my understanding, if you want to get into the semantics of it, is the following:
in a 3 phase electrical supply, each phase is 120* apart and 120V and since 3 phases = 360* they add up to 0; no neutral, return, or ground required (ground IS needed for safety and protection though obviously) if you mesure each phase to ground it's 120v and across the 2 phases is 240v.
look here for more details in 3 phase:
the standard house gets only 2 of the 3 phases, or poles. normally a device uses one or the other pole, neutral and ground again for safety. more power hungry devices, like air conditioners, use both poles.
so each pole has a breaker for 50A on a 50A circuit. theoretically you have 100A to play with. however both of these breakers are physically tied together. since the design is assuming both poles will be loaded equally, if theres a overload both breakers will pop together.
it's actually unsafe to use a 50A to 30A adapter. say theres a short before the the 30A breaker in the coach or it fails. since the 50A breakers are physically linked, it will take more current then 50A to trip one breaker and move the other. before the compromised 50A breaker can trip the cord melts or catches fire. it's much safer to use a 30A breaker on a 30A cord.
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