minimum breaker for 7.5HP 208V/3P motor?

[malachi constant]

Hi all,

A colleague asked me to help him answer this question. He specified a 30A/3P breaker for a 7.5HP 208V/3P fan motor. He asked me what I typically specify. My answer is:
* Refer to Table 430.250 for FLA -> 24.2A
* Multiply FLA x 125% -> 30.25A
* Round up to nearest breaker/fuse size -> 35A

I know 220.5 allows you to round down fractions of an amp (under 0.5), but not convinced that applies to motor protection.
I believe this is for a project where the 30A breaker is already purchased and in the field, so the question is do they need a larger breaker. I am advising "yes, unless someone can prove to me it is OK to round down".

Any help is appreciated. Thanks!

 

[mayanees]

430.52 for breaker size

430.52 for breaker size

You should be using 430.52 to size the short-circuit protection, which goes as high as 250% for an inverse-time breaker.
Your overloads protect the wire.
The 30-amp breaker could work if there was little to no inrush with the starting of the motor.

 

[infinity]

30.25 amps would be the minimum conductor size. As mayanees stated use table 430.52 for sizing the ground fault and overcurrent protection device. Would (B) be applicable?

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347,
480Y/277, 480, 600Y/347, and 600 volts shall be used.
(B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.

 

[petersonra]

30.25 amps would be the minimum conductor size. As mayanees stated use table 430.52 for sizing the ground fault and overcurrent protection device. Would (B) be applicable?

I don't recall there is a minimum rating for the OCPD. The tables only have the maximum ratings.

That would make the minimum rating a design decision.

 

[infinity]

I don't recall there is a minimum rating for the OCPD. The tables only have the maximum ratings.

That would make the minimum rating a design decision.

I agree, similar to sizing the OCPD for an AC unit at the MCA and not the maximum permitted by the nameplate.

 

[tom baker]

The square D motor data calculator, which I trust, calls for a 50 amp thermal magnetic breaker.
I am not a big CB fan, prefer dual element time delay fuses, for this application, if its about 25 A FLC, I would use a 35 amp fuse.
CBs don't protect the motor as well as a fuse. I have never lost a motor protected by fuses, but had several damaged where protected by CBs, of the MCP type. These are usually 35-50 HP motors, so they tend to be expensive

 

[ActionDave]

...
CBs don't protect the motor as well as a fuse. I have never lost a motor protected by fuses, but had several damaged where protected by CBs, of the MCP type. These are usually 35-50 HP motors, so they tend to be expensive

I've got to disagree with this. A breaker trips all the poles. Right now a fuse is blowing and single phasing a motor that is going to burn up.

It's all about the thermal protection, not about breakers or fuses.