ABYC E-11 Wire Gauge Calculator
Recommend wire AWG for DC circuits on boats and RVs. Follows ABYC E-11 voltage drop and ampacity guidance, with a matching fuse size for overcurrent protection.
Inputs
Critical circuit (3% drop)
Panels, navigation, electronics
Recommended gauge
4 AWG
Fuse / breaker
40 A
Voltage drop
0.30 V
Drop %
2.5%
Derated ampacity
160 A
Limit driver
Voltage drop
For 30A over 20ft at 12V (DC), 4 AWG copper with 90C insulation gives 2.5% drop at 160A derated ampacity (base 160A x 1.00 for 30C ambient, 1-conductor bundle). Use a 40A fuse. ABYC E-11 compliant.
Non-critical circuit (10% drop)
Bilge pumps, cabin lights, general loads
Recommended gauge
10 AWG
Fuse / breaker
40 A
Voltage drop
1.20 V
Drop %
10.0%
Derated ampacity
65 A
Limit driver
Voltage drop
For 30A over 20ft at 12V (DC), 10 AWG copper with 90C insulation gives 10.0% drop at 65A derated ampacity (base 65A x 1.00 for 30C ambient, 1-conductor bundle). Use a 40A fuse. ABYC E-11 compliant.
OCPD (fuse/breaker) placement — ABYC E-11 §11.10.1.3.1
Within 7" of the battery positive terminal (no sheath/conduit)
The 7" / 40" / 72" rule is the most-failed inspection item. The 7" rule applies to unprotected battery cables. Use a sheath or conduit and you can extend to 40" — add metal-protected bulkheads and 72" is allowed.
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The two ABYC E-11 drop limits
ABYC E-11 specifies a 3% maximum voltage drop for critical circuits — panels, bilge pumps, nav lights, electronics — where low voltage causes real-world problems. Non-critical circuits (cabin lights, accessory outlets) are allowed up to 10%, but you rarely want more than 5% for a system that feels solid.
At 12V, voltage drop adds up fast because a 0.36V loss is already 3%. Runs longer than ~10 ft at modest current often need a thicker wire than people expect. Switching to 24V or 48V reduces current by 2× or 4× for the same power — and voltage drop scales linearly with current — so higher system voltage is sometimes the right answer instead of thicker wire.
What the calculator uses
- Copper resistance per 1000 ft from standard AWG tables
- Bundled-wire ampacity values (engine-room derated) from ABYC E-11
- Round-trip length (one-way × 2) in voltage drop calculation
- Fuse selected as the next standard size ≥ 1.25× load current
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