Oversized wire and breaker?

solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
I recently bought a 2000 watt 24v magnum inverter and in the manual it says: Maximum DC fuse size 150 amps. Would it be okay for me to use my current 2/0 cable and a 250 amp DC breaker from my previous 12v system? I don't want to buy a new breaker and new inverter cables if possible.

Comments

  • mcgivormcgivor Posts: 2,046Solar Expert ✭✭✭✭
    I recently bought a 2000 watt 24v magnum inverter and in the manual it says: Maximum DC fuse size 150 amps. Would it be okay for me to use my current 2/0 cable and a 250 amp DC breaker from my previous 12v system? I don't want to buy a new breaker and new inverter cables if possible.
    If the design calls for a 150A fuse, that is what should be used, a 150A  fuse in series with the breaker would allow the breaker to be used as a means of disconnect while still protecting the conductors, 2/0 is rated for ~200A, depending on insulation,  but the inverter may have a restriction to protect itself in the event of an internal fault, not that of the conductors, in short it's always best to follow manufacturers recommendations.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited April 24 #3
    So, just to clarify... Using my old 2/0 battery and inverter cables is fine, so long as I introduce a 150 amp fuse before the breaker? This would be much better than buying a new breaker and cables. Thanks
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    The 150a fuse should go as close to practical to the battery positive. It will protect the controller internal wiring. The breaker can go close to the controller - it should never trip (the fuse should open in a controller short), but can be handy as a disconnect.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited April 25 #5
    Can I use a breaker instead of a fuse? the cheapest 150 amp fuse block with fuse is $55 https://www.solar-electric.com/mrcb-150-amp-dc-circuit-breaker.html
    I found this breaker used on Ebay for $25. My inverter cables have 3/8 lugs on them and this breaker has 5/16 studs but thats the only problem that I see with it.

    Here is another breaker I found with 3/8 studs, it says 12v but in the description it says Max 48v
    https://www.ebay.com/itm/12V-DC-150-AMP-Circuit-Breaker-for-Race-Car-Trunk-Mount-Battery-Switch-3-8-Studs-/351998833465 Would either of these work? I would rather use a breaker because if it trips I can just reset it and wont have to buy a new fuse. I am open to suggestions on which breaker to buy if anyone has any.
  • westbranchwestbranch Posts: 5,083Solar Expert ✭✭✭✭
    "solar'' breakers are usually rated to 150V ...  MidNite etc. brands.  If you expect to change your system in the future you will be money ahead with a 150V model.   Remember if you series your panels you could be well above 50 V
     
    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
  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    The breaker/fuse is for an inverter right? The key spec to match here is that it has Class T interrupt ability.
    This is just another reason why I always use the exact product that the manual specifies.
     I do not want to be sued and I want my clients to be safe :)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    Alrighty then, it looks like I will go with the 150 amp fuse block. I'd rather spend more money to do it the right way and not cut any corners. Thanks for all your help everyone.
  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited April 26 #9
    I have one more question for you guys... In my inverters manual it reads: "In some installations, if the combined short-circuit current of all the batteries in the bank is determined to be 2,700 amps or less, then an ANL type of fuse may be used---if in doubt, use a class-t fuse." I'll be purchasing eight Costco insterstate golf cart batteries (2 strings of 4) rated at 215 AH.

    My question is: How do I figure the short circuit current of all my batteries? I don't really plan on going much bigger with my system, seeing as how I just doubled it (overall system voltage/ batteries and solar modules). I may go bigger with my next battery bank in a few years, seeing I have 1600 watts of solar modules going to just the 8 golf cart batteries but I will not be adding any more PV
  • mcgivormcgivor Posts: 2,046Solar Expert ✭✭✭✭
    The overcurrent protection is based on the conductors maximum  current carrying  capacity , not the short circuit amperage of the batteries themselves, the purpose of the protection is to prevent the conductors from becoming the fuse, the fuse/ breaker would  protect the conductors in the event of a short circuit.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    My understanding is the ANL fuse will open in a short on the order of 100s of milliseconds to a second, whereas the class T should open on the order of 10s of ms. The risk is the inverter wiring may arc/burn before the ANL opens.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Posts: 27,682Super Moderators admin
    Also, the AIC rating (Amperes Interrupting Current) means that a 2,700 AIC fuse will catch fire, arc, explode if more than 2,700 amp short circuit.

    For example, all house hold circuit breakers (in North America) have an AIC rating of 10,000 Amps. That is also the maximum current rating of a "typical" pole mount transformer and some resistance of the electrical drop from the pole to the house.

    Lead Acid batteries output huge amounts of short circuit current. If you have a large bank and heavy copper cables, then there coudl be high short circuit current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    Just curious - would cranking amps be a reasonable order of magnitude guess for SC amps? I know they aren't so rated, but presumably a pair of GC battery would very roughly equal a typical auto battery CA at ~800-1000CA?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mcgivormcgivor Posts: 2,046Solar Expert ✭✭✭✭
    Estragon said:
    Just curious - would cranking amps be a reasonable order of magnitude guess for SC amps? I know they aren't so rated, but presumably a pair of GC battery would very roughly equal a typical auto battery CA at ~800-1000CA?
    The cold cranking amp rating is a rating of the battery’s ability to supply the rated current for a given duration/temperature  under load. The short circuit amperage would be significantly higher, in the thousands of amps,  depending on the size of the battery itself  as a short circuit is not a load designed to be applied to the voltage of the battery, unlike a starter motor.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited May 3 #15
    Can I use Blue Sea Systems MRBF terminal fuses like this? https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A  --- Apparently Samlex uses these fuses in their 2000 watt inverter install kit http://www.samlexamerica.com/products/ProductDetail.aspx?pid=91... You cant get any closer to the battery than these and like you say...
    Estragon said:
    The 150a fuse should go as close to practical to the battery positive. It will protect the controller internal wiring. The breaker can go close to the controller - it should never trip (the fuse should open in a controller short), but can be handy as a disconnect.

    The following types of fuses are included in the Samlex Kit:
    Marine Rated Battery Fuse (MRBF Series) made by Cooper Bussmann
    - Voltage rating of max 58 VDC
    - Current ratings of 100A (MRBF-100), 200A (MRBF-200) and 300A (MRBF-300)
    - AIC of 10000A at 14VDC, 5000A at 32 VDC and 2000A at 58 VDC
    - Ignition protected as per SAE J1171
    - Weather Proof (IP66)


    Hopefully these work because I found 2 fuse blocks and 4 fuses for the price of 1 Class-T fuseblock/fuse. I'd love to save the money but I'd rather be safe.

    ***UPDATE*** I emailed Magnum, my inverter manufacturer and blue sea. Blue sea told me this fuse rarely blows unless a major problem has occured and I should go with the class T seeing my inverter manual recommends it. Magnum tech support told me the MRBF fuse WILL work with this inverter... Now I am totally confused. I think I will go with the Class T before the inverter and MRBF fuse on my second string of batteries.
  • MrM1MrM1 Posts: 206Registered Users ✭✭
    Looking at those MRBF terminal fuses,  they say they are rated for 58v.  So would they not work on a 48v battery as the charging and EQ voltage would be higher?   

    Also,  if I have a Midnite Solar 250 amp breaker (24v system) on the inverter side,  Do I need a fuse on the battery side?  And would it be of benefit to down size the fuse to 175amps?  I am using 4/0 cable. between battery and inverter. 

    And one other issue,  the MRBF terminal fuses blocks say they connect to a 3/8's battery terminal stud,   but Trojan studs are 5/16.  Do I need an adapter? Or can one bolt the 3/8 ring to a 5/16 stud with no issue?

    REC TwinPeak 2 285W 3S/3P 2.6kW-STC / 1.9kW-NMOT Array
    MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware
    Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system
    Grid-Assist Off-Grid / Need 3200Whs Daily Off-Grid
    System went Online Oct 2017
  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited May 3 #17
    Jerry from donrowe.com customer service answered this same question for me, so here it is from the email he sent me...
    "With all the info you should be fine with 3/8’ on 5/16” posts. I would suggest using a good 5/16” ID hole washer on each side of the cable flats to provide a bit more support and to prevent slippage. Several customers have reported doing that and it works fine for them."

    As for your other questions, maybe someone else with more knowledge will chime in.
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    You should generally install the size of breaker or fuse recommended by the inverter manufacturer. Too small may not handle non-fault current safely, and too large turns the inverter and/or wiring into a fuse.

    With properly sized breaker and wire, fusing the battery side would normally not be needed. There are exceptions though, for example if more than two parallel strings/batteries in the bank each parallel connection should be fused.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Posts: 27,682Super Moderators admin
    Class T fuses (NAWS sells class T, not sure if all are Little Fuse LNN"S" series or not), but here are product page and the basic data sheets:

    http://www.littelfuse.com/products/fuses/industrial-power-fuses/class-t-fuses/jlln.aspx
    http://www.littelfuse.com/~/media/electrical/datasheets/fuses/industrial-and-ul-fuses/littelfuse_fuse_jlln_jlls_datasheet.pdf
    Specifications
    JLLN
    Voltage Ratings AC: 300 V
    DC: 160 V (1 - 60 A)
    125 V (70 - 1200 A)
    Ampere Range 1 – 1200 A
    Interrupting Ratings AC: 200 kA rms symmetrical
    DC: 50kA (1 - 30A)
    20kA (35 - 1200A)
    Approvals AC: UL Standard 248-15, Class T
    UL Listed (File: E81895): 1 – 1200 A
    CSA Certified (File: LR29862): 1 – 600 A
    DC: UL Listed (File: E81895): 1 – 1200 A
    Material 1-30 A: Melamine body, Bronze caps
    35-1200 A: Melamine body, Copper caps
    Environmental RoHS Compliant

    JLLS
    Voltage Ratings AC: 600 V
    DC: 300 V
    Ampere Range 1 – 1200 A
    Interrupting Ratings AC: 200 kA rms symmetrical
    DC: 20kA
    Approvals AC: UL Standard 248-15, Class T
    UL Listed (File: E81895): 1 – 1200 A
    CSA Certified (File: LR29862): 1 – 600 A
    DC: Littelfuse self-certified
    Material 1-30 A: Melamine body, Copper caps
    35-60 A: Melamine body, Bronze caps
    70-1200 A: Melamine body, Copper caps
    Environmental RoHS Compliant

    Took a bit of searching, but found MRBF fuse specifications:

    http://www.cooperindustries.com/content/public/en/bussmann/transportation/products/circuit_protection/fuses/marine_rated_batteryfuses.resources.html (not much here)
    http://www.peerlesselectronics.com//files/MRBF-100.pdf
    Applications: Full range circuit protection for automotive and marine
    applications. Break in capacity meets the requirements of conventional
    vehicle batteris and 42V electrical networks
    Voltage Rating: 58Vdc Maximum
    Amperage Rating: 30A - 300A
    Ingress Protection: IP66
    Ignition Protected: Per SAEJ1117
    Interrupt Rating:
    10000 AMP @ 14Vdc
    5000 AMP @ 32Vdc
    2000 AMP @ 58Vdc
    Color Coded (see next page)
    Torque Rating: Maximum 12 N􀁴m (106 in-lbs)
    Material:
    Body - Ceramic
    Housing & Cover: UL-rated 94V0 Thermoplastic
    Ring Terminals - Tin Plated
    Compliances: ISO 8820-6

    Rating
    30A - 300A
    100%
    > 100 h

    135%
    Max 900 sec
    200%
    max.
    60s

    350%
    min. max.
    0.1s 1 s

    600%
    < 0.2s

    And here is a LittleFuse design document... Not a bad place to start if you have detailed questions about how to pick the "right fuse/breaker":

    http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf

    With fuses and breakers--You really need to go back to the actual manufacturer's specifications to figure out if any particular choice will work for your needs.

    And you may never get an "official" answer to questions like can I use an MRBF fuse (58 volt max rated) on a 48 volt battery bank (which can see 60+ volts). My unofficial answer would be (don't sue me for this) is yes because actual battery bank voltage on a shorted circuit would be less than ~58 volts.

    HOWEVER, if you have a large Lead Acid (or Li Ion) battery bank and heavy cables, the 2,000 Amp interrupt current may not be high enough... And you then need to look at the Class "T" type fuse with 20,000+ Amp IC rating.

    And to make the decision, you need to look at secondary questions. For example, if the DC leads are long enough (and not huge diameter copper), the resistance of the leads may limit the short circuit current (V=IR; I=V/R). For example, to get 2,000 amps at 58 volts:
    • 58 volts / 2,000 Amps = 0.029 Ohms

    If you have a 2 AWG cable, that is around 0.16 ohms per 1,000 feet or 0.00016 ohms per foot

    http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html

    • 0.029 Ohms / 0.00016 Ohms per foot (2 AWG) = 181 feet round trip (or 91 feet one way)
    It appears that for many of our DC off grid System Battery Bank designs, I would guess there is no enough resistance in the copper wiring to limit a short to 2,000 Amps on a fully charged 48 volt battery bank (and using 48 VDC nominal would reduce the one way wire run to ~75 feet for 2 kAmps maximum).

    To get a feel of what in depth short circuit calculations look like, here is one for AC systems (search for "Cooper Bussman's Point to Point Method of Short-Circuit Calculation" if link is not permanent):

    http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html

    Here is a DC short circuit estimate from Schneider:

    http://www2.schneider-electric.com/documents/electrical-distribution/en/shared/interactive-catalogue/lvped2080006en/pdfs/page_019.pdf

    Short-circuit currents
    DB105060
    Calculation of the short-circuit current across the terminals of
    a battery
    During a short-circuit, the battery discharges a current equal to
    Vb = maximum discharge voltage (battery 100 % charged)
    Ri = internal resistance equivalent to all cells (a function of the capacity in amperehours).

    Isc=Vb/Ri

    Example
    consider a set of four 500 Ah batteries connected in parallel
    discharge voltage of one battery: 240 V (110 cells 2.2 V each)
    discharge current of one battery: 300 A with a run-time of 30 minutes
    discharge current of all four batteries: 1200 A with a run-time of 30 minutes
    internal resistance 0.5 mW per cell, i.e. for one battery:
    Ri = 110 x 0.5 x 10-3 = 55 x 10-3 W
    short-circuit current of one battery: Isc = 240 V / 55 x 10-3 W = 4.37 kA
    neglecting the resistance of the connections, for all four batteries discharging the
    short-circuit current in parallel, the total short-circuit current is four times that of one
    battery, i.e. Isc = 4 x 4.37 kA = 17.5 kA.
    Note: if the internal resistance is not known, it is possible to use the following rough
    approximation: Isc = kc where c is the capacity of the battery in ampere-hours and k is a
    coefficient close to 10 and always less than 20.

    Other typical examples
    PABXs: Isc from 5 to 25 kA at 240 V DC with L/R = 5 ms
    submarine: Isc from 40 to 60 kA at 400 V DC with L/R = 5 ms.
    So, looking at a "typical" worst case Lead Acid Storage Battery (large AH cell) with ~0.5 mOhms per cell, a 48 volt @ 1,000 AH or more? battery bank would have:
    • 24 cells * 0.0005 Ohms per cell = 0.012 Ohms total (and could be 2-5x higher vary easily)

    Some numbers measured/estimated for a 100 AH lead acid cell:

    https://www.blueboxbatteries.co.uk/blog/battery-internal-resistance-short-circuit-current-47

    Image 1

    For standard/typical Lead Acid storage batteries, they tend to have higher internal resistance. For AGM/GEL/Li Ion they tend to have lower internal resistance. And remember that a Lead Acid battery can only out peak current for a few minutes from full charge, and the current quickly drops after that.

    Has anyone found/done "real life" 12/24/48 volt lead acid storage battery short circuit tests here?

    -Bill


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Posts: 206Registered Users ✭✭
    Community Gold.   Thanks so much.  ;) 
    REC TwinPeak 2 285W 3S/3P 2.6kW-STC / 1.9kW-NMOT Array
    MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware
    Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system
    Grid-Assist Off-Grid / Need 3200Whs Daily Off-Grid
    System went Online Oct 2017
  • Dave AngeliniDave Angelini Posts: 3,884Solar Expert ✭✭✭✭
    edited May 3 #21
    I have done the tests and seen the results. I have used Class T fuses in my batteries since 1992.
    Watch a dead short on the power bridge of an XW6848+ and you may do this also. This was a test of a good unit BTW.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • solarpowernovicesolarpowernovice Posts: 129Solar Expert ✭✭
    edited May 3 #22
    Great information as always Bill and friends, thanks! As an aside here is a really good article i found on
    the subject of Over Current Protection Devices...
    http://www.electricityfromthesun.info/low_voltage_dc_fuse_and_circuit_breaker_applications.htm
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