Primary side short-circuit (s.c.) value — who cares! Bus bracing on 480V MCCs, as an example, comes in standard large increments of 42kA, 65kA, and 100kA. With a 2500kVA transformer and a typical 6% impedance, your plant s.c. value (using infinite bus near the transformer) will be about 50kA. Purchase MCCs with 65kA bracing. If you want to reduce these values so you can buy the 42kA MCCs, use fused breakers. It’s that simple.
If you do a detailed calculation, you will make an astounding discovery—The asymmetrical s.c. values with plant contribution will be near the infinite bus s.c. values. Extremely detailed calculations are usually unnecessary. They can get you into trouble. Would you purchase 42kA-braced equipment if your detailed calculation showed a symmetrical value of 38kA?
Of course, the transformer will care about primary s.c. values, but standard design and primary protection will usually save them.
While employed as a utility engineer, it seemed a week didn’t go by without someone requesting primary side short-circuit (s.c.) information. Considering these requests, the whole purpose of the article “What’s My Short-Circuit Value?” was to inform service-entrance designers of the reasons why the electric utility is reluctant to furnish any s.c. information.
Your use of the transformer s.c. value appears to serve you well. However, keep in mind the impedance-length of the feeder cable serving the MCC from the transformer. Factoring this into the s.c. equations might allow for the use of a lower rated MCC. An IEEE Transaction Paper (No. 55-442) summarizing procedures for determining s.c. currents in low-voltage systems depicts just how much the s.c. current reduces.
For example, let’s use a 2000kVA, 5.32% impedance, 12470/7200 – 480V transformer with four No. 750 kcmil copper conductors (75°C in magnetic duct) attached to the secondary terminals serving a load composed of 100% motors (25% synchronous/75% induction). The s.c. value is 53,218A at the secondary terminals of the transformer. However, just 200 ft away, the s.c. value reduces to 40,489A, and at 500 ft away the s.c. value drops to 29,479A.
Just this little amount of detail would allow you to specify a 42kA-braced MCC at a distance greater than or equal to 200 ft from the secondary terminals of the transformer.
Would I purchase 42kA-braced equipment if my detailed calculation showed a symmetrical value of 38kA? This all depends on the situation. If the only thing to consider in the decision were the presently available fault current, then I would say yes. I say this because having performed various s.c. calculations, I realize the conservative nature of the equations.
However, in making the final decision on the MCC bracing, I would also consider future expansion, outside contributions, cost, etc. and base my decision on all of these factors.
Jerry Borland, P.E.
EC&M Technical Editor