How to calculate motor full load amps?

Calculate motor full load amps from the NEC's own tables: 430.248 for single-phase and 430.250 for three-phase motors, entered by horsepower and voltage. For conductor and breaker sizing, the code requires the table value, not the nameplate.

That surprises people. The nameplate FLA governs the overload setting; the table FLA governs wire and breaker sizing. Two numbers, two jobs.

For a quick estimate outside the tables, single-phase amps equal watts divided by volts times efficiency and power factor, and three-phase adds the square root of three, but the tables already carry conservative real-world values.

Branch conductors then get sized at 125 percent of that table current, per 430.22.

The motor FLA calculator pulls the right table value and applies the 125 percent for you.