Conduits in a pipe

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smoothops10

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Like to get thoughts on running multiple conduits in a new ductile iron pipe installed with directional drill or jack and bore method.

Is it acceptable to install the pipe to get under a barrier that prohibits trenching, install the multiple sets of conduit in the pipe, and put a manhole on each side of the pipe (1600A, 480VAC, 4W). Manhole would have concrete encased ductbank exiting to a building on each side. Would probably use PVC 40 conduit in the pipe installed in 10' sections. Don't see how any conduit supports could be installed in the pipe so they'll be sitting on top of each other in the pipe. Thoughts?
 
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smoothops10 said:
Like to get thoughts on running multiple conduits in a new ductile iron pipe installed with directional drill or jack and bore method.

Is it acceptable to install the pipe to get under a barrier that prohibits trenching, install the multiple sets of conduit in the pipe, and put a manhole on each side of the pipe (1600A, 480VAC, 4W). Manhole would have concrete encased ductbank existing to a building on each side. Would probably use PVC 40 conduit in the pipe installed in 10' sections. Don't see how any conduit supports could be installed in the pipe so they'll be sitting on top of each other in the pipe. Thoughts?
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Since it will not be possible to "maintain spacing" between the PVC conduit runs, you would have to consider the whole set of them as a single thermal unit for the purposes of ampacity derating. If you are close to the code ampacity limit on any conductors based simply on the number of CCCs in a single conduit, you might end up having to get a calculation done on the thermal interactions of the conduits inside the iron duct.
You could probably avoid the need for a formal calculation by just considering the total number of CCCs across all of the conduits as being in one raceway and determining the ampacity adjustment based on that.

If the run were short enough, you could possibly take advantage of the rule exempting short sections of a circuit from derated ampacity limitations.
 
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So let's say the feeder is initially sized at 5 sets of 400-kcmil. Worst case scenario is that each set needs to be derated 50% per 310.15? Which would effectively then require 10 sets of 500-kcmil (45% derate) more than doubling the cost?

Where in the code does is a spacing required between the sets? What if 7.5" center-center spacers were fed in with the ducts as they were installed (if this is even possible)? Thoughts?
 
petersonra said:
why not just run the conduits separately and forget the pipe? what does it gain you?
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I would think it gains the ability to drive/drill just one hole under the obstruction instead of multiple smaller ones which would have to be drilled parallel to each other. And even if you ran the individual conduits independently, you would then have a duct bank that appropriate Annex B calculations must be applied to.

I think that a collar type spacer at intervals on the first few PVC conduits pushed into the pipe, then the last one or two without spacers might do the job.
The spacing would not need to be much more than the distance you would have between similar conduit runs on a wall (except for the fact that it will be a two-dimensional layering of course.)
 
petersonra said:
why not just run the conduits separately and forget the pipe? what does it gain you?
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Its a good question Bob but as gold digger eluded, it's to avoid having to do multiple runs. Site space and soil conditions are not good for the multiple runs which is what's driving the pipe option.

I like the idea gold digger but after several 10' sections are pushed through I'd imagine eventually it may become too hard to push the last sections through ( approx 200' run in pipe) with the spacers rubbing on the bottom of the pipe. What do you think?

And yes I'm starting to think this will require some special amp calc or similar derate calcs due to 6' depth but Would this software be able to model the pipe?
 
Why not run just one conduit (call it a pipe if you want) and pull your conductors through that?
 
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