Are there any exceptions to Article 440 to reduce conductor size to 460V-3P Motors?

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Five by Five

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I joined the forum this morning. I am newly in need of understanding UL and NEC Code. Searches reveal great content here and I hope I can contribute.

A compressor from a European country only lists the Full Load Amps at 170 for 460V-3P motor. Article 440 requires 4/0 conductors.

The motor control panel provides overcurrent protection set to 90 Amps. This does not conform with Article 440 requirements of 125% of FLA.

Is there any code allowing reduction of conductor size with limiting current protection in place? Thanks in advance for any comments.
 
If this compressor and its motor controller are a listed assembly, you should be able to size the branch circuit based on the nameplate amps of the compressor package. It likely needs to be considered a continuous load, so x 125%.
 
170A FLC and 90A over current protection is wrong somewhere. Is the 90A the setting of an overload relay by chance? And if so, is this a Wye-Delta starter? 90A is around 54% of 170, which is close to how you would set an OL on a Wye-Delta starter. If not, something is wrong here.
 
Sorry about the 90 A for 170 FLA. It is set for 165 A.

Facing a difficult time in my new position. Engineers can reduce current with mechanical change in the load. Seems it is percieved that Article 430 does not apply.

I searched in vane for any exceptions.
 
The NEC is about protecting the electrical system from a faulted motor, it is not really good for protecting a driven load or the motor itself as those are design decisions.
The conductors and branch circuit protection are based on amperage values found in NEC tables. The motor running overcurrent protection/overloads are sized based on the actual motor nameplate data and the protective device manufacturers instructions.

Your conductors need to be sized per the NEC table values based on the nameplate HP or equivalent HP based on nameplate amps.
 
Just guessing here, but suitably qualified engineers (eg those at the motor manufacturer) should be able to 're-rate' the motor to correspond to the actual use. Motors regularly have multiple ratings on their nameplate, and the manufacturer has to create that nameplate in the first place.

However this can only go so far. If you have a 10HP motor but your actual mechanical load is only 5 HP, you still have the starting and magnetizing current of a 10 HP motor.
 
winnie said:
Just guessing here, but suitably qualified engineers (eg those at the motor manufacturer) should be able to 're-rate' the motor to correspond to the actual use. Motors regularly have multiple ratings on their nameplate, and the manufacturer has to create that nameplate in the first place.

However this can only go so far. If you have a 10HP motor but your actual mechanical load is only 5 HP, you still have the starting and magnetizing current of a 10 HP motor.
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That could be the ticket.. We probably could specify the nameplate on our purchase order. The engineers say we only use 80 amps of ~140 RLA 170 FLA. This has led to a difficult design situation with choices made that do not conform with code.

That said.. the 150 HP VFD being installed doesn't have 2 1/2" conduit openings in the box. You would think the conduit box would be designed to meet the requirements of a 138 Hp motor as interpolated from Table 430.250.
 
Five by Five said:
That could be the ticket.. We probably could specify the nameplate on our purchase order. The engineers say we only use 80 amps of ~140 RLA 170 FLA. This has led to a difficult design situation with choices made that do not conform with code.

That said.. the 150 HP VFD being installed doesn't have 2 1/2" conduit openings in the box. You would think the conduit box would be designed to meet the requirements of a 138 Hp motor as interpolated from Table 430.250.
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VFDs must be fed with conductors and protection based on its maximum input not what is actually connected to it.
 
Five by Five said:
That could be the ticket.. We probably could specify the nameplate on our purchase order. The engineers say we only use 80 amps of ~140 RLA 170 FLA. This has led to a difficult design situation with choices made that do not conform with code.
Click to expand...

What is the reason for the oversized motor?

Jon
 
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