460V Motor Connection Leads, sizes

[GeorgeB]

True, but motor will rarely, if ever see SFA

YOUR motor, perhaps. I deal in the industrial hydraulics world ... it is not at all unusual for a machine to be loaded at 10% for 90% of a cycle, 100% for 9%, and 250% for 1%. That 1% is in the 0.1 to 1.0 sec range as oil is compressed, and a little compression of the workpiece after half of the 100% period.

 

[Isaiah]

Yep - I’ve always figured the manufacturers knew what they were doing and never worried about the size of the wires.

So, is it safe to assume motor connection leads for a typical Nema TEFC induction motor must carry at least SFA? And lastly, what standards do manufacturers use as a guideline?
I couldn't find anything in MG-1.

 

[retirede]

So, is it safe to assume motor connection leads for a typical Nema TEFC induction motor must carry at least SFA? And lastly, what standards do manufacturers use as a guideline?
I couldn't find anything in MG-1.

My answer to your 2 questions:

1) Yes (I’ve never seen a failure of motor leads).

2) I don’t know.

 

[LarryFine]

Yep - I’ve always figured the manufacturers knew what they were doing and never worried about the size of the wires.

Agreed. As Chico used to say, "Ees not my job!"

 

[RichB]

Does anyone out there know: Are motor connection leads (pigtails inside motor termination box), sized per NEC requirements of motor FLA x 1.25%? My understanding is motor leads are controlled by NEMA but shouldn't basic sizing requirements still apply according to motor HP, S.F., Insulation type etc?

Thanks in advance.

Isaiah.

Contact a motor rewind facility--I am sure they would know where and what the standards are

 

[ActionDave]

Contact a motor rewind facility--I am sure they would know where and what the standards are

Maybe some large shops that are highly organized, other places not so much I bet. We rewound motors for a long time and it was all cowboy engineering when it came to sizing of windings and leads. We would go up in wire gauge and insulation class so there was never a problem.

 

[Isaiah]

Maybe some large shops that are highly organized, other places not so much I bet. We rewound motors for a long time and it was all cowboy engineering when it came to sizing of windings and leads. We would go up in wire gauge and insulation class so there was never a problem.

Dave. When you say “we would go up in wire gauge and insulation...” you mean above NEC requirements?


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[retirede]

Dave. When you say “we would go up in wire gauge and insulation...” you mean above NEC requirements?


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Probably up from the gauge of the old windings/leads.

 

[Jraef]

The standard is for the MOTOR, not the wire size, meaning that if you can get your motor to pass the tests with wire that ends up smaller than the NEC would allow, that's fine, because the entire MOTOR is tested as an ASSEMBLY. But at the same time, a motor designer will not want to take too much of a chance because that testing is expensive and having something fail because you scrimped too much on the conductor size is the kind of thing that gets you a pink slip.

Motor wire manufacturers publish guidelines for their brands of wires to help those motor designers, here is one:
http://www.ecswire.com/mmspecs/Specsheets/Belden/leadWireSelectionChart.pdf

As you will see, using your example of #8s observed in a 40HP motor, #8 per the NEC is good for 50A without any adjustment factors, but for the motor lead wires, using the 3 conductor chart (#3), it's good for 76A if using EDPM 150C lead wire. Even this isn't that simple, there's a lot more to it. Bottom line, again, you can't equivocate the wire sizes used per the NEC to what is used INSIDE of the motor.

 

[Isaiah]

The standard is for the MOTOR, not the wire size, meaning that if you can get your motor to pass the tests with wire that ends up smaller than the NEC would allow, that's fine, because the entire MOTOR is tested as an ASSEMBLY. But at the same time, a motor designer will not want to take too much of a chance because that testing is expensive and having something fail because you scrimped too much on the conductor size is the kind of thing that gets you a pink slip.

Motor wire manufacturers publish guidelines for their brands of wires to help those motor designers, here is one:
http://www.ecswire.com/mmspecs/Specsheets/Belden/leadWireSelectionChart.pdf

As you will see, using your example of #8s observed in a 40HP motor, #8 per the NEC is good for 50A without any adjustment factors, but for the motor lead wires, using the 3 conductor chart (#3), it's good for 76A if using EDPM 150C lead wire. Even this isn't that simple, there's a lot more to it. Bottom line, again, you can't equivocate the wire sizes used per the NEC to what is used INSIDE of the motor.

I contacted Baldor-Reliance regarding this particular motor. According to their documentation the motor leads are #4’s which would vastly exceed the required FLA plus SF. However my observation, along with two other senior electrician tells me they’re really much closer to #8’s.


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[ActionDave]

Dave. When you say “we would go up in wire gauge and insulation...” you mean above NEC requirements?


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I meant compared to the existing wire of the motor.

There is no such thing as an NEC requirement for anything inside of a motor.

 

[kwired]

Why stop at the motor leads? What about the windings in the stator? They are seeing the same current.

So, is it safe to assume motor connection leads for a typical Nema TEFC induction motor must carry at least SFA? And lastly, what standards do manufacturers use as a guideline?
I couldn't find anything in MG-1.

Also consider your common dual voltage 208-240/480 volt motor has leads that only carry half the motor current when connected in the low voltage configuration. So you supplied the motor with a 4 AWG copper but those approximately 10 or 12 AWG leads are only carrying half the current that the supply conductor is carrying. In high volts connected application the supply conductor maybe only needs to be 8 AWG - closer to the size of the motor leads, which are carrying all the motor current in high volts configuration, but they are higher temp, short length, etc. as mentioned by others and designed for the application.

 

[Isaiah]

Also consider your common dual voltage 208-240/480 volt motor has leads that only carry half the motor current when connected in the low voltage configuration. So you supplied the motor with a 4 AWG copper but those approximately 10 or 12 AWG leads are only carrying half the current that the supply conductor is carrying. In high volts connected application the supply conductor maybe only needs to be 8 AWG - closer to the size of the motor leads, which are carrying all the motor current in high volts configuration, but they are higher temp, short length, etc. as mentioned by others and designed for the application.

This particular motor does not have a dual winding. There are only 3 leads.


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[Isaiah]

Also consider your common dual voltage 208-240/480 volt motor has leads that only carry half the motor current when connected in the low voltage configuration. So you supplied the motor with a 4 AWG copper but those approximately 10 or 12 AWG leads are only carrying half the current that the supply conductor is carrying. In high volts connected application the supply conductor maybe only needs to be 8 AWG - closer to the size of the motor leads, which are carrying all the motor current in high volts configuration, but they are higher temp, short length, etc. as mentioned by others and designed for the application.

Motors are not rated 240/480V - they are rated 230/460V.

 

[kwired]

Motors are not rated 240/480V - they are rated 230/460V.

Yep, I knew that:ashamed1:

 

[retirede]

Motors are not rated 240/480V - they are rated 230/460V.

That is standard. We used to have obtuse customers who insisted on 480V nameplate. We would have the motor manufacturer re-nameplate a standard motor and up charge accordingly.

Just pointing out that one may encounter some seemingly oddball ratings in the wild.

 

[kwired]

That is standard. We used to have obtuse customers who insisted on 480V nameplate. We would have the motor manufacturer re-nameplate a standard motor and up charge accordingly.

Just pointing out that one may encounter some seemingly oddball ratings in the wild.

Upcharge not only from you but from motor vendor/manufacturer. This makes every motor you purchase a special order.

 

[retirede]

Upcharge not only from you but from motor vendor/manufacturer. This makes every motor you purchase a special order.

Yes - we paid the manufacturer, marked it up, and passed on to the customer.

 

[Isaiah]

Upcharge not only from you but from motor vendor/manufacturer. This makes every motor you purchase a special order.

If you go back far enough, you'll find motors listed as '440V'. This has evolved over the years to the present day '460V' and sometimes, '480V'.

 

[Jraef]

That is standard. We used to have obtuse customers who insisted on 480V nameplate. We would have the motor manufacturer re-nameplate a standard motor and up charge accordingly.

Just pointing out that one may encounter some seemingly oddball ratings in the wild.

Made me think of a comedian I recently heard talking about growing up with a father who felt like everyone was trying to cheat him. He insisted that they never use low fat 2% milk because "Dammit, I want the WHOLE milk, not just 2% of it!"