60Hz and VFD Feeder in same conduit

[Senselessticker]

Does anyone know of a reason why I cannot run a 480V 3 Phase 60Hz feeder (to an AC unit) in the same conduit as a 480V 3 phase feeder from a VFD (to a Fan)? Are there any code reasons why this should not be done, or any magnetic interference reasons, practicaly reasons, etc..?

Thanks,

 

[dpc]

It's generally better to keep the VFD output circuit in its own steel conduit since it will have a lot of high frequency components and generate a good deal of electrical noise.

Since the other circuit is a 480V power circuit, you might not have too many problems, but some of this hash could get coupled into the A/C unit's low voltage circuitry and cause problems.

The only Code restriction would probably be the need to derate the ampacity of both circuits due to more than three current-carrying conductors in the same conduit.

 

[Skogsgurra]

It depends a lot on where in the world you are. The European EMC Directive says that you shall keep contaminated conductors (i.e. output from inverter) away from clean conductors. Capacitive coupling from contaminated to clean is against the EMC Directive. It is good practice not to mix the two.

Gunnar Englund

http://www.gke.org

 

[Laplacian]

What about VFD's installed in MCC's? Wireways are common for the whole assembly; control and power.

 

[jraef]

That has always been one of the arguments against putting VFDs in MCCs. I think the jury is still out on that issue. MCC debate link It is certainly not allowed in the EU, but then again their concept of MCCs is different than the NEMA concept. Placing VFDs in MCCs was a marketplace demand that was initially resisted on engineering grounds by MCC makers. But users and integrators kept doing it anyway, and eventually the manufacturers followed suit. If you carefully read the installation manuals they will tell you to separate the VFD output conductors from other cables, both power and control. Everyone knows however that this is not going to happen in the real world, and since it is rarely problematic, it has continued. I have seen many installations however where this was a big problem, and a lot of money was wasted trying to rectify it.

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[hrc]

I've done that...put VFD's in the same panel as regular starters BUT...like all control signals, etc I followed the best practices. Ran the signals away from the normal high power stuff, crossed at 90 degree (helps prevent cross coupling) and used shielded cables.

So alot comes down to just how well you build it. And how well it gets installed.

 

[dpc]

Metal-clad cable, such as Okonite C-L-X is a good option for VFD output circuits. It now comes standard with three separate interstitial ground wires.

 

[jraef]

Yes, that is the best solution when physical separation is unavoidable. Several MCC manufacturers mention that in newer installation giudes as well.

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