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scott minter

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Kennedy Space Center, Florida
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Associate Engineer
Good Evening

I was hoping someone could help me with a motor question. I sent a motor out for repair and when it returned

the leads were different. The motor is a 200hp 480v 3 phase 60 htz. Continuous duty use. The name plate says 237 amps.

When it left the leads coming out of the motor were #6 parallel.

When it returned it the leads were single #4.

I looked up the manufacture of the leads and the chart for the wire says 190 amps for the #4.

My question is does the wire for the leads need to carry the FLC of the motor i.e. 237 amps ?

Or do motor manufactures and repair shops use a percentage of the FLC on the leads? Thanks
 
Thank you for your reply.

I looked up the wire's amperage value on the maker of the wire's web page,

This is where i discovered the 190 amps.

With no NEC involved , is there any standard or best practices that would dictate the wire leads be able to carry

The FLC of the motor they serve ? Thank you for your time.

Scott
 
scott minter said:
Thank you for your reply.

I looked up the wire's amperage value on the maker of the wire's web page,

This is where i discovered the 190 amps.

With no NEC involved , is there any standard or best practices that would dictate the wire leads be able to carry

The FLC of the motor they serve ? Thank you for your time.

Scott
Click to expand...
The carrying capacity of the copper itself is so much higher than the insulation temperature rating of any common NEC wire type that the wire temperature (which is what the NEC ampacity tables are based on, and protecting the insulation, not the wire) is not an issue for motor windings or even any short leads leaving the motor.

The motor windings are sized to have a safe temperature in the core of the windings when the motor is operating at FLA with whatever ventilation is provided as part of the motor design. The motor shop has all of the information needed to make that evaluation, and the motor overloads will protect against a low level overcurrent far better than a standard breaker would on a branch circuit.
If the lead wires are short, they will also be cooled by conduction from the larger permanent wiring you attach to them
 
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scott minter said:
Thank you for your reply.

I looked up the wire's amperage value on the maker of the wire's web page,

This is where i discovered the 190 amps.

With no NEC involved , is there any standard or best practices that would dictate the wire leads be able to carry

The FLC of the motor they serve ? Thank you for your time.

Scott
Click to expand...

More detail on the value you looked up and how it compares to the environment it is used in are also needed to determine if this is acceptable. Your 190 amps is likely at a specific ambient temperature, but what is the max design operating temp of the motor it is used in? could make a difference. Some characteristics like an open air over design can also reduce the operating temp compared to a totally enclosed non ventilated design.
 
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