When an arcing fault occurs downstream from two 100A fuses in parallel, can the fuses modelled by a single 100A fuse by shifting the current axis to the right by a factor of two to represent the time-current curve for the two parallel 100A fuses ?
The above assumes that the current divides equally between the two 100A parallel fuses.

What voltage? Medium Voltage fuses have a parallel mounting for larger ratings. i.e. 540E which is made up of 2 - 300E fuses in parallel. 540E = (2 x 300E) x 90% The curve is specific for a 540. I did not overlay a shifted 300E TCC with a 540E but it looks like the shapes are similar.

In the U.S. we don't parallel LV fuses (except perhaps cable limiters on networks)

There is a small problem with this. The average fuse rating doubles but the fuse accuracy goes down. Fuse curves show a range of values to accomodate a +/-% factor. If you parallel them, you might get a batch which are both on the extreme to either the positive (bad from an arc flash point of view) or negative side.

I know this was post was made a long time ago, but maybe someone can still respond. so Jim Phillips gave the example of two fuses in parallel. But I have a situation where I have a medium voltage switchgear with 3 fuses in parallel with a total amperage of 1080A between the three. What would be the calculation for that; what would each fuse size be? Thank you

Here's another thread on this subject, low voltage version. It was never resolved though. Like a lot of posts on here, people try to help and answer the question but the OP disappears with no follow-up.

viewtopic.php?f=23&t=5609

You can model multiple fuses in parallel in SKM. Not sure about other programs. Never tried in etap and I don't have EasyPower.

That said, RE: your question, what are you trying to calculate or are you only trying to select fuses?

I am trying to calculate the fuse size. I have never seen a triple barrel per phase before. They want 1080A between the three fuses per phase, but I cannot determine what the E-rated fuse would be. I want to know if there is a calculation behind determining this.

To calculate fuse size you need to know what you are protecting. Are you trying to protect a transformer? A feeder? What size? What type of fuse or fuse holder? What type of switchgear will the fuses be installed in?

I haven't seen a triple barrel fuse before either but it appears that you can buy them in CL-14. I only see up to 300E though. If you think you need 360E (x3=1080??) then it must not be a CL-14.

Are you in the US or is this somewhere else? Is this existing switchgear or new? Do you have any cut sheets on the gear? Any other information?