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#73936 01/05/07 08:45 PM
Joined: Jan 2005
Posts: 109
Grover Offline OP
Have been reading the good book, and am confused....

1500A 3 phase 480 gen set. About 200' run. 3 x 750 MCM Copper per leg. PVC Sched 80, not RMC. How group? Each parallel run could be ABC + N. Or AAA, BBB, CCC, NNN, and then add EGC? Or, do I need to do ABC, ABC, ABC, NNN. EGC in additional pipe?

Can I do it in 4 4" runs, or do I need 5? How big does the ground need to be? Proposed ground is a good UFER - 3', 6' and 10' rebar grid - similar one gave me about 6 ohms. What about far (200') end?

Depending on which table I go to, I see 3 or 4 750 MCM's.....and then EGC...

Need to spec and bid....

Advice? Help?



Joined: Feb 2003
Posts: 939

The Best bet is run the large 750's in 5 inch PVC and run with ABC+N each and from my expernice do not run AAA BBB CCC NNN in separated pipes at all [ i treat this as RMC ]

for EGC i don't have the chart with me a monment but i am thinking at least 300KCM size but i might be wrong on this one [ check with the distance as well ]

Merci , Marc

P.S. for quick head up let you know i am doing fine for far just been busy for while

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Feb 2002
Posts: 182
Bob Offline
You can do it with 3 600 kcm per phase + N.
420 x 3 = 1690 amps.

Joined: Apr 2002
Posts: 7,387
Likes: 7
OK, 3 sets of 750 equate to 1605 amps, so that's good.

As to the PVC...ABCN in each is OK.

As to AAA, BBB,CCC,NNN you need 4 pipes. Acceptability of this method is dependent on the conduit terminations at each end. (Read)
This method is favored for the advantage of terminating straight to the landing lugs, as opposed to criss crossing the conductors.

Grounding decisions are dependant on the transfer switch operation, (3 or 4 pole) as to the gen being a sep. derived system.

As to your 'spec n bid' have you checked out LK's threads or chat rooms??


Joined: Apr 2002
Posts: 7,387
Likes: 7
Check your math above.


Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
When using Non-Metallic Raceways, the NEC allows the Parallel Feeders to be run either way -

*Option #1: A,B,C,N in each Duct,
*Option #2: All "A"s in one Duct, all "B"s in another Duct, all "C"s in a third Duct, etc...

Although I have never done it, nor ever specified it on Drawings, "Option #2" is valid for Non-Metallic Raceways, but you would need at least Three (3) for a 3 Wire Feeder - as in 3 Phase 3 Wire System,
and Four (4) for a 4 Wire Feeder - as in a 3 Phase 4 Wire System.

The EGC - if to be installed, will need to be the "Full Size According To Article / Table 250.122" in _EACH_ Duct.
So, if the Genny is _NOT_ to be an SDS (using a 3 Pole ATS), each Duct will need a 4/0 cu. (or 350 MCM al.) Equipment Grounding Conductor.

For an SDS, the EGCs (Equipment Grounding Conductors) will actually be GECs (Grounding Electrode Conductors), and therefore will need to be terminated at both the Genset + its local Electrode, and to the Building's Electrode system.

The size will reflect what is found in Article / Table 250.66.
In this case, you would be looking at a maximum size of 3/0 cu, or 250 al.

Good luck!


p.s. I rarely deal with any back-up Gensets over 100 KW, so if the information is lacking, feel free to fire away!!!
[Linked Image]

Dodging the flames...


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Mar 2005
Posts: 1,213
Personally, I like to see the phases run ABC+N together whenever possible for reduction of not only inductive losses in metal conduit, but also reduction EMI in non-metallic.

Even though it's cleaner for termination, running AAA BBB CCC NNN can create a lot of inductive loss in conduit or EMT.

This a 1MW Genset? Size doesn't matter, the runs just get heavier [Linked Image] The ground wire would be calculated like any other service. Be careful with the neutral!

[This message has been edited by SteveFehr (edited 01-05-2007).]

Joined: Jan 2005
Posts: 109
Grover Offline OP
Thanks for the replies guys!

One more question...

I planned on the run being entirely Sch 80 4", but am thinking at the far end I should go to RMC - gotta go up 20', over 10', and down thru the roof.... My thoughts are that the 4 750MCM Cu will be too heavy to make me comfortable with Sch 80.....

That says the pipes have all got to be A-B-C-N-G, but how do I handle the bonding? Just at the equipment cabinet under the roof?

And yes Scott, it is an SDS - Gen runs the plant - step-down provides 120/240 for some lighting etc.; when gen is down, transfer switch (with switched Neutral) flips over to PcCo. So the EGC is actually a GEC.


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