# 250 hp motor



## Controls Guy (Apr 18, 2008)

Where can I find what wire size is recommended for a 250hp 480v motor?


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## randomkiller (Sep 28, 2007)

By taking the FLA from the spec plate and matching the wire to the amperage.


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## randomkiller (Sep 28, 2007)

HP x 746
1.73 x E x Eff x PF


This will put you in the ballpark for amps.


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## raider1 (Jan 22, 2007)

Controls Guy said:


> Where can I find what wire size is recommended for a 250hp 480v motor?


The NEC requires that you size the branch circuit conductors for motors using the ampacity tables in the back of Article 430.

Take a look at 430.22(A) for a single motor.

You can't use the motor nameplate FLA's to size the branch circuit conductors. You are only permitted to use the motor nameplate FLA's for sizing the motor overload protection.(See 430.6(A)(2))

Chris


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## raider1 (Jan 22, 2007)

If you take a look at Table 430.250 you will see that a 250 HP 3 phase 460 volt motor has an ampacity of 302 amps. 

Chris


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## randomkiller (Sep 28, 2007)

raider1 said:


> The NEC requires that you size the branch circuit conductors for motors using the ampacity tables in the back of Article 430.
> 
> Take a look at 430.22(A) for a single motor.
> 
> ...


 
Take a look at the book and read that section, keeping in mind it's a 250 hp motor, more than likely a pump motor.
You maybe surprised.


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## pudge565 (Dec 8, 2007)

i thought it was HP (250) times 746 (watts per HP) 250 X 746 = 186500 then you divide that by the voltage (480) 186500 / 480 = 388.54 then you take 125% if it is considered a continuous load so 388.54 X 1.25 = 485.675 so rounded it would be 486 amps.


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## randomkiller (Sep 28, 2007)

pudge565 said:


> i thought it was HP (250) times 746 (watts per HP) 250 X 746 = 186500 then you divide that by the voltage (480) 186500 / 480 = 388.54 then you take 125% if it is considered a continuous load so 388.54 X 1.25 = 485.675 so rounded it would be 486 amps.


You are leaving out efficiency,power factor, and the fact its three phase on the divisor.

259x746
________
1.73(3phase factor)x 480xefficiency x power factor

then x 125%


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## waco (Dec 10, 2007)

Not having had to design any circuits for large HP motors, how does one determine values for "efficiency" and power factor?

Seems to me, the horsepower rating is meaningless without a value for amperage. Also, I wonder why the FLA can't be used since it reflects the current limiting effects of impedance.

My vote is that electricians work with what engineers have designed. Let's stop forcing the trade to serve as low paid engineers, huh?


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## randomkiller (Sep 28, 2007)

waco said:


> Not having had to design any circuits for large HP motors, how does one determine values for "efficiency" and power factor?
> 
> Seems to me, the horsepower rating is meaningless without a value for amperage. Also, I wonder why the FLA can't be used since it reflects the current limiting effects of impedance.
> 
> My vote is that electricians work with what engineers have designed. Let's stop forcing the trade to serve as low paid engineers, huh?


efficiency is attained from motor specs (usually catolog)and power factor is listed on the spec plate, if you look at the code section raider quoted it says you use the tables for blower type motors, multi speed or high torque (pump type direct coupled motors) the type you see more often in large hp you do use FLA. Sometimes you just want a formula to just double check the design engineer, you might be doing an install that is similar to others you have done and see a smaller or larger wire or breaker size and want to check. The motors close to that size I have done are either pump stations, chiller plants or something in that area never done a blower or belt sheave motor over 100 hp.


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## Ecopat (Apr 17, 2008)

randomkiller said:


> efficiency is attained from motor specs (usually catolog)and power factor is listed on the spec plate, if you look at the code section raider quoted it says you use the tables for blower type motors, multi speed or high torque (pump type direct coupled motors) the type you see more often in large hp you do use FLA. Sometimes you just want a formula to just double check the design engineer, you might be doing an install that is similar to others you have done and see a smaller or larger wire or breaker size and want to check. The motors close to that size I have done are either pump stations, chiller plants or something in that area never done a blower or belt sheave motor over 100 hp.


Wow you're good.
Was that all off the top of your head?


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## randomkiller (Sep 28, 2007)

Ecopat said:


> Wow you're good.
> Was that all off the top of your head?


 

I do a lot of that type of thing (chiller plants, large boiler plants, pump houses) all the time. I work for an electro mechanical company.


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## Ecopat (Apr 17, 2008)

Ahh, I see.
Now this is what I meant when I said that experience and not just learning is far greater a source of knowledge.
I'd rather have a guy like you on my team than some newly qualified 3 week course kiddie.


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## randomkiller (Sep 28, 2007)

Ecopat said:


> Ahh, I see.
> Now this is what I meant when I said that experience and not just learning is far greater a source of knowledge.
> I'd rather have a guy like you on my team than some newly qualified 3 week course kiddie.


 
Thanks for the compliment. I agree with the 3 week wonder boy issue. A multi year apprenticeship is what it takes to be a journeymen in my local. It helps if you are with a good contractor from the start.


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## Ecopat (Apr 17, 2008)

Random killer, I have seen the word "Journeyman on here a lot, what is a journeyman, is it an American term or do we brits use it too?
I must confess I have never heard of it.
The compliment is all yours, it is nice to know that there are still guys who take a pride in what they do and conviction in what they know. :thumbsup:


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## raider1 (Jan 22, 2007)

randomkiller said:


> Take a look at the book and read that section, keeping in mind it's a 250 hp motor, more than likely a pump motor.
> You maybe surprised.


The OPer never stated what type of motor he was installing so I posted the general requirement. You are correct if the motor in question is a multispeed motor, Wye start Delta run motor, Part winding or other than continious duty motor you use the motor nameplate.

Chris


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## chollapete (Apr 18, 2008)

randomkiller said:


> You are leaving out efficiency,power factor, and the fact its three phase on the divisor.


I could be wrong; but, if:
Efficiency is shaft mechanical-work output divided by electrical-work input, giving a percentage or ratio.
Power Factor is electrical-resistive power divided by total electrical-input power. I.e., it gives a percentage or ratio that measures what percent of total power is in-phase, resistive work while excluding inductive, VAR "work".
Is this correct so far? 

Then, the difference between efficiency and power factor is that efficiency measures the mechanical output as a percentage of the total EI, while power factor measures electrical output as a percentage of total EI.

In other words, efficiency is affected both by mechanical losses such as bearing friction and by electrical "losses", such as VAR, while power factor is only affected by electrical "losses", such as VAR.

Comments? Corrections?

Thanks,
Mark


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## Controls Guy (Apr 18, 2008)

Thanks for all the input. I currently design systems a lot smaller and have the challenge of a much larger machine. The motor is on a hydraulic pump with a swash plate control valve to prevent huge startup torque. Thanks for nec code insight.


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## randomkiller (Sep 28, 2007)

raider1 said:


> The OPer never stated what type of motor he was installing so I posted the general requirement. You are correct if the motor in question is a multispeed motor, Wye start Delta run motor, Part winding or other than continious duty motor you use the motor nameplate.
> 
> Chris


 
When was the last time you saw a motor over 75 hp that wasn't a pump, machine, or compressor motor?


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## randomkiller (Sep 28, 2007)

chollapete said:


> I could be wrong; but, if:
> 
> Efficiency is shaft mechanical-work output divided by electrical-work input, giving a percentage or ratio.
> Power Factor is electrical-resistive power divided by total electrical-input power. I.e., it gives a percentage or ratio that measures what percent of total power is in-phase, resistive work while excluding inductive, VAR "work".
> ...


 
You got it.


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## raider1 (Jan 22, 2007)

randomkiller said:


> When was the last time you saw a motor over 75 hp that wasn't a pump, machine, or compressor motor?


I don't deal with many large HP motors, so it has been a long time since I have had to deal with one.

The area I work in as an inspector doesn't have any industrial areas so large motors really aren't an issue. When I was working as an electrician I dealt with large pump motors, compressor motors ect...

Chris


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## Controls Guy (Apr 18, 2008)

One more thing, since I have never installed a motor this size with the pipe coming out of the concret. What would you recommend the distance of the stub up from the junction box of motor. Looking at article 373 if I'm using 400 or 500 mcm that turn is going to be a bear. I dont have the luxury of coming directly from the bottom because manufacturing and dimensions are not complete but slab is waiting to be constructed.


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## frenchelectrican (Mar 15, 2007)

Control Guy;

if you know what manufacter the motor i am sure they will have a drawing on line or get ahold of them drict they will give you the demison of the motor and my SOP with larger motor i useally ask them to give me oversized peckerhead.

i done quite few large motor installments both low and med voltage catory.

the med voltage that diffrent story i will leave this part out for now due you mention 480 volts here.

the size of conduit will be determed by size of conductor and distance.

and will this motor run with VFD or not ?

Merci,Marc


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## randomkiller (Sep 28, 2007)

Controls Guy said:


> One more thing, since I have never installed a motor this size with the pipe coming out of the concret. What would you recommend the distance of the stub up from the junction box of motor. Looking at article 373 if I'm using 400 or 500 mcm that turn is going to be a bear. I dont have the luxury of coming directly from the bottom because manufacturing and dimensions are not complete but slab is waiting to be constructed.


 
I would keep it back @30" if possible and stay pretty low so you can install a coupling and go to greenfield or sealtite from there to the jbox.


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## frenchelectrican (Mar 15, 2007)

randomkiller said:


> I would keep it back @30" if possible and stay pretty low so you can install a coupling and go to greenfield or sealtite from there to the jbox.


 
That is pretty much SOP i did see most area i work on med /large motor installment.

Merci,Marc


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## micromind (Aug 11, 2007)

Same here, it's best if you make the top of a rigid coupling stick about 1/2"above the slab. You'll need all you can get to bend the flex/sealtite from the coupling to the motor. On most of these motors, you can remove the entire box, and re-install it in 90 degree increments. 

Rob

P.S. I've connected some pretty good-sized motors on fans and conveyers, etc. the biggest fan I can remember was for a baghouse on an asphalt plant, 2-200HP driving the same fan shaft, (400HP total), the biggest conveyer was at a rock crushing plant, 300HP, about 1/4 mile long, and uphill. The biggest crusher motor was 1500HP, on a jaw-type crusher that would take a rock 6' in diameter and crush it to 3".


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## Controls Guy (Apr 18, 2008)

Thanks a lot for all the feed back and help. I had asked one of the local contractors and he had said at least 24" or more so thanks for confirming that. I have the motor specs and dims but not where the motor is exactly sitting on the skid was the reason for my concern. After this one I'll be hopefully helping someone.


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## Kletis (Jan 18, 2008)

Ecopat said:


> Random killer, I have seen the word "Journeyman on here a lot, what is a journeyman, is it an American term or do we brits use it too?
> I must confess I have never heard of it.
> The compliment is all yours, it is nice to know that there are still guys who take a pride in what they do and conviction in what they know. :thumbsup:


 
I could not tell you if Brits use this term as well, but a journeyman is someone who has completed the appropriate training and passed their tests to become an Electrician. When you are no longer an apprentice but instead a true electrician, you are a journeyman. They use this term in many fields. My dad, for instance, is a journeyman lineman.......

Hope that answered your question!!


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## Ecopat (Apr 17, 2008)

Ahhhhhhhh, got you now. Thankyou kind sir. :thumbsup:


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## wwilson174 (Apr 25, 2008)

In the absence of nameplate data I have always used 1.2 amps per horsepower @ 480v 3 phase times 125% for runs up to 70 feet. Bill W.


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## MechanicalDVR (Dec 29, 2007)

wwilson174 said:


> In the absence of nameplate data I have always used 1.2 amps per horsepower @ 480v 3 phase times 125% for runs up to 70 feet. Bill W.


 
How did you get the 1.2 amps?


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## wwilson174 (Apr 25, 2008)

40 years experience!


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## nap (Dec 26, 2007)

randomkiller said:


> When was the last time you saw a motor over 75 hp that wasn't a pump, machine, or compressor motor?


a 250 hp dust collector. would that fall into one of those catagories?


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## nap (Dec 26, 2007)

Ecopat said:


> Random killer, I have seen the word "Journeyman on here a lot, what is a journeyman, is it an American term or do we brits use it too?
> I must confess I have never heard of it.
> The compliment is all yours, it is nice to know that there are still guys who take a pride in what they do and conviction in what they know. :thumbsup:


three levels of skilled tradesman

apprentice

journeyman

master

In old days (back during the days of the guilds which were originally from GB), a person entered into a trade as an apprentice. He was taught by a master. Once the apprentice became skilled enough, he was considered able to _journey _about the country and ply his trade, hence the term, _journeyman_. When he became skilled enough, he would be considered a _master_ tradesman and considered to be well enough qualified to train _apprentices_. _Masters_ typically would be an owner of a shop and would employ _journeymen_ and _apprentices._


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## randomkiller (Sep 28, 2007)

nap said:


> a 250 hp dust collector. would that fall into one of those catagories?


 
Sure wouldn't, that would fit the code wording.


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## nap (Dec 26, 2007)

randomkiller said:


> Sure wouldn't, that would fit the code wording.


I was referring to you catagorization of:



> When was the last time you saw a motor over 75 hp that wasn't a pump, machine, or compressor motor


Not sure I would consider it any of these. Basically making the statement in opposition of your statement.


Now what are you referring to with "the code working"?


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## randomkiller (Sep 28, 2007)

nap said:


> I was referring to you catagorization of:
> 
> Not sure I would consider it any of these. Basically making the statement in opposition of your statement.
> 
> ...


I know what you meant and I was basically agreeing with you that it wasn't one of the quoted types, so it would fit into the wording of the code.


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## nap (Dec 26, 2007)

randomkiller said:


> I know what you meant and I was basically agreeing with you that it wasn't one of the quoted types, so it would fit into the wording of the code.


 
I guess it was late and it just didn't make sense at the time.:blink:


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## randomkiller (Sep 28, 2007)

nap said:


> I guess it was late and it just didn't make sense at the time.:blink:


 
I hear that, I worked until 2130 last night and couldn't go to sleep and ended up just about dozing off on the keyboard.


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## Electric_Light (Apr 6, 2010)

waco said:


> Not having had to design any circuits for large HP motors, how does one determine values for "efficiency" and power factor?
> 
> Seems to me, the horsepower rating is meaningless without a value for amperage. Also, I wonder why the FLA can't be used since it reflects the current limiting effects of impedance.
> 
> My vote is that electricians work with what engineers have designed. Let's stop forcing the trade to serve as low paid engineers, huh?


Here's a useful info on locked rotor current. 
http://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

Since locked rotor torque is affected by voltage, even if wiring supports enough current with acceptable voltage drop at FLR, but causes too much voltage drop at locked rotor, it may not be able to overcome static loads like a piston compressor or a hoist.


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## wildleg (Apr 12, 2009)

Electric_Light said:


> Here's a useful info on locked rotor current.
> http://www.engineeringtoolbox.com/locked-rotor-code-d_917.html
> 
> Since locked rotor torque is affected by voltage, even if wiring supports enough current with acceptable voltage drop at FLR, but causes too much voltage drop at locked rotor, it may not be able to overcome static loads like a piston compressor or a hoist.


hopefully the OP has figured it out, since the OP was april 2008


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## GDK 13 (Oct 6, 2009)

raider1 said:


> The NEC requires that you size the branch circuit conductors for motors using the ampacity tables in the back of Article 430.
> 
> Take a look at 430.22(A) for a single motor.
> 
> ...


:thumbsup:


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## Mr.C (Dec 20, 2009)

Hey if your wondering about conduit for this motor dont forget to run 2 additional 3/4" conduits to it. 1: for your rtd's, vibration sensors, and temp probes. and 1: for your 120v heater. Also if your going green and you dont need speed control then nix the vfd idea and go with a soft start. Dont forget to make sure your breaker will be able to handle the 1000+amps of inrush


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## Bob Badger (Apr 19, 2009)




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## sparky970 (Mar 19, 2008)

randomkiller said:


> When was the last time you saw a motor over 75 hp that wasn't a pump, machine, or compressor motor?


How about a cooling tower?


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## sparky970 (Mar 19, 2008)

Controls Guy said:


> One more thing, since I have never installed a motor this size with the pipe coming out of the concret. What would you recommend the distance of the stub up from the junction box of motor. Looking at article 373 if I'm using 400 or 500 mcm that turn is going to be a bear. I dont have the luxury of coming directly from the bottom because manufacturing and dimensions are not complete but slab is waiting to be constructed.


You may want to consider removing the peckerhead and replacing it with a J-box.


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## wildleg (Apr 12, 2009)

the op was 4/22/2008. If he hasn't installed it yet . . . .there might be a problem.


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