AIC Ratings, and Available Fault Current at Service

[bob]

My predicament is, there are 18,332A in available fault current from the 75kw transformer that supplies this house (200A Service, 120/240 3W 1P). The transfer switch has a "Amps Withstanding" rating of only 10,000kA
THANKS!

Can you show us how you came up with the 18382 amps. I have an estimate of 12500 using a %Z of 2.5.

 

[rcwilson]

Good point, Bob. Maybe the utility gave 18,332 amps as the 120V fault currrent.

If we use the assumption that the impedance on a single phase 240/120 V transformer feeding a 120V fault is about 20-50% higher than nameplate impedance, then 18.332 kA works out to a Z= 2.2%- 2.8 %. (Impedance on 120V is higher because only half of the secondary winding is in the circuit.)

18,332A x 120V = 2200 kVA short circuit.

75 kVA/ 2200 kVA = 3.4%

3.4% adjusted down for 20% = 3.4%/120%= 2.85%

3.4% adjusted down for 50% = 3.4%/150%= 2.23%

These numbers are more realistic.