Wire gauge for 24V x 420 AH battery bank

[Silver_Is_Money]

What wire (gauge and type) should be used to string 4 x 6V 420AH batteries in series for use with an inverter that is 24V and 1500 Watt, with 3,000 Watt surge capacity, and for which the recommended fuse is 100 amp. I'm thinking #2 THHN would be the minimum, but need an experts opinion.

[Sunking]

Assuming the one-way distance is less than 5-feet the size of the Over Current Protection Device (OCPD), wire insulation type, ambient temp, and raceway determine minimum wire size.

OK enough mumbo jumbo use 90 degree insulation type like RHH.RHW, THHN, THHW, THWN-2, XHHW, XHHW-2, ran in free-air, on a 100 amp OCPD requires a minimum of #6 AWG copper. 75 degree cable requires a #4 AWG. Go beyond 5 feet one-way and you start running into voltage drop losses the plague of low voltage circuits. In that case you have to upsize to over come losses, but not required by code.

Note building power and light wire come in three Temperature ratings, of 60, 75, and 90 Degrees C. 60 is never used except for underground feeders. You will finds a lot of both 75 and 90 in box stores.

#6 AWG 90 degree copper is rated for 105 amps in free air, and 75 amps in a race way with no more than 3 load conductors.
#4 AWG 75 degree copper is rated for 125 amps in free air, and 100 amps in a raceway with no more than 3 load conductors.

Take your pick. FWIW the 4 AWG will have the lower voltage loss regardless of insulation temp rating.

[lkruper]

Assuming the one-way distance is less than 5-feet the size of the Over Current Protection Device (OCPD), wire insulation type, ambient temp, and raceway determine minimum wire size.

OK enough mumbo jumbo use 90 degree insulation type like RHH.RHW, THHN, THHW, THWN-2, XHHW, XHHW-2, ran in free-air, on a 100 amp OCPD requires a minimum of #6 AWG copper. 75 degree cable requires a #4 AWG. Go beyond 5 feet one-way and you start running into voltage drop losses the plague of low voltage circuits. In that case you have to upsize to over come losses, but not required by code.

Note building power and light wire come in three Temperature ratings, of 60, 75, and 90 Degrees C. 60 is never used except for underground feeders. You will finds a lot of both 75 and 90 in box stores.

#6 AWG 90 degree copper is rated for 105 amps in free air, and 75 amps in a race way with no more than 3 load conductors.
#4 AWG 75 degree copper is rated for 125 amps in free air, and 100 amps in a raceway with no more than 3 load conductors.

Take your pick. FWIW the 4 AWG will have the lower voltage loss regardless of insulation temp rating.

I see you mentioning "codes."

If a solar or battery installation is not grid-connected, do most areas require pulling building permits? If so, any exceptions?


In my area there is verbiage about temporary/emergency/experimental installations with the installations being taken down within so many days. I was also thinking about setting up a place to mount a solar panel to charge batteries that could be manually relocated inside my cabin and connected to an inverter for an emergency situation... keeping the batteries on float chargers and having the panel stored for an emergency if a power outage was more than one day. Is it generally legal for a home-owner to connect a battery to an inverter if neither are permanently affixed to the building without a permit? I have a feeling it would be but want to make sure.

[Sunking]

I see you mentioning "codes."

Codes are minimum SAFETY RQUIREMENTS, not design criteria.

With low voltage systems like off-grid stand alone battery systems NEC code is not good enough, just minimum safety requirements. That is why I said if less than 5-FEET ONE-WAY. At 100 amps once you get further out than 10 feet you start to loose voltage quickly. In low voltage systems you want to keep voltage and power losses in wiring less than 2 to 3%. Well 2 to 3% i snot much of a problem at 120, 240, 480 and higher voltages. But at 24 volts you are talking .5 volts. So while 100 amps on #6 AWG is perfectly safe on open air conductors at 24 volts. Over say 20 feet one way you have lost 2 volts or 8.3% of your power and voltage lost as heat on the wire.

So to answer your question ignoring codes if not required by application can not only be not safe, but not functional either.

[Silver_Is_Money]

Is #3 AWG copper wire a viable (as in readily available) wire size. I see where many online charts and calculators jump straight from 4 AWG to 2 AWG as length is increased.

[inetdog]

Is 3 AWG copper wire a viable (as in readily available) wire size. I see where many online charts and calculators jump straight from 4 AWG to 2 AWG as length is increased.

3AWG is perfectly viable (as in useful, not just readily available) if 4AWG is just short of enough and 2AWG is overkill. The reason that you do not see it often is that, like 11AWG and 13AWG is just is not needed enough to justify stocking it.
In some case 3AWG will be just the right size for common service or other applications, and so it is made, but seldom stocked.
The voltage drop calculators recognize this and jump from one common size to the next in their sizing output.

Plus what Dereck says below....

[Sunking]

Is 3 AWG copper wire a viable (as in readily available) wire size. I see where many online charts and calculators jump straight from 4 AWG to 2 AWG as length is increased.

3 AWG is real but hard to find and expensive. As long as you use 6 AWG or larger you are good to go.

Go look and try to find 1, 3, 5, 7, 9, 11, 15..... gauge wires. There is no much difference between say a 3 and 4 AWG wire current capacity. So manufactures do not make the odd numbers as it makes for too much inventory with no real benefit.