Calculators › Motor Sizing
Motor Sizing Calculation
Motor circuits use four different numbers for four different jobs — and the trap is using the nameplate where you should use the table, or mixing up the percentages. This is core SP-PH territory (HVAC = motors). Here's the whole sequence.
The rules at a glance (Article 430)
| Conductor (430.22) | 125% of the table FLC |
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| Overload (430.32) | 125% of nameplate FLA (SF ≥ 1.15 or rise ≤ 40°C); else 115% |
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| Short-circuit/ground-fault (Table 430.52) | Inverse-time breaker 250%, time-delay fuse 175%, nontime-delay fuse 300% |
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| Disconnect (430.110) | At least 115% of FLC |
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Key distinction (430.6): use the table FLC (430.248/430.250) for the conductor and short-circuit protection; use the nameplate FLA only for the overload.
Example 1 — 5 HP, 230 V, single-phase, SF 1.15
- Table FLC (430.248): 5 HP @ 230V single-phase = 28 A.
- Branch-circuit conductor (430.22): 125% × 28 = 35 A.
1.25 × 28 A = 35 A → need a conductor rated ≥ 35 A
10 AWG Cu (75°C) = 35 A → use 10 AWG Cu
- Overload (430.32(A)(1), SF 1.15): 125% of nameplate FLA. If nameplate = 28 A:
1.25 × 28 A = 35 A max overload
- Short-circuit/ground-fault (Table 430.52, inverse-time breaker 250%):
2.50 × 28 A = 70 A → 70 A breaker (a standard size)
If 70 A won't let it start, 430.52 allows going up (next size, then up to 400% for an inverse-time breaker).
- Disconnect (430.110): 115% × 28 = 32.2 A → choose a disconnect rated ≥ 32.2 A (e.g., a 60 A safety switch).
Example 2 — 10 HP, 460 V, three-phase
- Table FLC (430.250): 10 HP @ 460V three-phase = 14 A.
- Conductor (430.22): 1.25 × 14 = 17.5 A → 14 AWG Cu (75°C = 20 A) satisfies it; many shops use 12 AWG minimum by preference.
- Short-circuit (inverse-time breaker 250%): 2.50 × 14 = 35 A breaker (standard).
- Disconnect (430.110): 1.15 × 14 = 16.1 A → rated ≥ 16.1 A.
Note: the motor small-conductor exception means 240.4(D) does NOT cap these motor conductors
Exam tip: always read whether the motor is single-phase (Table 430.248) or three-phase (Table 430.250), and which voltage column. Picking the wrong table is the #1 motor mistake.