Single conductors passing through steel enclosures

[rascal 26]

I am being ask to install parallel runs of 373 DLO on a cable tray to feed a 4000 amp free standing breaker. The cables will be attached to the gear with aluminum gland connectors.
Article 300.20 (B) requires aluminum or some other non ferrous metal for this application. Others are saying if each set is installed in a group it satisfies the code.
I have ask if I could install an aluminum gland plate for cable connection, but was turned down. Anyone have any suggestions?

 

[SceneryDriver]

Cut a slot between each hole, to effectively turn it into one large hole. 4000A will make a nice induction heater if the single conductors pass through the ferrous enclosure in individual holes.


SceneryDriver

 

[synchro]

And so you'd split up all the paralleled conductors into individual groups containing phases A, B, C, and N (if applicable) using a single conductor from each set, and then have the gland connectors arranged like in the top of the picture below. You'd need slots between the gland connectors in each such group so the magnetic fields of A, B, C, N will cancel within the larger hole formed by these slots (as SceneryDriver has noted). But having additional slots between one group of A, B, C, N and another group would have marginal benefits.

 

[rascal 26]

Thanks

 

[SceneryDriver]

And so you'd split up all the paralleled conductors into individual groups containing phases A, B, C, and N (if applicable) using a single conductor from each set, and then have the gland connectors arranged like in the top of the picture below. You'd need slots between the gland connectors in each such group so the magnetic fields of A, B, C, N will cancel within the larger hole formed by these slots (as SceneryDriver has noted). But having additional slots between one group of A, B, C, N and another group would have marginal benefits.

If the environment and spec allows, non-metallic cable glands and lock nuts would be even better. 373 DLO is just over 1" in diameter; cable glands (sometimes called domenuts) are easy to find in that size.


SceneryDriver

 

[rascal 26]

Yes I agree the non metallic would be better. The problem is that I have to match existing installations (hundreds of units) and I was searching in the code and other sources for something I had missed with this type of installation.

 

[roger]

I have ask if I could install an aluminum gland plate for cable connection, but was turned down. Anyone have any suggestions?

Out of curiosity who turned it down?

 

[rascal 26]

The project managers. I sent a copy of the code and was told not to change anything.

 

[roger]

Are these electrical project managers? If they are they need some theory education. Even though the NEC addresses the installation the designers and electrical PM's should know what the problem is and how to avoid it without needing direction from the NEC.

 

[rascal 26]

I think one is, they probably just don't want to rock the boat.

 

[synchro]

Gland connectors made of aluminum should not really be a problem in this case. The slots between the holes are for blocking induced magnetic flux from circulating around an individual conductor in a closed magnetic circuit. Although a metallic gland connector appears like it might be a "short" across the slot for this magnetic flux, it really isn't because the permeability of aluminum is quite small (nearly identical to that of air or free space).

Permeability in magnetic circuits is analogous to conductivity in electric circuits. And therefore induced magnetic flux will concentrate in areas with high permeability like steel instead of aluminum. Such magnetic flux will then create circulating eddy currents and produce I²R losses in the steel, and therefore heating. But the electrical resistivity of aluminum is substantially less than that of mild steel. And so any eddy currents that might get induced in aluminum would create proportionally less I²R heating. Aluminum also does not have any hysteresis effects like in ferrous metals that would produce heating.

 

[roger]

The gland connectors are not the problem, it's the individual holes in the steel enclosure.

 

[synchro]

The gland connectors are not the problem, it's the individual holes in the steel enclosure.

I was really responding to posts #5 and #6 that non-metallic gland connectors would be better than aluminum ones in this particular situation.

 

[roger]

I was really responding to posts #5 and #6 that non-metallic gland connectors would be better than aluminum ones in this particular situation.

Ohhhhh, I see

 

[infinity]

We use stainless steel plates and knock out the steel behind the plate to allow the locknuts to only touch the stainless.

 

[wwhitney]

We use stainless steel plates

If I understand correctly, that depends (physics wise) on using the correct type of stainless steel, I assume one of the kinds a magnet won't stick to.
And it wouldn't meet any NEC requirements that use the term "non-ferrous".

Cheers, Wayne

 

[infinity]

If I understand correctly, that depends (physics wise) on using the correct type of stainless steel, I assume one of the kinds a magnet won't stick to.
And it wouldn't meet any NEC requirements that use the term "non-ferrous".

Cheers, Wayne

It's the accepted method by the local AHJ. Personnally I wanted to see slots cut between the holes but we were told that it wasn't necessary due to the type of metal plates used.

 

[SceneryDriver]

Yes I agree the non metallic would be better. The problem is that I have to match existing installations (hundreds of units) and I was searching in the code and other sources for something I had missed with this type of installation.

Sounds like you have an opportunity to correct lots of improperly done installations. It's not right just because someone did it wrong hundreds of times.


SceneryDriver

 

[rascal 26]

That's why I've been searching to be sure there wasn't some kind of exception or some other code I didn't know about. I will make my connection plates from aluminum and paint to match existing. They probably won't notice the difference.

 

[synchro]

An aluminum plate allows you to mount individual gland connectors without restriction, but the steel enclosure under the plate still has to meet the requirements of 300.20(B).
Some options:
1. Pass all of the conductors though one single hole in the steel.
2. Pass all of the conductors in each set through its own hole in the steel. Slots must be cut in between these larger holes. This would be like in the bottom of the picture below but without the locknuts. You could use a single aluminum plate that covers all of the holes and slots.
3. Split up the conductors into individual groups, where each group contains all of the phases A, B, C, and N (if applicable) and it passes through its own hole. Each hole should have an equal numbers of conductors taken from each phase. Therefore their magnetic fields will cancel, and so no slots between the holes are necessary. If desired, the single "hole" for each group could be made using individual smaller holes with slots cut between them, as in the bottom of the picture below.
For best symmetry, I think it would be best to split up the paralleled conductors into groups of A, B, C, N, where each group has the same number of conductors as another. This would not be possible when there's an odd number of conductors in each set. But I don't think this is going to make any significant difference. 310.10(G)(3) requires that the number of conductors run in separate cables or raceways must be the same in parallel installations, but says nothing about conductors that are run in separate holes.