Estimate copper conductor size from voltage drop and adjusted ampacity, then find the largest common breaker or fuse rating that does not exceed the wire estimate.
Use the maximum continuous current and the one-way route from the source to the load.
Voltage-drop length is the one-way route because the calculator applies the round trip internally. Real routing around structure, service loops, switches, protection devices, and terminations can be longer than a straight-line measurement.
area = 2 × one-way length × current × copper resistivity ÷ allowed volts lostThe calculator independently finds the voltage-drop size and an ampacity size using the selected insulation rating, current-carrying conductor count, and engine-space condition. The larger conductor wins. Its breaker estimate is the largest common rating no greater than the adjusted wire ampacity.
The planning ampacities and correction factors are based on the public USCG conductor tables. Those federal bundling tables are expressly written for circuits of 50 V or more; this calculator conservatively applies their thermal corrections below 50 V as a planning assumption. ABYC E-11 and ISO 13297:2020 require licensed current copies for final verification.