Voltage Drop
Wire size, current and distance in — the volts you lose on the way, and whether that still passes.
Estimating only. Electrical work is permitted and inspected against the code edition adopted where you live — have a licensed electrician do or verify the work.
Drop over the run
0%
0 volts lost- Volts at the load
- —
- Verdict
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- Longest at this size
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- Next size up
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One-way distance — the formula counts the return leg for you.
What voltage drop costs you
A conductor is a resistor you did not intend to install. Push current through it and some of your supply is spent as heat in the wire instead of work at the load. The arithmetic is Ohm's law wearing a hat: volts lost = 2 × K × amps × feet ÷ circular mils, where K is the resistivity of the metal and circular mils is the conductor's cross-section.
Under about 3% nothing complains. Past 5% incandescent lights visibly dim, LED drivers buzz, motors draw extra current to make up the shortfall and run hotter for it, and a well pump at the end of a long run may not start at all on a hot day. The fix is always the same: fewer amps, a shorter run, a higher voltage, or more copper.
The 240 volt trick
If the load can take it, doubling the voltage is cheaper than doubling the copper. The same watts at 240 volts draw half the amps, and half the current over the same wire loses half the volts — measured against a supply twice as big. That is a quarter of the percentage drop, for the price of a two-pole breaker.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Resistivity
- K = 12.9 for copper, 21.2 for aluminum (ohm-circular-mils per foot)
- Run length
- One way — the formula doubles it for single phase, ×1.732 for three phase
- Target
- 3 % on a branch circuit, 5 % feeder plus branch — NEC informational notes
- DC resistance
- Reactance is ignored, which is the standard estimate for small conductors
Questions from the aisle
How much voltage drop is acceptable?
The NEC recommends 3% on a branch circuit and 5% total across feeder plus branch. Those figures are informational notes, not enforceable rules — but equipment is designed around them, and inspectors read them as the standard of care.
How far can I run 12 gauge wire on a 20 amp circuit?
About 45 feet at 120 volts before a fully loaded 20 amp circuit passes 3%. At 240 volts the same wire and load reach roughly twice as far, because the same volts are lost against double the supply. Most branch circuits are nowhere near fully loaded, which is why 12 AWG runs of 80 feet behave themselves in practice.
Do I measure the run one way or both?
One way — the distance from the panel to the load. The formula doubles it for you on a single-phase circuit, because the current goes out on one conductor and comes back on the other.
Why does 240 volts help so much?
Two reasons at once. Half the current carries the same watts, and the volts lost are measured against a supply twice as large. A 240 volt run drops about a quarter of the percentage a 120 volt run does for the same power.
The reasoning behind this one
Wire size, breakers and voltage drop
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.