Hello, I'm teaching some exam prep which includes sizing motor overloads. In 432.(A) 1 it tells us to use a 125% for a motor marked with a temp rise of 40 deg C or less. I know the motors usually have the ambient temp on the nameplate, but I don't see the temp rise on there. How can we tell the temp rise of the motor for application of this article?
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430.32(A)( 1) - 40 degree C temp rise or less?
- Thread starter jes25
- Start date
The maximum rise is determined by the insulation class. Class B insulation indicates a maximum rise of 80C in a 40C ambient, for example.
Sometimes you’ll see something like “Class F insulation, class B rise.” That means the motor is wound with class F insulation, but the temp rise at nameplate operating conditions won’t exceed 80C.
I’ve never seen a motor with a 40C rise, that’s usually the ambient temp rating.
ETA - I can’t find the code section cited. What year?
Sometimes you’ll see something like “Class F insulation, class B rise.” That means the motor is wound with class F insulation, but the temp rise at nameplate operating conditions won’t exceed 80C.
I’ve never seen a motor with a 40C rise, that’s usually the ambient temp rating.
ETA - I can’t find the code section cited. What year?
Been a long day....430.32(A)(1) 2023 NECETA - I can’t find the code section cited. What year?
It looks like even Class A insulation has a rise of 60 C. Therefore a "Motor with a marked temperature rise 40 C or less", per 430.32(A)1 must be some special motor with a special marking that we never / rarely run into?The maximum rise is determined by the insulation class. Class B insulation indicates a maximum rise of 80C in a 40C ambient, for example.
Sometimes you’ll see something like “Class F insulation, class B rise.” That means the motor is wound with class F insulation, but the temp rise at nameplate operating conditions won’t exceed 80C.
I’ve never seen a motor with a 40C rise, that’s usually the ambient temp rating.
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Article 430 NEC 2023 - Section 430.31 General - Part III Specifies overload devices intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start.
Tom Bishop, P.E.
EASA Technical Support Specialist (Article November 2003)
How do we know if a motor is operating within its temperature rating? The simple answer, and a good one, is that the National Electrical Manufacturers Association (NEMA) has defined temperature rise for electric motors in Motors and Generators, NEMA standard MG 1-1998.
We will begin by identifying some key terms. Temperature rise is the increase in temperature above ambient. Ambient temperature is the temperature of the air (or other cooling medium) in the area surrounding the motor, frequently termed “room temperature.” The sum of the ambient temperature and the temperature rise is the overall, or “hot,” temperature of a component. Insulation temperature classes are based on the overall temperature. For example, a Class B winding system is rated 130ºC. The normal maximum ambient, per NEMA, is 40ºC. The temperature rise limit for the Class B winding would be estimated at 90ºC (130-140)degrees.
What does all this mean when selecting Over-Current Protection?
A service factor of 1.15 means the motor is designed to operate periodically at up to 115% of its rated horsepower without damage to the Motor.
A Motor with a name plate temperature rise of (40 deg C) means the motor is designed to operate so it won't heat up more than (40 deg C) above its rated ambient temperature when operated at its rated load and voltage.
Motors that do not have a service factor rating of (1.15 and up) or a temperature rise rating of (40 deg C) and less, must have the Over Load protective device sized at not greater the 115% of the Motors ampere rating.
However if the (125%) allowances will not let the Motor come up to speed and run - it can be increased to (140%). Likewise if the (115%) Allowance will not let the Motor come up to speed and run - it can be increased to (130%).
Tom Bishop, P.E.
EASA Technical Support Specialist (Article November 2003)
How do we know if a motor is operating within its temperature rating? The simple answer, and a good one, is that the National Electrical Manufacturers Association (NEMA) has defined temperature rise for electric motors in Motors and Generators, NEMA standard MG 1-1998.
We will begin by identifying some key terms. Temperature rise is the increase in temperature above ambient. Ambient temperature is the temperature of the air (or other cooling medium) in the area surrounding the motor, frequently termed “room temperature.” The sum of the ambient temperature and the temperature rise is the overall, or “hot,” temperature of a component. Insulation temperature classes are based on the overall temperature. For example, a Class B winding system is rated 130ºC. The normal maximum ambient, per NEMA, is 40ºC. The temperature rise limit for the Class B winding would be estimated at 90ºC (130-140)degrees.
What does all this mean when selecting Over-Current Protection?
A service factor of 1.15 means the motor is designed to operate periodically at up to 115% of its rated horsepower without damage to the Motor.
A Motor with a name plate temperature rise of (40 deg C) means the motor is designed to operate so it won't heat up more than (40 deg C) above its rated ambient temperature when operated at its rated load and voltage.
Motors that do not have a service factor rating of (1.15 and up) or a temperature rise rating of (40 deg C) and less, must have the Over Load protective device sized at not greater the 115% of the Motors ampere rating.
However if the (125%) allowances will not let the Motor come up to speed and run - it can be increased to (140%). Likewise if the (115%) Allowance will not let the Motor come up to speed and run - it can be increased to (130%).
It looks like even Class A insulation has a rise of 60 C. Therefore a "Motor with a marked temperature rise 40 C or less", per 430.32(A)1 must be some special motor with a special marking that we never / rarely run into?
Agreed. I’ve never seen a standard 3 phase, integral HP, 3 phase induction motor with 40C rise.