Why not use the nameplate amps?

430.6(A)(1) requires the table values for conductor, switch and overcurrent sizing so that a replacement motor of the same horsepower fits the circuit. The nameplate current is used only for overload protection (430.32).

What breaker for a 10 hp 460 V motor?

FLC 14 A; inverse-time breaker up to 250 % = 35 A. A 35 A breaker is a standard size; if the motor will not start, 430.52(C)(1) Exception 2 allows up to 400 % for currents of 100 A or less.

The table has no 3 hp at 115 V single-phase; why?

Table 430.248 stops at 2 hp for 115 V (24 A); larger motors are not built for 115 V. The page prints “not listed” rather than a made-up figure.

Motor FLA calculator

Full-load current · Table 430.25014.0ATABLE VALUE10 hp, 460 V three-phase induction; use this table value, not the nameplate, for conductors and OCPD (430.6(A)(1)) · NEC 2017, identical to 2011

Phase

Horsepower

Voltage

Motor type (Table 430.52)

Branch-circuit conductors, minimum ampacity430.22: 125 % of the table FLC: 17.5 A
Inverse-time breaker, max 250 %Table 430.52; next standard size 35 A permitted by 430.52(C)(1) Ex. 1 where the value does not match a standard rating: 35.0 A
Dual-element time-delay fuse, max 175 %next standard 25 A: 24.5 A
Nontime-delay fuse, max 300 %: 42.0 A
Instantaneous-trip breaker, max 1100 %listed combination starters only: 154 A

Overload protection (430.32) uses the nameplate current, not the table; typically 115 % or 125 % by service factor and temperature rise. Multi-motor circuits, Design B exceptions and the 430.52(C)(1) exceptions are the electrician’s call.

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Motor branch circuits are sized from the NEC tables, not the nameplate: 430.6(A)(1) sends you to Table 430.248 for single-phase and 430.250 for three-phase motors. Pick the horsepower and voltage and read the full-load current, the conductor ampacity at 125 % (430.22), and the maximum overcurrent device by type from Table 430.52. Overload protection is the exception: it uses the nameplate (430.32).

Worked examples

10 hp three-phase at 460 V: Table 430.250: 14 A. Conductors 17.5 A (12 AWG at 75 °C carries 25 A). Inverse-time breaker up to 250 % = 35 A; dual-element fuse up to 175 % = 24.5 A, next standard 25 A. Open this run
1 hp single-phase at 115 V: Table 430.248: 16 A. Conductors 20 A (12 AWG); breaker up to 40 A, dual-element fuse up to 28 A, next standard 30 A. Open this run
25 hp three-phase at 208 V: 74.8 A. Conductors 93.5 A (3 AWG at 75 °C, 100 A); breaker up to 187 A, so a 175 A device, or 200 A by the 430.52(C)(1) Exception 1 next-size rule. Open this run
5 hp single-phase at 230 V: 28 A. Conductors 35 A (8 AWG at 75 °C, 50 A; 10 AWG is 35 A but 240.4(D) matters only for the OCPD); breaker up to 70 A. Open this run

How it is worked out

FLC is read from Table 430.248 (single-phase, 1/6 to 10 hp at 115, 200, 208, 230 V) or Table 430.250 (three-phase induction, ½ to 500 hp at 115 to 2300 V). Where the table prints a dash the page says so rather than interpolating.

Conductors: ampacity ≥ 1.25 × FLC (430.22). Branch-circuit short-circuit and ground-fault protection: ≤ FLC × the Table 430.52 percentage for the device type (nontime-delay fuse 300 %, dual-element fuse 175 %, instantaneous-trip breaker 800 % or 1100 % for Design B, inverse-time breaker 250 %), with the next standard size permitted by 430.52(C)(1) Exception 1 where the product is not a standard rating.

The figures behind it

NEC Tables 430.248 and 430.250: Full-load currents, 2017 text verbatim, cross-checked with the 2011 NECA-IBEW reproduction: identical. Synchronous-motor columns and the 90 / 80 % power-factor multipliers are in the data but not exposed in the picker. Source
NEC 430.22 and Table 430.52: 125 % conductor rule and the maximum OCPD percentages by motor and device type, 2017 text; standard ratings from 240.6(A). Source
Edition note: No open 2020 or 2023 copy was reachable; the 2011 and 2017 values match and “unchanged in 2023” is inferred. Source

Questions people ask

Why not use the nameplate amps?: 430.6(A)(1) requires the table values for conductor, switch and overcurrent sizing so that a replacement motor of the same horsepower fits the circuit. The nameplate current is used only for overload protection (430.32).
What breaker for a 10 hp 460 V motor?: FLC 14 A; inverse-time breaker up to 250 % = 35 A. A 35 A breaker is a standard size; if the motor will not start, 430.52(C)(1) Exception 2 allows up to 400 % for currents of 100 A or less.
The table has no 3 hp at 115 V single-phase; why?: Table 430.248 stops at 2 hp for 115 V (24 A); larger motors are not built for 115 V. The page prints “not listed” rather than a made-up figure.

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