On VFDs you are required to have the feed conductors sized at a MINIMUM of 125% of the VFD's maximum rated amps, the motor is NOT factorerd into it. So if you have a VFD rated for 100HP but it is connected to a 50HP motor, you MUST size the conductors for the max amp rating of the VFD, not the motor. You can then use a smaller breaker if you so desire, but most people just size the breaker for the conductors.
On soft starters, all of the motor circuit breaker rules that apply in 430.52 do not change just because there is a soft starter involved. Conversely you should be able to use whatever breaker is listed for the motor HP as per usual, there is usually no need to up-size the breaker because if the starting current is going to be long and high enough to cause the breaker to trip, more than likely you will have exceeded the rating of the soft starter first. If you were to up-size the soft starter to compensate, then make the breaker match the soft starter size (assuming you don't end up violating a 430.52 rule in the process).
Last edited by Jraef; 09-08-11 at 06:58 PM.
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