Marine Wire Size and Ampacity


By Tom Burden, Last updated: 5/7/2018


Use the charts here to determine the correct wire gauge, based on the amount of current in the circuit and the length of the wire run.

Even the experts have to check occasionally on the correct gauge and ampacity (maximum amount of electrical current a conductor can carry) of wire for a given marine DC load. The simplest method we’ve found uses the charts below.

  • Select either the 10% or 3% voltage drop chart, based on the type of load you are running.
  • Next, find the current consumption of the load on the horizontal axis of the chart.
  • Find the length of the circuit on the vertical axis of the chart, noting that the length is the “round trip” distance from the panel or battery to the load and back.
  • The wire size listed in the graph at the intersection denotes the gauge of wire to use.

We’ve included copper wire specifications which comply with the AWG standards at the bottom.

Of particular interest is the equation:
Voltage Drop = Current x Length x Ohms per foot

This simple equation allows you to calculate the voltage drop for a circuit of any length and any current flow, if you know the resistance of the wire.

Finally, note that the amp capacity of the wire curtails using very short lengths of wire for large current flows, as show by the “flat tops” of the 10% chart areas.

3% Voltage Drop

Use 3% voltage drop for critical applications affecting the safety of your boat and crew, such as running lights, blowers, electronics and panel board feeds. Remember that the Length is a "round trip" distance. This table is for 12-Volt systems only.

Current (Amps)
Length 5A10A15A20A25A30A40A50A60A70A80A90A100A
10' (3m)181412101086666644
15' (5m)16121010886644422
20' (6m)1410108666442222
25' (8m)121086664422211
30' (9m)121086444222211
40' (12m)10866442211/01/02/02/0
50' (15m)1066442211/02/03/04/04/0
60' (18m)106642211/02/03/03/04/04/0
70' (21m)8642211/02/03/03/04/04/0
80' (24m)8642211/02/03/04/04/0
90' (27m)842211/02/03/04/04/0
100' (30m)642211/02/03/04/0
110' (33m)642211/02/03/04/0
120' (36m)64211/02/03/04/0
130' (40m)62211/02/03/04/0
140' (43m)6221/02/03/04/0
150' (46m)6211/02/03/04/0
160' (49m)6211/02/03/04/0
170' (52m)6212/03/04/04/0

10% Voltage Drop

Use 10% voltage drop for non-critical applications such as windlass, cabin lights, circuits other than running lights, electronics or panel board feeds. Remember that the Length is a "round trip" distance. This table is for 12-Volt systems only.

Current (Amps) →
Length 5A10A15A20A25A30A40A50A60A70A80A90A100A
10' (3m)18181816161414121010101010
15' (5m)1818161414121210108888
20' (6m)181614141212101088866
25' (8m)18161412121010886666
30' (9m)1814121210108866664
40' (12m)161412101088666444
50' (15m)16121010886644422
60' (18m)1412108866644422
70' (21m)141088666422221
80' (24m)141088664422211
90' (27m)121086664222111/0
100' (30m)12108664422111/01/0
110' (33m)128866422211/01/01/0
120' (36m)128664422111/01/02/0
130' (40m)12866442211/01/02/02/0
140' (43m)10866422111/02/02/02/0
150' (46m)1086442211/01/02/02/03/0
160' (49m)1086442211/02/02/03/03/0
170' (52m)1066422211/02/02/03/03/0

These simple graphs assume:

  1. 105°C insulation rating: All Ancor wire uses 105°C insulation rating. Lower temperature insulation cannot handle as much current
  2. AWG wire sizes, not SAE: All Ancor wire uses AWG wire sizes. SAE wire sizes are 6%–12% smaller, carry proportionally less current, and have greater resistance
  3. Wires are not run in engine spaces: Maximum current is 15% less in engine spaces, which are assumed to be 20°C hotter than non-engine spaces (50°C vs. 30°C).
  4. Conductors are not bundled: If three conductors are bundled, reduce maximum amperage by 30%. If 4–6 conductors are bundled, reduce maximum amperage by 40%. If 7–24 conductors are bundled, reduce amperage by 50%.

To Complete Your Project, Don’t Forget:

  • Heat Shrink Tubing
  • Wire Strippers, cutters and crimpers
  • Terminals: ring, spade or butt connectors

AWG Wire Specs

Wire Size (AWG) Nominal OD Weight per 1000' Cir.Mil. Area Square mm Ohms per 1000' Max Amps
18 7/64" 12lb. 1,600 0.823 6.48 20
16 1/8" 16lb. 2,600 1.31 4.00 25
14 9/64" 23lb. 4,100 2.08 2.50 35
12 5/32" 31lb. 6,500 3.31 1.75 45
10 7/32" 44lb. 10,500 5.26 0.98 60
8 5/16" 86lb. 16,800 8.37 0.62 80
6 11/32" 118lb. 26,600 13.30 0.40 120
4 13/32" 178lb. 42,000 21.15 0.24 160
2 15/32" 277lb. 66,500 33.62 0.157 210
1 17/32" 350lb. 83,690 44.21 0.127 245
1/0 9/16" 437lb. 105,600 53.49 0.099 285
2/0 5/8" 549lb. 133,000 67.43 0.077 330
3/0 11/16" 675lb. 167,800 85.01 0.062 385
4/0 13/16" 837lb. 211,600 107.20 0.049 445