A standalone motor sizing calculator (independent of any protection-relay motor tab): NEC Article 430 or IEC general-method Full-Load Current, branch-circuit ampacity/OCPD/overload/disconnect sizing, a starting-method voltage-dip check against the source's short-circuit capacity, and NEC Part V multi-motor feeder sizing.
Standards followed
NEC Article 430 (motors, motor circuits and controllers)NEC Table 430.248 (single-phase FLC)NEC Table 430.250 (three-phase FLC)NEC 430.52(C)(1) (branch-circuit OCPD percentages)NEC 430 Part V (feeder sizing)
What it can do
- NEC or IEC Full-Load Current, branch-circuit minimum ampacity, maximum OCPD, and overload protection setting.
- Starting-method-aware voltage-dip check (DOL/star-delta use LRA directly; soft-starter/VFD use a current-limit multiplier) against source short-circuit capacity.
- Multi-motor feeder sizing per NEC 430 Part V — largest motor's percentage plus the sum of all other motors' FLC.
Example problem
A 25HP, 460V, 3-phase squirrel-cage motor. Find its FLC, branch-circuit sizing, and DOL starting voltage dip at a 5,000kVA source.
- NEC Table 430.250 FLC for 25HP/460V squirrel-cage = 34A.
- Branch-circuit minimum ampacity = 125% × 34 = 42.5A.
- Overload/disconnect sizing = 115% × 34 = 39.1A.
- Max OCPD (dual-element time-delay fuse, 175%) = 175% × 34 = 59.5A, rounded up to the standard 60A size.
- DOL start at 180A LRA → starting kVA = √3 × 460 × 180 / 1000 ≈ 143.414 kVA.
- Voltage dip = starting kVA / (starting kVA + source kVA) = 143.414 / (143.414 + 5000) ≈ 2.788%.
FLC 34A, branch ampacity 42.5A, 60A max OCPD, 2.788% voltage dip (passes a 15% limit).
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