VFD cable in concrete duct banks

[Brutus1055]

Apologies if this has already been addressed elsewhere...

I've seen multiple threads and white papers that indicate VFD cable in steel conduits is for the most part, redundant. What about VFD cable in concrete encased duct bank with PVC conduit (2" spacing between conduits)? Typically these cable runs end up being some of the longest onsite, so the added cost of VFD cable vs. vs. standard cable adds up real quick. Am I overbuilding if I put VFD cable in these underground concrete duct banks?

Additional info... (1) Our specs call for XHHW or XHHW-2 cable in all outdoor areas including below grade. (2) Other than a pair of small conductors for disabling the VFD if the local disconnect is opened, this is the only circuit in the conduit.

 

[Julius Right]

In my opinion, if the rebar does not passes between ducts-the rebar cage is only in the extern wall-no losses in the rebar it is expected.

 

[don_resqcapt19]

I don't know if the concrete provides the same shielding as would steel conduit or VFD cable from the RF emissions that the drive creates.
There are a couple of drive experts on the forum that may provide a better answer for you.

 

[paulengr]

EMC isn’t the big issue and concrete holds a lot of moisture so I don’t see it being an issue. It is nonferrous so reactance will be very different.

 

[petersonra]

Considering the millions of VFDs installed using regular conductors in both PVC or steel conduit that exhibit no negative issues, I don't see a problem with what you want to do.

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

 

[gadfly56]

Considering the millions of VFDs installed using regular conductors in both PVC or steel conduit that exhibit no negative issues, I don't see a problem with what you want to do.

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

Yeah, then you have to convince people like the Port Authority of NY & NJ, NYC, etc. as well as MEP firms that put it on drawings or in specs.

 

[Besoeker3]

Apologies if this has already been addressed elsewhere...

I've seen multiple threads and white papers that indicate VFD cable in steel conduits is for the most part, redundant. What about VFD cable in concrete encased duct bank with PVC conduit (2" spacing between conduits)? Typically these cable runs end up being some of the longest onsite, so the added cost of VFD cable vs. vs. standard cable adds up real quick. Am I overbuilding if I put VFD cable in these underground concrete duct banks?

Additional info... (1) Our specs call for XHHW or XHHW-2 cable in all outdoor areas including below grade. (2) Other than a pair of small conductors for disabling the VFD if the local disconnect is opened, this is the only circuit in the conduit.

We had a fairly large four quadrant variable speed drive - about 1300A, 600V. A bit of a special with the frequency being 357Hz. The output from the VSD was in a concrete type structure. It was called Preedcrete Ltd. That may what Brutus is looking for.

 

[don_resqcapt19]

Considering the millions of VFDs installed using regular conductors in both PVC or steel conduit that exhibit no negative issues, I don't see a problem with what you want to do.

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

Where does the code even suggest that raceway wiring methods are favored over cable wiring methods?

 

[Besoeker3]

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

What can I say. I think you are correct. I don't think we have used conduit for decades.

 

[petersonra]

Where does the code even suggest that raceway wiring methods are favored over cable wiring methods?

It doesn't specifically forbid it but it makes it much harder than necessary. For instance, why can't TC-ER be run fastened to the structure instead of some kind of raceway?

 

[don_resqcapt19]

It doesn't specifically forbid it but it makes it much harder than necessary. For instance, why can't TC-ER be run fastened to the structure instead of some kind of raceway?

Because no one has paid one of the NRTLs to do an outline of investigation for using TC-ER for that application.

 

[paulengr]

Considering the millions of VFDs installed using regular conductors in both PVC or steel conduit that exhibit no negative issues, I don't see a problem with what you want to do.

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

Have you ever heard of cable tray or bus duct?

At 40+ feet between supports vs. 6-10 feet and the fact that you can just pull one out and put another in, it’s already here in a lot of plants. Plus we have MC and TC-ER which are basically cable in conduit.

Not sure where NEC holds you back.

 

[kwired]

It doesn't specifically forbid it but it makes it much harder than necessary. For instance, why can't TC-ER be run fastened to the structure instead of some kind of raceway?

I think you need to talk to the engineers that design things more so than to code making panels.

If they want something protected by RMC might as well pull general purpose conductors through it instead of a cable though.

 

[Ragin Cajun]

Considering the millions of VFDs installed using regular conductors in both PVC or steel conduit that exhibit no negative issues, I don't see a problem with what you want to do.

To my way of thinking, using cable makes a lot of sense if you are not putting it in some kind of tubular raceway. IMO, the NEC needs to grow up and accept the idea of cabling in lieu of conduit as the preferred general purpose wiring method. It is something that is long overdue.

I have added many VFD's to existing "standard" motors using the existing wiring in a chemical plant. Not a single issue, and it's been decades!

 

[kwired]

I have added many VFD's to existing "standard" motors using the existing wiring in a chemical plant. Not a single issue, and it's been decades!

Same here.
Only problem I have had is IGBT reflected wave problems. Learned to install line reactors on drive output and been fine since doing that.