Free Steak Dinner Bet - please help

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GoldDigger said:
I see the raceway as a single turn inductor core, not a transformer with a shorted secondary. Heating in the conduit is the result of hysterisis losses, not current flow.

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winnie said:
That is a good point, though I think it is fair to say that both aspects are important.

Ferrous conduit will act like a core and you will see hysteresis losses. But if the structure of the conduit is such that it is connected at both ends, then you have a conductive loop acting as a shorted turn.

-Jon
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There could also be some losses from eddy currents circulating in the conduit that are induced by the AC magnetic fields surrounding the conductor. But I think this would be a smaller effect than the loss mechanisms already mentioned.
 
The conduit must be nonferrous but it must also be in underground

Exception: Conductors installed in nonmetallic raceways run underground
shall be permitted to be arranged as isolated phase, neutral,
and grounded conductor installations. The raceways shall be installed
in close proximity, and the isolated phase, neutral, and grounded
conductors shall comply with the provisions of 300.20(B).
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300.5(I) Exception applies to underground installations, but 300.3(B)(3) would apply more broadly. Seems to me that all of 300.5(I) is redundant with 300.3(B). The only difference I see is that (2020) 300.5(I) Exception 2 requires raceways to be nonmetallic, while 300.3(B)(3) doesn't differentiate between raceways and cables and just requires a nonmetallic or other nonmagnetic sheath. I'm not convinced the difference is intentional.

Cheers, Wayne
 
winnie said:
The legendary iwire posted about a 480V service in steel conduit that was installed 'isolated phase', and proceeded to burn up. I've not been able to find this post.
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I believe iwire is still here, using a different name.
 
Augie47 beat me to it... the BOX that they terminate in must also be non-ferrous....

PS: after the pic was posted:
...or,the penetration would have to be one big hole, not separate penetrations. This as shown would be OK.
 
Jraef said:
Augie47 beat me to it... the BOX that they terminate in must also be non-ferrous....

PS: after the pic was posted:
...or,the penetration would have to be one big hole, not separate penetrations. This as shown would be OK.
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Cutting slots between the holes effectively makes one hole for what we are trying to prevent from happening.

Also keep in mind if more than 3 conductors in a raceway ampacity adjustments must be made.
 
Another factor is that the entire raceway needs to avoid ferrous components completely. No steel bends, no steel locknuts, no steel stub-ups where subject to vehicle traffic, etc. The rule about not using steel locknuts could easily be overlooked, because it is common to use them in combination with PVC male adapters, rather than using plastic locknuts. Even passing the isolated phases through separate openings in a grid of steel rebar could introduce the issue of magnetising metal that isn't intended to be energized.
 
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