12V Wire Size Calculator

Twelve volts is where voltage drop does the most damage, because a 3% budget is only 0.36 V. The same current that runs happily through a 12 AWG house circuit needs cable three or four sizes bigger here, and past about 2 kW of inverter the current gets so high that the honest answer is a 24 V or 48 V bank rather than a bigger cable.

This is the solar cable size calculator set up for 12 V: the tool opens on the 12 V van circuit preset: 30 A over 15 ft at a 3% limit. Change anything you like; the tables below are fixed at 12 V.

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DC cable ampacity with the ambient and bundling corrections, how far each size runs at 12, 24 and 48 V, PV string fuse sizes from Isc, and battery-to-inverter current by inverter size. Three pages, made to pin by the battery bank.

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The three runs at 12 V

Every solar build has the same three DC legs. These are sized at the currents and lengths the calculator's own presets use, copper, 75°C conductor, 30°C ambient, three conductors, terminal rating per NEC 110.14(C). Change the current or the length and the answer moves; that is what the tool above is for.

Run Current One-way length Drop limit Copper size at 12 V Set by
Array to charge controller 30 A 40 ft 3% 1 AWG voltage drop (40 ft max)
Controller to battery 60 A 10 ft 2% 2 AWG voltage drop (10 ft max)
Battery to inverter 125 A 6 ft 2% 1 AWG both (6 ft max)

On a 12 V system almost every run is set by voltage drop rather than by ampacity, so the run length matters more than the current. The table below is the one to read first: find your current, then see how short the run has to be for each size. Shortening the run is cheaper than upsizing the cable, which is why van builds put the battery, the controller and the inverter within arm's reach of each other.

Worked example

A 2,000 W inverter on a 12 V bank at 92% efficiency draws about 181 A at nominal voltage and about 207 A at the 10.5 V low-voltage cutoff, which is the figure to size on. At 75°C terminations that needs 4/0 AWG copper for the ampacity alone, before any thought of run length. That is the practical ceiling for 12 V: the same inverter on 48 V draws a quarter of the current and lands on 2 AWG.

What each inverter size draws from a 12 V bank

The battery-to-inverter leg carries the highest current in the system, and it is sized on the current at the inverter's rated output and at the bank's lowest voltage, because that is the worst case the cable and the fuse have to survive. At 12 V nominal the low-voltage cutoff defaults to 10.5 V, which is 87.5% of nominal, and the inverter sizing calculator's own default efficiency is 92%.

DC input current, cable size and overcurrent device for each inverter rating on a 12 volt bank
Inverter DC amps at 12 V DC amps at 10.5 V cutoff Copper, 6 ft at 2% Device at 125%
300 W 27 A 31 A 6 AWG 40 A
600 W 54 A 62 A 4 AWG 80 A
1,000 W 91 A 104 A 2 AWG 150 A
1,500 W 136 A 155 A 2/0 AWG 200 A
2,000 W 181 A 207 A 4/0 AWG 300 A

This table is the argument for leaving 12 V behind. By 2,000 W the battery cable is at the largest single conductor in the NEC table and the fuse is a 300 A class-T, which is specialist kit at a specialist price. Most 12 V builds stop between 1,000 W and 2,000 W for exactly this reason, and a van that wants an induction hob is really a 24 V van.

How far each cable runs at 12 V

Maximum one-way run in feet before voltage drop reaches 3% (0.36 V) on a 12 V DC circuit, copper. Greyed cells are currents the cable cannot carry at a 75°C termination, so length is moot. Design the battery-to-inverter leg to 2%, which is two thirds of these figures.

Maximum one-way run length in feet at 3 percent voltage drop on a 12 volt DC circuit, copper conductors, by current
Copper size 75°C ampacity 10 A 20 A 30 A 50 A 100 A 150 A 200 A
10 AWG 35 A 15 ft 7 ft 5 ft — — — —
8 AWG 50 A 23 ft 11 ft 7 ft 4 ft — — —
6 AWG 65 A 36 ft 18 ft 12 ft 7 ft — — —
4 AWG 85 A 57 ft 28 ft 19 ft 11 ft — — —
2 AWG 115 A 95 ft 47 ft 31 ft 19 ft 9 ft — —
1/0 AWG 150 A 151 ft 75 ft 50 ft 30 ft 15 ft 10 ft —
2/0 AWG 175 A 178 ft 89 ft 59 ft 35 ft 17 ft 11 ft —
4/0 AWG 230 A 290 ft 145 ft 96 ft 58 ft 29 ft 19 ft 14 ft

Resistances are NEC Chapter 9 Table 9 values; DC resistance is marginally lower, so these lengths are slightly conservative. Ampacities are NEC 310.16 for not more than three current-carrying conductors at 30°C, before the ambient and bundling corrections the calculator applies when you change those fields. For the drop on a run you already have, use the DC voltage drop calculator; to protect the run, the PV fuse and breaker calculator.

Frequently Asked Questions

What size wire do I need for a 12 V solar system?

It depends on the run, not the system. At 12 V the array-to-controller leg at 30 A over 40 ft needs 1 AWG copper, and the battery-to-inverter leg at 125 A over 6 ft needs 1 AWG. A 3% budget at 12 V is only 0.36 V, so the run length decides the array cable and the current decides the battery cable. Enter your own current and length in the calculator above.

How far can I run 10 AWG at 30 A on 12 V?

About 5 ft one-way before the drop reaches 3%, and about 3 ft at a 2% limit. Beyond that, step up one size per further increment of length; the run-length table above shows where each size gives out at 12 V.

Where is 12 V the right system voltage?

Campervans, RVs, boats, small cabins and anything built around a single 12 V battery. Higher voltage means lower current for the same power, so the practical question is how much inverter you run: the DC current at the inverter's rating and at its low-voltage cutoff sets the battery cable and the fuse, and that number is what pushes bigger systems up to 24 V and 48 V.

Does this include ambient temperature and bundling derating?

The tables on this page do not; they are the NEC 310.16 base values at 30 C with three conductors. The calculator above does: change the ambient temperature and the conductor count and it applies the NEC 310.15(B)(1)(1) and 310.15(C)(1) factors, which matter on a hot roof or in an engine bay, and it caps the answer at the terminal rating per NEC 110.14(C).

Can I run 10 AWG from the panels to the controller on a 12 V system?

10 AWG carries 30 A comfortably, but at 12 V it only holds a 3% drop for about 15 ft one-way at 30 A, and about 45 ft at 10 A. A roof-mounted array on a van is usually inside that; a ground-mount array 40 ft from a cabin is not, and would need 4 AWG at 30 A. Run the actual length through the calculator rather than assuming the cable that came with the panels is long enough.

Why do people say 12 V tops out at 2,000 W?

Because of the current, not the power. A 2,000 W inverter at 92% efficiency pulls 181 A from a 12 V bank at nominal and 207 A at the 10.5 V cutoff, which is 4/0 copper and a 300 A fuse: the largest single conductor in the NEC table and a specialist class-T holder. Everything above that is parallel conductors, and the same inverter on 24 V draws 104 A into 2 AWG. The limit is where the parts stop being ordinary, and on 12 V that happens at around 2 kW.

Cable size at other system voltages

24 V wire size 48 V wire size Any voltage

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