Parallel Grounded Conductors

[mwh1023]

I have a quick question on what method is used to size the grounded conductor in a parallel conduit installation for a service feeder. Knowing also that 1/0 is the minmum size that can be used, these are the two methods I would like to discuss:

Installation: (2) 4" conduits will feed 600 amp 3-phase service. Each conduit has (3) 500 kcmil ALUMINUM ungrounded phase conductors. To size the minimum grounded conductor required in EACH conduit:

Method #1 - I would take the 500 kcmil x2 for the parallel feed. Go to Aluminum column on Table 310-16, 900 kcmil thru 1750 kcmil and a 4/0 aluminum conductor is required for BOTH conduits. I would convert the 4/0 to kcmil, 211,600, divide by 2 for the two conduits and 105,800 is the size required which is a 2/0 aluminum conductor (1/0 is 105,600 and not large enough)

Method #2 - Simply look at (1) conduit no matter what size of installation. So go the the 250 thru 500 column for the (1) 500 kcmil ungrounded phase conductor and you can install a 1/0 aluminum in each conduit.

I have went thru several installation scenarios and 99% of the time either method will give you the SAME wire size except for the one above.

Any thoughts on which method should be utilized?

Thanks,

Mike

 

[augie47]

From 250.24(c) (2) Parallel Conductors. Where the service-entrance phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 1/0 AWG.
FPN: See 310.4 for grounded conductors connected in parallel.(C)

 

[xCod3m4nx]

There are no exceptions to this rule? So if you are running two 4/0 calbes in parallel and there ground wires were #6 but with the run had to change to #3 because the 4/0 cables are used to keep VD under 3%, you still have to make the ground wires 1/0?

 

[augie47]

There are no exceptions to this rule? So if you are running two 4/0 calbes in parallel and there ground wires were #6 but with the run had to change to #3 because the 4/0 cables are used to keep VD under 3%, you still have to make the ground wires 1/0?

I believe in the case you are presenting, you are speaking of equipment grounding conductors which would be handled differently from the groundED conductor being installed with the service.