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Background: I'm considering installing a secondary battery, battery charger, and 120v inverter in my car, to provide power when I'm in remote locations, without having to worry about killing my car's main starting battery. (Specifically, eyeing the Renogy 12V 20A DC-to-DC charger, and configured to only charge the secondary battery while the engine is running.)

For this project, I'll quite likely need to run new (fused) wires from my car's battery (or the fuse box itself, if I can add a new one there) and I want to ensure that I size the wires appropriately.

From reading other questions on this site, with regards to household wiring, "amps is amps" regardless of voltage. For example, this fascinating answer indicating that you can go up to 600v (I'm guessing the limit of the insulation rating) to move a lot of watts, but keeping the amperage at 20 amps on 12 gauge wire. This information lines up table 310.16 in the NEC (2020 edition), stating that 12AWG copper wire is required for 20A circuits.

However, from searching online, most sites (such as this and this) claim that you need larger wires (such as 10AWG or even 6AWG) to supply 20 amps at 12V DC. (Looking at the 10-15 foot length, from the engine bay to the trunk.)

Why do these sites recommend larger wires for the same amperage at 12V DC? Are there reasons that the NEC wire size requirements are valid for higher voltages, but not lower voltages?

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  • The wire size requirements are for 120/240 volt circuits. The 600volts you see on insulation is just for that insulation, not the wire. The insulation should protect against electricity leaking(shorting) under 600 or sometimes 300 volts. Larger size wires are needed where distance will produce a voltage drop due to resistance of the wire.
    – crip659
    May 21, 2022 at 15:39
  • If you look at the NEC tables you can see that 14,12&10 awg wires can carry much higher currents but the NEC has a safety factor , so if you use stranded wire and use the NEC as a guideline you’ll be safe.
    – Ed Beal
    May 21, 2022 at 17:47

5 Answers 5

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Simple, and also easy with online "voltage drop calculators" to model and check for your specific wiring, amperage, and distance.

1.2 V lost on a 120V circuit is 1% of the applied voltage.

1.2V lost on a 12V circuit is 10% of the applied voltage.

1% is no big deal in most applications. 10% is often too much.

The NEC is mostly concerned with "will this wire overheat and start a fire" and on THAT basis, you're fine following their recommendations, normally - though you might need to pay attention to thermal derating for a hot car.

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  • Thank you! Your answer clearly illustrated the point that I was missing - that for a given wire length and size, the "subtracted number of volts" would remain constant.
    – maples
    Jun 22, 2022 at 18:12
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The thermal limits are the same.

The NEC ratings are based on thermal rise for any given foot (meter) of wire, at a given ampacity.

The only thing that matters to thermal rise is the wire size and ampacity.

Length doesn't enter into it. If the wire is 10 times longer, it has 10 times the voltage drop and thermal rise, yes. However it has 10 times the length to spread the heat around, so heat in any given foot of wire is exactly the same.

Voltage drop requires the same calculation.

However you will get different results because the voltage is lower.

  • Suppose you have a long 240V, 15A load at a shed, and you calculate out voltage drop at 1%. (2.4 volts).
  • If you re-use the same wire for 120V, 15A, then voltage drop will be exactly the same, 2.4 volts. However since your target voltage is lower, it takes a bigger bite out of it - 2.4V is 2% of 120V.
  • If you reuse that wire for a 12v, 15A circuit, what will voltage drop be? You guessed it, that same 2.4 volts again. But this time it's a real gut-punch because you only started with 12 volts. (so you're down 20%).

And I've seen that doom more than one off-grid home power project where the homesteader just didn't understand the math/physics of it.

So yes, with such a low voltage, voltage drop eats your face rather quickly. You have to account for it properly if you enjoy not failing.

You need to do the voltage drop calculation!

One of the biggest ways low voltage projects fail is from the designer "spitballing" the voltage drop, and not doing the proper calculation. Southwire's Voltage Drop Calculator works well enough, use actual circuit amps and remember to override the default "3%" value to something more sensible.

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  • Thanks for the explanations! I hadn't considered thermal limits, so thank you for the reminder on that :) I'll take a look at a voltage drop calculator before I buy the wire
    – maples
    Jun 22, 2022 at 18:23
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You can use them as a guide. Also stranded wire is used because of the vibration etc. When automotive harnesses are designed voltage drop, (controlled by wire size and load), is a big concern as is weight and ambient temperature. In some instances the ambient can reach over 170C.

A few of the factors that are considered when designing a wiring harness is the wire (sized to be as small as practical) for the given load. This is also influenced by the fuse rating, weigh, temperature rise of the wire and the load. Fuses used are automotive rated such as ATO for example.

The designers know shorter the wire the more current it can conduct for a given voltage drop, that is taken also into account. The fuses used normally one of the standard automotive type, Larger wire would be OK.

There are several great CAD programs that do the grunt work for the designer such as SaberES. If my memory is correct SaberES Designer will allow you to capture the vehicle wiring topology, including all the wiring sub harness paths and interconnection between various components as part of the package.

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  • Thanks! I wasn't aware that stranded wire would be better in conditions with lots of vibration, I'll look into that.
    – maples
    Jun 22, 2022 at 18:16
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Adding to Ecnerwal's excellent answer, I just want to point out that 20 amps at 12 volts into the inverter will give you only 2 amps or less at 120 volts out of the inverter. If you want 10 amps AC out of the inverter, you need to feed it more than 100 amps. Just remember that when you size your supply wire. As a rule of thumb, take the VA capacity of the inverter plus 10%, divide by 12 volts and that's how many amps the inverter can draw at maximum.

Example: 600VA +10% = 660VA and 660 / 12 = 55 amps maximum

Also, consider using "welding cable" for your supply wires. It's made with very fine wires for maximum flexibility, and it makes it easy to install the wire with the sharp bends and curves that are sometimes required in an automotive installation.

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    I think the 20A wire is for charging the battery, not running the inverter. But good point anyway, as both need to be done.
    – Ecnerwal
    May 21, 2022 at 23:55
  • Thanks for pointing that out! Ecnerwal is correct, the 20 amps will just be for charging the battery (over a longer period of time). I'll be using the short (but very fat, I think 6AWG?) wires that the inverter came with for the connection between the inverter and battery. Thanks for the tip about welding cables, I'll look into it!
    – maples
    Jun 22, 2022 at 18:14
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All good correct comments so far. Would like to clarify some bits. Given the distance you note is no more than 15 feet voltage drop will not be an issue even with DC. If you are running 12V to the inverter the Ampacity rating must be 10X the inverter output. Heavy cable. To help: 1,000 watts AC is 8.33 amps, 2,000 watts AC is 17 amps. Take into consideration the inverter Peak watts when you run the numbers. That said install the inverter close to the power source and choose the cable that has the highest temperature rating. Welding cable would work but so would XHHW wire,rated at 194degF, would work also and is easier to get. Then run the 120vac to where you want it with a 20amp extension cord. Run that inside the cab, use pvc conduit in exposed areas and in the truck bed. just like you would in outside areas.

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  • Thanks for the clarifications! The inverter and its battery will be very close together. The inverter came with some cables to use (6AWG I think?) that should be long enough, but I'll keep XHHW in mind if I need it. This is a sedan, so no outdoor conduits needed, though I hadn't considered routing an extension cord around the interior to provide a more convenient outlet location, thanks for the tip!
    – maples
    Jun 22, 2022 at 18:19

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