DC Wire Size Calculator
Size copper wire for a DC circuit — solar, RV, boat, or battery runs. Enter system voltage (12, 24, or 48 V), current in amps, one-way run length in feet, and a max voltage drop percent to get the AWG gauge plus the actual drop and watts lost.
Example: with System voltage (V) 12 · Current (A) 20 · One-way run length (ft) 15 · Max voltage drop (%) 3 → Recommended wire: 6 AWG copper.
- Actual drop at that gauge0.29 V (2.5%)
- Power lost in the wire5.9 W turned to heat
- Governing constraintVoltage drop governs the size
Computed by the calculator below using its default values. Change any input to see your own numbers.
Uses round-trip resistance: the current flows out and back, so resistance counts 2 × run length. Copper resistance per NEC Chapter 9 Table 8; ampacity floor per NEC Table 310.16 (75°C).
Why low-voltage DC needs fat wire
Voltage drop is the same Ohm's law everywhere, but at 12 V there is no headroom: a 0.36 V loss that a 120 V circuit would shrug off is a full 3% of a 12 V system. And DC loads at low voltage pull big currents — 240 W is 2 A at 120 V but 20 A at 12 V. High amps times round-trip resistance is why a modest solar or trolling-motor run can demand 6 AWG where house wiring would use 14.
The calculator works in round-trip terms: current flows down the positive and back on the negative, so resistance counts twice your one-way length. It then picks the smallest standard gauge that meets your drop limit and can legally carry the current.
Choosing a drop limit
Three percent is the usual target for critical circuits (solar charge lines, electronics); 5% is acceptable for lighting and general loads; 10% only for things indifferent to voltage, like resistive heaters. Battery charging deserves the tight end — a charger that sees 0.4 V less than the battery genuinely needs can leave it chronically undercharged. When a run comes out marginal, going one gauge up costs little and pays back in watts not turned to heat.
How it’s calculated
Required resistance = (V × drop% ÷ 100) ÷ amps over 2 × one-way length. Gauge = smallest AWG meeting both that resistance and ampacity ≥ load. Copper DC resistance from NEC Chapter 9 Table 8 (e.g., 14 AWG 3.07, 10 AWG 1.21, 6 AWG 0.491, 1/0 0.122 ohms/1,000 ft); ampacity from NEC Table 310.16 at 75°C (14 AWG 20 A ... 4/0 230 A). Actual drop = R/1,000 × 2L × I; loss = drop × I.
Assumes copper at typical operating temperature in free air or conduit per the 75°C column — engine rooms, bundled looms, and marine standards (ABYC) have their own derates, so verify against the standard that governs your build.
Copper wire quick reference
| AWG | Ohms per 1,000 ft | Ampacity (75°C) |
|---|---|---|
| 12 | 1.93 | 25 A |
| 10 | 1.21 | 35 A |
| 8 | 0.764 | 50 A |
| 6 | 0.491 | 65 A |
| 4 | 0.308 | 85 A |
| 2 | 0.194 | 115 A |
NEC Chapter 9 Table 8 (uncoated copper, DC resistance) and NEC Table 310.16 ampacities, 75°C column.
Common mistakes
- Using one-way length in a drop formula — the return conductor doubles the resistance, and forgetting it undersizes every run.
- Sizing on voltage drop alone: a short fat-load run can pass the drop test with wire too small to carry the amps safely. Ampacity always sets the floor.
- Assuming automotive "gauge" equals AWG; some imported wire is metric or overstated, and CCA (copper-clad aluminum) has about 60% of copper's conductivity.
- Ignoring fusing — this calculator sizes wire, but every positive DC conductor still needs overcurrent protection sized to the wire.
Frequently asked questions
What formula sizes DC wire?
Max loop resistance = allowed drop volts ÷ amps, spread over 2 × the one-way length. Convert to ohms per 1,000 ft and pick the smallest AWG at or below it from the copper table, then check the gauge can carry the current (ampacity).
Why does the run length count twice?
DC circuits are loops — out on positive, back on negative. A 15 ft run has 30 ft of conductor in series, and both halves drop voltage.
What voltage drop is acceptable on 12V?
3% (0.36 V) for charging circuits and electronics, 5% for lights and pumps. At 12 V there is little margin: many devices misbehave below about 11 V, which is only a 8-9% drop from a resting battery.
Can I use this for 120V household wiring?
The physics is identical, but AC branch circuits are governed by NEC rules on breakers, conductor types, and termination ratings. Use a dedicated AC voltage-drop tool and, for new circuits, an electrician.
Does wire size change for 24V or 48V systems?
Dramatically — doubling voltage halves the amps for the same watts and doubles the allowed drop volts, so a 48 V system can move the same power on wire several gauges smaller. That is exactly why solar arrays run at higher voltage.