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Joined: Jul 2004
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The correct way is to run a complete set of current carrying conductors in each pipe.
As for the equipment bonding conductor it can be sized to each set of conductors in each pipe
250.102(C)Where the service-entrance conductors are paralleled in two or more raceways or cables, the equipment bonding jumper, where routed with the raceways or cables, shall be run in parallel. The size of the bonding jumper for each raceway or cable shall be based on the size of the service-entrance conductors in each raceway or cable.
Greg Fretwell
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That's it ... each phase, neutral, and ground in each run of pipe.
Last edited by renosteinke; 11/20/08 03:50 AM.
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Joined: Jul 2004
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I am curious just how "hot" this installation was actually running. A FLIR picture would be great but simply looking with a non contact thermometer or a touch with a wet finger would still be interesting. I know when I moved in here and really got a good look at the panel (behind a pretty wood chase) they had three service conductors entering through 3 romex connectors. I didn't see any particular heating.
Greg Fretwell
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I've always had a worry about splitting the wiring like that. In an ordinary circuit, if one wire comes loose, you will usually know it because things don't work. But with parallel conductors, one of them coming loose doesn't break things (yet). So the remaining conductors carry more of the load. If they are spread over multiple conduits, it causes things to move towards a scenario like in the photo.
Do you depend on really good workmanship to reduce the risk? Would alternate methods like compression terminals help?
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Joined: Jan 2005
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pdh, check your NEC. The code has plenty of detailed specifications as to how parallel feeds are to be done. Yes, you're right, good workmanship is key.
The primary reason for parallel feeds is that eventually wire gets too heavy, too stiff to work with.
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Joined: Jul 2002
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The fault is not with the wiring, it is with the cabinet, it is too small!
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Joined: Jun 2006
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I'm currently working a job (I just moved into training for line voltage, still VERY new) where they have 7 or 9 MC cables paralleled to a 5000A 480V delta switchboard from a 35kV xfmr. Very nice work, cable tray and RMC 20' in the air (heavy industrial site). If I read the ampacity tables correctly, not only does wire get too heavy, there just isn't really any wire to handle 5000A.
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I would think that if the transfer switch frame and the cabinet come together as a single unit, or if they are intended for each other, and are listed for use at the current levels involved, that they would have engineered it, and checked it for listing, for the appropriate wiring installed.
But it certainly looks to me like it is stuffed on the right side.
A closer look and I see the top feeds have 1 phase (brown) and what I assume is neutral in the leftmost conduit, and the other 2 phases (orange and yellow) in the 2nd conduit from left. What comes from or goes to the conduits on the right are not marked within the frame of the picture.
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Yes, pdh, that's ecactly what you're supposed to do per 300.20(A). That way, the magnetic fields of all the phases cancel out, and there is very little inductive heating of the conduit.
The key to avoid the mistake is to take the # of conduit into account when sizing/selecting parallel conductors. With 2 conduits, they should have selected 2 or 4 wires per phase, and not 3. In some cases, the # of conduits may be dictated by the size and # of lugs. (5 wires does not appear to be an option in this case, and the lugs may not support the larger conductors required for 2 wires per phase.) And yes, derating and conduit fill must always be considered.
Reno: NEC 2008 is not clear in 310.4(D) as to whether they want EACH ground wire to be sized for the OCP, or that all ground wires in parallel add up to the OCP. From the wording, I believe the total simply has to match 250.122, not each parallel ground wire.
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Joined: Mar 2004
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I've seen a much smaller version of this: A UPS distribution board with all the phases and neutrals separated out. The UPS was very heavily loaded; most of the circuits were less than 50A, but near capacity. The four nipples between the wire-way and the panel were so hot you couldn't keep your hand on them for more than about a second. I'd wager something like 130-140 degrees F. I'd be interested to see how hot this installation actually does get....  -John
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