Single Motor - Disconnect: Fused or Non-fused?

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mrtom

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I am mostly a residential electrician that does not have much experience with motors, so apologies if I am asking a silly question.

I have a job for a family member (oh those jobs are fun LOL). It is in a two room office space that has two seperate main lug subpanels located in room #1 with a 3 phase 4 wire 120/120/208 volt service. One panel is a 100 amp single phase currently feeding lighting/outlets and AC/furnace. The second panel (which is currently not being used for any circuits) is a 100 amp 3 phase service. Both panels are being fed from the main electrical room located elsewhere in the building.

Room #2 will now need power to a new 3 phase 208 volt 10 HP motor feeding a large air compressor. The motor & compressor are installed in an existing enclosure/housing which has an existing start & stop control and an existing magnetic starter with overload protection.

I understand the motor calculations for circuit conductor size, short-circuit and ground-fault protection, and overload protection as well as when to use the NEC vs the nameplate for calculations. I also understand that since the panel with the circuit breaker and the motor are in different rooms, a disconnect will be required within sight of the motor.

My question: does this disconnect need to be fused or non-fused (safety switch) per NEC 2011? I just cannot seem to figure that one out LOL.

Thank you for any suggestions or guidance to code references.

(Today was the first day I came across this forum and I must say what a great resource of information and ideas.)
 
Unless the compressor has a requirement for "fuse protection", the NEC would accept a non-fuse disconnecting means.
 
Thank you for the reply. I thought that was the case since the motor/compressor has the magentic starter which provides overload protection.......and if the magentic starter was not present, then the fused disconnect would be required(?). I will double check to see if anything states that the compressor needs fused protection. And for a bonus, I believe the safety switch is cheaper LOL.
 
Basically, unless there is some circumstance such as manufacturer requirement or engineering requirement (such as fault current reduction) a fusible disconnect or breaker can be used for short-circuit protection or overload protection. If those issues are addressed then the only need is for a properly rated disconnecting means.

You may find the motor starter requiring fuse protection...some do.
 
Thanks again for the information, it was very helpful. I do believe that the disconnecting means should be rated for the amperage of the circuit AND rated for the HP of the motor. Thanks again. What an excellent forum!
 
mrtom said:
...located in room #1 with a 3 phase 4 wire 120/120/208 volt service. ....

Room #2 will now need power to a new 3 phase 208 volt 10 HP motor...
Click to expand...
When you say 120/120/208 do you mean that is the voltage from each ungrounded conductor to neutral (or ground)... or is it just a typo and you simply mean 208/120 3? 4W?

The reason I ask is because 120, 208, and 120 are what the voltages would measure to neutral or ground for a 240/120 3? 4W system.
 
Oh my, that was a typo! It is a 208/120 3? 4W service. Thanks for asking, I could see how that could be a major goof-up if someone messed that up on the jobsite LOL.
 
Lets just say a disconnecting means does not require overcurrent protection. If it is desirable or even required to have fuses as overcurrent protection, then a fusible disconnect as this local disconnect is often the most economical way to achieve that.
 
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