Fuse size calculator
Free fuse size calculator: get calculated maximum (flc × 175%) from motor flc from nec tables 430.247–430.250 (not nameplate). With the formula shown.
Project inputs
Updates as inputs change. Unit changes preserve the same physical values.
Method & assumptions
US (NEC) only. Motor branch-circuit short-circuit and ground-fault protection (Article 430 Part IV), not overload protection (430.32). Enter FLC from the NEC tables, not the nameplate. Percentages used: non-time-delay fuse 300%, dual-element fuse 175%, inverse-time breaker 250%. Heavy-start ceilings: 400%, 225%, and 400% for inverse-time breakers (300% where FLC exceeds 100 A); fuses rated 601–6000 A are limited to 300%. Verify against the NEC edition adopted locally. Engineering arithmetic, not installation approval.
Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.
Sources and validation →Worked example
Motor FLC from NEC Tables 430.247–430.250 (not nameplate): 27 A · Protective device: Dual-element (time-delay) fuse
Calculated maximum (FLC × 175%): 47.25 A · Next higher standard rating (430.52(C)(1) Exc. 1): 50 A · Conservative lower standard rating: 45 A · Heavy-start ceiling (FLC × 225%, Exc. 2): 60.75 A · Largest standard rating within heavy-start ceiling: 60 A
This example uses the stated inputs; results for the entered values appear above.Estimating and invoicing tool
Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.