sizing parrell feeds

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billdozier

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Hey guys can you point me in the right direction when it comes to sizing parellel feeds on a three phase 800a service. I know theres more than 310.16 that applies and thanks
 
infinity thanks for the code references. Now to make sure im on the right course. I see the code restrictions as how to run the conduit and wire. I also the reference to art. 250.122 which says I need a 1/0 cpr or 3/0 al egc wire. I also see the derating factor of 80% due to 4 current carrying conducters. Now how do I size the actual wire to feed this load. Thanks for the help. And am I tracting correctly?
 
A few questions:

Where did you come up with an 800 amp service?
What are the loads going to be? (HVAC, Motors, Computers, Lighting,etc.)
Where are you running the EGC?
 
its a hypothetical question. Just pulled an amprage out of the sky. Another crew was discussing parrellel feeds on their job this morn and were totally lost. I knew a lil more but not enough to really help them out. So im trying to understand this better for down the road.
 
billdozier said:
infinity thanks for the code references. Now to make sure im on the right course. I see the code restrictions as how to run the conduit and wire. I also the reference to art. 250.122 which says I need a 1/0 cpr or 3/0 al egc wire. I also see the derating factor of 80% due to 4 current carrying conducters. Now how do I size the actual wire to feed this load. Thanks for the help. And am I tracting correctly?
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If you look at table 310.16 you can come up with a few different scenarios for 800 amp. You can run 4 runs of 200 amp wire in different conduits preferably or you can run 2 sets of wire rated over 400 amps to deal with the derating. There are other options. Trying to do this with 2 sets of feeders in one conduit is not feasible.

If you have 8 CCC you must derate at 70%, thus 800 divided by .7= 1142 amps.

You need to now find a wire that has half the amp of 1142 or 571 amps. Probably something like 800 KCM copper. Depending on your calculated load this can vary along with 240.4
 
so to lock this in if I have a 600a service. And two parrelel conduits i would take 600 divided by .8 which would give 750amps. I then would take this 750 divide by 2 = 375a. I then would size accordingly to 310.16 factoring in ambient temp and come up with 500mcm Correct? Then size the ground per250.122 and run a #1 ground
 
Common Feeders

Common Feeders

As an estimator I see a lot of engineered feeder sizes that are actually incorrect. Our professional engineering community seem to forget about the volt drop and derating factors that need to be applied, or just don't care enough to provide them...

For an 800A service we follow the following rules as a guideline

3P3W - 2 sets 3" conduit with 3#500 and 1#1/0G
3P4W for general loads - 2 sets 3-1/2" conduit with 4#500 and 1#1/0G
3P4W for non linear loads - 3 sets 3" conduit with 4#350 and 1#1/0G

For most instances in the Chicago market we can get by with the above. Some engineers will increase the conduit up to 4" with 4#600 in lieu of the 500's, but in these times, we will estimate to the lower cable size where we can.

To answer your question, if you were looking to size a 1200A feeder, go to table 310.16 as your starting point, figure out what wire insulation and temp rating of the cable. and divide your desired ampere rating by the closets ampacity. i.e. a 600MCM THHN conductor at 75deg rating is good for 420amps, you would need at least three sets of this size to provide a parallel feeder for 1200A. Hope this makes sense.
 
j wire after studying the definition of non linear loads to the best of mine non ee abilitys :) I would tend to agree with you. Thus saving money on wire size. However I am interested in seeing what other people have to say on this subject. Thanks to all who helped me lock this info in
 
iwire said:
I would not count the neutral, you don't have to.

For a building service it would be most unusual for the majority of the load to be non-linear.
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I agree with Bob. Also for service entrance conductors you would not have an EGC.
 
iwire said:
I would not count the neutral, you don't have to.

For a building service it would be most unusual for the majority of the load to be non-linear.
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I also agree with this. I have yet to have to count a wye grounded service conductor as a CCC, and can't wait for the day I get to run my first super neutral. I was trying to make the point that I now take a much closer look at the loads served than I did say 10 years ago. I threw out 310.15(B)(4)(c) because it could apply, thats why it is in the NEC. If you are debating with the boys, bring this up.
 
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