Large AC motor wiring

[thinker]

For the project of automotive test facility we use a large AC dynamometer (1040 FLA).As per customer spec, this motor must be translating with significant amount of travel in both X and Y axes. To meet the spec, we have to use cattracs and flexible motor cables, and in this case we would need to use 10 cables in parallel, each cable (3 phase, 4 conductors, shielded) is VFD rated, Olflex/LAPP brand, conductor size AWG #2. These cables run from motor J-box through the cattrack and terminated at the end of the cattrack in fixed floor mounted enclosure. The problem is that NEC(2005 Edition) allows to connect conductors in parallel only for sizes AWG 1/0 and larger (exceptions listed in article 310.4 do not apply in our case). Any opinions how to handle this situation?

 

[jraef]

Why are you using smaller conductors? Availability? That is a lot of parallel conductors; the risk of imbalance is getting greater with that many. That's what is behind the code requirement.

 

[ScottyUK]

You should be able to find flexible welding cable up to about 240mm² without any problems. Two in parallel per phase would be more than adequate. You could use a single 630mm² flexible per phase, but it will be a monster to terminate. On a flexible I wouldn't go bigger than 300mm² before paralleling because the cable becomes unmanageable.

Is it the shielding requirement that is causing the problem?


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Sometimes I only open my mouth to swap feet...

 

[resqcapt19]

Scotty,
Welding cable is not permitted for use on power systems by the NEC...There is a Type W flexible power cable that is almost identical that is permitted for this use by the NEC.
Don

 

[jraef]

Sounds as though he has a VFD on this dyno (didn't say, but mentioned that the Lapp wire was VFD rated). Type W or G cable would not be suitable if that is the case.

thinker,
What you will likely need to do is to change to using a lower count of individual shielded conductors of a larger gauge. Lapp / Olflex makes it. 4 x 250kCMil per phase should probably do it.

http://www.lappusa.com/Spec_Template.asp?nGroupID=1320

 

[thinker]

We can not use individual wires, drive supplier insists on usage of 3 phase shielded VFD rated cables to guarantee the system performance. Th LAPP cable AWG #2 is the largest commecrially available VFD cable which is UL listed (a mandatory requirement), approved for Class I Div.2 locations (the case for engine test cell). So, our choice is quite limited, plus restrictions on cattrack dimensions. The length of this run is about 15 ft., I do not believe that imbalance under these conditions would be the issue.BTW, may be I could attach these 10 cables to the motor, and ship this motor with "pigtail" as a factory pre-assembled system thus ignoring NEC restrictions?

 

[Laplacian]

http://www.amercable.com/doc/catalogs/gexol/37-102VFD.pdf

 

[thinker]

Laplacian, thanks for reply. We know about this cable. Aside of prohibitive cost, they sell it in minimal quantity of 1000 ft.

 

[waross]

I would ask the supplier for acceptable, code compliant cables or a Merchandise Return Authorization number.
respectfully

 

[jraef]

I don't necessarily agree. It seems on the surface to be ridiculous that a motor manufacturer insists on the use of a product that apparently doesn't exist. But the American Cable link that Laplacian supplied shows that it is available, just unpalatable.

thinker,
You might try scouring the depths of the odd-lot wire brokers for some short lengths. Houston Wire and cable is one

http://www.houwire.com/

, Anixter is another

http://www.anixter.com

, but there are a lot of others out there.

Still, it seems as though the VFD manufacturer's engineers are living proof of my signature line...

JRaef.com
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