Voltage drop calculator
On any run over roughly a hundred feet, voltage drop rather than ampacity usually decides the conductor size. Undersize it and motors run hot, lights dim, and electronics misbehave in ways that are hard to diagnose.
Distance one way — the round trip is accounted for.
The 3% and 5% figures are recommendations, not requirements
This is worth being precise about, because a lot of sources imply otherwise. NEC 210.19(A) and 215.2(A) contain informational notes recommending a maximum 3% drop on a branch circuit and 5% total across feeder plus branch combined.
Informational notes are not enforceable code text. Exceeding 3% is not a violation in itself. It is, however, a genuine performance problem, and many local jurisdictions and specifications adopt the figures as hard limits — so treat them as the design target they're meant to be.
Fixing an excessive drop
In order of how often it's the right answer.
- Increase the conductor size — the usual fix, and drop falls roughly in proportion to circular mils
- Raise the voltage — running 240 V instead of 120 V halves the current and quarters the drop for the same power
- Shorten the run, or relocate the panel or subpanel closer to the load
- Split the load across two circuits
Questions
Do I enter one-way or round-trip distance?
One way. The calculation doubles it for single-phase because current travels out and back, and applies √3 for three-phase. Entering the round trip will double your answer.
Does this account for temperature?
Only approximately. It uses standard resistivity values for copper and aluminium at typical operating temperature. Conductor resistance rises as it heats, so a heavily loaded conductor in a hot space will drop slightly more than shown.
Why is aluminium worse?
Higher resistivity — roughly 21.2 against 12.9 ohm-cmil/ft for copper, about 64% more. For the same gauge and run, aluminium drops noticeably more voltage, which is why aluminium runs are sized up.