instantaneous bkr setting for breaker in combo motor starter 2

[electricpete]

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) ?

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[electricpete]

Question 2A - If the answer to #2 is yes (i.e. I am reading it correctly that I use the motor nameplate for the overload, but the table value for the breaker setpoint), then what is the logic behind choosing one value full load current (nameplate) for overload protection and another value full load current (table lookup) for breaker instantaneous setting?

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[davidbeach]

1. Arbitrary, if you don't like it submit a proposal to change it.

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.

 

[jraef]

1. Actually, in the 2005 edition it is now 1700% for "Design B energy-efficient motors", whatever they are... (meaning that there is no clear definition of that either). So really, it's just arbitrary as davidbeach says. They did add though that if you choose to use the higher values, it must be "demonstrated by engineering evaluation" and it goes on to state that you must FIRST apply the lower level devices and show that it will not work.

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://www.nfpa.org/itemDetail.asp?categoryID=860&itemID=21227&URL=Publications/&cookie_test=1

You need to register, but it's still free and they do not spam you.


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[electricpete]

Thanks David. The logic for which FLA to use makes good sense from that perspective.

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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[electricpete]

Actually, in the 2005 edition it is now 1700% for "Design B energy-efficient motors", whatever they are... (meaning that there is no clear definition of that either).

I have been looking a little more to understand where we can apply that energy efficient setpoint.

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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[electricpete]

As a clarification, that would be reading the "nominal efficiency" column of the tables.

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[electricpete]

Sorry - I misrepresented table 12-10. It does not present any minimum efficiencies. It presents the rules for what efficiency number can be placed on the nameplate.

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[jraef]

Well, unfortunately the NEC makes no reference to NEMA in that regard. They originally said "Design E" motors (if I remember correctly), but "Design E" was never adopted by anyone as a standard. Now it says "Design B energy efficient", which is nebulous at best. Energy efficient compared to what? If motor Mfr A has two product lines and one is more efficient than the other, does that qualify? But what if it is LESS efficient that motor Mfr B's non-efficient design? Does it still qualify?

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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[electricpete]

NEC 2005. Fine Print Note after exception 1 to 430.52(C)(3):

For additional information on the requirements for a motor to be classified as "energy efficient" see NEMA standards publication MG1-1993, Part 12.59"

NEMA MG-1 2003 Sectrion 12.59:

The nominal full-load efficiency of polyphase squirrel cage induction motors rated 600 volts or less determined in accordance with 12.58.1, identified on the nameplate in accordance with 12.58.2, and having a corresponding minimum efficiency in accordance with Table 12-10 shall equal or exceed the values listed in table 12.11 for the motor to be classified as "energy efficient"

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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[jraef]

I stand corrected. I missed that reference to NEMA MG-1.


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