I’m building a mobile battery box and am having an issue getting a consistent answer as to an appropriate wire gauge to connect the battery to the inverter. The battery is a 12V 230 Ah LiFePO4 and the inverter is 2000W with a 225A fuse in between them. Having looked online for a safe wire gauge to use in this circuit, I’ve seen seemingly reliable guides say everything from 1/0 to 4/0 and would appreciate some clarification. Obviously the thickest cable is the safest but there is a cost consideration here to. Any help would be appreciated
Appropriate wire gauge question
- Thread starter Wispy
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Zapper77
Renaissance Man
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- Jul 23, 2022
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I'm OK with 2/0 but that doesn't leave any room for future expansion.
I originally used 4/0 at 6' length for 1800w and in hindsight probably is a bit more than needed.
Today with the higher LFP voltages and the assumed very short length I recommend the 1/0 for the use as described.
My chart says good for 285 amps and should work fine in general use.
Today with the higher LFP voltages and the assumed very short length I recommend the 1/0 for the use as described.
My chart says good for 285 amps and should work fine in general use.
I appreciate the responses so far. To clarify, this system is not going to be expanded; the physical size of the box was determined by external factors and this is the most capacity I could fit into that space. Also I don’t have any crimping tools yet. And an addition question, what is a reliable source for this info? Is there a guide on the forum/site?
krby
Solar Enthusiast
Wire gauge vs current is really about heat dissipation. For wiring in a bundle or enclosed space where shedding heat may be a problem, current ratings are typically lower.
I typically use the recommendations on this chart: https://www.powerstream.com/Wire_Size.htm
On that chart "chassis wiring" means "wires run NOT in a bundle" where "transmission" means "bundled together" (and therefore heating each other up).
2000W at 12V is 166A. 1/0 might be ok, if the runs are short and not bundled. 2/0 would be better. Whatever you choose, you REALLY should put in a new fuse that is sized correctly for the wiring. If you wire is max rated for 150A (1/0, bundled) then your fuse should be sized for that. The fuse is there to protect the wire, not the equipment. You don't want the wire starting a fire.
You haven't said how long the wire runs are, if they're short, you might get away with smaller wire, but then the price difference for a short run of 1/0 vs 4/0 isn't that much. The other thing to think about is voltage drop. 166A on 2/0 will lose less power to heat than 1/0 (even better with 3/0 and 4/0)
If you're not crimping yourself, you can buy good pre-made cables from WindyNation on Amazon.
To give you a real world example. I built a portable power station designed for 3000W of output. I used a 24V battery bank (so 125A max) and built it with 2/0 cables. The cable runs are pretty short. Longest may be 2 feet, and not bundled together, but they are sealed up in a rolling Pelican-style luggage box. I chose 2/0 to minimize loss from resistance and to be sure I was running well within the limits.
I typically use the recommendations on this chart: https://www.powerstream.com/Wire_Size.htm
On that chart "chassis wiring" means "wires run NOT in a bundle" where "transmission" means "bundled together" (and therefore heating each other up).
2000W at 12V is 166A. 1/0 might be ok, if the runs are short and not bundled. 2/0 would be better. Whatever you choose, you REALLY should put in a new fuse that is sized correctly for the wiring. If you wire is max rated for 150A (1/0, bundled) then your fuse should be sized for that. The fuse is there to protect the wire, not the equipment. You don't want the wire starting a fire.
You haven't said how long the wire runs are, if they're short, you might get away with smaller wire, but then the price difference for a short run of 1/0 vs 4/0 isn't that much. The other thing to think about is voltage drop. 166A on 2/0 will lose less power to heat than 1/0 (even better with 3/0 and 4/0)
If you're not crimping yourself, you can buy good pre-made cables from WindyNation on Amazon.
To give you a real world example. I built a portable power station designed for 3000W of output. I used a 24V battery bank (so 125A max) and built it with 2/0 cables. The cable runs are pretty short. Longest may be 2 feet, and not bundled together, but they are sealed up in a rolling Pelican-style luggage box. I chose 2/0 to minimize loss from resistance and to be sure I was running well within the limits.
Last edited:
Order the needed cable premade.
https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot
https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot
chrisski
Solar Boondocker
- Joined
- Aug 14, 2020
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- 5,202
My calculations I use will be thicker wires. With 100k+ electrical fires per year, I intend to stay out of the up in smoke sub forum.
I looked in NEC 2023 code for formulas, but didn’t find it. Could be there or the formulas could be in a different doc.
I got my guidance on conservative postings off this forum.
4/0 wire with 75 c insulation can handle 10 volts sending 220 amps across it which also covers conversion loss. 1/0 even a thicker rating won’t cover.
For voltage loss, I go here:
www.calculator.net
I looked in NEC 2023 code for formulas, but didn’t find it. Could be there or the formulas could be in a different doc.
I got my guidance on conservative postings off this forum.
I use this ampacity chart, an older NEC chart
4/0 wire with 75 c insulation can handle 10 volts sending 220 amps across it which also covers conversion loss. 1/0 even a thicker rating won’t cover.
For voltage loss, I go here:
Voltage Drop Calculator
This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.
www.calculator.net
scrubjaysnest
Solar Enthusiast
What does the inverter manual recommend? Mine said to pair 2/0 at a length of 3 feet. That's 2 red 2/0 and 2 black 2/0. The inverter is 5000 watt @ 12 volt so socks a lot of current at full load. Max current would be 417 amps.
krby
Solar Enthusiast
I wish inverter companies would stop recommending this paired wired thing. IMO, substituting a pair of wires is asking for trouble. If one of the pair (on + or -) has a problem (loose crimp, bad connection at one end, etc) the other wire in the pair will have to handle the (at worst case) full load. I'm sure they do it to keep connector sizes smaller, but I really hate it.
scrubjaysnest
Solar Enthusiast
Agree
Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
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- 20,682
I doubt wire gauge usually the problem. Even if too small so it melts insulation and shorts, so long as you have OCP that shuts it off (guess how I know.)
Bad series connections can get very hot without exceeding current. We've seen pictures "What does this red light mean?"
But yeah, good to use large enough wires. No point in baking out insulation before its time.
I had thought 2/0 was proper for 200A service. Then I realized that is at 90C, but breaker terminals are rated 75C. Turns out it is - for split-phase (2 current carrying conductors not three), I found an NEC reference which says it is OK. But anyway, I just put 3/0 in my new service entrance.
scrubjaysnest
Solar Enthusiast
Yea! My bad that turns into a do over. Most likely go 48v like I should have in the 1st place.
Let's do the math:
A 2000 watt 12 volt inverter, if drawing that power output is using 2000/12 = 167 amps. Your battery is 12 volts and 230 amp hours (usually a rating like that is the C20 rating, meaning that drawing down the battery over 20 hours will provide 2760 watts of power). Lithium commonly have a C1 or C2 rating which may even be 100% capacity of the battery to charge or discharge in 1 hour for a C1 rating. C1 in this case would be 230 amps for 1 hour. Most batteries do not exceed a C1 of 100 amps. If your battery has a C1 of 100 amps (limited by the BMS of the battery), then 100A x 12V = 1200 watts per hour capacity. If this is the case, then your 2,000 watt inverter cannot receive 2,000 watts of power from your battery because it is limited by the math above, again if the BMS is 100 amps max discharge rate. So to output the full 2,000 watts of the inverter, you would need two 12 volt batteries in parallel, providing 12 volts and 200 amps combined of discharge power. But, again, the math says the inverter cannot draw more than 167 amps. The fine print may show a surge capacity for a very short period of time which is greater, but maybe not enough to trip a breaker or blow a fuse.
So, thinking about it, a wire capacity of 150 to 200 amps should do the trick. Bigger wires will only be of benefit if you enlarge your inverter capacity and add battery output.
A 2000 watt 12 volt inverter, if drawing that power output is using 2000/12 = 167 amps. Your battery is 12 volts and 230 amp hours (usually a rating like that is the C20 rating, meaning that drawing down the battery over 20 hours will provide 2760 watts of power). Lithium commonly have a C1 or C2 rating which may even be 100% capacity of the battery to charge or discharge in 1 hour for a C1 rating. C1 in this case would be 230 amps for 1 hour. Most batteries do not exceed a C1 of 100 amps. If your battery has a C1 of 100 amps (limited by the BMS of the battery), then 100A x 12V = 1200 watts per hour capacity. If this is the case, then your 2,000 watt inverter cannot receive 2,000 watts of power from your battery because it is limited by the math above, again if the BMS is 100 amps max discharge rate. So to output the full 2,000 watts of the inverter, you would need two 12 volt batteries in parallel, providing 12 volts and 200 amps combined of discharge power. But, again, the math says the inverter cannot draw more than 167 amps. The fine print may show a surge capacity for a very short period of time which is greater, but maybe not enough to trip a breaker or blow a fuse.
So, thinking about it, a wire capacity of 150 to 200 amps should do the trick. Bigger wires will only be of benefit if you enlarge your inverter capacity and add battery output.
zanydroid
Solar Wizard
If it's paired above size NEC allows for pairing, wouldn't the extra safety from large conductor size + intrinsic margin from using NEC ampacity charts instead of something more aggressive stack together to bring down the risk?
zanydroid
Solar Wizard
It's the single phase / single dwelling service (and feeder) exception. At various points in time it was a table or 83% undersize allowed.
OffGridForGood
Catch, make or grow everything you can.
The size of the cable is only half the answer - as Hedges noted, cables have various temperature ratings, and as krby noted, cables in free air vs bundled or in a conduit also matter. So you need to know the cable temperature rating to pick the correct size.
chrisski
Solar Boondocker
- Joined
- Aug 14, 2020
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- 5,202
I would say the max limit on the wires is 235 amps. Low voltage cutoff is what should be used.
I build these for the “guest factor.” The guy or gal who thinks the rules don’t apply or has no clue.
So, in the calculation there’s a couple things you left off. First is the low voltage factor.
2000/10=200 amps. Then there is the 85% conversion factor.
200/.85=235 Amps.
The real formula should be
Inverter wattage/low voltage cutoff/inverter efficiency factor*1.25 wire safety factor.
Things my guests have done that push these limits
1) ignore the low voltage warning from my battery monitor to let me get needed rest so they can cook with an electric appliance with the inverter maxed out.
2) run other high wattage appliances two at a time exceeding the rating.
With the inverter maxed out the NEC code wants the wire rated 1.25 times the load, so the wire should be rated for 294 amps. This is the 80% rule and is why Tesla wall chargers need a 60 amp breaker, but only push 48 amps. It’s also why a Tesla level 1 charger plugs into a 15 amp socket, but only charges at 12 amps.
For my inverter, I use welding wire which has an ampacity in excess of the NEC chart. I also know my wires have never gotten warm even with the inverter running at 66% hours on end or batteries charging.
OffGridForGood
Catch, make or grow everything you can.
are you recommending 300A class T fuse and 250mcm wire rated 90C ?
