DC Wire Size Calculator

Size copper DC wire or cable for 12V, 24V and 48V systems. Enter amps and one-way length to check AWG, voltage drop and NEC ampacity.

The formula, every assumption, and the primary sources behind this tool are published on this page.

Current (A)DC amps on this cable. From DC-side watts: A = W ÷ V. From inverter AC output watts: A = W ÷ (battery V × 0.90) at the default efficiency; substitute your efficiency or paste the DC current from Inverter Size Calculator

System voltageThe DC bus this cable sits on. Higher voltage means less current and a thinner wire for the same watts.

One-way length unit

One-way conductor length (ft)Battery to load, one way. The formula doubles this for the return conductor.

Voltage-drop budget (%)3% is the usual design target. Sensitive electronics sometimes use 2%; a short, cheap run can live with 5%.

Runs 3+ h at a stretchCheck only if this current lasts three hours without a break. That raises the ampacity demand by 125%; voltage drop still uses the amps you typed.

Reset

Results

Example — computed from the default inputs above. Change any field and press Compute.

Recommended copper wire: 6 AWG

Voltage drop is what sizes this cable.

One-way conductor length: 15 ft / 4.572 m

Geometric cross-sectional area: 13.3 mm²

mm² is converted from this AWG row's circular mils. It does not select a metric standard wire or establish its ampacity.

Circular mils the drop formula asks for: 21,500

Circular mils of the recommended gauge: 26,240

Voltage drop on the recommended gauge: 0.29 V (2.46%)

Voltage at the load: 11.71 V

Heat lost in the cable: 5.89 W

Ampacity the cable must cover: 20 A

Effective ampacity of the recommended gauge: 65 A

12V, 24V and 48V DC cable sizing examples

Each example carries 20 A over 15 ft one way, with a 3% voltage-drop limit and a non-continuous load. Results use the copper tables and assumptions below. Open an example to change the current, length or duty cycle.

Voltage	Copper wire	Actual drop
12 V	6 AWG	2.46%
24 V	8 AWG	1.94%
48 V	12 AWG	2.47%

How it works

A DC cable has to clear two independent checks: voltage drop on the round-trip run, and the current the copper (and its breaker) can carry. The circular-mil formula finds the drop-limited size; the pick is the smallest AWG that also passes the NEC ampacity table.

required_cm = 2 × K × I × L / VDVD = system_voltage × (voltage_drop_pct / 100)actual_drop_v = 2 × L × (R / 1000) × Iampacity_need = I, or I × 1.25 if continuousrecommended = smallest AWG with cmil ≥ required_cm AND actual_drop_v ≤ VD AND effective_ampacity ≥ ampacity_need

K: 12.9 — IAEI copper k-factor at 75 °C, derived from NEC Chapter 9 Table 8 (ohm/kFT × circular mils).
I: DC current on the run, in amperes. DC-side watts divide by voltage; inverter AC watts divide by battery voltage × 0.90 (same as the inverter-size tool). Voltage drop uses this number even when the load is continuous.
L: One-way length in feet. Both formulas multiply by 2 for the return conductor.
VD: Allowed drop in volts: system voltage × (budget percent ÷ 100). At 12 V and 3% that is 0.36 V.
required_cm: Minimum circular mils from the k-factor formula. A gauge still has to beat this number.
actual_drop_v: Round-trip drop using that gauge's Table 8 stranded resistance at 75 °C, not the rounded k-factor.
ampacity_need: Current the conductor must be rated for. Continuous loads (3+ hours at a stretch) are multiplied by 1.25. 14/12/10 AWG then take the 240.4(D) cap of 15/20/30 A.

Assumptions

1 international foot = 0.3048 m exactly. Length is one-way; resistance includes the return. mm² is geometric area, not a separate metric conductor rating.
Copper stranded cable only. Aluminum needs a different k-factor (about 21.2) and is not in v1.
Length is one way. Entering a round-trip distance would double the drop a second time.
Resistance is NEC Chapter 9 Table 8 at 75 °C, which matches 75 °C terminations (110.14(C)). Charts published at 20–30 °C (some marine tables) read about one size smaller.
14/12/10 AWG copper are capped at 15/20/30 A by NEC 240.4(D), even though Table 310.16 lists 20/25/35 A at 75 °C.
3% is the NEC 210.19 informational-note convention for a branch, not a mandatory Code rule. Feeder-plus-branch totaling 5% is the matching note.
The 125% factor applies only to ampacity, and only when you mark the run continuous. Voltage drop stays on the amps you entered.
A = W ÷ V is only correct for DC-side watts. Inverter AC output still has to cover inverter loss: A = W ÷ (V × 0.90), the same 90% default as the inverter-size tool. A 1,200 W inverter on 12 V is about 111 A on the DC cable, not 100 A.
This tool sizes the conductor. It does not pick a fuse or breaker, place the OCPD, or apply ambient / bundling derates from 310.15.
Boat and RV cable charts (ABYC E-11, some chassis-harness tables) are not the NEC table. They can recommend a different gauge for the same amps and feet.

FAQ

What size wire for 30 amps at 12 volts?

It depends on length. At the default 15 ft one-way and a 3% budget, 30 A on 12 V needs 4 AWG copper (6 AWG drops 3.7%). A 5 ft run of the same current is mostly an ampacity problem: 10 AWG would nearly hold 3%, but NEC 240.4(D) caps it at 30 A, so a continuous 30 A load still steps up.

Is 3% voltage drop a code requirement?

No. NEC 210.19's 3% / 5% figures sit in an informational note — they describe reasonable efficiency, they do not require a size. A 12 V inverter input is unforgiving of sag, so 3% is still the sensible default. Tighten it to 2% for electronics; loosen it only when you accept the extra heat.

Is wire length one-way or round trip?

One way — battery to the load. Both the circular-mil formula and the Table 8 resistance check multiply by 2 for the return conductor. If you type a round-trip tape measure, the drop is counted twice and the wire comes out a size (or two) heavy.

Sources

Related calculators

Some product links on this site may be affiliate links; we may earn a commission at no extra cost to you. Read the Affiliate Disclosure.