12 Volt Wire Size Calculator

Find the recommended AWG wire gauge for a 12V circuit from current draw, one-way wire length, and your allowable voltage drop, using the standard circular-mil sizing formula.

Quick Facts

Formula
Circular mils = (2 × K × Length × Amps) ÷ Allowable Drop (V)
K = 10.75 Ω·cmil/ft for copper, ≈17 for aluminum; length is doubled for the return conductor.
Typical limits
3% drop for critical circuits, 10% for general loads
Lower allowable drop always means a thicker (lower AWG number) wire.

Your Results

Calculated
Recommended wire gauge
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Smallest standard AWG that meets your limit
Required circular mils
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Minimum cross-sectional area needed
Actual voltage drop
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At the recommended gauge
Voltage at the load
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System voltage minus the drop

Ready

Enter current, length, voltage, and allowable drop, then press Calculate.

Understanding 12V Wire Sizing

Undersized wire on a 12-volt circuit causes a voltage drop — energy lost to the wire's own resistance before it reaches the load. Too much drop dims lights, weakens pumps and winches, and can overheat the wire's insulation. Wire size calculators solve this by working backward from how much drop you can tolerate to the minimum copper (or aluminum) cross-section needed for a given current and wire run.

The formula

The standard voltage-drop method used across marine, RV, and automotive 12V wiring guides calculates the required cross-sectional area in circular mils (CM):

CM = (2 × K × L × I) ÷ VD

  • K — the resistivity constant of the conductor: 10.75 Ω·cmil/ft for copper, roughly 17 Ω·cmil/ft for aluminum (aluminum resists current about 1.6× more than copper).
  • L — the one-way distance from the power source to the load, in feet. It is doubled in the formula because current must travel out through the positive wire and back through the negative (or ground) wire to complete the circuit.
  • I — the current draw of the load, in amps.
  • VD — the allowable voltage drop in volts, equal to the system voltage times your chosen drop percentage (for example, 12V × 3% = 0.36V).

Once the required circular-mil area is known, it is matched to the smallest standard AWG wire gauge whose circular-mil rating meets or exceeds it — larger circular-mil numbers correspond to lower (thicker) AWG sizes.

Choosing an allowable voltage drop

Marine and RV wiring guidance commonly recommends keeping drop under 3% for critical circuits — bilge pumps, navigation lights, engine controls — where a weak signal or slow pump matters. A looser 10% limit is generally considered acceptable for less sensitive loads like interior lighting or accessories. A lower percentage always demands thicker wire for the same current and distance.

What this calculator does not cover

This tool sizes wire strictly by voltage drop. It does not check the wire's ampacity (the maximum current its insulation can safely carry before overheating), which mainly matters on very short, high-current runs where voltage drop is negligible but heat build-up is not. For those cases, cross-check the recommended gauge against an ampacity chart for the wire's insulation type and installation conditions.

Frequently Asked Questions

Should I enter the one-way length or the round-trip length?
Enter the one-way distance from the power source to the load. The calculator automatically doubles it to account for the return conductor, since current has to travel out and back to complete a 12V circuit. Entering a length you already doubled will oversize the wire.
What voltage drop percentage should I use?
A commonly used guideline is to keep voltage drop under 3% for critical circuits such as bilge pumps, navigation lights, and engine wiring, and under 10% for less sensitive loads like interior lighting or accessories. Lower percentages are safer but require thicker, more expensive wire.
Does this account for the wire's amp rating, not just voltage drop?
No. This calculator sizes wire using the voltage-drop method, which is usually the limiting factor on long 12V runs. On very short, high-current runs the wire's insulation temperature rating (ampacity) can require a thicker gauge than voltage drop alone suggests, so check an ampacity chart for the wire and insulation type before finalizing short, high-current circuits.
Why does aluminum wire need to be thicker than copper for the same job?
Aluminum has higher electrical resistance than copper — roughly 1.6 times as much for a given cross-sectional area. To carry the same current with the same voltage drop, aluminum wire must have a larger circular-mil area, which is why the calculator recommends a thicker gauge when aluminum is selected.

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