Motor circuit protection with two different types

[jroman]

Hello,

I have a question that I haven't been able to get my mind around.

When protecting motor circuits we design both for short circuit (over current) and over load. Each has different requirements per NEC. If I have a panelboard that supplies one motor, how do I determine the size of the breaker via Short ckt propetection or over load protection? To elaborate further, the motor would be installed on the roof, and the panel board is on the floor below.

Assumming a Design B, 15 hp, 208V three phase squirrel cage motor with a 1.15 service factor. Nameplate current of 40.2 A and dual element fused disconnect switch for short circuit in weather proof enclosure.

Per table 430.250, the rated current is 46.2A. Per table 430.52 the sc setting would be 80.85A, we could then fuse it at 90A.

How to size the feeder coming from the panel board? Per 430.62 the feeder sc circuit protection shall not exceed the rating of the highest short circuit protection plus any additional loads. The handbook states that it is assumed the same type of proptective device is used. So, if I was protecting the feeder with another fused disconnect I would not have any confussion on the matter, it should not exceed 90A. Since I am using an inverse time breaker instead, how is it evaluated?

Are we to take 430.62 on a relative term? Meaning, if table 430.52 calls for 175%for dual element fuses and 250% for inverse time breakers, are we to "transform" the fused disconnect rating to its corresponding inverse time breaker and size the feeder protection accordingly? In this particular example, the 90A dual element fuses would have been 125A on an inverse time breaker. So my feeder protection breaker should not exceed 125A.

Or, as we to take 430.62 literally? On this example, with the 90A fuses, my breaker shall not exceed 90A. But since the short circuit requirement difference between these two devices is due to their ability to withstand the inrush current, am I risking the breaker to open during inrush?

Which one is it? Any thoughts?

Thanks.

Jose

 

[ike5547]

Check out Table 430.52 (2008)

You can generally go up to 250% with a inverse time breaker. With a dual element fuse it's 175%. There are a couple of exceptions down at the bottom of the table.

*edit*

I see you already know this, so now I can't understand your question.

It's sort of a tl;dr.

 

[ike5547]

are we to "transform" the fused disconnect rating to its corresponding inverse time breaker and size the feeder protection accordingly? In this particular example, the 90A dual element fuses would have been 125A on an inverse time breaker. So my feeder protection breaker should not exceed 125A.

Or, as we to take 430.62 literally? On this example, with the 90A fuses, my breaker shall not exceed 90A. But since the short circuit requirement difference between these two devices is due to their ability to withstand the inrush current, am I risking the breaker to open during inrush?

Which one is it? Any thoughts?

Thanks.

Jose

It's neither.

 

[ike5547]

Are we to take 430.62 on a relative term? Meaning, if table 430.52 calls for 175%for dual element fuses and 250% for inverse time breakers, are we to "transform" the fused disconnect rating to its corresponding inverse time breaker and size the feeder protection accordingly? In this particular example, the 90A dual element fuses would have been 125A on an inverse time breaker. So my feeder protection breaker should not exceed 125A.

I'm sorry. This looks right. unnecessarily verbose, but right.

 

[ike5547]

Why are you referring to 430.62 instead of just 430.52? You seem to be describing a branch circuit not a feeder.

 

[renosteinke]

Neither fuses nor breakers will provide you with overload protection - they're for short circuit protection. You've protected the wire, but still have not protected the motor.

Overload protection is provided (most often) by the 'heaters' in the motor starter. Maybe you call them the 'overload relays.' They're what sit under the reset button on the starter.

The NEC doesn't really provide guidance for sizing these. The general practice is to set them as close as you can to the actual current drawn by the motor under actual loads, without nuisance tripping. It can take some trial & error to get them 'right.' For something like a roof-top unit, you'll need a higher setting simply to allow for the higher temperatures up there on a sunny summer day.

 

[jroman]

Thanks for your replies.

ike5547 - You are correct, what I am discribing is a branch circuit and not a feeder per se. I got confused while looking at example D8, it is for a multi-motor and I simply eliminated the extra motors.

Thanks for confirming what I was thinking, to size the breaker according to table 430.52.

PS: You are also correct on my verbose statement. English is my second language and I have had issues in the past with people not understanding my ideas. Hence, I now have a tendency to write (or speak) in a verbose manner to get my point accross.

renosteinke- Thanks. As you can see, I was a little confused as to how to properly address a motor circuit.

Thanks.

Jose

 

[dana1028]

Neither fuses nor breakers will provide you with overload protection - they're for short circuit protection. You've protected the wire, but still have not protected the motor.

Overload protection is provided (most often) by the 'heaters' in the motor starter. Maybe you call them the 'overload relays.' They're what sit under the reset button on the starter.

The NEC doesn't really provide guidance for sizing these. The general practice is to set them as close as you can to the actual current drawn by the motor under actual loads, without nuisance tripping. It can take some trial & error to get them 'right.' For something like a roof-top unit, you'll need a higher setting simply to allow for the higher temperatures up there on a sunny summer day.

RE: "The NEC doesn't really provide guidance for sizing these [overload protection].

I believe Section III of Art. 430 - Motor and Branch-Circuit Overload Protection gives guidance in sizing motor overload protection with 430.32(A)(1) providing a starting point and 430.32 as the maximum OL protection setting permitted.

Please do correct me if I am mistaken as this is how I was taught.

 

[volt101]

This section needs to be re-written. Perhaps by an engineer such as yourself Jose.....

Imagine using an instantaneous trip breaker for a single motor within a small motor group, lets say at 1100%, than applying that to .62 for your feeder protection that is using a time-delay fuse. The conductors would not have short-circuit and ground-fault protection........

 

[ike5547]

Thanks for your replies.


PS: You are also correct on my verbose statement. English is my second language and I have had issues in the past with people not understanding my ideas. Hence, I now have a tendency to write (or speak) in a verbose manner to get my point accross.



Thanks.

Jose

My apologies, Hose. My reply was cranky.