instantaneous bkr setting for breaker in combo motor starter
instantaneous bkr setting for breaker in combo motor starter
(OP)
A combination motor starter includes an instantaneous-only breaker and a starter which starts/stops and provides overload protection.
I am looking at NEC 1999 (the latest version I have handy).
430-52.(c)(1) states the breaker should be set to 800%.
430-52.(c)(3) exception 1 states where the setting specified (800%) is not sufficient to start the motor, the setting may be increased but shall in no case exceed 1300% (excluding energy efficient motors which can go higher).
Question 1: What is the reason for the 1300% limit? It seems this may not be sufficient if the motor has kva code G, H or higher... and does not allow much margin for variability in the breaker trip point.
Question 2: I am looking at a 60hp motor. The nameplate FLA is 65.7A. The NEC table 430-150 FLA is 77A. That is a pretty big difference and it makes a big difference which value we choose as FLA in our 1300%*FLA calucaltion. Applying the 1300% to a higher number will give a higher setting. 430-6(a) states "For general motor applications, current ratings shall be determined based on (1) and (2). (1) Table Values - The values given in Tables 430-147 thru 430-150 shall be used to determine the ampacityh of conductors or ampere ratings of switches, branch short circuit and ground fault protection, instead of the actual current rating marked on the motor nameplate. (2) Nameplate Values - Separate Motor overload protection shall be based on the motor nameplate current rating".
Am I reading it correctly that I use the motor nameplate (65.7A) for the overload, but the table value (77A) for the breaker setpoint (up to 1300% * 77A) ?
I am looking at NEC 1999 (the latest version I have handy).
430-52.(c)(1) states the breaker should be set to 800%.
430-52.(c)(3) exception 1 states where the setting specified (800%) is not sufficient to start the motor, the setting may be increased but shall in no case exceed 1300% (excluding energy efficient motors which can go higher).
Question 1: What is the reason for the 1300% limit? It seems this may not be sufficient if the motor has kva code G, H or higher... and does not allow much margin for variability in the breaker trip point.
Question 2: I am looking at a 60hp motor. The nameplate FLA is 65.7A. The NEC table 430-150 FLA is 77A. That is a pretty big difference and it makes a big difference which value we choose as FLA in our 1300%*FLA calucaltion. Applying the 1300% to a higher number will give a higher setting. 430-6(a) states "For general motor applications, current ratings shall be determined based on (1) and (2). (1) Table Values - The values given in Tables 430-147 thru 430-150 shall be used to determine the ampacityh of conductors or ampere ratings of switches, branch short circuit and ground fault protection, instead of the actual current rating marked on the motor nameplate. (2) Nameplate Values - Separate Motor overload protection shall be based on the motor nameplate current rating".
Am I reading it correctly that I use the motor nameplate (65.7A) for the overload, but the table value (77A) for the breaker setpoint (up to 1300% * 77A) ?
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RE: instantaneous bkr setting for breaker in combo motor starter
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RE: instantaneous bkr setting for breaker in combo motor starter
2. Yes.
2a. So you can change out the motor for another of the same HP without having to change more than the overloads. The tables are intended to be worst case for general purpose motors.
RE: instantaneous bkr setting for breaker in combo motor starter
There is a practical issue at hand though as well. Breakers typically come with certain ranges of settings. Most Thermal-Mag breakers that come with adjustable magnetic trips will only go to a maximum of 10X their thermal rating. That means that in most cases, in order to be able to take advantage of the higher trip settings provided for in the code, you must use Instantaneous Trip (aka Mag-Only or MCP) breakers. The code then states that they can only be used in a "listed combination motor controller having coordinated motor overload and short circuit and ground fault protection...". So essentially you cannot build your own anyway. The settings available to you then become dictated by what the starter manufacturers have had UL listed.
E-pete, you may want to review a newer version of article 452, it has had a lot of changes since 1999. You know that you can view it on-line for free right? Here is the link in case you didn't.
http://w
You need to register, but it's still free and they do not spam you.
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RE: instantaneous bkr setting for breaker in combo motor starter
The motor was purchased in early 1980's, so I'm guessing it is not energy efficient.
It is a Mag-only breaker, part of a combination starter. The breaker is Cutler Hammer HMCP 100. We are currently on the 900A setting ("G") and have available 1000A setting ("H") on the breaker which is within the bounds of 1300% times the table value (but not the nameplate value).
We have some literature from Cutler Hammer. They have a table that lists 800% table FLA values and 1300% Table FLA values.. but they don't exactly say what we're supposed to do with those numbers (I would presume use them in accordance with the NEC).
We have an ungrounded system, so we are applying the instantaneous only breakers outside the code I guess. NEC does not strictly apply at this facility. (By the way, I am not the one who does setpoint changes... just want to be able to talk intelligently to the people that do).
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RE: instantaneous bkr setting for breaker in combo motor starter
Review of NEMA MG-1 - 2003 shows 3 levels of efficiency: Table 12-10, 12-11, and 12.12.
12-12 is premium efficiency (highest)
12-11 is "energy efficienct (middle)
12-10 doesn't have a designation, but I would call it "standard" or "EPACT" efficiency.
I believe that as long as the efficiency stamped on the nameplate exceeds table 12-11, we can call it energy efficient for purposes of setting the instantaneous?
Would you guys agree?
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RE: instantaneous bkr setting for breaker in combo motor starter
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Eng-tips forums: The best place on the web for engineering discussions.
RE: instantaneous bkr setting for breaker in combo motor starter
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RE: instantaneous bkr setting for breaker in combo motor starter
My feeling though is that the spirit of the exception is an allowance for the fact that different motor mfrs will approach energy efficiency in different ways, leading some to have higher inrush currents than in the past. So the NEC allows for you to take an exception (within limits) IF you demonstrate that it won't work otherwise. I therefore think your assessment is as good as any; makes perfect sense to me. Now if you can just convince an AHJ...
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RE: instantaneous bkr setting for breaker in combo motor starter
NEMA MG-1 2003 Sectrion 12.59:
This is a little more info than I had before. It seems more certain to me now that I have the correct interpretation. But still let me know if anyone disagrees my my conclusion, summarized as follows: The way to determine if a motor can be treated as energy efficient for NEC instantaneous settings is to compare the motor nameplate efficiency to NEMA MG-1 table 12-11.
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RE: instantaneous bkr setting for breaker in combo motor starter
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