15 Amp Wire Size at 300 Feet (12V DC (Solar / Automotive / Marine))
Complete conductor sizing, Ohm's law voltage drop, $I^2R$ line power loss, and circuit breaker specification for a 15 A load over a 300 ft (91.4 m) one-way run.
Copper Wire (≤3% Drop)350 kcmil177.3 mm² · Drop: 2.76%
Aluminum Wire (≤3% Drop)600 kcmilDrop: 0.32V (2.65%)
Voltage at Load End11.7 VLoss: 0.33 V (5.0 W)
Power & Breaker (125%)0.18 kW20A Breaker Recommended
All AWG & kcmil Wire Gauges for 15 Amps at 300 Feet (12V)
| Wire Gauge | Metric Area | NEC 75°C Cu Ampacity | Copper Voltage Drop | Load Voltage | Aluminum Drop | Line Loss ($I^2R$) | NEC Status |
|---|---|---|---|---|---|---|---|
| 14 AWG | 2.08 mm² | 15 A | 28.22 V (235.19%) | 0.0 V | 46.56 V (387.96%) | 423.3 W | Drop/Amp High |
| 12 AWG | 3.31 mm² | 20 A | 17.74 V (147.8%) | 0.0 V | 29.26 V (243.79%) | 266.0 W | Drop/Amp High |
| 10 AWG | 5.26 mm² | 30 A | 11.16 V (93.0%) | 0.8 V | 18.41 V (153.41%) | 167.4 W | Drop/Amp High |
| 8 AWG | 8.37 mm² | 50 A | 7.01 V (58.45%) | 5.0 V | 11.57 V (96.41%) | 105.2 W | Drop/Amp High |
| 6 AWG | 13.3 mm² | 65 A | 4.41 V (36.78%) | 7.6 V | 7.28 V (60.67%) | 66.2 W | Drop/Amp High |
| 4 AWG | 21.15 mm² | 85 A | 2.78 V (23.13%) | 9.2 V | 4.58 V (38.15%) | 41.6 W | Drop/Amp High |
| 3 AWG | 26.67 mm² | 100 A | 2.2 V (18.34%) | 9.8 V | 3.63 V (30.26%) | 33.0 W | Drop/Amp High |
| 2 AWG | 33.62 mm² | 115 A | 1.75 V (14.55%) | 10.3 V | 2.88 V (24.0%) | 26.2 W | Drop/Amp High |
| 1 AWG | 42.41 mm² | 130 A | 1.38 V (11.54%) | 10.6 V | 2.28 V (19.03%) | 20.8 W | Drop/Amp High |
| 1/0 AWG | 53.49 mm² | 150 A | 1.1 V (9.15%) | 10.9 V | 1.81 V (15.09%) | 16.5 W | Drop/Amp High |
| 2/0 AWG | 67.43 mm² | 175 A | 0.87 V (7.25%) | 11.1 V | 1.44 V (11.97%) | 13.1 W | Drop/Amp High |
| 3/0 AWG | 85.01 mm² | 200 A | 0.69 V (5.75%) | 11.3 V | 1.14 V (9.49%) | 10.4 W | Drop/Amp High |
| 4/0 AWG | 107.2 mm² | 230 A | 0.55 V (4.56%) | 11.5 V | 0.9 V (7.53%) | 8.2 W | ~ ≤5% |
| 250 kcmil | 126.7 mm² | 255 A | 0.46 V (3.86%) | 11.5 V | 0.76 V (6.37%) | 7.0 W | ~ ≤5% |
| 300 kcmil | 152.0 mm² | 285 A | 0.39 V (3.22%) | 11.6 V | 0.64 V (5.31%) | 5.8 W | ~ ≤5% |
| 350 kcmil | 177.3 mm² | 310 A | 0.33 V (2.76%) | 11.7 V | 0.55 V (4.55%) | 5.0 W | ✓ PASS (≤3%) |
| 400 kcmil | 202.7 mm² | 335 A | 0.29 V (2.41%) | 11.7 V | 0.48 V (3.98%) | 4.3 W | ✓ PASS (≤3%) |
| 500 kcmil | 253.4 mm² | 380 A | 0.23 V (1.93%) | 11.8 V | 0.38 V (3.18%) | 3.5 W | ✓ PASS (≤3%) |
| 600 kcmil | 304.0 mm² | 420 A | 0.19 V (1.61%) | 11.8 V | 0.32 V (2.65%) | 2.9 W | ✓ PASS (≤3%) |
| 750 kcmil | 380.0 mm² | 475 A | 0.15 V (1.29%) | 11.8 V | 0.25 V (2.12%) | 2.3 W | ✓ PASS (≤3%) |
| 1000 kcmil | 506.7 mm² | 545 A | 0.12 V (0.97%) | 11.9 V | 0.19 V (1.59%) | 1.7 W | ✓ PASS (≤3%) |
Frequently Asked Engineering Questions
What size copper wire do I need for 15 amps at 300 feet (12V)?
To stay within the NEC recommended 3% maximum voltage drop for a 15A load over 300 feet one-way at 12V (DC), you need 350 kcmil copper wire. This produces a voltage drop of 0.33V (2.76%) and delivers 11.7V at the load.
What size aluminum wire is required for 15 amps at 300 feet?
Using aluminum conductors (75°C rating), you should use 600 kcmil to maintain voltage drop at 0.32V (2.65%).
What breaker size is needed for a 15A continuous load?
Under NEC 210.20(A), continuous loads require a circuit breaker rated at 125% of the continuous current (15A × 1.25 = 18.8A), which corresponds to a standard 20A circuit breaker.
Compare Other Distances & Amperages
15 Amp Wire Size by Distance (12V)
15A at 20 ft (12V)15A at 30 ft (12V)15A at 40 ft (12V)15A at 50 ft (12V)15A at 60 ft (12V)15A at 75 ft (12V)15A at 80 ft (12V)15A at 100 ft (12V)15A at 120 ft (12V)15A at 125 ft (12V)15A at 150 ft (12V)15A at 175 ft (12V)