Voltage drop and ampacity

Size a conductor against both heating and voltage drop, and state which constraint governs and why.

Two separate constraints decide conductor size. Ampacity is a thermal limit: how much current the conductor can carry continuously without exceeding its insulation temperature rating, after correction for ambient temperature and for bundling. Voltage drop is a performance limit: how much voltage the run consumes before it reaches the load.

They govern in different regimes. On short runs at high current, ampacity usually governs. On long runs, voltage drop takes over and often forces a conductor several sizes larger than the thermal limit alone would require. Correction and adjustment factors can flip the answer, which is why both checks are always run.

Percent drop is what matters to the load, not volts alone: 5 V is 4.2 % on a 120 V circuit and 1.0 % on a 480 V circuit. That is one reason longer runs are pushed to higher voltage rather than to larger copper.

Predicting before you experiment is what makes the result stick. There is no penalty for being wrong here.

Find where the governing constraint flips from heating to voltage drop on the same load.

1. Enter a 40 A continuous load at 208 V, single-phase, on a 50 ft run and note the required size and the percent drop.
2. Extend the run to 250 ft with everything else held constant and note the percent drop.
3. Increase conductor size until the drop returns to an acceptable percentage.
4. Record the size ampacity alone would have allowed versus the size the drop required.

Pick a run you might actually install. State its length, load, and voltage, and say which constraint you expect to govern before you calculate it.