proper fusing for big motor/small motor combination

[eeinpa]

Howdy. I'm an EE who has worked in the design of industrial control products. I'm now looking at the application of these products in some power designs. Please pardon my ignorance due to this fact!

I'm looking at a control panel for a machine (pump) which has a 100 hp electric motor, 3 hp cooling fan motor, and 5kW electric heater. I emphasize these are all parts of one machine. I am confused as to fuse requirements for this arrangement, because the small loads are such a tiny fraction of the load current for the entire panel.

As all devices are part of "one machine", I believe NEC would permit one disconnect to serve the entire panel. That could be a fusible disconnect for the entire panel, with 175A fuses feeding a large power distribution block. The small motor and heater could come off the PDB (conductors 1/3 ampacity of main) and have small (e.g. CC) fuses before their respective starter and contactor.

Must the 100 hp motor have a redundant set of fuses before its starter? It seems rather silly (and expensive) to have a main fused disconnect with 175A fuses followed by a set of 175A fuses for the main motor just to be able to tap <10 amps off for the heater or cooler (they generally would not be on together).

Thank you for helping me better understand the requirements.

 

[motorsdirect]

Hi,
You only need the one main set of fuses to cover the total load, and need a seperate overload trip for each motor to protect them seperately. The fuses being mainly to protect the supply line and the overload units to protect the motors.
Hope this helps
Jeff

http://www.motors-direct.co.uk

 

[eeinpa]

Jeff
Thank you for your reply (I was wondering if anyone would answer!

I believe I do need separate fusing for the small loads. The cooling fan starter is rated for a maximum fuse size of 25 amps, which is obviously far less than the ~150 amps required for the 100 hp motor. Also, I would not think there is any reason to put an overload relay onto the heater load...? A contactor with fast-blow fuses is what I would expect there.

Further comments appreciated.
Thanks
Paul

 

[motorsdirect]

Sorry Paul
I was only talking about the motors...I would put seperate fusing in for the heater, this can safely be fitted after your main fuses,without the need for more fuses for either motor.

regards
Jeff

http://www.motors-direct.co.uk

 

[mc5w]

For the small amount of auxiliary loads, you do not need a redundant set of fuses before the controller for the large motor.

However, BOTH motor overload relay contacts need to be connected in the hot side of the control circuit so that if either or both overloads trip, both motors will stop. NEC 430.73 essentially requires this and well as 404.2(B). If say the cooling fan motor overload trips out and the larger motor continues to run you will have a BIG mess.

Also, a 100 HP motor controller that has a group rating could be used with 200 amp class J time delay fuses if that is within the rating of the controller. The reason for restricting the maximum size of the fuses of circuit breaker is so that a short circuit will not destroy the motor overload relay.

Mike Cole

 

[armandh]

fuse appropriately to protect any wiring lighter than the mains