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
DC
Set by the system voltage above. AC adds an impedance correction (~5-8%) to the resistance-only drop calc.Critical circuit (3% drop)
Panels, navigation, electronics
Recommended gauge
4 AWG
Fuse / breaker
40 A
Voltage drop
0.31 V
Drop %
2.6%
Derated ampacity
135 A
Limit driver
Voltage drop
For 30A over 20ft at 12V (DC), 4 AWG copper with 90C insulation gives 2.6% drop at 135A derated ampacity (base 135A x 1.00 for 30C ambient, 1-conductor bundle). Use a 40A fuse (125% continuous margin, NEC-derived practice). Sized per ABYC E-11 ampacity and voltage-drop tables — marine outputs are DRAFT pending licensed sign-off.
Non-critical circuit (10% drop)
Bilge pumps, cabin lights, general loads
Recommended gauge
8 AWG
Fuse / breaker
40 A
Voltage drop
0.78 V
Drop %
6.5%
Derated ampacity
70 A
Limit driver
Voltage drop
For 30A over 20ft at 12V (DC), 8 AWG copper with 90C insulation gives 6.5% drop at 70A derated ampacity (base 70A x 1.00 for 30C ambient, 1-conductor bundle). Use a 40A fuse (125% continuous margin, NEC-derived practice). Sized per ABYC E-11 ampacity and voltage-drop tables — marine outputs are DRAFT pending licensed sign-off.
OCPD (fuse/breaker) placement — ABYC E-11 §11.10.1.3.1
Within 7" of the source of power, measured along the conductor (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
- Conductor resistance from circular-mil area, ABYC E-11 constant K = 10.75
- ABYC E-11 free-air ampacity for a singleconductor outside engine spaces, at 30 °C. Bundling and engine-space derates are applied only when you set them under advanced — the defaults apply no derate at all.
- Round-trip length (one-way × 2) in voltage drop calculation
- Fuse selected as the next standard size ≥ 1.25× load current
- Gauges from 18 AWG to 4/0 — ABYC E-11 does not publish 3 AWG or kcmil sizes
Marine (ABYC) outputs are DRAFT — and read the assumptions
The ABYC E-11 tables behind this calculator are transcribed from published reproductions and have not yet been signed off by a licensed marine electrician. Treat every marine result as a draft to be checked, not a specification.
Assumptions matter more than the number. This tool defaults to one conductor in free air, which applies no bundling derate. Our sister tool, the CircuitProof calculator, defaults to two conductors (duplex cable), which takes ABYC's 0.70 bundling factor, and its marine table starts at 14 AWG rather than 18. On the same inputs the two tools can therefore return different gauges — CircuitProof returns the larger, more conservative conductor. Neither result is a substitute for the published standard. If you are specifying a real vessel, size to the larger of the two and have a licensed marine electrician confirm it.
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