Sizes overcurrent protective devices (breakers/fuses) two ways: from a general continuous/non-continuous load current, or from a motor's starting-current withstand requirement. The general path checks the device protects the downstream cable; the motor path reuses the same NEC 430.52(C)(1) logic as the standalone Motor Calculator so the two tools always agree.
Standards followed
NEC 210.19(A)(1) / 215.2(A)(1)NEC 240.4 / 240.4(B) / 240.6(A)NEC 430.52(C)(1) & 430.52(C)(1)(b)IEC 60364-4-43
What it can do
- General load OCPD sizing at 125% continuous + 100% non-continuous, rounded to the nearest NEC standard size.
- Checks the selected OCPD actually protects the cable, including the 240.4(B) next-size-up exception.
- IEC 60364-4-43 path with fuse-vs-breaker-specific conventional operating current (I₂).
- Motor starting-current withstand sizing by NEC device type or a configurable IEC margin factor, with the 430.52(C)(1)(b) exception ceiling shown separately.
Example problem
A feeder carries an 80A continuous load plus a 20A non-continuous load, protected by a cable rated 130A. What's the minimum standard OCPD, and does it protect the cable?
- Minimum OCPD = 125%×continuous + 100%×non-continuous = 1.25×80 + 20 = 120A.
- Round up to the nearest NEC 240.6(A) standard size: 125A.
- Check against cable ampacity (240.4): 125A ≤ 130A cable ampacity, so the breaker protects the cable directly.
125A breaker/fuse, cable protected — matches the calculator's default NEC scenario exactly.
Want the full step-by-step version?
On the IEC path, a fuse's higher conventional operating current (I\u2082=1.6\u00d7In) can fail the 1.45\u00d7Iz check on a tight cable even when an equivalent-rated breaker (I\u2082=1.45\u00d7In) passes \u2014 the calculator surfaces this device-type difference rather than treating breakers and fuses as interchangeable.