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Old 25-01-2021, 18:59   #1
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Large (3000w) inverter/charger cables size?

For anyone with a larger inverter, what cable size are you using for wiring? And how long are your cables?

From a little research it looks like 1/0 is "recommended" (link) for up to a 3500w inverter, and 4/0 for anything larger.

BlueSea recommends 2/0 cables for 0-6' for 200amp loads.

Some calculators I use show a 2.58% drop with 2' (one way) cables on 250 amps at 12v.

I'm not planning on installing an inverter yet, but if I do I will likely choose a 3000w inverter/charger with the expectation of charging a 280ah LFP bank. I see no reason I can't keep my inverter within 2 feet of the bank, but I don't want to under-size wires and create more than needed heat or less than desirable losses.
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Old 25-01-2021, 19:29   #2
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Re: Large (3000w) inverter/charger cables size?

There are two issues you need to consider here. First is voltage drop, and especially if you use a 3% voltage drop, it is quite rare that you need to go further. But in this case, with large currents and short wires, you ALSO need to be sure you look at the ampacity of the wire. Even a low voltage drop with huge currents, can overheat a wire.

A 12V 300W inverter is likely to need a 400 Amp fuse on its DC supply, and that 400Amp rating is likely to be the controlling factor in your installation if you keep the wires as short as you have planned. If fused for 400 Amps, the wire NEEDS to be able to carry that without overheating. I am pretty sure if you run the numbers you will find the "recommended" AWG 1/0 wire is much too small to safely carry these currents--even if the voltage drop seems to be OK at the normal operating load.

Here is an extreme example: a 3 inch length of 12AWG wire will calculate an acceptable voltage drop for well over 100 Amps, but it will rapidly overheat if used in such a case.
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Old 25-01-2021, 20:06   #3
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Re: Large (3000w) inverter/charger cables size?

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Originally Posted by BillKny View Post
There are two issues you need to consider here. First is voltage drop, and especially if you use a 3% voltage drop, it is quite rare that you need to go further. But in this case, with large currents and short wires, you ALSO need to be sure you look at the ampacity of the wire. Even a low voltage drop with huge currents, can overheat a wire.

A 12V 300W inverter is likely to need a 400 Amp fuse on its DC supply, and that 400Amp rating is likely to be the controlling factor in your installation if you keep the wires as short as you have planned. If fused for 400 Amps, the wire NEEDS to be able to carry that without overheating. I am pretty sure if you run the numbers you will find the "recommended" AWG 1/0 wire is much too small to safely carry these currents--even if the voltage drop seems to be OK at the normal operating load.

Here is an extreme example: a 3 inch length of 12AWG wire will calculate an acceptable voltage drop for well over 100 Amps, but it will rapidly overheat if used in such a case.

That's an exceptionally good point I had overlooked, and explains why I find differing recommendations.

This site recommends 2/0 wire has an ampacity of 330 amps and is acceptable for up to a 214 amp load, and recommends 4/0 wire with an ampacity of 445 amps for loads over 214 amps. It also recommends a 400 amp breaker.


That definitely helps reconcile the difference between the calculators and the recommendations, as well as my own expectations.
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Old 25-01-2021, 20:56   #4
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Re: Large (3000w) inverter/charger cables size?

Iíve got a 2500 watt inverter with 2/0 and about six feet between the 12v and ground combined. Donít forget you need to look at the combined length.
And my fuse is 250A class T.
Look at the interrupting capacity of the fuse and the current your batteries can deliver into those cables. Not all fuses can clear a short under those circumstances. Dc metal vapor arcs are brutal.
As well the AC ground wire for the inverter should be heavy enough that if the 12v shorts to the AC ground (likely tied to dc ground) the fuse will blow before the AC ground bursts in to flames.
Or so says ABYC Iíve read.
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Old 25-01-2021, 21:15   #5
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Re: Large (3000w) inverter/charger cables size?

Quote:
Originally Posted by ForeverDes View Post
I will likely choose a 3000w inverter/charger with the expectation of charging a 280ah LFP bank.
If you use anywhere close to 3000W for very long with such a small bank, you will drain those batteries very, very quickly.
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Old 26-01-2021, 00:49   #6
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Re: Large (3000w) inverter/charger cables size?

Read the manual. Normally 2/0 for 2000w and 4/0 for 3000w. Cables less then 5’
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Old 26-01-2021, 08:17   #7
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Re: Large (3000w) inverter/charger cables size?

And don't forget the case ground.
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Old 26-01-2021, 11:06   #8
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Re: Large (3000w) inverter/charger cables size?

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If you use anywhere close to 3000W for very long with such a small bank, you will drain those batteries very, very quickly.
And I can't believe that the smallish lithium bank would be able to absorb the full 250A output for any significant length of time before saturation voltage is reached and amperage acceptance falls dramatically. And if anything in the sensing circuitry goes amiss, you've got a firebomb in the bilge...
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Old 26-01-2021, 11:26   #9
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Re: Large (3000w) inverter/charger cables size?

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And I can't believe that the smallish lithium bank would be able to absorb the full 250A output for any significant length of time before saturation voltage is reached and amperage acceptance falls dramatically. And if anything in the sensing circuitry goes amiss, you've got a firebomb in the bilge...

I'm not sure what you mean by that. Most LFP batteries are rated at 1C discharge, and 250 amps is only about 0.9C for a 280ah bank. That would only give just over 1 hour of discharge time at that current, maybe a bit less if maintaining the LFP bank between 20% and 90% SOC.

And what do you mean by "Saturation voltage"? That makes some sense for charging but not for discharging. It's possible the voltage will drop below the safe limits for the bms, but LFP batteries have a remarkably stable discharge curve, so discharging at less than 1C shouldn't cause the voltage to drop prematurely.

And why would LFP batteries cause a "firebomb" in the bilge? Li-Ion batteries have a reputation for combustion, but LFP batteries do not.

For the record, I don't even have any 120v loads on my boat currently, and I don't plan to in the future. But I can foresee a change in those plans if, for instance, I add another 400w of solar to my current 380w array and then have an excess of energy. And I do like to oversize electrical components, so if I decide to add a 3000w inverter charger I might as well have wiring capable of delivering all 3000w, continuously.
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Old 26-01-2021, 11:29   #10
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Re: Large (3000w) inverter/charger cables size?

Quote:
Originally Posted by dfelsent View Post
Iíve got a 2500 watt inverter with 2/0 and about six feet between the 12v and ground combined. Donít forget you need to look at the combined length.
And my fuse is 250A class T.
Look at the interrupting capacity of the fuse and the current your batteries can deliver into those cables. Not all fuses can clear a short under those circumstances. Dc metal vapor arcs are brutal.
As well the AC ground wire for the inverter should be heavy enough that if the 12v shorts to the AC ground (likely tied to dc ground) the fuse will blow before the AC ground bursts in to flames.
Or so says ABYC Iíve read.

That's a very good point on the AC ground side! I hadn't considered the AC ground would need to be sized for the DC current, but it makes a lot of sense. Thanks!
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Old 26-01-2021, 12:15   #11
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Re: Large (3000w) inverter/charger cables size?

Let's step back a bit. I think the OP was saying he planned to use and "inverter/charger" to charge his batteries. Surely there is no rush to charge the batteries and this can be done at a modest amperage?

So i suppose when he wants to use 110/220V on the boat from the inverter it is just a matter of the amperage that the inverter will need from the batteries. Not that i would favour a 3,000W inverter because of the constant idle current even if no load.

Did I misunderstand something?
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Old 26-01-2021, 12:34   #12
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Re: Large (3000w) inverter/charger cables size?

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Originally Posted by SaltyMetals View Post
Let's step back a bit. I think the OP was saying he planned to use and "inverter/charger" to charge his batteries. Surely there is no rush to charge the batteries and this can be done at a modest amperage?

So i suppose when he wants to use 110/220V on the boat from the inverter it is just a matter of the amperage that the inverter will need from the batteries. Not that i would favour a 3,000W inverter because of the constant idle current even if no load.

Did I misunderstand something?
Andrew

Well, you're mostly there. I was actually just trying to understand why wire size calculators didn't match generic inverter wire size recommendations at higher wattages. That question was thoroughly answered by BillKny in the very first response. (Wire Ampacity)

However, if I do add an inverter/charger, it's most likely I'd go with a 3000w unit.

The few I've glanced at don't charge at 250 amps though, but that doesn't matter for wire sizing because they'll still pull 250 amps in an extreme case. And I'd rather have everything sized properly for maximum amperage.

But generally speaking, I was just looking to reconcile wire size calculators, inverter recommendations, and my own expectations.
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Old 26-01-2021, 12:51   #13
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Re: Large (3000w) inverter/charger cables size?

Quote:
Originally Posted by ForeverDes View Post
For anyone with a larger inverter, what cable size are you using for wiring? And how long are your cables?

From a little research it looks like 1/0 is "recommended" (link) for up to a 3500w inverter, and 4/0 for anything larger.

BlueSea recommends 2/0 cables for 0-6' for 200amp loads.

A typical 3000 watt inverter will draw around 300 amps at full load.


Ampacity is the problem and the difficulty is that ampacity is a squishy concept that varies depending on ambient temperature, whether the cables are bundled, whether they are in conduit, whether there is air movement, and how much heat the insulation can take.



U.S. building wiring standards will put you at 350 MCM (which is about 1.5x the diameter of 4/0) but assume the wire is in conduit or bundled with other wires. If you keep the wires separated and exposed to air then a higher rating applies, because the air movement helps cool the wires. For individual conductors in free air you would be at 3/0 for 300 amps.



Here is the marine ampacity table from the ABYC, which has slightly different values than the building wiring one.


350 MCM cable is heavy and inflexible. Its inflexibility puts large strain and torque on terminals. Availability of cable, terminals, and crimping equipment is limited. For these reasons many inverter (and bow thruster) manufacturers recommend using two cables in parallel at these sizes, though that has its own problems (as it can result in overheating if one of the conductors develops a high-resistance connection somewhere).


I would look closely at running a 24v house bank as that would cut the current in half and cut the losses by a factor of 4. You could then do it in 1/0 which is far easier to work with.
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Old 26-01-2021, 14:22   #14
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Re: Large (3000w) inverter/charger cables size?

I second the 24v. I had 48v on my last boat & wish this one had it rather than 24v. (but it's a houseboat & I live on it)
25A or so converters are not that expensive for running your 12v gear.
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Old 26-01-2021, 17:24   #15
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Re: Large (3000w) inverter/charger cables size?

A 24v system is a real possibility. I had one on my last boat and liked it a lot. However, 24v inverter/chargers are considerably more expensive than 12v. (I didn't have an inverter or 24v charger on my last boat.) Maybe not a deal breaker, but the added complexity and cost might be a tipping factor to keep a 12v system.
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