Sizing overcurrent protection for motor control panel ?

[Da Electrician]

I have a motor control panel that contains controls, contactors and thermal overloads for 3 different motors as fas as I understand by the wiring diagram only one motor can run at a time. This control panel does not contain a disconnect switch nore any sort of overcurrent protection (fuses, breaker, ect)
all three motors are 460V 3 phase
one is a 10hp, one is a 7.5hp, the last is a 1.5hp
I am trying to figure out how to select a proper size overcurrent protection breaker to put on the feeder to the control panel . the manufacturer engineer has told us to add up all the loads and just simply get a breaker that will handle the load. But this can not be correct if only one motor can run at a time. As i would think that adding all of the load up would create an overcurrent that would possibly never trip because only one motor can run at a time. Am I understanding this incorrectly ?

 

[erickench]

If the control circuit is set up so that only one motor at a time can be turned on then you must size the short circuit-ground fault protection for the largest motor in this case being the 10HP. I would look at NEC 430.52 since you must size it as an individual motor circuit.

 

[Jraef]

...This control panel does not contain a disconnect switch nore any sort of overcurrent protection (fuses, breaker, ect)...

Each motor circuit must contain a short circuit protection device; fuses, CB, or possibly Motor Protective Switches* (those IEC devices that are the SCPD and OL in one). If you have none of those then you would have to feed it with a breaker sized to the NEC table 430.52 for the SMALLEST motor fed by that panel. You cannot protect a 1.5HP motor with a breaker big enough to allow a 10HP motor to start. But you also will not be able to start a 10HP motor with a 15A breaker, the smallest you can use at all. You might have no other choice but rebuilding that panel or feeding each motor circuit with it's own breaker.

* If your panel does have those and you thought they were just the overloads, then they are fine, but they themselves have rules about the upstream protective devices, so you will have to study those specific components to learn what applies to them. Some are rated to be used alone, some are not. If you must rebuild the panel though, that may be a viable option. Use those devices to replace the thermal overloads.