I have a old FPE switchboard that has two fuses per phase. In the attached image you have 300-amp fuses with 300 MCM cable. I am assuming for proper results in SKM this would need to be modeled as a 600-amp fuse with parallel 300 MCM. Any thoughts or suggestions on modeling this?

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Thanks bbaumer, I suppose I should have asked my question with more detail. I am really concerned more with the TCC as I have a 300-amp breaker in the panel downstream of the 300-amp fuses. We really have a 600-amp feed to the panel. I can make the drawing look like the picture, but with a 300-amp fuse the TCC does not tell the story.

Is this your facility or a customer's?

If it's yours, and you only have a 300 amp rated panel downstream, remove one set of fuses and disconnect the corresponding set of conductors at the panel.

Or

Replace the fuses with 150's, although I think removing one set of fuses would be more appropriate.

I just re-read your last post. Is the 300 amp breaker in the downstream panel a main?

The panel itself would be protected from overload by the main in that case.

MLO panel the 300-amp is a feeder out of the panel.

Questions and Answers:

1. Why are there five conductors on the load side of the fuses.

2. The Buss NOS, is only a 10kA fuse, these should be changed. Best choice would be a Mersen A6D or Buss LPS.

3. A 400A fuse clip will accept fuses in the range of 225-400A.

4. In your case the ampacity rating of the parallel fuses would be .85 X 600 or 510 amps- this accounts for proximity heating and sharing.

5.This part is a stretch. since the NEC does not allow (in general) parallel fuses, it is hard to define the parallel ampacity rating. However, if I use the normal UL/NEC rules, your parallel rating would be .8 X 510 or 408 amps. Note 1: the UL test does not allow a UL98 rated switch to be loaded more than 80%. Note 2 - this method should keep the fuse terminals at or under 167F when loaded to 408 amps.

6.This means the wire ampacity must equal or exceed 408 amps.

7. Let me know what is happening on the load side with the five conductors, then, I can give you a real answer.

I see 6 conductors on the load side.