Calculating the HP rating of a disconnect switch

[little guy]

When figuring out the HP rating of the switch for a 3-phase Design B, C, or D motor you simply take the calculated LRC to the conversion table 430.251(B). My question is how would you figure the HP rating of the switch for a Design A motor? Can anyone point me in the right direction?

 

[little guy]

crickets ehh?

 

[retirede]

I started in the industry in 1979 and have never encountered a design A motor. From the perspective of the load, they’re not appreciably different than a standard design B, so they aren’t used that often.

That may be why no one has replied. I can’t answer your question.

 

[little guy]

Thanks for your reply. Since I am an instructor I prefer to not only be prepared when asked that question from other journeyman or apprentices, but while writing a code question to prep our apprentices for a state exam I saw this question and it begged me for an answer. Anyway thanks for your time.

 

[retirede]

Thanks for your reply. Since I am an instructor I prefer to not only be prepared when asked that question from other journeyman or apprentices, but while writing a code question to prep our apprentices for a state exam I saw this question and it begged me for an answer. Anyway thanks for your time.

Was the question on a practice exam? If so, maybe there’s an answer guide somewhere. Now I’m curious, myself!

 

[little guy]

It is an exam that I am writing based on a Motor Nameplate image I found where I have the students figure the Minimum ampacity of the Branch Circuit conductors and minimum size THWN copper conductors, Maximum rating of overloads using separate overload devices, Max rating of OCPD using Dual Element fuses, and both the minimum ampacity of separate disconnect and the minimum HP rating of the switch. All I give them is a picture of the nameplate and the voltage they are using and they figure the rest out. However while figuring the HP rating of the switch I saw that 430.251(B) is only for Design B, C, and D motors and this one is a design A which prompted this question because I cant answer it myself.

 

[drcampbell]

If the disconnect switch is not used to turn the motor on & off, I think a horsepower rating is not applicable.

 

[Besoeker3]

When figuring out the HP rating of the switch for a 3-phase Design B, C, or D motor you simply take the calculated LRC to the conversion table 430.251(B). My question is how would you figure the HP rating of the switch for a Design A motor? Can anyone point me in the right direction?

Wouldn't it would be current you measure rather than power (HP)? LRC is current.

 

[little guy]

Wouldn't it would be current you measure rather than power (HP)? LRC is current.

Locked rotor current calculations are calculated first and then a conversion chart is used (430.251(B) for 3-phase) to calculate the horsepower rating of the disconnect switch. So to answer your question Yes you need current to figure the HP rating of the switch if the switch is rated in HP

 

[Besoeker3]

Locked rotor current calculations are calculated first and then a conversion chart is used (430.251(B) for 3-phase) to calculate the horsepower rating of the disconnect switch. So to answer your question Yes you need current to figure the HP rating of the switch if the switch is rated in HP

Ah yes. We Brits don't use horsepower any more - we use SI. That said we still use HP for car engines and pints for beer in pubs.

 

[drcampbell]

Ah yes. We Brits ... still use ... pints for beer in pubs.

But different pints, in accordance with the Universal Principle of Maximum Confusion.

When I first started spending a lot of time in Canada, I was flummoxed by Canadian cars achieving 20% more miles per gallon than their apparently-identical American counterparts.