motor short-circuit/ground fault OCPD with respect to wire size and transfer switch

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malachi constant

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Minneapolis
Hi all,

We have a new lift station pump with 39.2 FLA / 49 MCA. We specified this being fed from a 60A manual transfer switch, which in turn is fed from a 60A circuit breaker on the normal side, and on the "emergency" side a 60A twist lock receptacle for portable generator to plug into.

We are getting close to installation and the motor manufacturer states they require a 100A breaker. This is easy enough to accommodate (there is an existing breaker that size in the panel). Normally I would be comfortable specifying #6 wire (60A) for this size motor even with the larger OCPD, because code allows it, overload protection is built into the motor, etc.

1. But does the presence of the transfer switch change things? I'm thinking that needs to increase to 100A to match the breaker size.

2. Once you've done that do you need to also upsize the wire between breaker and motor (through the transfer switch) to #3s/100A?

3. At that point you should probably be consistent and upsize the wiring and twist lock receptacle on the "emergency" side of the transfer switch to 100A. (Though I do not believe this is required by code.)

Would appreciate some code clarification on this, especially on items #1 and #2. I'm not sure if I explained this very well. Let me know if you need anything clarified. Thanks!

Thanks!
 
tom baker said:
100 A breaker allows for motor starting.
Your xfr switch would be sized at 125% of the NEC table value.
Click to expand...

I think that would be the case if this MTS is a standard double throw safety switch that has a HP rating. If it was an ATS the type and size of OCPD would be dictated by the required labeling on the switch and might mean a larger ATS.
 
Looking at a couple of Double Throw safety Switches (as the MTS), they both say their 60A switch is rated for 20HP @ 230V / 50HP @ 460V, or 10HP @ 230V single phase. So even though the OP didn't state the HP or voltage, that tracks as sufficient given the 39.2A FLC. So the remaining questions are, what kind of MTS is there, and why is the pump mfr. insisting on 100A? 250% of the FLC is the MAXIMUM size breaker you can use, which would be 100A, but there is nothing saying it HAS to be that high. Motor Locked Rotor Current will be roughly 600% of FLC, so 235A is fine. A 100A T-M breaker will have mag-trips set for 1,000A, roughly 4x what is necessary. A 60A T-M breaker will have 600A mag trips, still more than enough to allow that motor to start.

It it were me, I would leave it as is. IF for some bizarre reason that 60A breaker trips, you can change it to the 100A.
 
I'll check to see if there is a HP rating for the transfer switch. Assuming there is, we will likely proceed with 100A breaker, feeding 60A (15HP rated) transfer switch, feeding 39.2FLA (14.1HP) motor. All with #6 / 60A wire. If you see any red flags there let me know.

Thanks all!
 
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