Wire Size & Voltage Drop Calculator
By Frank Zale · Last updated
Size DC cable for a van, RV or caravan. This checks both requirements that matter, ABYC ampacity and a 3% voltage-drop limit, then gives you the thicker of the two along with the fuse that protects it.
Measure one way. The calculation doubles it, because current travels down the positive conductor and back through the negative.
Cable size
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- Continuous current
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- Fuse
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- Round trip
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- Ampacity needs
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- Voltage drop needs
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This run is too long for a single cable. Even 4/0 AWG cannot hold voltage drop under 3% here. Shorten the run, move to a higher system voltage, or use parallel conductors. Do not simply fit 4/0 and hope.
Show the math
Why two calculations, not one
Undersized DC cable is the most common serious fault in a self-built van electrical system, and it fails in two different ways.
The first is heat. A conductor carrying more current than it is rated for gets hot, and in a vehicle that is a fire risk. Ampacity, the current a cable can carry safely, has to cover the fuse rating rather than the normal running current, because the fuse is what decides how much current can ever flow.
The second is loss. Every foot of cable drops a little voltage, and at 12V there is very little to spare. A 3% drop on a 12V system is only 0.36V, which is why long runs need surprisingly thick cable even at modest current. This is measured on the round trip, out and back, not the one-way distance.
The single biggest saving
Moving from 12V to 24V halves the current for the same power, which cuts the required cable area dramatically. If you are planning a large inverter or a long run, changing system voltage often costs less than the copper it saves.
Common runs, worked through
Cable size is unintuitive because it depends on three things at once. These are typical battery-to-inverter runs at 12V, using the same method as the tool:
| Load | Current | Fuse | 3 ft | 10 ft | 20 ft |
|---|---|---|---|---|---|
| 600 W | 56 A | 70 A | 8 AWG | 4 AWG | 2 AWG |
| 1,000 W | 93 A | 125 A | 4 AWG | 2 AWG | 2/0 AWG |
| 2,000 W | 185 A | 250 A | 1/0 AWG | 2/0 AWG | too long * |
* A 2,000 W inverter 20 ft from the battery at 12V needs about 221,000 circular mils to hold 3% drop. The largest cable in the ABYC table, 4/0 AWG, is 211,600. No single conductor is adequate: shorten the run, move to 24V, or use parallel cables. The calculator flags this case rather than printing a gauge that would not do the job.
Read across a row and the pattern is clear: the same inverter needs progressively thicker cable as the run lengthens, even though the current has not changed. That is voltage drop taking over from ampacity as the governing requirement. Read down a column and doubling the load roughly doubles the conductor area.
The practical lesson is to shorten the run before you buy copper. Moving an inverter three feet closer to the battery is usually free at the design stage and can drop you two full cable sizes. After the build it is expensive.
Why 12V punishes long runs
A 3% drop budget is a percentage of system voltage, so it shrinks with the voltage. At 12V you have 0.36V to spend across the whole circuit. At 24V you have 0.72V, and you are carrying half the current to begin with. Those two effects compound: the same power over the same distance needs roughly a quarter of the conductor area at 24V.
This is why large inverters and 12V systems fight each other. If your build wants 3,000 W of AC, the cable and fuse costs alone are a strong argument for moving the whole system to 24V before you buy anything.
Fusing
The fuse protects the cable, not the appliance. It belongs within a few inches of the battery positive terminal, because the cable between the battery and the fuse is unprotected and live whenever the battery is connected. Size the fuse to the load, then size the cable to the fuse.
For the reasoning in full, see wire gauge for the battery-to-inverter cable, or size the inverter first if you have not chosen one yet.
Frequently asked questions
＋ What size wire do I need for a 1000W inverter?
At 12V a 1,000 W inverter draws about 93 A continuously and is fused at 125 A. Over a 5 ft run that needs 4 AWG. The same inverter at 24V draws half the current and needs only 8 AWG, which is why higher system voltage saves money on cable.
＋ How do I calculate voltage drop?
Voltage drop depends on current, cable length and conductor area. This calculator works to a 3% limit measured over the round-trip length, because current flows out along the positive conductor and back through the negative. A run measured at 5 ft is 10 ft of conductor.
＋ Why does my cable need to be thicker than the ampacity table says?
Two separate requirements apply. Ampacity says the cable must safely carry the fuse rating without overheating. Voltage drop says it must deliver usable voltage at the far end. On long runs the voltage-drop requirement is usually the thicker of the two, and the thicker answer always wins.
＋ Should I use the ABYC or NEC wire table?
For 12V and 24V vehicle and marine wiring, use ABYC E-11 at 105 °C, which is what this calculator applies. The household NEC table assumes different conditions and gives thinner results that are not appropriate for a van or boat DC circuit.