Short Circuit Current Calculator
Calculate transformer full-load amps, available fault current, motor contribution and the minimum equipment interrupting rating.
Step-by-step
- Enter valid values to begin.
Infinite-bus (utility source impedance neglected) method: this gives a conservative upper-bound fault current at the transformer secondary terminals. Cable and bus impedance downstream reduce the fault current; use a full short-circuit study (IEEE 551 / point-to-point) for final equipment ratings.
Short-Circuit Current: What Your Breaker Must Be Able to Stop
Core Engineering Principles
When a fault happens, nothing limits the current except the impedance between the source and the fault. A transformer’s percent impedance tells you how much: a 5.75% transformer will let about 17 times its full-load current flow into a bolted fault at its terminals, assuming the utility behind it is an infinite source. That number is the starting point for the available fault current at your panel. It’s a worst-case number and it’s the one that matters, because a breaker that can’t interrupt that much current doesn’t just fail to trip. It can explode.
Running motors feed the fault too. For a few cycles after the fault, the spinning motor acts like a generator. A common rule of thumb is four times the motor full-load current, and some engineers use six. This page adds that to the transformer contribution so you see the total at the secondary terminals. Cable and bus impedance downstream bring the number down, so for real equipment ratings the panel schedule should come from a proper short-circuit study.
NEC & Standard References
NEC 110.9 says equipment intended to interrupt current at fault levels shall have an interrupting rating at least equal to the current available at its line terminals. 110.10 requires the protective devices and component ratings to work together so a fault doesn’t destroy the equipment. 110.24 requires service equipment (except dwelling units) to be field-marked with the maximum available fault current and the date it was calculated. Standard interrupting ratings run 10, 14, 18, 22, 25, 35, 42, 50, 65, 100 and 200 kA. Series ratings under 240.86 are a separate topic and need tested combinations.1. IFLA = 500,000 / (1.732 × 480) = 601.4 A.
2. Isc = 601.4 / 0.0575 = 10,459 A (about 10.5 kA).
3. Standard ratings step up to 14 kA, so that’s the minimum.
4. Add 100 A of running motors: 4 × 100 = 400 A, so the total goes to 10,859 A and 14 kA still covers it.
- A breaker has two ratings. A 20 A, 10 kAIC breaker will interrupt 10,000 A. Put it where 25,000 A is available and you’ve built a grenade.
- Voltage changes everything. The same kVA at 208 V gives more than twice the fault current of 480 V.
- Utility changes it too. When the POCO swaps a transformer, your available fault current can double and your labels become wrong.
- Label it. 110.24 and 110.16 require markings, and NFPA 70E arc-flash work depends on the study.
- Don’t guess for a real job. Ask the utility for the available fault current at the service and get a licensed engineer to study anything critical.