Voltage drop calculator

How the calculation works

Voltage drop comes down to one relationship: the further the current travels and the smaller the conductor, the more voltage you lose on the way. The K method packs conductor resistance into a single constant — 12.9 for copper and 21.2 for aluminum, both at 75 °C — and pairs it with the circular mil area of the conductor from Chapter 9 Table 8.

The multiplier at the front accounts for how far the current actually travels. Single-phase and DC circuits use 2, because the current runs out and back. Balanced three-phase circuits use 1.732, the square root of 3. Getting that factor wrong is one of the more common mistakes, and it moves the answer by about 13% — enough to change which conductor you pull.

The 3% figure is a target, not a rule

It is worth being precise about this, because it gets stated as code far more often than it is one. The widely quoted 3% branch circuit and 5% total figures live in informational notes. Informational notes are explanatory — they are not enforceable requirements, and an inspector generally cannot fail an installation on those numbers alone.

That does not make them optional in practice. Specific applications do carry firm limits, project specifications routinely impose their own, and some jurisdictions amend the code to make a limit enforceable. And a circuit that drops 8% will have equipment problems regardless of what any book says. Use 3% as the design target, and check what your job actually requires.

Ampacity comes first

This page answers a performance question, not a safety one. A conductor also has to be able to carry the load without overheating, and that requirement is mandatory. Size for ampacity first, check drop second, and install whichever conductor comes out larger. You can look up the ampacity side on the Table 310.16 ampacity chart.

Common questions

What is the maximum voltage drop allowed by the NEC?

The NEC does not set an enforceable limit on voltage drop for ordinary branch circuits and feeders. The familiar 3% branch circuit and 5% total figures come from informational notes, which are advisory rather than mandatory. Some specific applications — sensitive electronic equipment, fire pumps, elevators — do carry firm requirements, and many specifications and local amendments impose limits of their own. Treat 3% as a design target you should be able to justify, not as a rule you can cite.

Does this use the K method or the exact R and X method?

The K method: a single resistance constant per material (12.9 for copper, 21.2 for aluminum) at 75 °C, with circular mil areas from Chapter 9 Table 8. It is the approach most field references use, and it is accurate enough for sizing decisions on typical circuits. The exact method uses the AC resistance and reactance values from Chapter 9 Table 9 together with a power factor, and matters most on long runs, large conductors, and low power factor loads.

Why does three-phase use 1.732 instead of 2?

A single-phase circuit drops voltage on both the outgoing and returning conductor, so the current travels twice the one-way length — hence the factor of 2. In a balanced three-phase circuit the return current is shared across the phases rather than flowing in a dedicated conductor, and the line-to-line relationship works out to the square root of 3, about 1.732. DC behaves like single-phase and uses 2.

Should I size a conductor for voltage drop or for ampacity?

Both, and then take the larger. Ampacity is the mandatory requirement — the conductor must be able to carry the load without overheating. Voltage drop is a performance concern that often governs on long runs, where a conductor with plenty of ampacity still delivers unusable voltage at the far end. Size for ampacity first, check voltage drop second, and install whichever comes out bigger.