if somebody says i have a 100amps powerbox 3phase does that mean that i can plug 300amps to it?

Answer 1

Assuming a wye connected service, you can run 300 amps of single phase load by splitting the devices so there are 100 amps worth on A, B and C phase.

With a clipon ammeter you'd measure 100 amps on A phase, 100 amps on B, and 100 amps on C phase.

Technically this is not quite correct since the NEC only allows you to load the main switch to 80% of its rating. In theory you could run 240 amps of single phase load.

Answer 2

No. There will be 3 main circuit breakers (or fuses with switches), one for each phase, rated at 100 Amps each.

You can't run an unbalanced (or even a balanced) load at 3 times the rating -- the breaker (or fuse) will blow.

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Answer 3

A three phase 100 amp panel won't do what you are asking. It will not provide you with 300 amps.

I hope this is a school question and not a real world endeavor.