AC Branch Circuit Conductor Sizer
Sizes interior 120V and 240V mobile AC branch wiring for RVs, campervans, and boats, comparing marine triplex boat cable vs Romex NM-B. For campground pedestal and marina shore power cords, use the Shore Power Cable Sizer.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Marine Triplex Boat Cable vs. Residential Romex
Calculation Assumptions
- Standard 120V/240V AC 60Hz branch circuits with NM-B (Romex) or marine triplex boat cable.
- Ampacity matched to 15A, 20A, 30A, or 50A standard overcurrent protective devices.
Engineering Limitations
- Mobile installations require stranded conductor wire; solid copper Romex is prohibited on marine vessels due to vibration metal fatigue.
A continuous 12.5A 120V water heater branch requires 12 AWG stranded copper paired with a 20A breaker to meet continuous duty requirements (125% of load).
In vehicular and marine environments, AC branch circuits are subjected to constant vibration, mechanical flexing, and temperature extremes.
- Solid Copper Risk: Residential building wire (Romex / NM-B) uses solid copper conductors. Under road or wave vibration, solid copper work-hardens and suffers fatigue fracture at terminal connection points. ABYC E-11 explicitly forbids solid conductor AC wiring on boats.
- High-Strand Marine Triplex: Marine AC wire consists of Type III stranded copper (each conductor has dozens of individual hair-like strands) bundled inside an outer protective sheath. Stranded wire withstands continuous movement without cracking.
- Thermal Derating in Sheathed Cables: When three current-carrying conductors (Hot, Neutral, Ground) are enclosed in a common outer jacket, heat cannot escape as easily as individual wires in free air. Ampacity ratings are derated accordingly.
Frequently Asked Questions
What size wire do I need for a 15A, 20A, or 30A AC branch circuit?
Under NEC 240.4(D), standard small conductor rules specify: 14 AWG copper for 15-amp circuits, 12 AWG copper for 20-amp circuits, and 10 AWG copper for 30-amp circuits. If the one-way distance exceeds 50 to 70 feet, upsizing the conductor by one gauge size is recommended to maintain voltage drop below the NEC recommended 3% limit.
What is the NEC 80% continuous load rule for AC circuits?
NEC 210.19 and 210.20 mandate that an overcurrent device and branch conductor must not be loaded beyond 80% of its nominal rating for continuous loads (defined as loads running continuously for 3 hours or more). For example, a 20A circuit breaker cannot carry more than 16A of continuous load (20A * 0.80 = 16A, or 16A * 1.25 = 20A minimum breaker sizing).
How do you calculate voltage drop on a 120V vs 240V AC circuit?
AC single-phase voltage drop is calculated as: V_drop = (2 * K * I * L) / Circular Mils, where K is copper resistivity (~12.9 ohms-cmil/ft), I is current in Amps, and L is one-way distance in feet. Because 240V operates at half the amperage of 120V for identical wattage, voltage drop percentage is reduced by a factor of 4.
Can you use 14 AWG wire on a 20-amp breaker?
No. NEC 240.4(D)(3) strictly prohibits protecting 14 AWG copper wire with a circuit breaker or fuse rated higher than 15 Amperes in standard branch circuit applications. Doing so poses a direct fire hazard under sustained overload.