Looking at the service wires for a 20 unit multifamily dwellings, all fire rated units. To cover the 1600A, I'm looking at 4 - 600kcmil at 420A = 1680A, looks OK until I put in the derating for the 4 current carrying condutors; then I come up with, using THHN wire, 475 x .8 = 380A per wire x 4 = 1520A, not enough. I'm limited to only 4 lugs on my service MCB so now doing the math for 700kcmil THHN wire I get, 520 x .8 x 4 = 1664A and can now cover the 1600A MCB. I really hate using the 700kcmil wire, but it looks as I have no choice here. Anybody agree or disagree. It's a 1600A 3PH 4 wire service.
1600A service
- Thread starter Btrox
- Start date
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bphgravity
Senior Member
- Location
- Florida
Why would be required to derate? Each of the 4 raceways will only contain 3 current carrying conductors and a grounded (neutral) conductor.
bphgravity
Senior Member
- Location
- Florida
Are you planning on installing the paralleled sets per the exception to 300.3(B)(1) or will each set be installed in a single raceway?
Read my first post again and think about the installation.
Read my first post again and think about the installation.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
4-sets of parallel conductors should be run with Phase A, B, C, N grouped per conduit. Don't run all A in one, B in another, and C in another.
With this being a multifamily dwelling occupancy do you have to count the neutral as a current carrying conductor? Now if it was an office building chuck full of computers and variable frequency drives on air handlers, etc. etc. yes you would. I guess this comes down to 310.15(B)(4)(a) versus 310.15(B)(4)(c)?
wirenut1980
Senior Member
- Location
- Plainfield, IN
celtic said:
Ideally, yes. In King's defense, he did say "should."
This will be run in PVC conduit so I will be running all phase conductors in one pipe, A - B - C & N for the service. This is OK as long as it's in PVC to prevent the overheating, I know it at first it looks wrong but it's OK to do it in PVC. 300.20A. As for the current carrying conductors, it's a 3PH 4 wire system, look at 310.15b4. Neutral conductor can carry the load of the ungrounded conductors, I realize the system should be balanced but will still have 4 current carrying conductors in each pipe. The distance is about 15ft into the building where the service will sit, all underground piping. I will need 4 condutors/PH to carry the load so right there puts me into derating if I'm looking at it right. What size conductors to use?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I think you need to read that section again. See in particular 310.15(B)(4)(a). You do not have to count the neutral. I will carry ONLY the unbalanced current from the phase conductors, so it does not have to be counted. I don?t expect a residential unit to have a significant percentage of harmonic loads.Btrox said:
You are certainly free to do more than the code requires. In this instance, the NEC does not require derating.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
celtic said:
Very clever!!!
Btrox
Be careful installing the Isolated phases...the utility company will have plenty to say about this installation. I know that the utility company here is not happy with Isolated phases and tends to shun against those installations.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
You do realize that, as a Moderator, I could go back and edit my typo and make it look as if it had never happened, then delete the post in which you "couldn't resist" pointing it out. I might even find a way to edit your post, to make it look like your typo. :grin: :grin:celtic said:
P.S. An amp weighs two fig Newtons, that's watt. Do you know your weight in Newtons?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Don't forget to read 300.20(B).Btrox said:
I'm still curious about why you want to group each phase's paralleled conductors, rather than splitting them, which would eliminate the derating.
First, my weight in Newtons last I checked I was about 950N. I put each phase together in a single pipe so when they come into the service teminal box you avoid all the criss-crossing of conductor when terminating. I realize the Neutral may or may not be a current carrying conductor, in a 3PH 4 wire Y system is isn't, but I still have 4 current carrying phase conductors in my 4" conduit..................this is only if I put all phases together in a single conduit, A-B-C phase and a Neutral conduit..... The light just went on with all the confusion. If you put each phase conductor into a conduit, (as previously stated), with the Neutral then I agree the Neutral isn't a current carrying conductor and you only have 3 current carrying conductors and no derating is neccessary. I'll have to weight the pros and cons of each and either use 4- 600kcmil or 4 - 700kcmil. I hope I didn't confuse everybody too much by throwing all phase conductors into one pipe, it does make a nice clean look coming out, but only in PVC. Does that clear it up?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I think we had it already. If you decide to keep the paralleled conductors in one pipe, don't forget the section I mentioned before, about cutting slots hole-to-hole in any metal you run through, for the same reasons as metal conduit being un-doable.
I'd still mix them up and put up with the criss-crossing, and skip the derating issues.
I'd still mix them up and put up with the criss-crossing, and skip the derating issues.
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
The issue of having the phases separate is the generation of magnetic fields. Now using PVC means that there are do inductive heating effects of conduit to consider, but the issue is wider than that; a TV in the wrong place could be interfered with by those magnetic fields, for example.
Creating unnecessary EMF fields is not good practice.
Creating unnecessary EMF fields is not good practice.
This system will be installed in a heated garage in a electrical room under the 20 unit Condo complex. I've done this before in the engineering company I worked at and they signed off. It does seem a little strange when you look at it and I've even had one engineer from Canada say they couldn't, (or wouldn't) do it, but the only place in the code is 300.20 A & B that I can find anywhere that talks about the only reason you can't is because of heat effect, if you watch out for that your fine.... you can also use an insulated wall barrier instead of cutting the slots. It's really nice when your using big wire and landing them into a tight place. Also many manufacturing plants have been done this when you run your PVC under the floor and up into switchgear, I've had no problems with it. I would really like to use 5 - 500kcmil but the limitation with the 4 hole connection at the breaker forces me to up the wire size, so I will most likely criss-cross them and go done a wire size. The parallel argument always comes up when someone does the one phase system and I would like to here some more thoughts on it. I've seen it done quite a few times but it isn't the normal way you think about it when you see it for the first time, but I haven't really had a good argument from the code that you can't do it, except watch our for heat effect. I probably wouldn't ever do it on a large single family dwelling service, but industrial I would. Until tomorrow, thanks for all the help.
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