Trolling motor wire size
Use the motor's maximum current draw from its manual and the one-way distance from the battery to the motor's plug. A bow-mount motor with the batteries at the stern can need a long run. The table uses 50 A; change the calculator to your motor's figure.
Use 4 AWG (21.2 mm²) copper or larger.
Voltage drop sets the size. 6 AWG would drop 3.29%, over your 3% limit. Ampacity alone would allow 8 AWG.
Check: 4 AWG (21.2 mm²)
| Load current | 50 A |
| Wire in the circuit (out and back) | 2 x 20 ft = 40 ft |
| Resistance of 4 AWG | 0.2485 ohm per 1000 ft; 0.00994 ohm for this circuit |
| Voltage drop | 0.497 V = 2.07% within 3% |
| Voltage at the load | 23.5 V |
| Power lost in the wire | 24.8 W |
| Ampacity of 4 AWG | 85 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 50 x 2 x 20 x 0.2485 / 1000 = 0.497 V, and 0.497 / 24 = 2.07%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 12.77 | 53.21 | 11.23 | 10* | drop and current |
| 16 | 1.31 | 8.032 | 33.47 | 15.97 | 13* | drop and current |
| 14 | 2.08 | 5.05 | 21.04 | 18.95 | 20 | drop and current |
| 12 | 3.31 | 3.176 | 13.23 | 20.82 | 25 | drop and current |
| 10 | 5.26 | 1.998 | 8.32 | 22 | 35 | drop and current |
| 8 | 8.37 | 1.256 | 5.24 | 22.74 | 50 | too much drop |
| 6 | 13.3 | 0.79 | 3.29 | 23.21 | 65 | too much drop |
| 4 | 21.2 | 0.497 | 2.07 | 23.5 | 85 | smallest that passes |
| 3 | 26.7 | 0.394 | 1.64 | 23.61 | 100 | passes |
| 2 | 33.6 | 0.313 | 1.3 | 23.69 | 115 | passes |
| 1 | 42.4 | 0.248 | 1.03 | 23.75 | 130 | passes |
| 1/0 | 53.5 | 0.197 | 0.82 | 23.8 | 150 | passes |
| 2/0 | 67.4 | 0.156 | 0.65 | 23.84 | 175 | passes |
| 3/0 | 85 | 0.124 | 0.51 | 23.88 | 200 | passes |
| 4/0 | 107 | 0.098 | 0.41 | 23.9 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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Wire size for a 50 A trolling motor, by length and voltage
| One-way length | 12V, 50 A, 3% | 24V, 50 A, 3% | 36V, 50 A, 3% |
|---|---|---|---|
| 10 ft (3 m) | 4 | 8 | 8* |
| 15 ft (4.6 m) | 3 | 6 | 8 |
| 20 ft (6.1 m) | 2 | 4 | 6 |
| 25 ft (7.6 m) | 1 | 4 | 6 |
| 30 ft (9.1 m) | 1/0 | 3 | 4 |
Copper, minimum AWG. * ampacity, not voltage drop, sets this size. "over 4/0" means no single copper conductor up to 4/0 passes: shorten the run, use a higher system voltage or run conductors in parallel. Length is one way; the math uses twice that, out and back.
Worked example
50 A at 24 V, 20 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 20 = 40 ft of wire, and the allowed drop is 24 x 3 / 100 =
0.72 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 6 AWG:
50 x 40 x 0.3951 / 1000 = 0.79 V, which is 3.29% of 24 V; over the 3% limit. - 4 AWG:
50 x 40 x 0.2485 / 1000 = 0.497 V, which is 2.07% of 24 V; passes both checks.
Answer: 4 AWG (21.2 mm²), rated 85 A, the smallest size that passes both.
Marine wiring
On a boat, ABYC E-11 is the usual standard for DC wiring. It asks for stranded copper, rated for the location, and an overcurrent device near the battery. Motor makers often list a cable size and breaker by length; if theirs is thicker than this result, use theirs.
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.
Questions
Why does a 36V trolling motor use thinner wire than a 12V one?
Higher voltage motors draw less current for the same thrust, and the allowed drop in volts is larger. Both mean thinner wire for the same run.