50 Amp Motor Feeder

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FrancisDoody

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Durham, CT
Motor feeder services two motors. One motor is a 5 HP, the other is a 3 HP. The power is 3 phase 240 volt Delta. The conductors are #10 AWG CU. The OCPD is a 50 Amp CB. Each motor is controller by a contactor with the properly sized heaters for OL protection. The coil for each contactor is rated 240 volts.

In an emergency situation one of the contactors has to shut one of the motors down. This is done via a NC relay. The way this circuit was designed was that one of the coil leads was intercepted and re-routed via the NC switch. The question I have is; Should this control circuit have additional OCPD before it leaves the contactor? Maybe an inline fuse rated for 30Amps.
Thanks,
Fran
 
You need to protect the complete control system, not the jumper to the contacts. A 30 amp fuse at 240 VAC is way to large for most control circuits, especially since the starter coils are operating 3 and 5 hp starters.

Check the control XFMR. Some have the input or output fused. With the output being the most common. If it is fused you do not need any more fuses. That would be over kill.
Size the fuse to either the primary or secondary of the XFMR if one does not exist. A small fuse block mounted inside the panel will work fine if required.
 
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Power

Power

John,

The panel has 120 volts to Phase A, and Phase B, and 208 on Phase C. The coil is a 200-208 volt system. Each leg of the coil is 120 volts. The motors support an exhaust hood and make-up intake air system. In the case of a fire the exhaust system must run and the intake has to shut down. The controller for the fire supression system will engage the NO contact and release the coil and the system will shut down, however the CB is still based on calculations from Table 430.52 for the motor feeder. 50 Amps. The #10 AWG is piped over to a junction box next the fire system box and emt connects the two boxes. I am sure that this would or should not be called a motor control circuit in the typical sense, but it may control it if there is ever a fire.
Thanks,
Fran
 
Fran, my point was that you do not need the extra fuse. I made no mention or was concerned about the application.
One properly sized fuse for a standard everyday control circuit is all that is needed. Sized to the XFMR volt amps.
Could I be missing something in your question?
John
 
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