Best practice for multiple motors on a branch curcuit

[cal7212]

I have searched the forum and have not come up with a solid answer for my situation so here goes...

I work for a hydraulic systems OEM and we are re-designing the control panel that we use for testing our units. We want to be able to run up to 2 motors between 50 and 100HP each. There is a 200A circuit breaker supplying this panel. What is the best method to allow this? The only guidance I have seen is 430.53(C), however I am not sure that this meets the group installation criteria. Am I correct? Is there another reference? For each of the 2 starters how do they need to be rated if a variable range of motor sizes can be run on each?

 

[kwired]

If this is a test bench and you would never run more than one at a time then size things for the largest possible. If the units all have their own overcurrent protection that is about all you need to do as your conductors you connect to the unit would essentially be feeder taps.

Otherwise you may need to rig up proper overcurrent protection for each instance or set up so you do have multiple outputs to match the needs of what you may be connecting to.

 

[cal7212]

If this is a test bench and you would never run more than one at a time then size things for the largest possible. If the units all have their own overcurrent protection that is about all you need to do as your conductors you connect to the unit would essentially be feeder taps.

Otherwise you may need to rig up proper overcurrent protection for each instance or set up so you do have multiple outputs to match the needs of what you may be connecting to.

To clarify, overcurrent protection is the circuit breaker not motor overloads?

 

[petersonra]

50 hp is 65 amps from the charts
100 HP is 125 amps

The max size cb is 250%. That would be 162 A, so you can't make this a group motor installation with the 200A cb.

You also have to supply overload protection for each size motor appropriate to that motor.

Make a chart of what the motor sizes you plan to test are and have separate connections for each motor size that are properly protected.

 

[kwired]

To clarify, overcurrent protection is the circuit breaker not motor overloads?

Breakers typically only can provide short circuit/ground fault protection and is the reason they are sized up to 250% of the motor full load current. Starting current is normally too high for a standard inverse time breaker to hold if it's setting were also at the overload protection level.