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Instrument Conduit Spacing in Duct Bank Arrangements

Instrument Conduit Spacing in Duct Bank Arrangements

Instrument Conduit Spacing in Duct Bank Arrangements

In designing a duct bank for a chemical plant, I was going by our company's standard detail for underground ductbank which calls for 6" separation between conduits with instrumentation & low level communication circuits and low voltage ducts (in our case 480 V conductors for lighting and 120V for instrument power). My signals will be 4-20 mA.

I have an 18" width to stay within. Some of the cables may have to run 2000 ft inside the duct. I'll be using 2" or 3" conduits.
This requirement restricts greatly how much I can get into a 18" ductback section and I'd really like to not have to use more than 3". I can't find anything in the code that requires this, and the spacers available for stacking the conduits before the concrete is poured don't seem to allow for this situation,  so I like to ask people in this forum if anyone has personal experience with putting a mix of power and instrument conduits into a ductbank section with 3" spacing to no bad effect.
I was planning to use RGS for the signal cable and PVC for the power, as is also called for in the detail.

I have a lot of experience in instumentation but have never designed duct arrangements before.

RE: Instrument Conduit Spacing in Duct Bank Arrangements

You won't probably find code on that as the code doesn't care if you instrumentation works or not just the safety and fire aspects.

Three inches huh..
Is that three inches of concrete?

I suspect you won't have any problems unless you're talking VFD or some other harmonic monster running in the LV power conduit.

What you don't make the mistake of running three phase components in different conduits.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Instrument Conduit Spacing in Duct Bank Arrangements

Thanks Keith. Yeah, I'm sure you are right about the code. After talking to a few people around here, I find that 3" separation like that is commonly used with no problem- it just bugged me that someone thought it was a good enough idea to build it into the standard detail, and I began to think well if it's for a long run, maybe it could be a problem.

4-20mA signals are not all that delicate and the other signals I have are in optical fiber so I know there is no problem there - so I'm not going to worry about it any longer.


RE: Instrument Conduit Spacing in Duct Bank Arrangements

Don't forget to use shielded, LOW IMPEDENCE (Z) instrument cable.

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