# Underground rebar in duct bank



## oldsparky52 (Feb 25, 2020)

Is this a parallel circuit or 5 different circuits or ??


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## oldsparky52 (Feb 25, 2020)

My gut says it would be better if all 5 conduits were inside the rebar cage w/out rebar between any of the conduits.


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## oldsparky52 (Feb 25, 2020)

If the engineer doesn't have a specific reason for the bent rebar, I would send and RFI about if it would be acceptable bend a U and then after the conduits are installed, put the cross piece on top.


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## joe-nwt (Mar 28, 2019)

gpop said:


> Any thoughts on how this would react to a un-balanced 3 phase feed in one conduit.


Are you asking if the rebar would react like metallic conduit?


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## gpop (May 14, 2018)

joe-nwt said:


> Are you asking if the rebar would react like metallic conduit?


Would it act like a inductance coil.


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## oldsparky52 (Feb 25, 2020)

gpop said:


> Would it act like a inductance coil.


I thought the 4/0 would deal with that (IDK).


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## MikeFL (Apr 16, 2016)

The bent rebar you're speaking of are called Stirrups and from a structural perspective they create a structural beam. Same thing we do in a tie beam over a door opening.

Is it the EE's detail on the "E" sheets that shows it? 
Or are you getting that from an "S" sheet?

Either way they don't mind questions, most of the time.
Call the phone number on the sheet and ask they guy whose name is on it.


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## joe-nwt (Mar 28, 2019)

gpop said:


> Would it act like a inductance coil.


Maybe I'm not understanding your layout.

How about some red, black, blue (A,B,C) circles in the conduits to show your conductor layout.


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## wiz1997 (Mar 30, 2021)

gpop said:


> We have a engineered job that calls for rebar in the duct bank
> 
> Now im wondering if this is a bad idea. Rebar running with the conduit seems fine but they called for a ring every 12" which makes you wonder how it reacts to a electrical field
> 
> ...


Over thinking.

There will be no induced voltage on the rebar because the rebar is grounded.

Any induced voltage is bled off.

You would have to remove the ground wire, to be able to measure an induced voltage on the rebar.


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## splatz (May 23, 2015)

I don't think grounding the metal matters, it's an issue with parallel sets coming through KOs in ferrous metal enclosures that are bonded. 

If you had plates instead of rings, and parallel sets with the phases separated in each conduit, you'd worry about inductive heating between the holes when eddy currents flow in the sheet metal. You're supposed to use a nometallic or nonferrous box, or remove the ferrous metal between the conduits and patch with something non-ferrous. I can't cite the code for that though. 

I don't think this is an inductive coil but it may technically qualify as a minimal / trivial choke. Let's not go all kenny clamp here. 

With parallel conductors and phases separated into each conduit, I am pretty sure it's going to happen with a ring like this too, and you technically should not do it. However and I am guessing here, I think at 480V it would be something you could measure but nothing that would damage the rebar or the concrete, just a little warm spot between the conduits out of sight and out of mind.


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## gpop (May 14, 2018)

splatz said:


> I don't think grounding the metal matters, it's an issue with parallel sets coming through KOs in ferrous metal enclosures that are bonded.
> 
> If you had plates instead of rings, and parallel sets with the phases separated in each conduit, you'd worry about inductive heating between the holes when eddy currents flow in the sheet metal. You're supposed to use a nometallic or nonferrous box, or remove the ferrous metal between the conduits and patch with something non-ferrous. I can't cite the code for that though.
> 
> ...



That makes sense.


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