Hello,

I am trying to lower the incident energy of a 208 panel. The current incident energy is 6.16 cal. It is being supplied by a 45 kVA transformer. I thought that adding a disconnect might help reduce the incident energy, but that did not work. Do you have any other suggestions?

You need to also be looking at the line side of the MCB and using the worst case between line and load sides.

Did you try a fused disconnect or just a disconnect? Where did you add it? On the secondary? What type of fuses?

You might also have a tap rule violation with a 100' transformer tap but I don't know if you meet any or all of the exceptions.

Plus, a 45 kva transformer is normally used to feed a 150A panel, not 175A, at 208V with #1/0. FLA of the transformer is only 125A.

Hey,
I used a Fused disconnect with FRS-R 175A. I was able to bring it down to 6.01. The customer wants to bring it under 4 cals if possible. I did play around and add an adjustable trip breaker. that did bring it under 4.

Run the report with line and load side and see what the line side is with the fuses in place. SKM will look at the MCB because it is the first upstream from the buss you are looking at. If the line side is lower with the fuses then that means the fuses are going to act quicker than the breaker at the AFC level and will blow while the MCB doesn't trip. Verify by looking at the TCC. In that case, the line side, even though lower, would be more accurate. Again, verify this this by looking at the TCC with the fuse and MCB overlaid.

Also, you are using 600V fuses on the 208V side. I expect the curves are the same but you should be using FRN-R not FRS-R fuses on the 208V side.

Run the report and look at your clearing time. I bet you are more than 2 seconds on the MCB. Also, I am having a hard time re-creating your model and getting 3000A of AFC with a 45 kva transformer at that %Z. I can only get like 3400A of bolted fault current.

When I overlay the curves, you are barely in the instantaneous range of the breaker, if you even are.

If you want to jump on a Teams call with me and share your screen right now I can look at it with you. Or some other time. Email me or call me.

My AFC is much lower than yours. Same transformer, same cables. Not sure why. Mine times out at 2 seconds because I'm not in the instantaneous region of the MCB curve.

One last thing I'll add, I expect that in reality your AFIE is very low despite what the software says due to the voltage and very low available fault current. It wasn't that long ago (prior to 1584-2018) that you could exclude calcs on under 240V fed from under 125 kva transformers because it is unlikely there is enough snot there to sustain an arc.

The new rule is 2000 amps. You are probably around 3000 amps bolted but, I think still, very unlikely you'd get 6 cals in reality, if you could even sustain an arc. I doubt you could sustain an arc for 2 seconds which is what I think it being simulated by your IE value. In my model, that is what my results show. Timeout at 2 seconds, no breaker trip before then.

There are some things that need looked at in your model, as I pointed out above.