / crewzer
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    Glad to be of help...

    To answer a few of your questions... Just to be clear, the 2x36 is 2 parallel sets of 3 series modules is the minimum voltage at which the MX 60 can recharge your battery system (24 VDC)... Basically, the voltage of the solar panels has to be 2 volts (or more) higher than the battery bank's maximum voltage (generally during equalization--or "overcharging" to make sure all cells are fully charged--required for flooded cell type batteries--and you have to check the water every month or so as equalization converts water into hydrogen (and oxygen) gas--so it is normal for the batteries to "lose" a bit of water over time).

    You can use either 3 panels in series, 4, or 5 and meet all of the MX 60/24 VDC banks requirements. However since you have only 6 panels at the present time, you need to divide them into "even" numbers of panels per string... In this case, 3xseries in 2xparallel strings. If you had 8,9, or 10 (etc.)--you would have to choose other configurations.

    Whether 6 panels in series would work or not--it is close. You are on a tropical island, and if you had a light frost and bright early morning sun, you 6x1 panel string could reach ~144 volts DC. The MX 60 won't start over 140 VDC--but where you are building your home it will probably work (assuming never under 32F with the sun up).

    Jim brings up the issue that MPPT controllers (Maximum Power Point Tracking charge controllers, like the MX 60) are slightly less efficient the higher the input voltage from the solar cells (you can ignore the rest if the details are not of interest--not real important)...

    It is true, but I like to do a second check... Assume that we lose 1.5%
    power at 6x1--that is (6x130watts * 0.015) 11 watts or, at $5 a watt for solar panels, a $55 dollar loss in solar output.

    But, we need to compare the current bettween a 3x2 set of panels and a 6x1 set. And if you look at my analysis you will see that for your setup that difference is the requirement for 8 AWG wire or 12 AWG wire for the main solar panel run back to the MX 60 controller.

    Using Wind-Sun's wire pricing (because it is handy), 2 conductor 8 AWG is $1.60 per foot and 2 conductor 12 AWG is $0.64 per foot. The cost savings is 45'*$0.96=$43.20 for your wiring--pretty much a wash (loss in panel "value" vs loss power in MX 60 controller).

    Obviously, the longer the wire runs, the larger the price cost differences will be between a "high voltage" vs a "low voltage" run (you may also need larger conduit and other issues).

    Of course, it is always best to save energy everywhere you can in designing/building a solar panel system--once you lose the energy in wasted heat (inefficiencies), that will be lost every day for the rest of the system's life.

    Back to question of Breaker vs Fuses--In general, the weak point of any electrical system are the fuses and breakers. Good breakers are expensive, but nice to have if you need to break the circuit to work on something. Fuses (and good fuse holders too) are nice, small, and cheap, for example if you need series protection fuses out at the combiner boxes (fuses will probably never "blow" and you don't have to pull the fuse often to service an individual solar panel.

    Sizing of the fuses and breakers? Basically, they are there to protect the wiring. Remember, you have sized your (for example) 15 amp load/source * 1.25 = 18.75 amps... You round up to the next standard circuit size (20 amps in this case). You want to have larger breakers and wire than the actual load for safety and to prevent un-needed trips (breakers/fuses).

    For solar, in many cases, the wiring is designed to handle more load because we want low voltage drop (higher efficiency). So, for your 15 amp load, you are using 8 awg wire--which works for well over 15/20 amps--but you would use a 20 amp breaker just because you don't need the extra current (and may help protect the attached devices--MX 60, Panels, etc.).

    There is also one other issue with Breakers and Fuses, make sure that they are rated for protecting DC circuits at you rated voltage. DC is much harder for a breaker or fuse to stop in the event of a short circuit (AC is much easier--does not sustain an ARC across contacts like an equivalent DC voltage can).

    If you have a good electrician there--they can probably help you with some of the planning of the system wiring and installation.

    Hope that was clear.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses
    I finally bought all the equipment (almost): 6 Kyocera 130 (junction box); 1 MX60; 1 Exceltech 1100 (24 volts); and 4 GC2 batteries (6volts, 216 amps).

    I need some advice regarding cable size, fuses and anything else I may need to put everything together.

    Distance from panels to MX60: 45 feet
    Distance from MX60 to batteries: 6 feet
    Distance from batteries to inverter: 6 feet
    Distance from inverter to small sub-panel (circuit breakers): 50 feet
    OK… Here’s “a way” to do this per the NEC and Wiles' Appendix I, "Selecting Overcurrent Devices and Conductors in PV Systems":

    PV Source Circuit Calculations (wiring from PV modules to combiner box, plus circuit breakers)

    Step 1: Determine Maximum Current [690.8(A)(1)]:
    = Isc x 125%; 8.02 A Isc x 125% = 10.025 A

    Step 2: Determine Ampacity of Wire and Wire Size [690.8(B)(1)]:
    = maximum current x 125% = 10.025 A x 125% = 12.53 A.
    Free air ampacity of THHN / THWN-2 wire is 14 A; 14 A > 12.53 A, so OK (for now).

    Step 3: Determine Rating for Circuit Breaker [690.8(B)(1)]:
    DC PV array circuit breakers from OutBack and Midnite are rated for 100% continuous duty, so 690.8(B)(1) multiplier is not required per exception. Next standard size above 10.025 A is 11 A [690.9(C)]. The closest I’ve seen is a 12 A model from Midnite.

    Step 4: Derate Wire Ampacity for Temperature and Conduit Fill, and Determine Wire Size
    Assume maximum temperature at/near PV array is 70 C.
    Assume conduit fill is four wires (one pair for each array string)
    70 C temp derate factor THHN/THWN in conduit (</= three conductors) is 0.58 (Table 310.16)
    Conduit fill factor for four wires in a conduit is 80%

    Required nominal ampacity is 12.53 A / (0.58 x 80%) = 27.004 A. Min THHN/THWN-2 wire size required is 12 AWG (30 A nominal ampacity)

    Step 5: Overcurrent Device Protection Check
    Derated ampacity of 12 AWG wire at 70 C with four conductors in conduit is:
    30 A x 0.58 x 80% = 13.92 A. 13.92 A is larger than 12 A circuit breaker, so 12 AWH THHN/THWN-2 is still OK.

    Step 6: Circuit Breaker Terminal Temperature Check. Circuit breaker terminal are rated for 75 C, but conductor >3 fill factor not required in combiner box (I'm not sure that I understand why 75 C wire ratings are use for this step when 90 C wire is used... there's some logic to it, but it fails me at the moment... :blush:).

    Nominal ampacity of 12 AWG wire rated for 75C is 25 A. Temp derate factor for 75 C wire at 70 C is 0.33. 25 A x 0.33 = 8.25 A, which is < 12.53 A required, so FAIL.

    Nominal ampacity of 10 AWG wire rated for 75C is 35 A. Temp derate factor for 75 C wire at 70 C is 0.33. 35 A x 0.33 = 11.55 A, which is < 12.53 A required, so FAIL.

    Nominal ampacity of 8 AWG wire rated for 75C is 50 A. Temp derate factor for 75 C wire at 70 C is 0.33. 50 A x 0.33 = 16.5 A, which is >/= 12.53 A required, so OK.

    So, using the data and assumptions above, this process says that you’ll need #8 AWG THHN/THWN-2 wire from the PV array to the combiner box. Calculating the voltage drop in this wiring requires an additional step.

    More later… I’m tired!
    Jim / crewzer
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    The above by Jim is why I hate code checks and NEC. It was bad enough to do it once when designing a new computer system--but to do it for every solar system install--ugh...

    You are a better man than I, Jim.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • morrisrvmorrisrv Registered Users Posts: 13
    Re: Cable size and fuses

    Crewzer:
    very nice explanation, you put in some time on that! To maybe help explain your statement in step 6. Most all of electrical equipment terminations are UL tested with wire size based on 75c. In other words, the wire would be larger during the test than if you used 90c wire with the higher allowable ampacity, thus higher breaker size. The equipment terminal is the "weak link", wire insulation technology has progressed over the years but equip. UL testing is not rated for 90c Wow I hope that is clear and not more confusing!!
    Bob
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses

    Thanks, Bill... I dunno about better, but crazier, maybe... ;)

    Bob: That was the "logic" that I forgot... thanks! :cool:

    Yeah, that exercise is a mess... and, just think: That's just to get from the array to the combiner box! :grr

    Whew!! :cry:
    Jim / crewzer
  • VolcanoSolarVolcanoSolar Solar Expert Posts: 56
    Re: Cable size and fuses

    I am also wiring my arrays at 72 volts, with an MX60, and live in a warm climate (Hawaii). According to the Outback MX60 manual, this is okay to do, because the lack of cold means there won't be the damaging high-output spikes one gets in a cold climate.

    In my particular configuration -- 70' distance (140' of wire), 8 amps, 72 volts (Three 24-volt Kyocera 170GTs in series)-- I'm using 10-gauge wire. The PV Voltage Drop Calculator I downloaded here (Thanks!! Very useful!) shows that I could even use 12 gauge wire and still remain under the goal of maximum 3% loss.



    Nevertheless, I have to ask: I think both of you prefer the "two x 36 volts configuration". Any particular reason? Is 2 x 36 "better" than just one string of 72 nominal volts?

    Second: The system will be installed in Puerto Rico (In fact, the panels are ALREADY INSTALLED, but obviously not connected; my house is built completely of concrete).
    Temperatures in my town (Aguada) in Puerto Rico never fell below 75 or 80 degrees.
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    I would question the need for derating of the 75C breaker wire/terminals in a normal solar installation...

    I can see 90C rated wires/environment for exposed wires/wires run in conduit/box on a hot roof or exposed wall...

    However, if this is for a breaker installed next to a MX 60 inside a building or shaded lean-to, then it would seem that the whole exercise for terminal rating would be 100% at 75C (doubt that the MX-60 is rated for a 90C environment).

    How does the NEC address the difference between a 90C and a 75C environment? (don't have a need for an NEC book anymore--trying to avoid buying one for typing on this forum).

    Cascading requirements (like Jim's post) was another reason I avoided using breakers/fuses/terminals/gas discharge tubes/etc. all over my design. Made my regulatory engineers/UL/CSA/etc. prove that a protective device was needed (other than as just a handy power off switch for the system).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VolcanoSolarVolcanoSolar Solar Expert Posts: 56
    Re: Cable size and fuses

    To get familiar with the Excel wiring-size calculator (downloaded from the sticky note, above), try using it for one simple calculation first:
    - input on the blue numbers (these are your numbers):
    - Array voltage: 72
    - Battery bank voltage: 24
    - String 1: Feet: 90 (45 feet x two wires)
    - Panel Amps: 7.39

    Now play with different wire sizes and look at the results: your goal is to find the smallest wire you can use that will still keep the "Voltage drop" cell (on the right) under 3.0%.
    - For example: input "12" on the wire size: it shows 1.64%.
    - try "14" on wire size: it shows 2.6% -- still good
    - try "16" on wire size: it shows 4.17% -- no good.

    So, according to the calculator, you could use 14-gauge wire and be fine.

    Or, you could try series-parallel wiring the entire six panels, so that they are set up for 72 volts and now double the amps, 14.78 amps:
    - wire size "12" -- 3.27% -- no good
    - wire size "10" -- 2.06% -- this will work.

    The advantage of this is half as much wiring.


    Question for the group: why does Outback specify a minimum 8-gauge wire in the manual? And why a maximum 1.5% voltage drop? Obviously lower is better, in terms of efficiency -- is there an agreed-upon number in the industry?

    In my own particular configuration, 10-gauge wire puts me right around 1.5% voltage drop, so I'm especially curious about their "minimum 8-gauge." I'm not cheap -- I'll go with 8-gauge if needed -- just curious as to why.






    Amilkar wrote: »
    I finally bought all the equipment (almost): 6 Kyocera 130 (junction box); 1 MX60; 1 Exceltech 1100 (24 volts); and 4 GC2 batteries (6volts, 216 amps).

    I need some advice regarding cable size, fuses and anything else I may need to put everything together.

    Distance from panels to MX60: 45 feet
    Distance from MX60 to batteries: 6 feet
    Distance from batteries to inverter: 6 feet
    Distance from inverter to small sub-panel (circuit breakers): 50 feet

    Is it better to connect the Kyoceras in series or series-parallel?

    I would REALLY APPRECIATE any and every advice.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses

    Bill,

    My head is swimming… My quickie assessment, as I really haven’t studied this is great detail:

    Nominal wire ampacity specs are based free air (no conduit), 30 C ambient, and temperature rating of the insulation (NEC Table 310.17). Enclose the wire in a conduit, increase the ambient temperature, and/or lower the temperature rating of the insulation, and effective ampacity is reduced. [i.e., Tables 310.16, 310.15(B)(2)(a)].

    It’s possible that PV array circuit breakers could operate in a fairly hot environment in a PV combiner box. Both OutBack and Midnite Solar sell PV combiner boxes that are rated for outdoor use. I could see some combination of ambient temperature, direct sunlight, and wiring/terminal loss pushing a box and its innards to 60 C and beyond.

    But, you bring up a very good point nonetheless. If I get around to the next exercise (PV output circuits from the combiner box to the controller), I plan to use 40 C for the maximum temperature.

    I’m still woozie…

    Regards,
    Jim / crewzer

    P.S. I have a spare copy of the 2005 NEC… hint, hint, nudge, nudge…
  • morrisrvmorrisrv Registered Users Posts: 13
    Re: Cable size and fuses

    Well at great risk, let me add something. About the temp issue. Those three col. in table 310.16 60c 75c and 90c, they are not representative of allowed installation envirnment. They are MAX. temp. rating of insulation of conductor types. We generally always want to use a wire insul. from the 90c col. then when it comes to correction factors, ie, 4 or more cond. and ambient temp. above 86F you correct from the 90c col. ampacity of that particular wire size. Also, just because, say a #3 RHW-2 cond. is good for 110A, it cannot be protected by a 125A breaker(240.4(B) next higher device is allowed) you must go to 75c col. to decide overcurrent prtection, 100A.Again because the equip. is rated 65/75c. and used those wire sizes to get that rating. It is the wire insulation type that requires derating, not the equipment.
    As an electrician, generally we don't use table 310.17 as our conductors are always in some form of raceway or cable.
    Does this make sense? I hope so!!!
    Bob
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses
    crewzer wrote: »
    Bill,
    ...
    Regards,
    Jim / crewzer

    P.S. I have a spare copy of the 2005 NEC… hint, hint, nudge, nudge…

    Jim,

    That is very kind and generous of you... Hmmm--Now I have to think if I want to interpret the NEC as a hobby when I "hated" being paid to do that at work (hate is probably too strong of word--but balancing multiple requirements and their impact on the design sure was probably the equivalent of a good chess game--at least this NEC/NRTL/IEC/etc. requirements had a real/physical outcome--something that is missing, for me, in board games).

    One of the issues that I had to always go back to was trying to determine, for this example, if an outdoor box was rated at, say 40C ambient and it included the safety factors for temperature rise due to internal power and full sun exposure--or if I had to factor those other influences into the design requirements.

    Frequently, I was going back and asking the question if I was getting multiple levels of "safety" when only one level was really required. For example (don't remember if this exact issue was a problem--but I had issues like this): Using 90C wire in my system--I would have my safety engineers derating using those NEC tables, even though I could have used 75C/60C (or whatever lower time wire) rated wire and they would not have pushed the derating (my system was designed for 50C ambient). They where pushing the deratings just because I was using a higher temperature wire (because I could get the gauge/color/flame rating) as a stock item instead of special order the lower temp rated insulation.

    Jim, let me think about it. Again, thank you very much for the offer.

    Sincerely,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses

    Question for the group: why does Outback specify a minimum 8-gauge wire in the manual? And why a maximum 1.5% voltage drop? Obviously lower is better, in terms of efficiency -- is there an agreed-upon number in the industry?

    In my own particular configuration, 10-gauge wire puts me right around 1.5% voltage drop, so I'm especially curious about their "minimum 8-gauge." I'm not cheap -- I'll go with 8-gauge if needed -- just curious as to why.

    The MX60’s wire size chart only goes down to #8 wire. But, that’s not a “minimum specification”; that chart just doesn’t include #10…

    It's my view that the "1.5% voltage drop" target is intended to cost-effectively maximize the MPPT gain. The higher the input voltage at the MX’ PV terminals, the higher the output current from the charger.

    Remember to double the distance from the array to the controller when calculating voltage drop in the wiring, as there are two wires, one for (+) and one for (-).

    HTH,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Thanks again.
    Sorry for the delay (I do not have a computer in my home)

    Just curiosity: How did you come to Know so much?

    I begining to think this is much more difficult than what I first thought.

    Nevertheless, your advice is welcome...and very useful!

    Two more questions/comments:

    1. I will use #10 cable, just to be sure (I am not cheap, either!)
    2. NOBODY told me whether "fuses" are better than "breakers", nor the differences between them
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    Fuses and Breakers...

    Fuses are normally one-time devices that have a resistive element that is calibrated to burn open when a particular current is reached. They tend no to be too accurate and the actual time it takes to open with over current depends on the over current amount (higher current, faster open) and ambient temperatures (cold fuses take longer to blow than hot ones).

    Typically, they are cheap, but can be a pain to replace (open panels, trying to pry a glass tube out of a metal holder--that may be energized by solar panels/batteries/etc... Good to use where you don't expect problems or need a switch anyway to cycle power (breakers are many times used as on/off switches).

    Big issue that many people forget about fuses is that they are maximum voltage rated and DC voltages tend to arc more than AC voltages/currents. So the DC rating of switches/fuses/breakers tend to be much lower.

    Also, protective devices have maximum current limits too... A shorted solar panel is not going to put out that much more current that a non-shorted one... However, a shorted Lead Acid battery can output enough current that a normal automotive fuse can arc and cause more damage because of the high current.

    A circuit breaker--there are two main types. One is thermal--more or less like a fuse, the higher the current, the warmer they get, until they trip.

    And there are magnetic breakers--these will usually open more reliably and closer to the rated current as they measure that magnetic field (current) directly instead as heat.

    And for both Fuses and Circuit breakers, there are fast acting and slow acting type safety devices. Fast acting, just like the name says, as soon as there is an over current the device will open. But for some devices (like motors, well pumps, etc.), they draw high currents for a few seconds, and once running, the current drops to "normal levels". A time delay fuse/breaker can be used here to allow short term heavy currents and protect against longer term overloads.

    So, to your question:

    I would tend to use fuses in distribution boxes with solar panels (cheap--if using a lot of them, small, no reason to disconnect under load) and tend to use Circuit Breakers at the Battery/charge controller/inverter interfaces.

    Both fuses and breakers are a tad bit on the "unreliable" side--in that anything sensitive enough to break/open on over current cannot be overbuilt to not open. A breaker is much easier to "reset" and switch back on vs finding a bad fuse, (somewhere) pull, and replace.

    Neither fuses or breaker do well in temperature/humidity/salt spray/fog/etc. extremes.

    If I were balancing breaker/fuse costs vs adding more solar panels--I would tend towards fuses. If because of space/cost/size rating issues, I would tend towards fuses and adding a big old marine switch to switch power off when needed.

    If you want a nice, easy to service installation, and you have the $$$+space--an "all breaker" situation looks the best and is easier to service if/when something goes wrong.

    Help?
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Bill:

    Great!

    I will go with breakers!

    Thanks
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Cable size and fuses

    you may overlook the obvious that bill mentioned and that it must be dc rated as an ac rated breaker is NOT dc rated unless it is stated to be so. now qu and qou breakers are dc rated to i believe 48vdc, but are not the only breakers dc rated either.
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Niel:

    Thanks.

    Does that mean that an "AC breaker" WILL NOT work in a DC system? Even if AC breakers are rated for a larger voltage? Or will they do the job anyway?
    Is it absolutely necessary to use DC brakers, or just a recommendation?
  • GreenerPowerGreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Cable size and fuses
    Amilkar wrote: »
    Niel:

    Thanks.

    Does that mean that an "AC breaker" WILL NOT work in a DC system? Even if AC breakers are rated for a larger voltage? Or will they do the job anyway?
    Is it absolutely necessary to use DC brakers, or just a recommendation?
    NEC specifially requires breakers on DC side to be "DC rated". The AC breaker contacts might be fused together when using for DC. DC breakers are specially designed to avoid this.

    GP
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: Cable size and fuses

    Must be DC rated for safety.... Problem is that DC current sustains arcs (sparking across the switch contacts) much better than AC currents (which turn off and switch directions at 120x per second). DC approved breakers are approved at much lower voltages than AC or, for the same voltage ratings, much larger contacts with much more distance between contacts when off.

    (large/professional arc welders usually have a DC setting because of DC's several advantages when arc welding).

    And not all breakers/switches are rated for DC applications. Using the wrong breaker can result in welded contacts, melted breakers, and failing to trip open when protecting against shorted outputs.

    It will be listed on the breaker or with the data sheet the AC and DC ratings for each breaker.

    Purchasing breakers and panels from companies (like Wind-Sun) that specialize in Solar/Battery systems (especially if this is your first time) should be useful.

    There are a some other manufacturers you can look at (Xantrex, Outback, Midnight Solar) to see what they offer.

    Other people here can give you some more definitive links for specific solutions that they use or manufacture.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 7,281 ✭✭✭✭
    Re: Cable size and fuses
    Amilkar wrote: »
    Is it absolutely necessary to use DC brakers, or just a recommendation?


    AC breakers are NOT reliable under DC conditions, unless the breaker also caries a DC rating.
    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 ,

  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Greetings:

    O.K., DC breakers will be.

    Now, what rating?
    The MX-60 Manual recommends their OBDC 60 breaker, but since total current from pv array is just 15 amps (2 strings of three Kyoceras each), that seems not to be the right breaker. Shouldn't I use a 30 amp breaker instead?
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Cable size and fuses

    Amilkar,

    We haven't forgotten you...

    The ratings for the breaker between the PV array and the combiner box were discussed in my October 20 post:

    Step 3: Determine Rating for Circuit Breaker [690.8(B)(1)]:
    DC PV array circuit breakers from OutBack and Midnite are rated for 100% continuous duty, so 690.8(B)(1) multiplier is not required per exception. Next standard size above 10.025 A is 11 A [690.9(C)]. The closest I’ve seen is a 12 A model from Midnite.


    If you decide on a breaker between the combiner box and the controller, it'll need to be =/> 20.05 A. 20 A is probably close enough, but 25 A is the next size up. Again, 100% continuous duty DC breakers are available from OutBack and Midnite.

    You’ll also need breakers between the controller and the batteries, and between the batteries and your inverters.

    The typical output breaker from an MX60 is 60 A (or 63 A) continuous duty. This matches the MX60’s factory output spec. However, it’s highly unlikely that your array’s output current will ever exceed 40 A, even allowing for an edge of cloud event (brief burst of magnified sunshine resulting in brief output current surge). So, you may wish to set the MX60’s output current limit at 40 A and use a 40 A (continuous duty) DC breaker.

    The XP1100 inverter’s minimum operational voltage is 19 V, and it’s 87% efficient at full power. So, it’ll need a continuous duty circuit breaker rated at ((1100 W / 19 V) / 87%) = 66.5 A. I think that 70 A is the next standard size up.

    Check the XP1100’s manual (page 12) for cable size recommendations.

    A nice, tidy way to combine all of these items (breakers, inverter, etc.) might be to use an E-Panel “Lite” from Midnite Solar. See: http://www.midnitesolar.com/MidNite-Products.html

    HTH,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Cable size and fuses

    Crewzer, et. als.:

    Thanks again for all your help.
    I will be buying fuses and breakers today (at Arizona Wind and Sun, of course!)
    Though my system is a small one, my ignorance is (was) very big. Hence, I could not have put everything together without your help.

    I will inform you shortly about my experince with the system.

    Amilkar,
    Aguada, Puerto Rico

    P.S. By the way, my email is clopez@aguadilla.inter.edu, just in case anyone needs to contact me.
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