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.
On this page
Why the code uses volt-amperes
Because it refuses to assume anything about the load. Volt-amperes is what the conductor actually has to carry regardless of how efficiently the load uses it.
Watts is what gets converted into useful work. On a heater those are the same. On a motor they are not, because current and voltage are out of step with each other.
A conductor does not care about the phase relationship. It heats up according to current, which is why the code sizes from volt-amperes.
Power factor in one paragraph
Power factor is the ratio of watts to volt-amperes. A power factor of one means they are equal. A power factor of 0.8 means the circuit carries more current than the useful power would suggest.
So a load with poor power factor needs a bigger conductor for the same amount of work. That is the practical consequence and it is the reason the concept exists on this exam at all.
The single-phase and three-phase forms
Single phase: volt-amperes is voltage times current.
Three phase: volt-amperes is voltage times current times the square root of three.
Watts, in both cases, is volt-amperes times power factor.
Those three lines cover every power question on the paper. The mistakes are omitting the square root of three, or applying power factor where the question wanted volt-amperes.
Reading which one is wanted
- A question mentioning conductor size, load calculation or service size wants volt-amperes.
- A question mentioning efficiency, output, heat or useful work wants watts.
- A question giving you a power factor wants you to convert between them.
- A question giving you a horsepower rating is about a motor, and motors have their own rules that supersede this arithmetic entirely.
That last one matters. Do not compute a motor conductor from power. Go to the full-load current table.
What this page cites
- NEC 120.13 The volt-ampere per square foot figure for branch circuits.
- NEC 430.6 Motor currents come from a table, not from power arithmetic.