Bilge pump wire size
A bilge pump that sees low voltage pumps less water, so treat it as a critical load and use 3%. Take the current from the pump's label or manual and measure the run from the battery or panel to the pump.
Use 10 AWG (5.26 mm²) copper or larger.
Voltage drop sets the size. 12 AWG would drop 4.23%, over your 3% limit. Ampacity alone would allow 18 AWG.
Check: 10 AWG (5.26 mm²)
| Load current | 8 A |
| Wire in the circuit (out and back) | 2 x 20 ft = 40 ft |
| Resistance of 10 AWG | 0.9989 ohm per 1000 ft; 0.03996 ohm for this circuit |
| Voltage drop | 0.32 V = 2.66% within 3% |
| Voltage at the load | 11.68 V |
| Power lost in the wire | 2.56 W |
| Ampacity of 10 AWG | 35 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 8 x 2 x 20 x 0.9989 / 1000 = 0.32 V, and 0.32 / 12 = 2.66%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 2.043 | 17.03 | 9.96 | 10* | too much drop |
| 16 | 1.31 | 1.285 | 10.71 | 10.71 | 13* | too much drop |
| 14 | 2.08 | 0.808 | 6.73 | 11.19 | 20 | too much drop |
| 12 | 3.31 | 0.508 | 4.23 | 11.49 | 25 | too much drop |
| 10 | 5.26 | 0.32 | 2.66 | 11.68 | 35 | smallest that passes |
| 8 | 8.37 | 0.201 | 1.68 | 11.8 | 50 | passes |
| 6 | 13.3 | 0.126 | 1.05 | 11.87 | 65 | passes |
| 4 | 21.2 | 0.08 | 0.66 | 11.92 | 85 | passes |
| 3 | 26.7 | 0.063 | 0.53 | 11.94 | 100 | passes |
| 2 | 33.6 | 0.05 | 0.42 | 11.95 | 115 | passes |
| 1 | 42.4 | 0.04 | 0.33 | 11.96 | 130 | passes |
| 1/0 | 53.5 | 0.031 | 0.26 | 11.97 | 150 | passes |
| 2/0 | 67.4 | 0.025 | 0.21 | 11.98 | 175 | passes |
| 3/0 | 85 | 0.02 | 0.16 | 11.98 | 200 | passes |
| 4/0 | 107 | 0.016 | 0.13 | 11.98 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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Bilge pump wire size by length at 12V, 3% drop
| One-way length | 3 A | 5 A | 8 A | 10 A | 15 A |
|---|---|---|---|---|---|
| 5 ft (1.5 m) | 18 | 18 | 16 | 14 | 12 |
| 10 ft (3 m) | 16 | 14 | 12 | 12 | 10 |
| 15 ft (4.6 m) | 14 | 12 | 10 | 10 | 8 |
| 20 ft (6.1 m) | 14 | 12 | 10 | 8 | 6 |
| 25 ft (7.6 m) | 12 | 10 | 8 | 8 | 6 |
| 30 ft (9.1 m) | 12 | 10 | 8 | 6 | 6 |
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
8 A at 12 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 12 x 3 / 100 =
0.36 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 12 AWG:
8 x 40 x 1.588 / 1000 = 0.508 V, which is 4.23% of 12 V; over the 3% limit. - 10 AWG:
8 x 40 x 0.9989 / 1000 = 0.32 V, which is 2.66% of 12 V; passes both checks.
Answer: 10 AWG (5.26 mm²), rated 35 A, the smallest size that passes both.
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
Can bilge pump wire run through the bilge?
Only wire rated for wet locations, with connections kept as high as you can. Marine standards such as ABYC E-11 set the rules for that; follow them.