Controls Guy

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

randomkiller

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

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randomkiller

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

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

raider1

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

randomkiller

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

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.

randomkiller

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

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?

randomkiller

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

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

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randomkiller

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

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

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

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.

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raider1

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

chollapete

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

Controls Guy

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.

randomkiller

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

You got it.

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