Checked against primary sources 2026-08-24

Watts and volt-amperes are the same number until they are not

The code calculates in volt-amperes. Equipment is often rated in watts. Questions are built in the gap, and the three-phase formula is where the gap gets expensive.

Why the code uses volt-amperes

Because it refuses to assume anything about the load. Volt-amperes is what the conductor actually carries, regardless of how efficiently the load turns it into anything.

Watts is what gets converted into useful work. On a heater those are the same. On a motor they are not, because the current and the voltage are out of step with each other.

A conductor does not care about the phase relationship. It heats according to current, and current follows volt-amperes, which is why the load calculation rules are written in that unit and not in watts.

Power factor in one paragraph

Power factor is the ratio of watts to volt-amperes. At one they are equal. At 0.8 the circuit carries more current than the useful power would suggest, and the amount more is worth working once.

Take 8000 watts on a 240 volt single-phase circuit. At unity, the current is 8000 over 240, or 33.3 amperes. At 0.8, the apparent power is 8000 over 0.8, or 10,000 volt-amperes, and the current is 10,000 over 240, or 41.7 amperes. Same useful work, a quarter more current, and the conductor is sized on the current.

So a load with poor power factor wants a bigger conductor for the same amount of work. That is the practical consequence and the reason the concept is on this exam at all.

The single-phase and three-phase forms

Single phase: volt-amperes is the voltage across the load times the current through it.

Three phase: volt-amperes is the line-to-line voltage times the line current times the square root of three. The word line is load-bearing. Hand that formula a phase voltage instead and the answer is wrong by a factor of 1.73, which is the single most common wrong answer in this topic.

Written from the other side, the same quantity is three times the phase voltage times the phase current, with no square root of three in it anywhere. Two forms, one result. If yours disagree by roughly 1.73, you have taken one quantity from each side.

Watts, in both cases, is volt-amperes times power factor.

Worked, so you can see the two forms close

A 480 volt delta with 100 amperes in each winding. The line current is the square root of three times 100, about 173.2 amperes. From the line side, the square root of three times 480 times 173.2 is 144,000 volt-amperes. From the phase side, three times 480 times 100 is 144,000. They are the same expression, because the square root of three appears twice on the line side and squares to the three on the phase side.

Those forms cover the arithmetic. They do not cover every power item on the paper, because motor items are not arithmetic items, which is the next section.

Reading which one is wanted

• An item working toward a conductor size, a feeder or a service is working in volt-amperes, because that is the unit the load calculation rules are written in.
• An item about output, heat or useful work is working in watts.
• An item about efficiency has two watt figures in it, input and output, and the trap is answering with the one it did not ask for. Efficiency is output over input, so input is always the larger.
• An item that hands you a power factor wants you to move between watts and volt-amperes.
• An item that hands you a horsepower rating is a motor item, and motors have their own rules that supersede this arithmetic entirely.

That last one carries the most weight. Horsepower describes what leaves the shaft, about 746 watts of it per horsepower. What enters at the terminals is larger by the efficiency, and the current is larger again by the power factor, so a chain of three assumptions sits between a horsepower rating and an ampere figure.

Which is a large part of why the code sends you to a full-load current table instead of letting you calculate it. Do not compute a motor conductor from power. Go to the table.

What this page cites

• NEC 120.13 Dwelling unit minimum unit load for branch-circuit calculations, in the 2026 edition. This material was Article 220 through the 2023 edition. source
• NEC 120.41 The feeder and service counterpart to 120.13. It carries a different figure for a different purpose, and using one where the other belongs produces a plausible wrong answer.
• NEC 430.6 Which motor current applies to which calculation. Motor conductor sizing comes off the full-load current tables, not off nameplate arithmetic and not off horsepower.