Short-Circuit Current and Interrupting Capacity — Basics for Estimators

Every breaker and panelboard has an interrupting rating. If the available fault current exceeds it, the device can fail catastrophically during a fault. Estimators need a working feel for the numbers.

What sets the fault current

The dominant source at low voltage is the supply transformer. Its percent impedance (%Z) limits how much current flows into a bolted fault on its secondary.

Worked example

500 kVA, 400 V, %Z = 5%:

Assuming an infinite utility source is conservative (real fault current is slightly lower). Add motor contribution (~4–5 × motor FLA) for the first few cycles where large motors are present.

Fault current decays downstream

Cable resistance and reactance reduce the fault current at each point further from the transformer. A panel 40 m away on 38 mm² feeder might see 9–10 kA instead of 14.4 kA. Long, small feeders bring it down quickly.

Selecting interrupting ratings

• Main distribution panel near the transformer: specify breakers rated ≥ 18 kAIC (next standard above 14.4).
• Remote sub-panels: 10 kAIC is often sufficient once you have run the downstream numbers.
• Series-rated combinations can be used where tested and listed, but fully-rated is safer to specify and estimate.

The AIC rating is a real cost driver — 22 kAIC breakers cost noticeably more than 10 kAIC. Getting the fault study right avoids both danger and over-specification.