AmpSizerWhat size wire for a 12 volt run?
12 V DC wire size
6 AWG copper
on a 15 A fuse or breaker · 2.5% drop, leaving 11.7 V at the load
2.5% drop at 30 ft
- Ampacity wants14 AWG
- Voltage drop wants6 AWG
- Current the conductor has to carry
- 10 A
- Usable ampacity after derating
- 15 A (table 15)
- Longest run before the drop passes 3%
- 37 ft
- In metric sizes
- 13.3 mm²
— had the ampacity and 240.4(D) still rejected it, at 0 A.
The run is one-way and the formula doubles it. Measure the wire you are actually pulling.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
One circuit check a month
MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What size wire do you need for a 12 volt run?
The AmpSizer 12-volt result for 10 amps over a 30 foot run at a 3% target is 6 AWG copper. Ampacity alone would have accepted 14 AWG — four sizes smaller — because at 12 volts a 3% budget is only 0.36 of a volt.
That is the whole difference between low-voltage and mains wiring. The volts lost in a conductor depend on current and distance, not on system voltage, so the same loss that is a rounding error at 240 V is a crisis at 12.
Doubling the system voltage quarters the problem. The same 10 amps at 24 V over 30 ft needs 10 AWG instead of 6, which is why serious off-grid and RV wiring moved to 24 and 48 volt systems.
The table below is the shape of the problem: the same 10 amps, the same 3% target, at three system voltages.
| Run | 12 V | 24 V | 48 V |
|---|---|---|---|
| 10 ft | 10 AWG | 14 AWG | 14 AWG |
| 20 ft | 8 AWG | 12 AWG | 14 AWG |
| 30 ft | 6 AWG | 10 AWG | 14 AWG |
| 50 ft | 3 AWG | 8 AWG | 12 AWG |
| 100 ft | 1/0 AWG | 4 AWG | 8 AWG |
At 10 amps and a 3% target, copper. The right-hand column is the argument for higher-voltage battery banks in one glance: a 100 ft run at 48 V wants 8 AWG, and the same run at 12 V wants 1/0.
The four places this bites
Solar to charge controller. The controller measures panel and battery voltage to decide what to do. Voltage lost in the cable is voltage it never sees.
Battery to inverter. The highest current in the whole system, usually over the shortest distance, which is why these cables are enormous and short. An inverter drawing 100 A at 12 V over 6 ft still wants 2 AWG.
Lighting runs on a boat or a van. Long, low current, and the failure is visible: the far light is dimmer than the near one.
Trolling motors and winches. Short, brutal current. Here the wire is sized for the drop under load, and the load is not the average, it is the stall.
Frequently asked questions
Why does 12 volt wiring need such thick wire?
Because the percentage is measured against a small number. A 30 ft run at 10 A loses about 0.37 V in 6 AWG copper — an amount nobody would notice on a 240 V circuit, and 3% of a 12 V system. Halving the acceptable loss means doubling the copper, and at 12 V the acceptable loss is tiny to begin with.
Will a 100 Ah battery run my fridge overnight?
That is a capacity question rather than a wire question, but the wire is what ruins the answer: undersized cable between the battery and the inverter drops voltage under load, the inverter compensates by drawing more current, and the battery empties faster than the arithmetic said it would. Size the cable first, then work out the run time.
Is 3% the right target for 12 volt, or should it be tighter?
3% is a sensible general target and 2% is better for anything that measures voltage to decide what to do — a solar charge controller, a battery monitor, an inverter’s low-voltage cutout. For lighting, 5% is usually invisible. For a winch, nobody cares about the volts, only the amps.
Does the chassis return count as part of the run?
It counts as resistance and it is usually less than a copper conductor would be, but it is not free and it is not predictable — the path runs through bolted joints that corrode. Vehicle and boat wiring standards commonly ask for a dedicated negative conductor for exactly that reason, and then the run is out and back in copper.
Can I use the same wire size for 24 or 48 volts?
You can use much less. For the same power, doubling voltage halves the current, and the drop is proportional to current and inversely proportional to voltage — so it improves by four times. A 48 V run needs about a sixteenth of the copper of the same 12 V run at the same power.
How we calculate this
2 × amps × R × one-way feet ÷ 1,000 — the DC case, where the factor of two is the
out-and-back conductor — using the direct-current resistance of stranded uncoated conductors in NEC
Chapter 9, Table 8. That resistance figure is a DC resistance, so unlike alternating-current work
there is no impedance correction to argue about; this is the exact calculation rather than an
approximation of one. Ampacity uses the 60 °C column of Table 310.16 as a conservative floor for
general-purpose building wire. Note that vehicle, marine and photovoltaic wiring are governed by their
own standards, which have their own ampacity tables for cable in free air and their own protection
rules, and those can permit more current than a building-wire table.Worked example
With Load current 10, One-way run 30, System voltage 12 V, Metal Copper, this page works out 6 AWG copper. on a 15 A fuse or breaker · 2.5% drop, leaving 11.7 V at the load
- Current the conductor has to carry10 A
- Usable ampacity after derating15 A (table 15 )
- Longest run before the drop passes 3%37 ft
- In metric sizes13.3 mm²
Sources:
Where low-voltage runs go wrong
Terminations matter more than they do at 240 V. A crimp with 20 milliohms of resistance is nothing in a mains circuit and is a measurable share of a 12 V budget. Crimp properly, use marine-grade terminals where there is damp, and keep joints out of the run.
Fuse for the wire, not for the appliance. In a DC system the wire is often the only thing between a battery capable of thousands of amps and a fault. The fuse belongs at the battery end, sized to the conductor.
Inrush is not the running current. A compressor fridge, a winch and an inverter all draw far more at start than they do a second later. Size the wire for the running load and the protection for the surge.