Motor circuit protection

[a10jp]

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.

 

[rbulsara]

Yes. Do not forget the overload protection, not exceeding 125%.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Overload 125% for 1.15SF motors, else 115% for SF = 1.0. But can increase 130% (SF = 1.0) or 140% (SF = 1.15) if problems with tripping. I don't know if this type increase would affect min conductor sizing (125%).

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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[electricpete]

I see the 250% is listed in NEC table 430.52 under the column "inverse time breaker". I assume if you use a combination starter including instantaneous trip breaker and overload protection that this requirement is irrelevant (?)

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[electricpete]

Also there is provision in 430.52(C).1 Exception 1 to go to the next higher setting if you land in between the device settings. We are definitely inclined to do this with instantaneous trip settings for hi-efficiency motors (round to the next setting above 17*FLA) and in a very few cases even that does not seem high enough.

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[resqcapt19]

Pete,
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.

 

[electricpete]

Thanks for your comments resqcapt.

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.

I have not run accross that at out plant, but it seems consistent with what the code is saying. Thanks.

It is my opinion that Exception #1 to 430.52(C)(1) does not apply to instantaneous trip devices.

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.

(C) Rating or Setting
(1) In Accordance with Table 430.52 A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52
shall be used.
Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal pro- tective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that doesnot exceed the next higher standard ampere rating shall be permitted.

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[jraef]

Actually I think it applies, but that is irrelevant in the case of an IT breaker anyway. Even if you use the "next size up" and that next size allows you to turn up the instantaneous trips higher than the size below, there STILL is no provision allowing the use of a setting over 1700%.

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...

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[electricpete]

ok, I see how you get there reading the other exception that allows you to get to 1700 to begin with. I was trying to take the best of both exceptions. As you said this instantaneous setting can cause problems.

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[electricpete]

Just to put it all together as I see it (tell me if I'm wrong)

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.

Where the setting specified in Table 430.52
is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current for other than Design B energy-efficient motors and no more than 1700 percent of full-load motor current for Design B energy-efficient motors. Trip settings above 800 percent for other than Design B energy-efficient motors and above 1100 percent for Design B energy-efficient motors shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous-
trip circuit breaker at 800 percent or 1100 percent.

The "in no case" words perhaps imply that this requirement trumps the other exception ("next higher").

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[electricpete]

The other interpretation which would be more of a stretch is that 430.52(C).3 exception 1 effectively modifies the table 430.52. So if we apply 430.52(C).3 exception 1 FIRST and 430.52(C).1 exception 1 SECOND, then we could get to the next higher setting above 1700%. I'm not saying that is correct, but one could make the argument.

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[a10jp]

Thank you all for all these comments. One thing I am puzzlled when I post the question was, the purpose of Table 430-52, at least for general polyp[hase AC motors, was to compenstate for motor starting current, which could be as high as 6 times the FLA. Although it states the exception for energy efficient motor (cat Design B) that can push the range of protection from 1300% - 1700%, why do we use 250% in this table to start off, and then for HVAC compressor motors under Art 440, it limite to no more than 175% which at first glance I thought they are referring to the fuse elements. I guess I am confuses about the inconsistnecy on the range of the protection.

 

[rbulsara]

There is no inconsistency, just allowance for letting the motors start as not all motors are created equal.
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

 

[resqcapt19]

Pete,
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.