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Motor Full-Load Current Calculator

Look up motor full-load current from NEC Tables 430.248 and 430.250, plus the 125% conductor minimum per 430.22 — the table value the code requires.

Phase
15.2A

Minimum conductor ampacity 19 A — 125% of table FLC (NEC 430.22)

5 HP, 230 V, 3φ → FLC 15.2 A (NEC Table 430.250) · conductor ≥ 1.25 × 15.2 A = 19 A (NEC 430.22)

Sized from the table per 430.6(A). The nameplate FLA is for overload protection only — never for conductors or the branch-circuit device.

What this does not check

  • It gives the table FLC and the 125% conductor minimum only. It does not size the motor branch-circuit short-circuit and ground-fault protection — that device uses the 430.52 multipliers (up to 250% of FLC for an inverse-time breaker) and is a different calculation.
  • It does not size overload protection either. Overloads are the one place the code DOES use the nameplate — 430.32 works from the nameplate full-load current and service factor, not from these tables.
  • Only the 115 V and 230 V single-phase columns and the 230 V and 460 V three-phase columns are carried here. The full NEC tables have more columns — 200 V, 208 V, 575 V — and a 208 V motor draws more current than the 230 V value, so the 230 V column is not a substitute.
  • The tables apply to motors running at usual speeds with normal torque characteristics. Multispeed, high-torque, and other special motors are outside them, and no ampacity derating, voltage-drop, or terminal-rating check happens here.

How to use it

Pick the phase first — that decides everything else. Single-phase motors come from NEC Table 430.248, which runs from 1/6 HP to 10 HP at 115 V or 230 V. Three-phase motors come from Table 430.250, which runs from 1/2 HP to 200 HP — this tool carries its 230 V and 460 V columns. The horsepower list changes when you switch phase because the two tables cover different ranges; a 50 HP single-phase motor is not a thing the table knows about, so the tool will not pretend otherwise.

Then pick the horsepower and voltage off the motor. You get two numbers: the table full-load current, and 125% of it — the minimum ampacity the branch-circuit conductors must have under 430.22.

The table, not the nameplate

This is the whole reason the calculator exists, so it goes first.

NEC 430.6(A) says conductors and overcurrent protection for a motor circuit are sized from the table full-load current — the values in 430.248 and 430.250 — and not from the current marked on the motor’s nameplate. The nameplate has exactly one sizing job in the code: overload protection under 430.32 works from the nameplate FLA and service factor.

Most people’s instinct runs backwards. The nameplate is right there on the motor, it is specific to that machine, and it feels more accurate than a generic table — so it gets used for the wire. But nameplate values are usually lower than the table, because your motor is one particular design and the table covers the worst ordinary motor sold at that horsepower. Size the conductors from a low nameplate and the wiring is undersized the moment that motor gets swapped for a hungrier replacement — which happens without anyone reopening the circuit design. The table value is the code’s insurance against that swap. A calculator that just echoes whatever nameplate you type into it is worse than useless, because it launders the wrong number into an official-looking answer.

Worked example

A 5 HP three-phase motor on a 230 V system.

Table 430.250 gives 15.2 A for 5 HP at 230 V. The conductor minimum is 125% of that:

15.2 A × 1.25 = 19 A

So the branch-circuit conductors need at least 19 A of ampacity. From the 75°C copper column of Table 310.16, 12 AWG carries 25 A and covers it in ordinary conditions — run the ampacity derating calculator if the circuit shares a raceway or sits in a hot space.

Now suppose that motor’s nameplate reads 14.0 A. Size from the nameplate and you get 14.0 × 1.25 = 17.5 A — a smaller number, a smaller wire, and a code violation. Same motor, same job, wrong starting value.

One more, single-phase: a 1/2 HP motor at 115 V is 9.8 A in Table 430.248, and 9.8 × 1.25 = 12.25 A of required conductor ampacity. Note the arithmetic keeps two decimal places — the same rounding the FieldDojo app uses, so the page and the app always agree.

Where it goes wrong in the field

Nameplate wire sizing. Covered above, and worth repeating because it is the mistake: the nameplate number is the one physically in front of you on the job, and it is the wrong one for conductors.

The reverse mistake. Having learned “use the table,” some people then size the overloads from the table too. That is backwards the other way — overloads protect this motor from cooking, so 430.32 sizes them from this motor’s nameplate. Table-based overloads set higher than the nameplate warrants let a motor run overloaded without tripping.

208 V systems. A shop fed at 208 V does not get to use the 230 V column. The real Table 430.250 has a 200 V column with higher currents — lower voltage, more amps for the same horsepower — and this tool refuses rather than guesses. If your system is 208 V, look up the 200/208 V column in the printed table.

Treating 125% as the breaker size. The 19 A in the example is the conductor minimum, not the overcurrent device. The motor branch-circuit short-circuit and ground-fault device comes from 430.52 and can legally be far larger than the conductor ampacity — up to 250% of FLC for an inverse-time breaker — because the overloads, not the breaker, protect the wire from sustained overcurrent on a motor circuit. A breaker bigger than the wire’s ampacity looks wrong to anyone trained on ordinary branch circuits, and on a motor circuit it is normal. That sizing is a separate calculation this tool deliberately does not do.

Special motors. The tables assume usual speeds and normal torque. A high-torque or multispeed motor, or anything odd enough to carry its own engineering data, is outside them — for those, the code itself points you away from the table.

Code references

Section numbers and table values move between editions, and jurisdictions amend them. Confirm against the edition your authority having jurisdiction enforces.

Common questions

Where does a motor full load amps chart come from?
Every legitimate one is a reprint of NEC Table 430.248 (single-phase) or 430.250 (three-phase) — standardized full-load currents by horsepower and voltage. That is what this calculator looks up, from the 2023 edition. If a chart you find online disagrees with the NEC table, the chart is wrong or it is quoting a different voltage column than you think.
What size wire do I need for a 5 HP motor?
Start from the table, not the nameplate. A 5 HP three-phase motor at 230 V is 15.2 A, so the conductor needs at least 19 A of ampacity — 12 AWG copper at 75°C carries 25 A and covers it in ordinary conditions. Single-phase at 230 V the same motor is 28 A, needing 35 A of ampacity. Then check ambient and bundling derating separately before you commit to a size.
Do I use the nameplate FLA or the table FLA?
Both, for different jobs. Conductors and the branch-circuit breaker or fuses are sized from the table value — 430.6(A) requires it. Overload protection is sized from the nameplate — 430.32 requires that. Using the nameplate for the conductors is the classic motor-circuit mistake, and it almost always undersizes the wire.
Why is motor FLA different from the nameplate?
The table values are standardized, conservative currents for each horsepower and voltage, covering the range of motor designs sold at that rating. Your particular motor's nameplate reflects its own tested draw, which is usually lower. The code sizes the circuit from the table so the wiring survives the day someone swaps in a less efficient replacement motor of the same horsepower.