Electronic – Ampacity of Conductive Tape

This is a question of resistive heating, so it's about RMS, not average current. You need to maintain an RMS current of 12.5 amps. That depends on the shape of the pulses and the duty cycle. If we assume your pulses are perfectly rectangular:

$$ I_{RMS}=I_{PK}\sqrt{D} $$

Plug in 12.5A RMS and 25A peak, and your duty cycle is .25.

Now, this is only really useful if your pulses are short and close together. If you're on for ten minutes, off for thirty, you may have a 12.5A RMS, but it's an RMS over a much longer period than the thermal time constant of the wire.

The thermal time constant of the wire will be on the order of 30 minutes to a few hours. Welders use a 10 minute cycle for "short enough to use RMS".

3 times 100A at 120V means you've got 36KW coming into your fuse board.

3 times 57.7A at 208V means you've got 36KW going out of your fuse board.

Assuming your load is balanced, anything more than 57.7A on the load side will be more than 100A on the supply side.

Where I work, we step down the wiring and the circuit breakers so the end points break before the supply points, so for 208V circuits we'd be looking at a 3 phase (3 pole) 50A circuit breaker to break all three circuits if there is an overload on one, or 3 single phase 50A circuit breakers if you can allow single phase loads to fail independantly