1 phase per conduit mandate

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There is a spec for non-metallic conduit over 800A.




THREE PHASE PADMOUNTED T
RANSFORMER SPECIFICATIONS
8
4.
Service conductors supplying 480Y/277 volts shall conform to the following
specifications:
a)
Radial services shall be limited to supplying one meter or one main
disconnect switch.
b)
Conductors shall be of type USE – 2, XHHW – 2, or RHW – 2.
c)
Isolated phase service run shall not exceed 50 feet.
d)
Services
up to 800 amperes
can be installed in metallic or non-metallic
conduit in an integrated phase configuration.
e)
For services

above 800 amperes
, all secondary conductor runs between
the transformer enclosure and the customer’s meter or main disconnect
switch shall be run in an isolated phase configuration in non-metallic

conduit.


Bold/underline text added by me.
 
mbrooke said:
Thank you, I missed it! :)

But why mandate 1 phase per conduit as apposed to 3?
Click to expand...

If you look at this thread and especially towards the end we had massive discussion on the topic.....

Isophasing is allowed by NEC but there is a strong culture that is opposed to it.

Like you, my position is that it makes for beautiful wiring.

Start at about page 5 and you will get every possible opinion on this topic......

http://forums.mikeholt.com/showthread.php?t=178796
 
mbrooke said:
Thank you, I missed it! :)

But why mandate 1 phase per conduit as apposed to 3?
Click to expand...
The requirement to use an isolated phase installation only applies to services over 800 amps. For a larger service, it make a much neater installation and it is much easier to make the conductors of one phase the same length when you put them in one conduit.
 
dionysius said:
If you look at this thread and especially towards the end we had massive discussion on the topic.....

Isophasing is allowed by NEC but there is a strong culture that is opposed to it.

Like you, my position is that it makes for beautiful wiring.

Start at about page 5 and you will get every possible opinion on this topic......

http://forums.mikeholt.com/showthread.php?t=178796
Click to expand...

It makes geometrically beautiful wiring, thanks to being able to group the phase conduits with the phase busbars, but magnetically ugly wiring.

Concentrating simultaneous/same-direction current, means that you amplify the magnetic field surrounding the conductors. Due to AC's time variance in current, it also has heating effects in all iron-based material around it. If you can remain in nonmetal conduit with nonmetal locknuts, and have only bottomless enclosures where you don't need to slit between the openings, you can do it according to the NEC.

I'm uncomfortable with being required to cut a slit between openings in the bottom of steel enclosures for isophase installations, because it causes you to lose the enclosure's mechanical integrity. I'm aware that is what you have to do to reduce the undesired magnetism effects, and it is good that most equipment in ampacities 1000A and larger will have an open bottom anyway. Switchboards, transformers, transclosures, etc..
 
Carultch said:
I'm uncomfortable with being required to cut a slit between openings in the bottom of steel enclosures for isophase installations, because it causes you to lose the enclosure's mechanical integrity.
Click to expand...

I have never seen that method used.

If I had to do an isolated phase installation through a steel enclosure I think I would opt for cutting that section of steel out entirely and installing a nonferrous replacement as we do with MI cable. :)
 
Carultch said:
It makes geometrically beautiful wiring, thanks to being able to group the phase conduits with the phase busbars, but magnetically ugly wiring.

Concentrating simultaneous/same-direction current, means that you amplify the magnetic field surrounding the conductors. Due to AC's time variance in current, it also has heating effects in all iron-based material around it. If you can remain in nonmetal conduit with nonmetal locknuts, and have only bottomless enclosures where you don't need to slit between the openings, you can do it according to the NEC.

I'm uncomfortable with being required to cut a slit between openings in the bottom of steel enclosures for isophase installations, because it causes you to lose the enclosure's mechanical integrity. I'm aware that is what you have to do to reduce the undesired magnetism effects, and it is good that most equipment in ampacities 1000A and larger will have an open bottom anyway. Switchboards, transformers, transclosures, etc..
Click to expand...

I agree, which leads me to ask: isn't voltage drop also greater?
 
iwire said:
If I had to do an isolated phase installation through a steel enclosure I think I would opt for cutting that section of steel out entirely and installing a nonferrous replacement as we do with MI cable. :)
Click to expand...
Can you elaborate on that? I have wondered about this. What would you use for material and where would you get it? I would be a bit paranoid about getting called on a 110.3 "listing violation"
 
electrofelon said:
Can you elaborate on that? I have wondered about this. What would you use for material and where would you get it? I would be a bit paranoid about getting called on a 110.3 "listing violation"
Click to expand...

Getting a brass plate isn't that difficult, and as far as a listing violation isn't it the same as when a cutout is made for MI cable, knock outs, or flanging a wireway to an enclosure?
 
tkb said:
Getting a brass plate isn't that difficult, and as far as a listing violation isn't it the same as when a cutout is made for MI cable, knock outs, or flanging a wireway to an enclosure?
Click to expand...

That would work. Aluminum too, easy to get. I was mixing up "non-metallic" and "non-ferrous". I am not saying I think it would be a 110.3(B), just that I would be paranoid an inspector could hassle me.
 
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