parallel conductors

[dhsvcs]

If an underground service from a delta high leg transformer feeds a fusible disconnect in a single existing conduit, and 2 of the conductors ( a phase, c phase, parallel 700 mcm ) must all of the conductors in the same conduit be parallel also, even if the loads are different ? Calculated loads are different.....

A & C phases 2 pairs of 600 mcm ( existing ) for 700 amp class L fuse
B Phase 1 - 250 MCM for 250 amp class L fuse
1 - 700 mcm for neutral ( meets calculated neutral load )
in 1 existing 4" conduit.

Going over 6 current carrying conductors will cause me to derate to 70% instead of 80% where the conductors will no longer fit and will require an additional conduit to be installed under an existing building or 2 new conduits around the building perimeter, abandoning the existing one. The situation is existing, as If it were new I would have run 2 parallel conduits from the beginning.

I also have to consider the neutral having a lesser ampacity than the A & C phases, even though the neutral calcs would allow a smaller mcm than the ones on A & C phases...

I will have an electrical P.E. review my plans and wonder if he has the power to approve this before presenting plans to the LJHA.
Same as my earlier post today but in simpler terms...
Thanx, dhsvcs

 

[augie47]

As long as the conductors are sized to carry the load you should be OK. In one conduit, tjhere would be no need fro you to paralle b or neutral.
(You sledom encounter a load requiring the service neutral to be considered a CCC

 

[dhsvcs]

thanx

 

[Carultch]

As long as the conductors are sized to carry the load you should be OK. In one conduit, tjhere would be no need fro you to paralle b or neutral.
(You sledom encounter a load requiring the service neutral to be considered a CCC

So in otherwords, if you parallel A & C (& possibly N) in a shared conduit, it is OK to have only one high leg B-phase conductor, given that it would only needs fewer amps than phases A & C.

By contrast, if you were to parallel in separate conduits, you'd have to parallel the high leg enough to divide it among the raceways, even if the amps it needs in its own right wouldn't cause you to parallel. Since that's what you'd have to do, to balance the magnetic fields within both/all raceways.

 

[dhsvcs]

I was told by a friend of mine who is an inspector it would not be allowed in his city in Broward County, that if any conductors in a single conduit is run in parallel, all the runs in that conduit has to be in parallel. If they were 1 pair, all runs 1 pair. If triplicate, all runs in triplicate. And then derated for the amount of current carrying conductors. The size of the conductors is not the factor....I could have 2 600 mcm each for A + C, 2 1/0 for B phase ( min size of runs in parallel ), and 2 4/0 for the neutral....But then since I have 8 current carrying conductors ( they would count the neutral as current carrying conductors in a high leg delta ), I would have to derate to 70% instead of 80% and the conductors would not fit.
I have seen single and paralleled conductors in Miami Dade. In fact this job was supposed to have been inspected, but the neutral was undersized and they had a 500 mcm conductor feeding a 700 amp fuse on the B phase, although the total load on B phase was 90 amps.

 

[roger]

Sounds like the Broward County inspector needs some schooling, he doesn't have a good grasp on the way things work.

Roger

 

[kwired]

Sounds like the Broward County inspector needs some schooling, he doesn't have a good grasp on the way things work.

Roger

+1

 

[kwired]

Individual "components" of a "parallel conductor" need to be same size, type, length, and any other characteristic that may effect impedance, this is to assure each individual component has as close as possible to same overall impedance and will make the current as balanced as possible through each component. Get too far off balance and you may have too much current on a particular component and overheat it.

Each conductor or "set of parallel conductors" need not be same material, type, length, etc. though it still may be desirable from some design perspectives to keep them within a certain range. For example if you have higher impedance on one line to a three phase motor, you unbalance the current to that motor and may have other undesired effects because of it.

Otherwise there is nothing wrong per NEC with Phase A being copper, Phase B being aluminum, Phase C being parallel aluminum and the neutral being parallel copper - if that is what you want to run and have met minimum ampacity required on each conductor or set of parallel conductors.


Another thing you can run into and may be even more common on high leg delta with minimal load on the high leg is that you can run a smaller conductor for that high leg - but if your overcurrent protection is a typical three pole breaker - you will normally have exceeded overcurrent protection that is required if you run that smaller conductor. If your overcurrent protection is fuses, you possibly can do this sort of install by putting the proper size fuse on the smaller conductor.