WYE DELTA MOTOR TAP

[ohmhead]

Question art 430-22 conductors for a wye delta motor from controller to motor they shall be 58% of full load current . So each conductor which in my case is 6 wires will be reduced in size . In example I have a 100 hp 3 phase 480v motor fla is 124 amps x 58% = 71.92 a x 1.25= 89.9 amps wire size is no. 3# awg . Is this correct because I have a engineer who turn me down and said I need 1/0 awg to motor from controller . And reference me to any paralleled run of conductors must be 1/0 awg. I dont see this as a paralleled feeder .

 

[kwired]

Question art 430-22 conductors for a wye delta motor from controller to motor they shall be 58% of full load current . So each conductor which in my case is 6 wires will be reduced in size . In example I have a 100 hp 3 phase 480v motor fla is 124 amps x 58% = 71.92 a x 1.25= 89.9 amps wire size is no. 3# awg . Is this correct because I have a engineer who turn me down and said I need 1/0 awg to motor from controller . And reference me to any paralleled run of conductors must be 1/0 awg. I dont see this as a paralleled feeder .

The conductors carry 58% of motor rated current, you still must increase that by 125% for continuous loading purposes which means the conductors need to be 72% of motor rated current.

Informational Note: The individual motor circuit conductors of a wye-start, delta-run connected motor carry 58 percent of the rated load current. The multiplier of 72 percent is obtained by multiplying 58 percent by 1.25.

They are not parallel conductors, each goes to an individual winding lead - only a portion of the entire load.

 

[Smart $]

I agree with you.

 

[ohmhead]

The conductors carry 58% of motor rated current, you still must increase that by 125% for continuous loading purposes which means the conductors need to be 72% of motor rated current.



They are not parallel conductors, each goes to an individual winding lead - only a portion of the entire load.

That's what I have to tell engineer but he is using NEC feeder ART- 310-4 No. 1/0 cu on this motor iam going to write a response to him and I want to be 100% correct.
1/0 would be 150 amps and two runs would be 300amps . Plus the conduit size would be too large for the pickerhead on motor and wires would be too large for terminations .

 

[Smart $]

That's what I have to tell engineer but he is using NEC feeder ART- 310-4 No. 1/0 cu on this motor iam going to write a response to him and I want to be 100% correct.
1/0 would be 150 amps and two runs would be 300amps . Plus the conduit size would be too large for the pickerhead on motor and wires would be too large for terminations .

More importantly, I believe, is to refute his interpretation of parallel. Code uses the terminology "electrically joined at both ends". Through a load (motor windings in this case) is not considered electrically connected at the load end for the purpose of a determining a parallel connection. The other statements can be used as secondary logic to back up the primary.

 

[kwired]

The leads in question are not electrically joined at both ends. They do supply separate windings that ultimately drive the same mechanical load but that is not the same thing as parallel conductors.

 

[Jraef]

More importantly, I believe, is to refute his interpretation of parallel. Code uses the terminology "electrically joined at both ends". Through a load (motor windings in this case) is not considered electrically connected at the load end for the purpose of a determining a parallel connection. The other statements can be used as secondary logic to back up the primary.

Yep, that's where he (the Engineer) is going wrong. Besides that, if it is a motor circuit, 430 trumps 310.

Now, the tricky part is to find a way to let him know he is wrong without bruising his ego and turning him into an enemy... Been there, it's never pretty.

 

[kwired]

Yep, that's where he (the Engineer) is going wrong. Besides that, if it is a motor circuit, 430 trumps 310.

Now, the tricky part is to find a way to let him know he is wrong without bruising his ego and turning him into an enemy... Been there, it's never pretty.

I'm not afraid to set him straight, and if it is a one time encounter with this individual kind of don't care if his ego gets bruised.

 

[Phil Corso]

OhmHead...

You must also must increase it for the fact there are 6 conductors in the same conduit!

On a related subject V-Drop must be determined for twice the circuit-distance because the conductors are connected in series during start when the motor is Wye-connected!

Phil Corso

 

[don_resqcapt19]

....
On a related subject V-Drop must be determined for twice the circuit-distance because the conductors are connected in series during start when the motor is Wye-connected!

Phil Corso

I would not really care about that, because the very intent of this type of starting is to drop the voltage across the motor windings to reduce the starting current.

 

[Sahib]

I would not really care about that, because the very intent of this type of starting is to drop the voltage across the motor windings to reduce the starting current.

But it should not drop so much that the motor starting torque is less than required load torque,as torque is approximately proportional to the square of the applied voltage!

 

[kwired]

But it should not drop so much that the motor starting torque is less than required load torque,as torque is approximately proportional to the square of the applied voltage!

But it is only connected in wye configuration very briefly to reduce the "inrush" current that occurs when first energizing the windings, and then is switched to the delta connection which will provide much better torque to accelerate the load.

 

[Jraef]

But it should not drop so much that the motor starting torque is less than required load torque,as torque is approximately proportional to the square of the applied voltage!

If that happens, a Wye-Delta starter would not have been the right choice for the application. So one can assume that if the Wye-Delta starter was selected, someone understood this in advance so that possibility is not an issue.

 

[Sahib]

But it is only connected in wye configuration very briefly to reduce the "inrush" current that occurs when first energizing the windings, and then is switched to the delta connection which will provide much better torque to accelerate the load.

This brings in the question of how long an induction motor could be stalled.

 

[kwired]

This brings in the question of how long an induction motor could be stalled.

That one is easy, until to smoke comes out

 

[Jraef]

That one is easy, until to smoke comes out

And it is so hard to get that smoke back in once it escapes...?

However, I've seen motors run for years under incomplete sequence with Wye-Delta starters, it totally depends on the load. A common occurrence is that on a centrifugal pump, the motor load is reduced by flow restriction to the point where the loss of 2/3 of the motor torque is still sufficient to run the pump at that reduced flow. I've even heard of people in some remote "third world" areas doing this a lot to save energy because often times the pumps were originally sized for large industrial processes that are gone, so the now residential users don't need all the flow, but can't afford new smaller pumps. So they reconnect the large motors in Wye to Save energy costs by reducing the fixed magnetic losses because the voltage across the coils is reduced to 58%, just like an Energy Saver does. It seemed odd to me until someone explained the reason why their motors were so over sized and why they couldn't just replace them.