Parallel Feeders

[troywibb]

This question is regarding a new installation that didn't look right. The building has a 600amp 3phase 408/277V service. The slab was originally roughed in with 2-3.5" pvc conduits. It was discovered that one of the conduits had an impass. The single 3.5" conduit now contains 6-phase conductors, 2-neutral conductors, and 2-grounding conductors. The conduit is not overfilled and the derating factors work out OK.
My question is this; can the parallel feeders from the service disconnect to the main distibution panel in the building be in the same conduit per NEC?

 

[charlie b]

You already looked at the two problems you might have: conduit fill and conductor derating. If those two work out for you, then you can put all the conductors in the same conduit. By the way, you don't need two grounding conductors.

But I have my doubts about these two working out. Are you sure that you meet the conduit fill requirement? I don't know what type of conduit or wire you are using, but I checked a couple possibilities. I didn't find a combination that allows 8 conductors sized for a total of 600 amps, plus a ground wire, in a 3.5 inch conduit. This is especially true since derating for 6 current-carrying conductors gives you at least 500 MCM.

 

[BAHTAH]

Conductors in parallel

Conductors in parallel

Yes. 300.3(1), 310.4. At 310.4 there are five things listed to comply with and then a statement about the raceways used. " Where run (does not mean you must) in separate receways or cables, the raceways or cables shall have the same physical characteristics."

Grant

 

[don_resqcapt19]

Charlie,
I didn't check for the ground wire, but 9 400kcmil THWNs will fit in 3.5" EMT. The derated ampacity for 6 current carrying conductors would be 305 amps each. Assuming this is a wet location, the conductors would have to be THWN-2.
Don

 

[mikehughes8]

Parallel conductors

Parallel conductors

The NEC Article 392.8(D) states:
Where single conductor cable comprising each phase, nuetral, or grounded conductor of an alternating-current circuit are connected in parallel as permitted in 310.4, the conductors shall be installed in groups consisting of not more than one conductor per phase, nuetral, or grounded conductor to prevent current imbalance in the parallel conductors due to inductive reactance.


Using this article I would say aforementioned installation was done incorrectly.

 

[mikehughes8]

Charlie,
I didn't check for the ground wire, but 9 400kcmil THWNs will fit in 3.5" EMT. The derated ampacity for 6 current carrying conductors would be 305 amps each. Assuming this is a wet location, the conductors would have to be THWN-2.
Don

Actually, and if I'm reading this right, this installation violated NEC article 392.8 (D). In short it states that when conductors are run in parallel they must be grouped so that not more than one conductor per phase or nuetral are grouped together to prevent inductive reactance.

 

[mikehughes8]

Charlie,
I didn't check for the ground wire, but 9 400kcmil THWNs will fit in 3.5" EMT. The derated ampacity for 6 current carrying conductors would be 305 amps each. Assuming this is a wet location, the conductors would have to be THWN-2.
Don

I believe 392.8 (D) of the NEC prohibits this. When conductors are ran in parallel only one conductor per phase, nuetral, should be grouped together to avoid inductance is basically what it says.......


SORRY about triple posting folks. Didn't realize there would be such a time delay between actually writing my post and it making it to the board. New member error.....oops

 

[gregorsc]

I think there are eight current carrying conductors in the installation making the deration factor 70% so even with THWN-2 insulation you will need to use 500's. As for the conductors being run in the same conduit I will have to read up on that but the code article "Mikehughes8" refers to is talking about cable tray so I don't think you can apply that code article to this installation.

 

[sceepe]

Are you assuming that a major portion of the load is non linear? How else do you get 8 current carrying conductors? 310.15-B-4:-?

 

[gregorsc]

Without knowing all of the factors involved in this service it colud be a possibility that the neutrals will crry current. Still with six current carrying conductors and using 400 THWN-2 wire you can't install nine wires that size in a 3.5" conduit.

 

[don_resqcapt19]

I see that I missed the fact that the raceway is PVC. Charlie is correct that you cannot get than ampacity in a single 3.5" PVC raceway.
Don

 

[don_resqcapt19]

Actually, and if I'm reading this right, this installation violated NEC article 392.8 (D). In short it states that when conductors are run in parallel they must be grouped so that not more than one conductor per phase or nuetral are grouped together to prevent inductive reactance.

How does Article 392 have anything to do with this installation? That article only applies to cable tray installation. In a conduit installation, the conductors are confined by the raceway and are close enough to prevent problems caused by inductive reactance. Of course, you must have multiple of full sets in the raceway.
Don

 

[benaround]

They must be using 'compact conductors', you can hardly get a 400a in

a 4" conduit and this is a 600amp ?

 

[don_resqcapt19]

Frank,
Six 400kcmil THWNs fit in a 3.5" PVC conduit, 7 in rigid and 8 in EMT. If you are using compact strand conductors, then the numbers are 7, 7 and 9 based on the Annex C tables.
Don