Wiring

Marine Wire Gauge Chart

Find the wire size for a 12-volt boat circuit from the current it carries and the length of the run, out and back. The charts follow the method in ABYC E-11, the American Boat and Yacht Council standard for boat wiring.

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Size the wire for the current and the round-trip length.

How to use the chart

Find the current in amps on the device's label or in its manual.
Measure the run out and back: from the power source to the device and back to the negative connection. A device 12 feet from the battery is a 24-foot run.
Pick the table: 3% for circuits where voltage drop matters, 10% for the rest (see below).
Between columns, round up to the longer length and the higher current.

3% or 10% voltage drop?

ABYC E-11 sizes panel feeders, bilge blowers, electronic equipment, navigation lights and other circuits where voltage drop must be kept to a minimum for no more than 3% drop. Lighting other than navigation lights, and circuits where drop does not matter, may drop up to 10%. When in doubt, use the 3% table. It costs a size or two of wire.

12-volt marine wire size chart, 3% voltage drop

AWG wire size by current and round-trip length.

Current	10 ft	15 ft	20 ft	25 ft	30 ft	40 ft	50 ft	60 ft	80 ft	100 ft
5 A	16	16	14	12	12	10	10	10	8	6
10 A	14	12	10	10	10	8	6	6	6	4
15 A	12	10	10	8	8	6	6	4	4	2
20 A	10	10	8	6	6	6	4	4	2	2
25 A	10	8	6	6	6	4	4	2	2	1
30 A	10	8	6	6	4	4	2	2	1	1/0
40 A	8	6	6	4	4	2	2	1	1/0	2/0
50 A	6	6	4	4	2	2	1	1/0	2/0	3/0
60 A	6	4	4	2	2	1	1/0	2/0	3/0	4/0

12-volt marine wire size chart, 10% voltage drop

AWG wire size by current and round-trip length. On short runs with high current, heat sets the size before voltage drop does.

Current	10 ft	15 ft	20 ft	25 ft	30 ft	40 ft	50 ft	60 ft	80 ft	100 ft
5 A	16	16	16	16	16	16	16	14	14	12
10 A	16	16	16	16	14	14	12	12	10	10
15 A	16	16	14	14	12	12	10	10	8	8
20 A	14	14	14	12	12	10	10	8	8	6
25 A	12	12	12	12	10	10	8	8	6	6
30 A	12	12	12	10	10	8	8	6	6	4
40 A	10	10	10	10	8	8	6	6	4	4
50 A	8	8	8	8	8	6	6	4	4	2
60 A	6	6	6	6	6	6	4	4	2	2

The voltage drop formula

ABYC E-11 gives the formula behind the chart: circular mils = 10.75 × amps × round-trip feet ÷ allowed voltage drop. At 12 volts the allowed drop is 0.36 volts at 3% and 1.2 volts at 10%. Then convert circular mils to a gauge and round up.

At 24 volts the allowed drop in volts doubles (0.72 volts at 3%), so the same circuit needs half the circular mils for voltage drop. Heat limits do not change with voltage, so check the wire still carries the current and use the larger of the two sizes.

Marine wire and automotive wire

ABYC E-11 calls for stranded copper on boats, at least 16 AWG, with oil-resistant insulation rated at least 75 °C in engine spaces. Same gauge number, less copper: ABYC lists 12 AWG at 6,530 circular mils and 12-gauge SAE automotive wire at 5,833, about 11% less.

Protect the wire with a fuse

ABYC E-11 calls for a fuse or breaker on the positive wire within 7 inches of where it connects to the battery or power source. Two exceptions apply when the wire runs in a sheath or conduit: up to 72 inches if it connects directly to a battery terminal, and up to 40 inches if it connects to another power source.

The fuse protects the wire. Use the rating the device maker specifies, and never one above what the wire can carry.

Common questions

Round trip. Measure from the power source to the device and back to the negative connection.

Use the pump's current draw and the round-trip length in the 3% table. Rule's float switch instructions add that wire added to a pump's leads should be at least the same gauge, and one size larger beyond 20 feet.

SAE automotive wire carries less copper than AWG wire of the same gauge number, and ABYC E-11 calls for stranded copper sized in AWG. Marine primary wire is made for this.

Trolling motors draw high current over long runs from the stern battery to the bow. See the trolling motor wiring page for sizes by motor.

The American Boat and Yacht Council, which writes the standards US boat builders follow. E-11 is its standard for AC and DC electrical systems on boats.

Sources

ABYC E-11, AC and DC Electrical Systems on Boats, as excerpted by Paneltronics; Rule's float switch instructions.

Specifications come from manufacturers' published documents and can change. Check your engine or boat maker's current manual before you change a setting or fit a part.