Sparky,
You need to find the time-current curve for the device that you are using. The only one that I could find this morning for a 15 amp FRN-R fuse. It shows that this fuse will hold on average for 90 seconds at 20 amps, 30 seconds at 30 amps, and 2 seconds at 60 amps.
Other fuses and breakers will have different curves.
Don(resqcapt19)
