A 25 HP motor's branch circuit sized entirely off its NEC table full-load current, not nameplate — plus checking the voltage dip its direct-on-line start produces at the site's short-circuit capacity.
| Motor | 25 HP, three-phase squirrel-cage induction, 460 V |
| Full-load current source | NEC Table 430.250 |
| Overcurrent device | Time-delay fuse |
| Starting method | Direct-on-line (DOL), locked-rotor current 180 A |
| System short-circuit capacity | 5000 kVA |
| Maximum acceptable starting voltage dip | 15% |
NEC Article 430 deliberately requires using the table FLC value for sizing calculations, even if the nameplate current differs slightly — the table value is what standardizes protection coordination across motors of the same rating.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Branch-circuit conductor ampacity | ≥ 42.5 A | 42.5 A (sizing target) | ✓ PASS |
| Branch-circuit OCPD | n/a (this is the sizing result) | 60 A time-delay fuse | ✓ PASS |
| DOL starting voltage dip | ≤ 15% | 2.79% | ✓ PASS |
Key insight: Every downstream protection figure in this calculation traces back to the single NEC table FLC value (34 A), not the motor's actual nameplate current — this is a deliberate feature of NEC Article 430, not an approximation, since it keeps branch-circuit sizing consistent and predictable across motors of the same horsepower/voltage/phase rating regardless of small manufacturer-to-manufacturer nameplate variation.
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Open Motor Calculator calculator →Voltage dip depends on the ratio between starting kVA and the system's short-circuit capacity at that point — the same 143.4 kVA starting demand against a weaker 1000 kVA system (instead of this example's 5000 kVA) would produce a dip around 12.5%, getting close to the 15% limit, and against an even weaker system could exceed it, at which point a star-delta, soft starter, or VFD start would be needed to reduce starting current.
NEC 430.52(C)(1)(b) permits going up to a higher percentage of FLC (225% for a time-delay fuse, versus the standard 175%) specifically when the standard-size device genuinely can't carry the motor's starting current without nuisance tripping — it's not a size to default to, but a documented exception to use only when the lower standard size has actually proven insufficient.