Parallel Feeders Underground and Above Grade

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ggdennfru

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We have a 2000 amp, 480 volt feeder from a Unit substation to MCC in the same room.
One line diagram shows 6 conduits for this feeder. Four conduits are presently installed underground for this feeder but six conduits are required. Could these 4 underground conduits be used plus 2 overhead conduits for the 6 conduit feeder and still conform to the NEC as long as wires in all conduits are of equal length? Distance underground and overhead is around 50 feet.
 
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My initial reaction was to say ?no,? as Ron has done. But now I am not so sure. Look at the second sentence in that paragraph, and focus on the words, ?shall apply separately to each portion.? Does that mean that as long as one set of A,B,C,N,G are run together in the same trench, and another set of A,B,C,N,G are run together in the same conduit (outside the trench), then we have met the ?shall apply separately? part of the rule? :confused: :confused:
 
charlie b said:
My initial reaction was to say ?no,? as Ron has done. But now I am not so sure. Look at the second sentence in that paragraph, and focus on the words, ?shall apply separately to each portion.? Does that mean that as long as one set of A,B,C,N,G are run together in the same trench, and another set of A,B,C,N,G are run together in the same conduit (outside the trench), then we have met the ?shall apply separately? part of the rule? :confused: :confused:
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Charlie - I also was thinking the same thing; the "shall apply separately" makes me think each separate parallel section can be run differently.
 
As far as 300.3(B)(1) is concerned, there is no requirement that the conduits be the same, just that each conduit contain all of the circuit conductors...and this requirement doesn't even hold if the conduit is non-metallic.

However 310.4 has a separate requirement that all conductors for a given _phase_ have the same characteristics, and this includes conduit characteristics. You could not run some of the conductors of a phase underground and others overhead.

-Jon
 
Thanks, Jon. That was what I had been missing. 310.4 would forbid the proposed installation, because the raceways would have different characteristics. That's one ATTABOY for you. :smile:
 
charlie b said:
Thanks, Jon. That was what I had been missing. 310.4 would forbid the proposed installation, because the raceways would have different characteristics. That's one ATTABOY for you. :smile:
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Charlie,...... Raider/ Chris hit it first time. Jon clarified it.
 
OP has not stated what type of raceway is existing and what he/she plan to install for the additional two how could you come to the conclusion they don't physically the same characteristics aside from be above and below grade I dont know how the changes there characteristics. Never done it that way nor saw it done that way can't say right or wrong
 
I would say as long as the conductors are the same length it shouldnt matter how they are run, thats my opinion only.
 
Dean83169 said:
I would say as long as the conductors are the same length it shouldnt matter how they are run, thats my opinion only.
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If the conductors were of _exactly_ the same length, but some were run in metal conduit and some run in plastic conduit, then more current would flow on the conductors in the plastic conduit.

-Jon
 
Conduit characteristics

Conduit characteristics

If all the conduit material was the same for the above and below ground runs, how would the conduit characteristics be changed between the two locations?
 
If all the conduit material was the same for the above and below ground runs, how would the conduit characteristics be changed between the two locations?
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The ambient temperatures would be different between the two locations. The underground conduit would have an ambient temperature quite a bit less then the conduits that are run overhead in the room.

Chris
 
dana1028 said:
If all the conduit material was the same for the above and below ground runs, how would the conduit characteristics be changed between the two locations?
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Allowing with what Chris added IMO characteristics covers everything.

The fact the raceway does not follow the same route means they have different characteristics.

The fact the raceway is above and one is below ground means they have different characteristics.

The fact the raceways will be made of different materials means they have different characteristics.
 
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The conduit above ground will be better able to release heat that is generated by its conductors, then will the conduit below ground. As Chris pointed out, there will be a difference in ambient temperature. The net result is that the conductors will reach a different steady-state temperature. That means a they will have a different value of resistance, and will carry a different amount of current.
 
charlie b said:
[...] the conductors will reach a different steady-state temperature. That means a they will have a different value of resistance, and will carry a different amount of current.
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IMHO this aspect of the discussion is not a significant safety issue.

The cooler conductors will have lower resistance, and will thus carry a greater portion of the current. But they would not carry so much current as to run warmer than the other conductors, because they would then have a higher resistance.

If the entire system is designed such that the warmer conductors would not overheat when carrying their fair share of the load, then having the cooler conductors in parallel will only make things better, because the cooler conductors will never get as hot as the warmer conductors, and the cooler conductors will be taking more than their fair share of the load.

I guess that in transient conditions the cooler conductors could momentarily take more of the load than the warmer conductors, and be damaged during this transient, eg a short circuit.

-Jon
 
I agree with Iwire, and Charlie b on this. When I was on a Submarine tender in the Navy, we used to have at least 10 shore power cables (400A 480VAC 3 phase) ran to city power on the pier. Amazing how the cables would be in the same ballpark. Typical readings at busy times, would be from 260A to about 270A for each cable. each cable was same length, size, and material, with a 90 pound "head" to plug in.

Per tables 8 and 9, in chapter 9 of the NEC, suggests smart money is to get them all in same environment equivically. I would think that following existing route of other conductors would be the right way to go. If it is designed different, and has an engineer's stamp on it (thinking that there would CT's for monitioring), then build it.
 
winnie said:
IMHO this aspect of the discussion is not a significant safety issue.
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I think the key point is that it is not practical to analyze every possible variation of installation scenarios. Nobody can say, without an analysis, whether having two conduits underground and one above, with a specific set of conductors and loads, might be safe, whereas having three conduits underground and two above, with a different set of conductors and loads, might cause a problem. This is, IMHO, a situation in which the simplest approach, from a regulator’s point of view, is to forbid any differences in the parallel runs, including differences in the types or locations of raceways.
 
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