Motor T-Lead Sizing

[Laplacian]

What is the basis for large motor t-lead sizing? We have a 7000HP, 4000V, 900FLA motor that had to be rewound. During the repair, parallel 4/0 conductors were used per phase. The motor originally had 3/0 conductors. This seems like it would but a large thermal load on termination lugs for the guy used to sizing such things per NEC.

 

[waross]

Well the replacement conductors are bigger, and the NEC doesn't apply to internal motor leads. They typically have a much higher current and temperature rating than the NEC allows for branch circuit wiring.
You may not have a problem
respectfully

 

[aolalde]

The cable manufacturer tables should be consulted. Be aware that elevated ambient temperature into the motor frame will affect the ampacity. We have successfully used the following ratings for 7.5 kV, 150°C EPDM Cables on motor terminal leads.
4/O AWG---- 275 Amperes
3/O AWG---- 235 Amperes
2/O AWG---- 203 Amperes
1/O AWG---- 176 Amperes

According to above values, for 900 Amperes you should have 4 cables 3/O in parallel or any other practical combination per phase.

 

[Laplacian]

I knew that seemed a little small. This is a motor from a pretty well known and respected large motor manufacturer. We started having problems after a rewind. A random lug in the main terminal box would burn off every few months to few years. This was thought to be from bad crimps at the repair facility. I cut the insulation back on one of the t-leads and found heavy VPI contamination all the way to the lug. We cut off the remaining unfailed lugs and found that the lugs were installed after the VPI process.

The motor has been sent to a different repair facility and is ongoing complete lead replacement. I want to specify a larger lead size or more than 2 parallel 4/0 per phase, but didn't know what the manufacturer used as the basis for sizing. I asked, but wasn't given this information.

The last lug failure resulted in the catastrophic destruction of the terminal box; after the lug burned off, the open ended lead contacted a grounded component in the box.

 

[petronila]

Laplacian This seems like an ampacity wire problem,because the amperes rating used by each manufacturer depends of designs and the material .Normally for electric motors you could find Hypalon, EPDM or Silicone Glass Class F & H.

Ussually in our Shop for 4160 Volts motors we use EPDM Class F - 7500 V manufactured By BELDEN for this manufacturer wire the 3/0 Ampacity is 457 and 4/0 wire: 529 Amps.(Maximum values for single conductor in free air @ 30 C) If the motor was designed by the maufacturer with parallel 3/0 per phase, I think this could be a short design. If the motor was rewound with two Parallel 4/0 and failled maybe the motor´s temperature is too high and this could be derrating the wires.

the shops could use 13.2 KV 250KCMIL wire.

Regards

Petronila

 

[Laplacian]

The motor runs pretty cool even at full load. The parallel 4/0 installed after rewind were impregnated with the stator VPI resin; insulating the inner strands from the outer strands resulting in much less current carrying contact at the lugs.