Wireway use with Multicore cables and NEC requlations

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keeby371

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Hi. I am an engineer in the UK looking to design a control system for a customer in the US but I am having some difficulty with the use of wireways with regard to fill and de rating. The customer has specified the use of NEMA 12 wireway which is fine but when I read up on the NEC guidelines things start to worry me?
I have a control panel supplying numerous motors and supplies on a machine. My largest supply requirement is 32A. Now I would typically use multicore cables, 3c+E, 5c+E etc to wire the supplies directly from the control panel but everything I read makes reference to single cores?

Is it common practice in the US to wire machines in single core cables away from the control panel?
Are multicore cables used in this way and wired in wireway?
How are multicores perceived in this situation, I think I have read somewhere that a multicore is treated as a single cable, is this correct and if so how is this taken into account as far as de rating and fill?

For example if the multicores are seen as single cables then i will have in excess of 40 which then means a de rating of 35% if I am not mistaken. This would mean that my 6mm square cable for my 32A supply has now increased to a 90mm square cable?

Any feedback would be appreciated.

Thanks
 
We use a lot of conduit here, so typically the conductors in a wireway would be single core conductors.

If the project was using cable tray and tray cable, we would probably not use a wireway, but just use the cable tray.

As soon as you have over 30 current carrying conductors at any cross section of the wireway you are required to derate. If the wireway is short and to total length of the cables are fairly long you may find some relief from the derating in the exception to 310.15(A)(2).
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
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Thanks for the reply.

When you refer to TC(tray cable) are you referencing what I would call multicores? Does the de rating apply to cable tray installations or does it come down to segregation only?

I have to wire a machine which is in excess of 25m from my main cabinet and I have probably 10 or more 3 phase circuits of varying current rating which I need to supply so I don't want to believe multiple 1"or 2" conduits are the answer and I would prefer not to use single core cables but this may be my only option?
I am not sure of the reason the customer is specifying NEMA12 trunking instead of cable tray, which given what I am beginning to find out I would probably prefer to use.

It may be time to rethink?

Any more comments would be appreciated.
 
keeby371 said:
When you refer to TC(tray cable) are you referencing what I would call multicores? Does the de rating apply to cable tray installations or does it come down to segregation only?
Click to expand...

The NEC lists many flexible wiring methods that would count as multi-core, TC being one. There are rules about where you can/cannot use some of them. Also, some methods, such as AC or MC cable are flexible enough for installation but aren't suitable for continuous flexing as they use solid conductors inside. (You have to take the actual construction into account.)

keeby371 said:
I have to wire a machine which is in excess of 25m from my main cabinet and I have probably 10 or more 3 phase circuits of varying current rating which I need to supply so I don't want to believe multiple 1"or 2" conduits are the answer and I would prefer not to use single core cables but this may be my only option?
Click to expand...

Is this machine going to be shipped in pieces and assembled on-site? A 25m NEMA 12 wireway is going to be expensive, especially compared to conduit. IME the wireway and wiring from control panel to most of the machine would be supplied, and probably be installed, by the customer or their local contractor. Provide terminal blocks in a NEMA12 enclosure at each end and let someone else connect between by whatever method they want. (We're equipped for pulling lots of single-core wiring into conduit, that's a common way of doing things here :D, especially in industrial settings.)

You can read the NEC online for free at www.nfpa.org, it's code "nfpa 70". Look at the sections for tray cable and poke around in the 300's. Article 400 flexible cord is probably not an option for this use.
 
keeby371 said:
Thanks for the reply.

When you refer to TC(tray cable) are you referencing what I would call multicores? Does the de rating apply to cable tray installations or does it come down to segregation only?

I have to wire a machine which is in excess of 25m from my main cabinet and I have probably 10 or more 3 phase circuits of varying current rating which I need to supply so I don't want to believe multiple 1"or 2" conduits are the answer and I would prefer not to use single core cables but this may be my only option?
I am not sure of the reason the customer is specifying NEMA12 trunking instead of cable tray, which given what I am beginning to find out I would probably prefer to use.

It may be time to rethink?

Any more comments would be appreciated.
Click to expand...

Yes, tray cable would be a multi-core cable.

There are various derating rules for cable installed in cable tray also. They are found in Article 392. With spacing you can get down to not having to derate.

How much current do the loads pull? You would be permitted to put more than one load in a single conduit in many cases.
 
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