Overloads for Motors

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fifty60

fifty60

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Is it a requirement that motors have overloads? I understand why motors have overloads. The branch OCPD is only providing short circuit protection, and without it there is absolutely no overload protection. I have a 1/2HP single phase motor that does not have an internal overload, and the question has been asked "I know I need one, but it is an absolute requirement that I have an overload."

I've explained the "why's" to them, but is there an NEC or NFPA79 code that absolutely requires it?
 
I seem to remember reading that a single thermal magnetic circuit breaker could be used in place of the short circuit OCPD and separate overload....
 
I've read through section III of article 430 (NEC 2011) and do not see the exception that allows you to use a single thermal-magnetic breaker or dual element fuse as short circuit AND overload protection....does this exception exist anywhere inside the code?
 
fifty60 said:
I've read through section III of article 430 (NEC 2011) and do not see the exception that allows you to use a single thermal-magnetic breaker or dual element fuse as short circuit AND overload protection....does this exception exist anywhere inside the code?
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Section III of Art 430 permits a "Separate Overload Device." This can be fuses or a circuit breaker. But keep in mind that there is no "next size up" rule for overload devices.
 
So a dual element fuse or a thermal magnetic breaker would qualify, even though it would be serving the dual purpose of short circuit protection and overload protection?
 
430.32(1) says "Motors with a marked temperature rise 40 C or less 125%". I would get this information from the insulation rating right? In this case it is Class F insulation. The temperature rise for this motor, with a 1.25 service factor, is much more than 40 C right? It would be closer to 115 C? Or would that not be the information that is being asked for in NEC 430.32(1)?

Also, I cannot tell if the "Motors marked with service factor 1.15 or greater" and "Motors with a marked temperature rise 40 C or less" are anded together, or if they are or'd. Since I know the service factor is 1.25, can I then ignore the temperature rise requirement for this particular clause?

I actually found the temperature rise in the data sheet. It is 67 C at rated load and 80 C at service factor load....this is above the 40 C, does that mean I have to size at 115% or can I size at 125% based on the "service factor greater than 1.15"...
 
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The answer, IMO, is not a simple one. Details matter.
Possibilities: Is it Automatically started ? If so, 430.32(B)
For non-automatic start, see 430.32(D)
Is it on a General-Purpose branch circuit ? If so 430.42(A)
(which also directs you to G P circuit cord & plug connected 430.42C)
 
It is an automatic start motor. It will be an air circulator motor inside of a refrigeration/oven chamber. Refrigeration equipment itself is not a general purpose equipement. Would the motor inside be considered a general purpose branch circuit inside a specific purpose application? I believe regardless, I am directed back to 430.32 (1). Service factor 1.25, temperature rise is above 40...is 125% rounded down permissible?
 
augie47 said:
The answer, IMO, is not a simple one. Details matter.
Possibilities: Is it Automatically started ? If so, 430.32(B)
For non-automatic start, see 430.32(D)
Is it on a General-Purpose branch circuit ? If so 430.42(A)
(which also directs you to G P circuit cord & plug connected 430.42C)
Click to expand...

I agree. Especially when you are considering anything less than 1 HP. Above 1 HP things are a little more straight forward.
 
I have it mostly figured out except for the fact that it is a 1.25 service factor motor, which allows 125% rounded down, but its temperature rise is above 40 C, so I would have to go to 1.15%. Is this "and" or "or"? Does the service factor of 1.25 alone allow 125% rounded down, or does the temp rise being above 40 C mandate then that I would have to use 115% rounded down?
 
I know that they exist, but single phase motors that don't contain internal thermal protection are quite rare. Have you verified that this motor doesn't have this?

Bob
 
fifty60 said:
I have it mostly figured out except for the fact that it is a 1.25 service factor motor, which allows 125% rounded down, but its temperature rise is above 40 C, so I would have to go to 1.15%. Is this "and" or "or"? Does the service factor of 1.25 alone allow 125% rounded down, or does the temp rise being above 40 C mandate then that I would have to use 115% rounded down?
Click to expand...

I see it as an "or".
 
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