Sizing Motor Conductors and Overload/Short Circuit-GFP

[jar546]

Let's make this simple. We can look at a typical 3HP 115/230v motor that will be used for continuous duty and will be the only motor on a circuit. It is not a DC motor, nor is it a multi-speed motor, or a Wye-Start/Delta Run Motor or a Part-Winding Motor or even anything other than a continuous duty motor. A typical, everyday motor. Nice and easy to follow. The questions are:

What size copper wire do we need and what size overload & overcurrent protection. We can look at both of those separately but first, here is the info that will be on the nameplate:



First let's look at conductor (wire) size. Part II of NEC 430 is Motor Circuit Conductors and for a single motor as described 430.22 tells us that conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125% of the motor full-load current rating as determined by 430.6(A)(1).

Full-Load Current, otherwise know as FLC is found on in the NEC tables. Remember, 430.6(A)(1) lets us know that in order to find the amperage for the conductors, we use the charts and not the label of the motor which normally only gives the FLA. For this example, we will be using the charts and assuming the 3 exceptions to not apply.

NEC 430.6(A)(1) sends us to Tables 430.247, 430.248, 430.249 & 430.250. Let's go take a look.

Our motor is clearly labeled as 3HP and we will be wiring this with 230v from a double-pole breaker so we will be using 430.248 which gives us 17A. Looking at Table 310.15(B)(16) we see that we will need a 12awg wire for the conductors.

We now have to figure out what size overload protection we need so let's head over to Part III, NEC 430.32(A) Continuous Duty Motors. This motor is greater than 1HP and per the nameplate, does not have any integral Motor Thermal Protection. Overload protection is to protect against an overheating motor which can be from various reasons from startup to a locked rotor. It is NOT for short circuit.

OK we are now know we have to use 430.32(A)(1) a separate overload device such as fuses or an overload device used in conjunction with a contactor as two examples. We use the data plate, specifically the Full Load Amps (FLA) and Motor Service Factor to determine the overload protection. Here we see that the service factor is 1.15 and the full load amps for 230V is 16.0. We now have to multiply 16A x 125% per 430.32(A)(1) for an overload protection of 20A. We will need 20A fuses for overload protection.

Lastly we have to worry about a short circuit, ground-fault, etc. which is how we decide our circuit breaker size. Part IV of NEC 430 covers this starting with 430.51 which states that the purpose of this section is to protect the conductors, motor controls and motors against overcurrent due to short circuits or ground faults.

430.52 applies to us as we are using this for an individual motor circuit. Within this section we are directed to Table 430.52 which has us use the Full Load Current (FLC) again and NOT the nameplate of the motor. Our FLC as determined by the NEC was 17A. Remember we found that in Table 430.248. We will be using an Inverse Time Breaker (typical of panelboards) for this single-phase motor which gives us 250% of FLC. Therefore, 17A x 250% would be 42.5A and since we are allowed, we would use a 45A breaker.

So this motor circuit will have 12awg wire with 20A OL protection and fed from a 45A breaker which is the next standard size above 40A. Now you see it is possible to legally have a 12awg wire connected to a 45A breaker.

There are a ton of other variables that could have affected these numbers but keeping is simply, typical and straight forward, I believe, is the best approach. You can branch out from here.

I hope this helps and provides useful information for the forum.

 

[jar546]

3-1/2 years later after providing educational, useful information on this thread and nothing. I suppose no one inspects electrical on this forum.

 

[wwhitney]

Part II of NEC 430 is Motor Circuit Conductors and for a single motor as described 430.22 tells us that conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125% of the motor full-load current rating as determined by 430.6(A)(1).

. . .

Our motor is clearly labeled as 3HP and we will be wiring this with 230v from a double-pole breaker so we will be using 430.248 which gives us 17A. Looking at Table 310.15(B)(16) we see that we will need a 12awg wire for the conductors.

17A * 125% = 21A. So #12 AWG with 60C insulation (or subject to that limit, such as NM cable) would not comply. But #12 AWG with 75C insulation would be fine, as it has an ampacity of 25A.

Also worth pointing out that 240.4(G) is what relieves motor applications from the limits of 240.4(D), the rule that #12 AWG can usually be protected at no more than 20A.

Cheers, Wayne

 

[jar546]

17A * 125% = 21A. So #12 AWG with 60C insulation (or subject to that limit, such as NM cable) would not comply. But #12 AWG with 75C insulation would be fine, as it has an ampacity of 25A.

Also worth pointing out that 240.4(G) is what relieves motor applications from the limits of 240.4(D), the rule that #12 AWG can usually be protected at no more than 20A.

Cheers, Wayne

Finally some feedback! Thank you. Yes, NM cable would not comply. I agree.

 

[ICE]

Finally some feedback! Thank you. Yes, NM cable would not comply. I agree.

It’s obvious that you are not appealing to the masses with esoteric questions. And I would like to know if that formula works with medium size electrons or just the large variety.

 

[jar546]

It’s obvious that you are not appealing to the masses with esoteric questions. And I would like to know if that formula works with medium size electrons or just the large variety.

Esoteric questions? This is a code forum and this is common knowledge to all competent electricians, electrical engineers and qualified electrical inspectors and plans examiners. No difference between this thread and any building code thread.

 

[ICE]

Esoteric questions? This is a code forum and this is common knowledge to all competent electricians, electrical engineers and qualified electrical inspectors and plans examiners. No difference between this thread and any building code thread.

And it only took three years to get an answer.

 

[ICE]

Jeff,
I understand your motive. There are members that can decipher the questions that you present. The trouble with it is that few of us are tasked with the, yes esoteric, situations. In twenty-five years I have never had to deal with it other than what's on an approved plan. I have always had engineers for that and the commercial stuff gets plan checked.

There are undoubtedly many smaller AHJs that have the odd motor to deal with but I haven't seen any evidence of that here at your forum. I imagine that most would take it to an electrical forum and get pushed around by the primadonnas. For me, I would look for one of your tutorials ....so there is a reason for being here even if you are really anal about the electrical trade.

 

[jar546]

Jeff,
I understand your motive. There are members that can decipher the questions that you present. The trouble with it is that few of us are tasked with the, yes esoteric, situations. In twenty-five years I have never had to deal with it other than what's on an approved plan. I have always had engineers for that and the commercial stuff gets plan checked.

There are undoubtedly many smaller AHJs that have the odd motor to deal with but I haven't seen any evidence of that here at your forum. I imagine that most would take it to an electrical forum and get pushed around by the primadonnas. For me, I would look for one of your tutorials ....so there is a reason for being here even if you are really anal about the electrical trade.

Even as an electrician, this is a daily scenario when you do commercial work. Electrical inspections in a dwelling unit are like kindergarten and the most basic of all electrical. As I try to move this forum forward, I can't dumb it down while Mike Holts's forum has questions significantly harder than the one I posted with a detailed explanation to make it educational. I've taken this forum in a forward, positive direction over the past 13+ years and want to continue to do so. So no, you don't understand my motive. I want good, accurate, professional content for those in the industry at all levels.

 

[steveray]

Going to be tough to draw a lot of sparkies in from those other crappy forums....I'm trying with some of my CT folks...

 

[Mr. Inspector]

I could have answered it 15 years ago when I was studying for the test but haven't needed it on the job. Always had engineered plans or manufactures instructions. But I do have an app on my phone that does the cals but never needed it.

 

[jar546]

I could have answered it 15 years ago when I was studying for the test but haven't needed it on the job. Always had engineered plans or manufactures instructions. But I do have an app on my phone that does the cals but never needed it.

Did you ever do electrical plan review for drawings that came from an electrical engineer? This is where it really comes in handy.

 

[ICE]

So no, you don't understand my motive.

I can read and you spelled it out here years ago. You didn't follow through.

A long winded complicated post for the college crowd every few years just puts us kindergartners in a stupor.

This thread has generated almost 700 views yet it went nowhere for three years.

Rick and I share the same experience only mine was twenty-five years ago. I sat in a classroom learning about every last word in every code. In those days the certification exam was half closed book so we had to know this stuff. It was an exercise that proved nothing.

I can't dumb it down while Mike Holts's forum has questions significantly harder than the one I posted with a detailed explanation to make it educational.

You are not in a competition with Holt.

This is a code forum and this is common knowledge to all competent electricians, electrical engineers and qualified electrical inspectors and plans examiners.

Well then Jeff, apparently you are back in the bubble. Out here in the jungle it is rare to find anyone that has such "common knowledge." Ask yourself why they would be common when there's a thousand residential jobs for every commercial job.

Electrical inspections in a dwelling unit are like kindergarten and the most basic of all electrical.

Really? That is what you believe? Trust me, I have done it all and residential presents far more of a challenge than commercial. For one, there is never a plan. Seldom does anyone know any code. And by the way, it is readily apparent when a commercial electrician has done a residential job....and they always say, "Well I don't do residential" ...when what they mean is "I don't know residential."

So instead of spouting off about how simple I am perhaps you should step away and assess what you actually have. Compare your occasional, convoluted thread that gets 700 views and no response to the Average Day thread that is 99% residential electrical with a half million views and over 5000 replies....What does that tell you about your forum?

 

[steveray]

Really? That is what you believe? Trust me, I have done it all and residential presents far more of a challenge than commercial. For one, there is never a plan. Seldom does anyone know any code. And by the way, it is always readily apparent when a commercial electrician has done a residential job....and they always say, "Well I don't do residential" ...when what they mean is "I don't know residential."

This^^^^^

The other stuff is super important and hopefully we have good guys looking at that stuff but it is not all day every day for most of us like IRC stuff...And I still have licensed electrician that can't get the GEC to utility ground right and the EGC/ grounded separation on a sub-panel.....I would bet North of 80% of transformers around here are wired wrong, but hey, I don't see a bunch of people dying, so until we are adequately staffed to inspect everything, oh well...

I am lucky and I have some good electricians on staff and some great guys to ask questions of when required...

 

[Mr. Inspector]

Never got a plan that showed the name plate of a motor.

 

[jar546]

Never got a plan that showed the name plate of a motor.

Which is why we ask for equipment specs during plan review. You don't work with what they give you. They are required to give you what you need. Ask and you shall receive.