← Electrical Systems Studio
⚡

Short Circuit Current Calculator

IEEE 551 · NEC 110.9 · OCPD Interrupting Rating

When to use: Required at every stage of electrical system design. NEC 110.9 requires that every overcurrent protective device (breaker, fuse) have an interrupting rating at least equal to the available fault current. Undersized OCPDs can explode during a fault, causing fires, equipment destruction, and injury. Use this calculator when selecting circuit breakers and fuses, and when performing equipment labeling for arc flash studies.

Transformer
%
Utility Source (Optional)
kA
Leave 0 to use transformer-only (conservative)
Standard Interrupting Ratings
10,000 A
14,000 A ← Minimum required
18,000 A
22,000 A
25,000 A
35,000 A
42,000 A
65,000 A
100,000 A
200,000 A
Available Fault Current
10,459
Amperes (A)
Minimum OCPD Interrupting Rating
14,000A
All breakers/fuses on this bus must meet or exceed this rating
Calculation Details
Transformer FLA (Secondary)601.4 A
Transformer Fault Contribution10,459 A
Available Fault Current10,459 A
Min. Interrupting Rating14,000 A
References
NEC 110.9 — OCPD interrupting rating requirement
IEEE 551 — Short circuit analysis methods
ANSI/IEEE C37 — Breaker ratings standard
Formula: I_fault = I_FLA / (%Z/100)