Motor circuit protection
Motor circuit protection
(OP)
I just want to consult others on the following. In general for motor protection(typical 3-phase motor) using circuit breakers, reading the NEC, I got:
Branch ckt ground fault & overcurrent protection = not to exceed 250% FLA per Table 430.52
Disconnect device = min 115% - 125% FLA
Branch ckr conductor = 125%
HVAC (motor compressor) = not to exceed 175% HACR-type breakers..(with exception, under certain circumstances shall not exceed 225%)
I realized in reality one needs to confirm with the specific equipment being protected and adjust these protection factors accrodingly, but as a rule of thumb
are these factors typically used? Perhaps there might be different opinions, and I like to hear them. Thanks.
Branch ckt ground fault & overcurrent protection = not to exceed 250% FLA per Table 430.52
Disconnect device = min 115% - 125% FLA
Branch ckr conductor = 125%
HVAC (motor compressor) = not to exceed 175% HACR-type breakers..(with exception, under certain circumstances shall not exceed 225%)
I realized in reality one needs to confirm with the specific equipment being protected and adjust these protection factors accrodingly, but as a rule of thumb
are these factors typically used? Perhaps there might be different opinions, and I like to hear them. Thanks.





RE: Motor circuit protection
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor circuit protection
Instantaneous trip at 800% FLA or 1100% for high-efficiency motors. For tripping concerns, when evaluated can increase to 1300% or 1700% for high efficiency motors. I'm not familiar with the 250% number mentioned above (we don't have ground fault protection on individual motors).
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(2B)+(2B)' ?
RE: Motor circuit protection
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(2B)+(2B)' ?
RE: Motor circuit protection
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(2B)+(2B)' ?
RE: Motor circuit protection
Some combination type starters use inverse time breakers and not instantaneous trip devices. In some cases using an inverse time breaker at 250% of the motor full load current gives you a breaker with the "instantaneous" trip point higher than what is permitted with a instantaneous type device.
It is my opinion that Exception #1 to 430.52(C)(1) does not apply to instantaneous trip devices. The rule that applies to those devices is 430.52(C)(3) and its exceptions. I see no provision, in the code, that would permit a setting in excess of 1700% of the motor current. Yes, I am aware that some motors will not start at that setting and that higher settings must be used to get the motor to run, however, I am not aware of any provision of the code that permits these higher settings.
RE: Motor circuit protection
I have not run accross that at out plant, but it seems consistent with what the code is saying. Thanks.
You lost me on that one. I have quoted the relevant portion below with bold emphasis added to show the way I read it. To me it looks strightforward that it does apply to instantaneous trip devices (which of course provide branch short circuit protection). Please comment if you disagree.
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(2B)+(2B)' ?
RE: Motor circuit protection
So let's look at an example:
Most IT breakers with adjustable mag trips have adjustments that go to 1000% of the breaker frame rating. So a 125A breaker has an IT setting that will go to 1250A. If you have a motor where the FLA is 73A, the maximum IT setting can be 1241A. If you select the next size IT breaker up from 125A, a 150A breaker that has an IT setting that goes to 1500A, you still cannot set the IT setting above 1241A. So even if you interpret that it can be done, there is no point when it comes to IT breakers.
Unless you care to ignore the code...
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RE: Motor circuit protection
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(2B)+(2B)' ?
RE: Motor circuit protection
430.52(C).1 Exception 1 quoted above (next higher...) DOES apply to instantaneous setting listed in 430.52. Jeff you are talking about changing breaker rating but I'm not... I'm just talking about going to the next higher setting on existing breaker instantaneous (which come in only descrete settings). As I read it you could go to higher frame size or higher setting on existing frame, right?
Table 430.52 lists 1100% FLA for instantaneous trip. Surely no-one thinks it is code violation to go above 1100% even without invoking provisions of the other exception 430.52(C).3 exception 1?
Now lets look at 430.52(C).3 exception 1.
The "in no case" words perhaps imply that this requirement trumps the other exception ("next higher").
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(2B)+(2B)' ?
RE: Motor circuit protection
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(2B)+(2B)' ?
RE: Motor circuit protection
RE: Motor circuit protection
NEC has stood test of time.
For better understanding plot the time current curve of a motor starting current vs. the TCC of the breaker and fuses in question and study them.
For example, 250% of a IT breaker would normally have a INST setting up to 10X. So a motor with 100A FLA could have 2500A INST setting. Check the time it takes for a 250A thermal magnetic breaker to trip at 600A.
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor circuit protection
It is still my opinion that the only exceptions that applies to IT breakers are the exceptions that follow 430.52(C)(3).
I agree that the code is not clear and can understand it being read to apply the exception in (C)(1) to IT breakers. That is just not how I read it or how I was taught.