Question

Question: Trying to figure out available fault current for a transformer. Utility has not responded to any of my calls.Transformer 500KVA,208/120,3ph,4wire,assuming Z=1.2%500K/(208x1.73x.012)=115792Existing switch gear is all fused and want to set an MDP within 50'.Am I correct in the fact breakers must have a 100KAIC rating, without talking to utility.

Answer 1

Assumptions

Fault current is symmetrical (balanced three-phase fault). Z = 1.2% (transformer impedance). Distance from transformer to MDP is 50'.

Fault Current Calculation

Using your formula:

Fault Current = 500KVA / (208V  1.73 * 0.012) = 115,792A

Breaker Rating

For a 50' distance between the transformer and MDP, the minimum required breaker AIC rating is typically 100 kA. This is because the fault current may increase as the distance from the fault location to the breaker increases.

Conclusion:

Based on the assumptions, the existing switchgear fuses should have a 100 kAIC rating to protect against potential fault currents. However, it is always recommended to verify the fault current rating with the utility company. Flag for review

Answer 2

Assumptions

Fault current is symmetrical (balanced three-phase fault). Z = 1.2% (transformer impedance). Distance from transformer to MDP is 50'.

Fault Current Calculation

Using your formula:

Topic: Fault current calculation