Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of voltage drop. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Voltage drop is a function of current, conductor resistance and length — and the length is the round trip for a single-phase circuit, because the current returns on the neutral.
Common limits are 3% on a branch circuit and 5% overall. The usual temptations are to notice it only on long runs, when in fact a short run at high current drops just as much.
Common misuse
Using the one-way length in a voltage-drop calculation. For single-phase the return path doubles the resistance, so the drop is twice what a one-way calculation gives.