Voltage Drop CalculatorBoat wire size calculator
Wire size for 12 V and 24 V boat circuits, checked against the ABYC E-11 voltage drop limits: 3% for critical circuits, 10% for the rest.
Smallest wire for 3%
Copper| Wire size | 8 AWG |
| Metric, same or larger | 10 mm² |
| Area | 8.37 mm² |
| Voltage drop | 0.31 V |
| Percent drop | 2.60 % |
| At the load | 11.69 V |
Within 3% target
At other limits
- 8 AWG at 3%
- 12 AWG at 10%
Worked out with
- Copper at 75 °C (K = 12.9)
- 20 ft one way; the return wire is added
- 10 A, the current the load draws
- DC, 12 V
8 AWG. Drop 0.31 volts, 2.60 percent. Within 3% target.
- The fuse or breaker doesn't get bigger. Voltage drop sets the wire size, not the protection. Keep the fuse the circuit needs.
- Check the wire fits. Look up the largest wire the terminals and lugs take. If 8 AWG won't fit, ask an electrician how to connect it where you live.
- Cheaper ways to the same result: the same power at 24 V (half the current) needs 14 AWG.
How this is worked out
V = 2 × K × I × L ÷ CM
2 × 12.9 × 10 A × 20 ft ÷ 16,510 CM = 0.31 V
0.31 V ÷ 12 V × 100 = 2.60 %. At the load: 12 V − 0.31 V = 11.69 V.
- L is the one-way length; the 2 counts the return conductor.
- K is 12.9 for copper and 21.2 for aluminum, in ohm·CM per foot on a 75 °C basis.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
ABYC E-11 voltage drop limits
- 3% for panel main feeders, bilge blowers, electronics, navigation lights and other circuits where the drop must be kept low. That is 0.36 V on 12 V and 0.72 V on 24 V.
- 10% for other lighting and non-critical loads: 1.2 V on 12 V, 2.4 V on 24 V.
- In practice, many installers treat bilge pumps as 3% circuits, and makers such as Blue Sea and Ancor list windlasses as 10% loads. When in doubt, size to 3%.
- The standard is ABYC E-11, AC and DC Electrical Systems on Boats; the current edition is the 2025 revision (as of November 2026). ABYC standards are voluntary industry standards that most US boatbuilders follow, not federal law.
Worked example: electronics 20 ft from the panel
A chartplotter and radar draw 10 A on a 12 V circuit, 20 ft from the panel one way (40 ft of wire in all). Electronics are a 3% circuit:
| Circuit | Smallest size | Drop |
|---|---|---|
| Electronics, 3% | 8 AWG | 2.60 % |
| Lighting, 10% | 12 AWG | 6.59 % |
The 3% limit needs 8 AWG (0.31 V of drop), where a 10% lighting circuit of the same current and length would allow 12 AWG (0.79 V). On a 24 V system the same 3% circuit needs only 10 AWG, because the same drop in volts is half the percentage. The calculator above starts with these numbers.
Measuring the length
- ABYC counts the whole circuit, from the positive source to the device and back to the negative. Enter the one-way distance here: the calculator doubles it.
- Unequal runs: if the negative goes back to a different point, add the positive and negative runs and enter half the total.
- Branch circuits are measured from the panel that feeds them, not from the battery.
Marine wire
- Stranded copper only, never solid. Finer stranding is required where the wire flexes, and is usual in marine wire.
- 16 AWG minimum, under both ABYC E-11 and the Coast Guard rule for gasoline boats; 18 AWG only in limited cases, such as inside a multi-conductor sheath.
- Tinned copper is not required, but it resists corrosion and is recommended, especially in salt water. Most marine wire sold is tinned UL 1426 boat cable.
- AWG, not SAE. Boat cable is sized in AWG. Automotive SAE wire of the same gauge number can have about 5–12% less copper, so it drops more. This calculator assumes AWG copper.
This calculator and ABYC’s tables
This calculator uses copper resistance at 75 °C (K = 12.9, details on the method page), a warm-wire figure. That is about 20% above the room-temperature copper constant in the sizing formula printed in older editions of E-11, so the results here lean cautious and may suggest one size larger than an ABYC table near the limit. Tinned wire’s slightly higher resistance, a few percent, is well inside that margin.
Fuses and current rating
- Current rating. The wire must also carry the current safely. Its rating depends on the insulation, engine-room heat and bundling; where the voltage drop and the current rating disagree, use the larger wire. Enter the rating in the optional ampacity field and the calculator checks your current against it.
- Fuse placement: put the fuse or breaker within 7 in of the battery or other power source, measured along the wire. Up to 72 in is allowed for a sheathed or enclosed wire straight from the battery terminal, and up to 40 in for a sheathed wire from another source such as a switch. Engine cranking cables are exempt.
- Coast Guard rules: 33 CFR 183 Subpart I covers boats with gasoline inboard, sterndrive or generator engines (not outboards): ignition protection, batteries, wiring and fuses. It sets no voltage-drop limit; the 3% and 10% figures come from ABYC.
Frequently asked questions
What voltage drop does ABYC allow?
ABYC E-11 sets 3% for panel main feeders, bilge blowers, electronics, navigation lights and other circuits where the drop must be kept low, and 10% for other lighting and non-critical loads. On 12 V that is 0.36 V and 1.2 V; on 24 V, 0.72 V and 2.4 V.
Do I measure the wire one way or round trip?
ABYC counts the whole circuit: from the positive source to the device and back to the negative. This calculator asks for the one-way distance and doubles it, which is the same when both runs are equal. If the negative run is a different length, add the two runs and enter half the total. For a circuit fed from a panel, measure from the panel.
Is a bilge pump a 3% circuit?
ABYC names bilge blowers, not pumps, in its 3% list. Many installers size bilge pumps to 3% anyway, and when in doubt 3% is the safer choice.
What is the smallest wire allowed on a boat?
16 AWG. ABYC E-11 and the Coast Guard rule for gasoline boats both set it as the minimum; 18 AWG is allowed only in limited cases, such as inside a multi-conductor sheath.
Does marine wire have to be tinned?
No. ABYC requires stranded copper, with finer stranding where the wire flexes, but not tinning. Tinned wire resists corrosion and is recommended, especially in salt water, and most marine wire sold is tinned.
Can I use automotive wire on a boat?
Marine boat cable (UL 1426) is sized in AWG. Automotive SAE wire with the same gauge number can have about 5–12% less copper, so it drops more voltage. If you use SAE wire and are close to the limit, go up a size.
Related: 12V wire size calculator for vans and RVs, voltage drop chart (pick 12 V), wire gauge chart (AWG to mm²), ampacity vs voltage drop, how the calculator works.