Multiple Parallel Raceways

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jkflipflop67

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When paralleling conductors for larger ampacities, is there any minimum spacing between raceways (conduits) or a derating that needs figured? The situation is a 480VAC feed between a substation and motor control center. No more than 3 current-carrying conductors and a ground will be in each raceway (conduit). The raceways will not be in a concrete encasement or direct buried but passing in "free air" through a large tunnel space.
 
Re: Multiple Parallel Raceways

There is no minimum spacing, and no need to derate from the values in Table 310.16. The only related statement I could find is a Fine Print Note under 310.4. It "suggests" that there is an advantage in selecting an installation method that minimizes the differences in inductive reactances. One way to do that is to maintain, as closely as possible, whatever spacing you choose. For example, if the conduits are 4 inches apart at the start of the run, try to keep them 4 inches apart throughout the run.
 
Re: Multiple Parallel Raceways

Check out 310.15(B)(2)(b)__quote__
More than one conduit,tube,or raceway. Spacing between conduits, tubing, or raceways shall be maintained.

This pertains to your question. I just have no idea what to do with it. There is no direction as to how much spacing is required. Is it just me or does this section seem incomplete, like they forgot to finish it.
The explanation in the handbook is no help.
 
Re: Multiple Parallel Raceways

It goes back to the FPN that I had mentioned above. For example, if you have a long run, and if the conduits are 4 inches apart for most of the run, but are 1 inch apart for (let us say) 10% of the run, then the impedance per linear foot of that section will be different than that of the rest of the run. You could create a hot spot in that section, even though you keep the total load below the tabulated ampacity.

That is the basic reason for the rule that ?spacing . . . shall be maintained.?

It does not tell you how much spacing, because that is not the issue. The issue is that whatever spacing you choose, you maintain throughout the run.
 
Re: Multiple Parallel Raceways

Charlie
Your explanation should be in the handbook. Until now, we never new what to do with it.

Is the problem different between ferrous and non-ferrous conduits? Seems like more of a problem with ferrous conduits? Conduit acts as a magnetic core like a transdformer?

We're talking parallel conductors, but how about two different feeders or even branch circuits next to each other?? This section doesn't differentiate between them.
 
Re: Multiple Parallel Raceways

Originally posted by sandsnow:Is the problem different between ferrous and non-ferrous conduits?
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The physics is certainly different, for the very reason that you gave. Is the code different? I think not. Should the code be different? In order to answer that, I would have to go through the math. And things are getting too tired around here for me to do the math. ;) (Actually, that is a clever way to disguise the fact that I am not certain I still know how to do that math. :confused: But I still have the necessary textbook.)
We're talking parallel conductors, but how about two different feeders or even branch circuits next to each other?? This section doesn't differentiate between them.
Click to expand...
Very good point. I had not noticed that before. I think the same rule should be followed: Pick a separation distance, and maintain that same distance throughout.

There are two commercially-available software programs that I have used to perform Neher-McGrath calculations. Both were smart enough to take into account having conductors from different circuits installed in parallel conduits within the same concrete encasement. Put three #1s for one circuit in the first conduit, Put three #1/0s for a different circuit in the second conduit, and put parallel sets of three #2/0 for a third circuit in the third and fourth conduits, and finally leave the other two conduits empty, and these programs would give you a calculated ampacity for all three circuits. However, I never tried to use these programs for conduits in free air. So I don?t know how they would treat this situation.
 
Re: Multiple Parallel Raceways

Thanks to everyone that has chipped in. I have since been told by an electrical contractor friend that 1 inch between conduits is a minimum and from what I gathered it's important to maintain that distance throughout the run(s).
 
Re: Multiple Parallel Raceways

Charlie b: I'm trying to recall some of the discussions that took place over a few Code cyles back in the 1980's in code panel 6 pertaining to mutual heat from adjacent conduits. I believe, the intent was that if the raceways were far enough apart to install the locknuts on adjacent conduits, it was not required to apply the adjustment factors of Table 310.15(B)(2)(a).
 
Re: Multiple Parallel Raceways

That sounds reasonable to me. But only because the conduits are in "free air." The air surrounding any one conduit should be able to carry away its heat. This is important for two reasons:
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  • <font size="2" face="Verdana, Helvetica, sans-serif">The heat generated within any one conduit will not be carried into a nearby conduit, effectively raising the ?ambient temperature? internal to the second conduit.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">The heat generated within any one conduit will not ?hang around in the area,? causing a higher ambient temperature around all the conduits, making it harder for each of them to dissipate its own heat.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">If you start talking about having the conduits side-by-side direct buried or encased in concrete, I would no longer offer the same reasoning, nor would I come to the same conclusion. The NEC does not give a minimum spacing for underground conduits (despite what you might see in some of the figures in 310 and in B310). But I don?t think ?just far enough away to install locknuts? would be enough.
 
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