High amperage parallel power feeders phase wires run in separate conduits AAA-BBB-CCC-NNN??

[bdn2004]

A Contractor was to splice two 1000 Amp busways together. The busways are about 100 feet apart. He used conduit but put each phases conductors in a separate conduit. I'm aware of the NEC Handbook that says it's essential to run A-B-C-N in each conduit, but it doesn't really state it like this in the actual NEC. Is there a big problem here?

 

[jraef]

Possibly. Read the NEC, article 300.20(B) Depending on whether the conduit is magnetic or not, it could allow inductive coupling to take place and current to flow in the conduits. In addition where the conduits enter or leave something made of magnetic material (i.e. steel) there could potentially be magnetic heating that could start a fire. Other than that though, it's technically not "illegal" as long as you do things like cutting slots in steel enclosures to decouple the magnetic fields and use non-magnetic (i.e. aluminum or PVC) conduit or use a special non-magnetic penetration plate where the conduits enter and leave steel enclosures.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[waross]

If it is ferrous conduit, just set your coffee pot on the conduits. The coffee will be ready long before you can prepare the order for the switches and bus connections to replace the heat damaged equipment now in place and cooking.
I have seen this several times with much less current.
A 400 Amp switch destroyed by heat corrosion by a 200 Amp load.
I have seen the paint burned off of a buss-way with uneven current.
In the Canadian Code it is called "Magnetic Encirclement".
In the Canadian Code steps must be taken to mitigate the effects when the unbalance is greater than 200 Amps. Personally I think that that limit is a little high.
That is why single conductor cables are entered into panels through aluminum plates or non-conducting plates.
If the conduits are non-ferrous, there may still be issues with higher than needed reactive impedance and excess voltage drop.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

See NEC 310.10(H)(2), particularly requirement (6).

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bdn2004]

Wow. This isn't our project - we were just having to document this change to the system. They've not flipped the switch yet. There's little choice here but to bring it to a higher authority. I've got no problem doing that but citing a Code section violation is eay easier to pass on than "best practice" tips where you're seen as meddling.

 

[waross]

What is the material of the conduits? If the conduits are ferrous, the problem will soon immediately become obvious.
If the conduits are non-ferrous it will be more an issue of inductive reactance and higher than expected voltage drops.
This rule will prohibit the installation in Canada:

12-108 Conductors in parallel (see Appendix B)
(1) Ungrounded and grounded circuit conductors of similar conductivity in sizes No. 1/0 AWG and larger,
copper or aluminum, shall be permitted to be installed in parallel sets provided that each parallel phase or
grounded conductor set is individually comprised of conductors that are
................
(4) The orientation of single-conductor cables in parallel, with respect to each other and to those in other
phases, shall be such as to minimize the difference in inductive reactance and the unequal division of
current.

Is there a similar rule in the NEC?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

I'm not sure what the conduits are made of, I was assuming rigid steel as most of the plant , I'll find out.

 

[waross]

If the conduits are ferrous you only need to send a short memo questioning the installation. That will cover your ASSets. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

I found out today that we wrote it down wrong, there's only 3 conduits, and it's installed correctly. So false alarm but it was worth it for the learning experience here.Thanks again.