Battery Bank Wire Gauge Sizes

[Nate Vizzi]

hello,

keeping it basic, I am designing a system that will have a 12 volt bank composed of two 12 volt batteries connected in parallel.

I am going to be using a 1000 watt inverter. assuming i use 1000 watts at some point in time at 120 volts AC i would be drawing 8.3 amps AC. If i convert that to 12 volts the amperage coming out of my batteries in DC could be up to 83 amps so naturally i know that i need to use very thick cables from that battery to the inverter.

my question is the following:

do i likewise need to use the same gauge wire that connects the two batteries in the parallel configuration (the + to + and the - to - of the bank). Or could i downsize those wires since half of the current would be coming from one battery and the other half would be coming from the second battery to combine the parallel circuit's current to the above mentioned 83 amps (voltage stays the same but the currents add up).

83 amps scares me to death but as it is i can barely afford the 2 batteries that i am getting and to have to get another 2 on top of that to bump up my bank to 24 volts with the same capacity would not be feasible at this point.

thanks

[Rob Beckers]

Hi Nate,

Don't be scared about the 80-or-so Amps. At 12 Volts it's just current, there is no chance of feeling the Voltage. If you want to make it safe it is a good idea to put a large fuse in line with the positive coming from the batteries. That's what we do for the very simple "cottage kits (http://www.solacity.com/product/1-panel-cottage-kit/)" we well; I've written up a wiring diagram and you can see it at this link (http://www.solacity.com/docs/2-Panel%20Cottage%20Kit.pdf). We use what's called "class-T" fuses, they are so big that one side can simply be bolted directly to the battery, and all other wires go to the other side. Normally that large fuse should never blow, but it's there in case of a short-circuit, and it's better to blow the fuse (even if they are somewhat expensive) than to burn the house down.

Normally inverters can deliver about twice their rated output when it comes to starting large loads such as fridge compressors, so sizing is usually done for that (160 Amp in your case). That would be 2/0 AWG wires for your inverter. Pretty thick stuff!

For that current (160A) the fuse size would be a 300 Amp class-T fuse (http://www.solacity.com/product/littelfuse-jlln300/). Again, so it never blows under normal conditions, but only in case of a severe short. Batteries, even small batteries, can deliver amazing currents for short periods. I have, quite literally, see tools melt when they accidentally shorted the battery terminals. Regular wiring would be turned (briefly) in to a light bulb in case of a short, not something you want to risk IMO.

To get back to the battery wiring: With two batteries in parallel each battery would share half the current, and the wiring can be sized for that. So, for 160 / 2 = 80A it would be 4 AWG.

There is a trick to wiring up multiple strings of batteries in parallel so they share current better. A crude drawing looks like this:


inverter (+) ----------+-B-+
| |
+-B-+---------- inverter (-)


It also holds for more than 2 strings of batteries. Doing this makes sure that there are the same number of interconnects between positive and negative to the inverter, regardless if the current flows through one battery or the other.

Hope this helps!

-Rob-

[Nate Vizzi]

Rob thank you for your reply ... very helpful.

I was going to limit my system use to devices that don't require surge such as lights and basic electronics (hoping to never go above say 900 watts max), but I see what you are saying. With the points you make I see how using the 160 amp as a basis for choosing 2/0 from my bank to inverter would then let me say run a coffee pot or something with a start up surge. I think I will do just that along with the recommended fuse.

Doesn't 4 gauge sound a little light to handle the 80 amps from each battery ... would 2 gauge by the correct choice ?

Thanks again

[Rob Beckers]

Nate, code says 4 AWG is good for 95A (the 90 Centigrade rated wire, which is nearly all wire these days). That's from the Internet; I don't have my code book here, it's at work, so who knows, maybe the Internet is wrong (yeah! never happens! :suspicious:). That said, yes, I'd go with 2 AWG so there's less Voltage drop at high currents as well.

By the way, a coffee pot doesn't have a surge current when you switch it on; it's just resistive heating. Things that do have (large) surges are pumps, motors, compressors, fridges/freezers etc. Rule of thumb there is that their startup current is about 3x to 4x the running current!

-RoB-

[Nate Vizzi]

Thank you SI MUCH ! Very very helpful