Motor FLA Calculator

A motor circuit is protected by two devices doing different jobs, and almost everything confusing about Article 430 follows from that split.

The overload device — typically the heaters or electronic overload block in the starter — protects the motor and its conductors from running too hot over minutes and hours. It is sized under 430.32 from thenameplate current, because that is a property of the actual machine installed.

The branch-circuit short-circuit and ground-fault device — the breaker or fuse in the panel — clears faults in milliseconds and otherwise stays out of the way. It has to be big enough to ride through a starting inrush of six to eight times running current without tripping. It is sized under 430.52 from thetable value.

That is the whole reason for 430.6(A)(1). The nameplate belongs to one motor; the circuit outlives it. Standardising the sizing basis means a compliant replacement motor of the same rating still works on the same conductors in ten years.

From the table figure, two calculations run in opposite directions.

430.22, the conductor: 125%, rounded up. Branch-circuit conductors must have an ampacity of at least 125% of the table full-load current. That is a minimum, so you take the next conductor that meets it.

430.52, the device: a table percentage, rounded down. How far above full load the device may be rated depends on what kind of device it is — Table 430.52 gives a different figure for each, and 250% is the column for the inverse-time circuit breaker this page assumes. That figure is a permittedmaximum, which is what sends the rounding the other way. The FAQ below sets the two directions side by side.

A 5 HP three-phase motor on a 230 V system. Squirrel-cage induction, started across the line, protected by an ordinary inverse-time circuit breaker.

Step one, the lookup. Table 430.250, the 5 HP row, the 230 V column: 15.2 A. Nothing is calculated here and nothing is read off the motor. If the nameplate on this particular machine says 14.2 A, or 16.0 A, it makes no difference to any of what follows — that number is for the overload relay.

Step two, the conductor. 430.22 wants at least 125% of that:15.2 × 1.25 = 19 A. Reading up Table 310.16 at the 75 °C copper column, the smallest conductor carrying 19 A is 14 AWG, rated 20 A. At the 60 °C column the answer would be 12 AWG, which is what to use where the terminations at either end are not rated higher.

Step three, the breaker. Table 430.52 permits an inverse-time breaker at up to 250% of full-load current: 15.2 × 2.5 =38 A. That is a ceiling, so rounddown to the largest standard rating underneath it — which is35 A, with 3 A of headroom unused.

40 A is a standard rating too, and sits only 2 A above the ceiling — which is precisely why this step is worth doing deliberately rather than by eye.

So the circuit is 14 AWG conductors behind a 35 A breaker, with the overload device set separately once the nameplate is in front of you. Every figure above came from one row of one table.

Both tables in full, because they are the point of the page — a lookup tool whose table is hidden behind a form is less useful than the table. Use these when you are checking someone else's figure, pricing a job from a schedule of motors, or working without a signal on site.

Read across a row and the inverse-voltage relationship is visible: the 115 V and 230 V columns are exactly 2:1, as are 230 V and 460 V on the three-phase table. Read down a column and note where each table stops. Single-phase ends at 10 HP, which is genuinely the practical limit for single-phase motors rather than an omission. Three-phase starts at 1/2 HP and runs to 200 HP.