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#13201 08/27/02 08:37 PM
Joined: May 2002
Posts: 202
J
Member
I heard a story at work today that a guy that worked at our shop a while back before i was hired. Ran 2 wires of a 3phase system in one section of conduit and the third wire in another section of conduit about a foot or less apart and when he threw the switch the system shook the conduit apart and it fell apart before they got it shut down or it tripped out. that was the last time he ran electric they said. is this due to the magnetic effects and the phases or is this bull. i have 8 yrs of experience but i have never heard of this. but i always run my wire to code also.

Joined: Apr 2002
Posts: 914
E
Member
I've heard of similar stories. One that sticks in my head happened at Turner Field(Atlanta Braves). They kept having severe problems( don't know the specifics) and the problem turned out to be 3 conduits each with 3 wires. The problem was each conduit had only one phase each(3 parallel A's in 1, 3-B's in another & 3-C's in another).

Joined: Jan 2002
Posts: 324
A
Member
THIS IS TRUE! A new company (first and last big job) wired a Kwickset doorknob factory down here and thought it would be easier to keep each set of phase feeders in their own conduit. Blew the whole back wall out of the plant!

And yes this is caused by the magnetic field that is generated by the conductor as current flows through it.

[This message has been edited by arseegee (edited 08-27-2002).]

Joined: Aug 2002
Posts: 25
J
Member
One big electro magnet, works like a big solenoid.

Joined: Aug 2002
Posts: 402
J
Member
I can remember some years back assiting in a 800a 3 phase plant service install. The feeders wires were run in pairs into the bottom of the panel. We had to do it something like this I think.
a b c
c b a
And cross them back inside the panel. The wire was 250 kcm I think and bending was a real pain in the butt. The guy leading the job said this is nuts and said just go
a b c
a b c
Inspector cam back and no way magnetic fields need to be canceling so wires don't flex.

Joined: Oct 2001
Posts: 597
E
Member
In addition to the electromagnet effect, the magnetic field off the conductors, as it moves out and collapses 120 times a second, combines with the steel of enclosures and raceways to act as a generator. "Eddy currents" are generated in the steel. Higher eddy current creates higher I²R losses in the steel, turning the steel into an electric heater, melting insulation at pressure points. . .and BANG!


Al Hildenbrand
Joined: Oct 2000
Posts: 2,749
Member
See the 2002 NEC, where this concept is allowed in Exception No. 2 of 300.5(I) Conductors of the Same Circuit.

All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors shall be installed in the same raceway or cable or shall be installed in close proximity in the same trench.

Exception No. 2: Isolated phase, polarity, grounded conductor, and equipment grounding and bonding conductor installations shall be permitted in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic sheath in close proximity where conductors are paralleled as permitted in 310.4, and where the conditions of 300.20(B) are met.


Joe Tedesco, NEC Consultant
Joined: Oct 2001
Posts: 597
E
Member
Following the 2002 NEC 300.5(I) Ex. 2, the NEC Handbook adds the folowing:
Quote
Isolated phase installations contain only one phase per raceway or cable. The spacing between isolated phase raceways and cables should be as small as possible and the length of the run limited to avoid increased circuit impedance and the resulting increase in voltage drop inherent in an installation involving ac circuits. Isolated phase installations may be used in ac circuits to limit available fault current at downstream equipment.
Isolated phase installations present an inherent hazard of overheating, a risk that must be understood and carefully controlled. This hazard results from induced currents in metal surrounding a raceway that contains only one phase conductor. {See 300.20(A) and 300.20(B) for more information on induced currents in raceways.} The surrounding metal acts as a shorted transformer turn. In underground installations, a single conductor is unlikely to be installed in a metal raceway or, if it were, it is unlikely to present a fire hazard. This is not true, however, for aboveground raceways, and it is the reason isolated phase installations have limited application for aboveground installations.

[This message has been edited by ElectricAL (edited 08-29-2002).]


Al Hildenbrand
Joined: Oct 2000
Posts: 2,749
Member
Does anyone have any installation information related to the permission in the exception to the 2002 NEC rule mentioned above?


Joe Tedesco, NEC Consultant
Joined: Apr 2002
Posts: 2,527
B
Moderator
Joe, I have only anecdotal information. Apparently from an engineering standpoint, it is acceptable for medium-voltage circuits limited to ~200 amperes. Used with precast-concrete manholes, there is a small problem with ferrous-metal reinforcing cloth, but for cast-in wire diameter of ~3/16-inch, wall heating between adjacent raceway penetrations is minimal.

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