Parallel feeders single phase

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wayne123

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I was invited to donate some of my free time to a local school to help wire a new athletic complex. There are a number of local electricians involved in the project and we are all there at different times of the day( haven't even seen some of them). Anyway, one electrician built the service the other day 120/240V single phase 400 A service. He ran parallel feeders from the meter base to the 400A panel. All looked good to me until I noticed that he had run both A phase conductors in one conduit and both B phase in the other conduit. I thought that with parallel feeders A@B run together. Am I correct? I have never delt with parallel single phase feeders before and was wondering if it is different than 3 phase parallel feeders.
 
Re: Parallel feeders single phase

And which conduit did he put the Neutral conductors in? Look at 300.3(B) and 300.3(B)(1).

[ May 13, 2005, 09:13 AM: Message edited by: infinity ]
 
Re: Parallel feeders single phase

Originally posted by wayne123:Am I correct? I have never delt with parallel single phase feeders before and was wondering if it is different than 3 phase parallel feeders.
Click to expand...
You are. It's not.
 
Re: Parallel feeders single phase

Wayne123
Charie is correct. He should have ran A,B & the Neutral in one conduit and the same in the other conduit. All wires should be the same length. You might still be able to fix this problem how he has it ran. If you have a neutral in each pipe you can simply take the pipe with the 2 A phases and now use one as B. Now you will have A&B in that pipe. You will have to still change the other pipe with the B phases also. Hope this will help.
Jim
 
Re: Parallel feeders single phase

I had an incident like this once before:

Basic Electromagnatism
In a phase conductor, there is a magnetic field around that conductor. If you wrap that conductor around a metal core, OR if you encircle the conductor with metal, you end up with an electromagnet.

A hot Conductor (ungrounded) has that potential to create electro magnets. However when we run the neutral (grounded) that also has electromagnetic field, but the two cancel each other out. Therefore, no electromagnet is created.

We are allowed to run multiwire branch circuits (3 or 4 wire networks) because of the same principle, that all the electromagnetic fields will cancel the others out, and if all wiring is correct, no one conductor would have current without any others.

If all conductors (for parallelling) for the same phase are in the same conduit without a balance, then you would create a very large electromagnet, which creates vibration and heat, and will start a fire.

In my case the 'foreman' at the time said that it would be OK since we ran PVC conduit. FORGETTING that the metal lockring and metal cabinets would CHOKE the heck out of that conductor.
 
Re: Parallel feeders single phase

It is my understanding that a slit can be cut into the wire way or junction box that the conduit terminates if all the same phase wires are in the same conduit. I personaly think it is a hack way to fix the problem. Would this also apply to parallel feeders as described, though the neutrals are in each raceway? Thank you.

Justin W.
 
Re: Parallel feeders single phase

Originally posted by justinjwalecka:
Would this also apply to parallel feeders as described, though the neutrals are in each raceway?
Click to expand...
If you have line-to-line loads, then the neutral would not be bearing current. That's why both (or all three) phases and the neutral must be in the same conduit. ;)
 
Re: Parallel feeders single phase

Isolated phase, isolated neutral, isolated equipment ground, and any combination of the above are allowed with NONMETALLIC underground conduits that are in the same duct bank. This makes it easier to make the conductors that are on the same phase the same length in some instances.

However, you have to cut slots between the holes in any steel box wall to eliminate induction heating. A good idea would be to grind the sharp burr(s) off of the slot(s) and then fill the slot(s) with red type 1 room temperature vulcanizing silicone sealant to keep out bugs and to alert the inspector that you did indeed take care of any induction heating problem.
 
Re: Parallel feeders single phase

Mc5w,
A good idea would be to grind the sharp burr(s) off of the slot(s) and then fill the slot(s) with red type 1 room temperature vulcanizing silicone
Click to expand...
what is the deal with you and silicone? :D

Roger
 
Re: Parallel feeders single phase

Roger: :D

Mike: I thought the slots were to allow air to pass freely over the conductors and cool them? If you plug the slots with silicone, isn't that defeating the purpose? :confused:
 
Re: Parallel feeders single phase

As everyone said, the phases and neutral should be in the same conduit.

A colleague of mine was once called to a university campus to investigate excessive heating on a classroom floor. He found out that a contractor installed something similar...but of much higher ampacity, about 1,000AMPS. The heat generated was so much that the vinyl tile adhesive was simply melting and ozzing out between the tiles and the room was very hot, even with the air conditioning going full blast.
 
Re: Parallel feeders single phase

Molotov, if the rules of theException to 300.3(B)(1) are followed which takes us to 300.20 (pointed out by Bob) there is nothing wrong with individual phases in their own raceways.

Now this being the case, If these parallel runs are in fact in PVC, I can't see where there would be any problem with the instalation described by Wayne other than the fact it is not an everyday run of the mill installation.

George, cutting slots are to reduce to effect of inductive heating, this is not really a result of air flow. When I have used this method, it has always been in open bottom (floor mounted)switchgear.

Roger
 
Re: Parallel feeders single phase

Bottom line.
It is incorrect & must be corrected. I agree with Kiloamp7.A ,B and the neutral must be ran together. No cutting corners to save a buck.
Bye now,
Jim
 
Re: Parallel feeders single phase

James,
Originally posted by james wuebker:
Bottom line.
I agree with Kiloamp7.A ,B and the neutral must be ran together. Bye now,
Jim
Click to expand...
they are run together, just in seperate raceways. If they are in PVC there is no problem, like I said, I have done it. Have you read 300.3(B)?

Roger
 
Re: Parallel feeders single phase

I went through all of my reference books to look this up and was quite surprised that only one of the books explained this in detail.
Soares Book explains this and also has an illustration.
For underground nonmetallic raceways installed in parallel installation, the phase conductors and the grounded (neutral) conductor all have to be installed in their own raceway. This makes sense to me from all that I have learned.
 
Re: Parallel feeders single phase

Roger, I read 300.3(B) then looked at 300.3(1). It does stay that you can run them like you explained if it is an underground run. I think he's running this somewhere inside the building which what you are staying doesn't apply now. If you can show me I'm wrong please let me know. A good learning process for this old guy.
Thanks!
Jim
 
Re: Parallel feeders single phase

In the case of running the underground installations as described in PVC, these raceways DO NOT enter a metal cabinet throug a knock out or with the use of a metal lockring. These installations are ONLY into open bottom switch gear.
That way you do not encircle the phase conductor with a metal circle.

Again, basic electromagnatism. That is what a current transformer (CT) is, a metal circle around a phase conductor.
 
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