how do you calculate the fault current? 200Amp service 208 3 phase, 4 wire going to a 42 circuit panel?

Technically, fault current can get up to the 200 amp limit.  It is just that the return path of the current does not go back through the neutral, but through everything else like the ground wire, earth, and, unfortunately, people.

The National Electrical Code (NEC) addresses fault current by talking about grounding at the service.  The purpose of grounding is to provide a path big enough back to the service ground so that the breakers open quickly so that no one get hurt.  If the ground at the service is too small, fault current will not be big enough to trip the breakers and a fire can start.

According to the NEC, table B.310.1, 4/0 THHN wire has an ampacity of at least 200 amps, so that is probably the wire size at the service.  Using table 250.66, a 2 AWG copper ground wire is required, which has an ampacity of 115 amps.  That means that 115 maximum ampacity would be enough to trip the breakers fast enough to prevent fires/injury.

There is not enough information to calculate the fault current. Fault current ratings normally range up to 10k amps, but can also reach 20k amp. It depends on how close you are to the substation, and the substation´s fault current capacity.

It is important to know what the fault current capacity at your circuit panel is, to ensure that you have circuit breakers able to handle it.

As a rule of thumb, if you are far away from the substation, the 10k rating should do. If you are close, choose 20k. If you know who to ask, ask.
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