New 3000 ampere service

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Sean B

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Mechanicsburg PA US
Hello. I am new to this forum. I am writing here because I can't get a clear answer from people I work with around here.
Looking at a new 3000 ampere service for a school building. The plans specify 8 sets of 4 #500 kcmil THHN copper conductors for the service.
The calculations on the plans give a total load of 3740A and a demand load of 2651A.
All sets of 4 plus grounds are run in 8 separate conduits.
The main transformer for the building is Delta to Wye.

Based on the 2008 NEC, since this is a wye system and the majority of the loads are non-linear, the 'neutral' wire is counted as a current-carrying conductor.

If I use the 90 degree column for #500 kcmil I get 430A x 8 = 3440A. De-rated to 80% gives 2752A.

Can I protect these service conductors with a 3000 ampere breaker? Over 800 amperes, can't round up. Based on the demand factor, it looks like
the conductor ampacity is okay but I'm not clear about the overcurrent protection.

Any help is appreciated.
 
I didn't check your math but you cannot protect them at 3000 amps-- over 800 amps you can not up size the overcurrent protective device.
 
I really don't believe the majority of the loads are non linear. That would mean that over 50% is non linear-- your heat/ac probably uses most of the loads but I would be surprised if there was 51% lighting and computer loads but maybe.... You may want to up size the neutral if that were true
 
Just curious are these service entrance conductors or feeder conductors? Are you sure that your load is more than 50% non-linear? IMO that seems somewhat unlikely for a school.


For a 3000 amp service with an 80% derating factor for each raceway you would need a minimum total ampacity of 3750 amps. {3000/.8=3750 amps}
 
I agree with the others in that I would not think the major portion of the load was non-linear. If it is not, based on a 75? termination, your (8) 500s would be compliant.
If in fact, the major portion in indeed non-linear, then, as Rob stated, you would need 3750 ampacity and (8) 750s would suffice.
 
Thanks for all of your responses. I am going to have to take another look at the loads tomorrow to see about the percentage of non-linear loads.
It looks to me from your responses that if it turns out that more than 50% of the loads are non-linear, the service conductors are not sufficient as sized on the plans for a 3000 A main breaker.

BTW, it is the service from the main transformer coming into the building, not the feeders. From the MDP there are 14 feeders going to 480/277 panels and each one of those panels feed various loads and
14 transformers.
 
As Joe mentioned what these conductors actually are can make a difference. If they're service entrance conductors then you wouldn't need the EGC's in the raceways.
 
Another factor to consider: just how many of the 'non-linear' loads are on the downstream side of 480:208/120 delta:wye transformers?

Triplen harmonic loading on the secondary of a delta:wye transformer will not add neutral current on the primary side.

-Jon
 
You would be hard put to find a facility with over 50% neutral loading or over 50% non linear loads, much less a school.

You can likely reduce the neutral.

I see the 8 - 500's OK.

IMHO

RC
 
david said:
Just curious as to why you are using the 90 deg. column
Click to expand...


You can always start at the 90C column if your insulation is rated 90C. You derate from there and the final ampacity cannot be more than the weakest link in the circuit-- usually the breakers-75C
 
Dennis Alwon said:
You can always start at the 90C column if your insulation is rated 90C. You derate from there and the final ampacity cannot be more than the weakest link in the circuit-- usually the breakers-75C
Click to expand...

yes I know that's why I posted never mind, I didn't edit that part because I always tell guys that inspectors can get tunnel vision too at times. just read to fast
 
Your comment about the non-linear loads from the 14 transformers helps a lot. Most of the lighting and
computer circuits are on the 208/120 circuits. It seems that counting the neutrals on the incoming service
will not be required so de-rating the 500s to 80% is not necessary.
 
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