Induction

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steve11777

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I ask the question to my PM about induction does anybody have any thoughts on this set up? I think I need to cut a slot between each phase so there is no induction. This is going to be used for temp power for events at a collage. The idea is they would drag over cables with cam locks on the end to hook up power and be able to feed what ever event they would have going on.
 

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...no you dont need to cut slots, it like that at the terminating end where it is acceptable. Would we need to cut slots in every panel that has the neutrals grouped for terminating? (No)
 
steve11777 said:
I ask the question to my PM about induction does anybody have any thoughts on this set up? I think I need to cut a slot between each phase so there is no induction. This is going to be used for temp power for events at a collage. The idea is they would drag over cables with cam locks on the end to hook up power and be able to feed what ever event they would have going on.
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Would be a code violation, couldn't you use a PVC or alum. box ?

There will be heat generated there with a heavy enough load if one of those plastic insulators
melts and causes a fire you will be up the creek without a paddle.

Ronald :)
 
I don't understand the question about neutrals grouped at terminating. From what I can remember if your conductor run threw separate holes in metal you would get heat from induction. So if you where to cut a slot between the holes it would cancel out the induction and you would have no heat.
 
Factory setup

Factory setup

This setup is an all plastic housing. Probably for the reasons you stated.
 

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jusme123 said:
...no you dont need to cut slots, it like that at the terminating end where it is acceptable.....
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Sure you do.

NEC 300.20 said:
....
(B) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit.
Click to expand...
 
steve11777 said:
I don't understand the question about neutrals grouped at terminating. From what I can remember if your conductor run threw separate holes in metal you would get heat from induction. So if you where to cut a slot between the holes it would cancel out the induction and you would have no heat.
Click to expand...

In an experiment that was run here it took 250A to get a 10 degree C rise on the steel barrier.
 
I would cut out the bottom and use an aluminum plate. I have seen slots in theater company switches.
 
Here a set of slots connect the five conductor entries. You can see the steel plate that is on the underside of the enclosure which is needed due to the various prepunched KOs and weakening of the enclosure from the slots.
Camlocks with slots.png
Quick, dirty and cheap to match a tight budget.
 
Volta said:
Here a set of slots connect the five conductor entries. You can see the steel plate that is on the underside of the enclosure which is needed due to the various prepunched KOs and weakening of the enclosure from the slots.
View attachment 9086
Quick, dirty and cheap to match a tight budget.
Click to expand...

Just a small point, and one that can be ignored in the interests of simplicity in directing fabrication:
The EGC, the green wire in that picture, does not normally carry current and so would not be involved in any inductive heating effect. To the extent that it will carry fault current, that should not last long enough for heating to be a concern. The neutral, on the other hand, may be current carrying conductor under normal use.

Now if there were a ground-neutral bond at both ends of that wire run, the EGC could be a current carrying conductor.

Now the language of 300.20 states that the effect arises from passing a current carrying conductor through a ferrous plate, but then goes on to say that all of the circuit conductors must be isolated magnetically or go through the same opening. Either that was an oversight or was done for simplification. :)

PS: Again referring to the OP's photo, the inner box, without a cover, would need only a total of 4 or 5 slots, each cut from the terminal hole to the front edge of the box. But if the cover plate is added and is also ferrous, it would end up bridging those slots magnetically and causing problems. The issue is not circulating currents, but rather magnetic fields with hysteresis losses.
Using an aluminum cover plate would fix that.

PPS: And finally, mounting each individual terminal on a separate non-ferrous plate would not be sufficient if each conductor still passed through a separate larger hole in the ferrous enclosure.
 
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GoldDigger said:
Just a small point, and one that can be ignored in the interests of simplicity in directing fabrication:
The EGC, the green wire in that picture, does not normally carry current and so would not be involved in any inductive heating effect. To the extent that it will carry fault current, that should not last long enough for heating to be a concern. The neutral, on the other hand, may be current carrying conductor under normal use.

....
Click to expand...

I agree.
 
PS: Again referring to the OP's photo, the inner box, without a cover, would need only a total of 4 or 5 slots, each cut from the terminal hole to the front edge of the box. But if the cover plate is added and is also ferrous, it would end up bridging those slots magnetically and causing problems. The issue is not circulating currents, but rather magnetic fields with hysteresis losses.
Using an aluminum cover plate would fix that.
Click to expand...

So if cutting a slot from the cam lock hole out would fix the induction problem but installing the cover of the trough would create the problem again. So what if you where to cut the slots from cam lock to cam lock? Then the ferrous material would not be continues around the cam locks. Sorry if I messed up the quote thing still new at it.
 
steve11777 said:
So if cutting a slot from the cam lock hole out would fix the induction problem but installing the cover of the trough would create the problem again. So what if you where to cut the slots from cam lock to cam lock? Then the ferrous material would not be continues around the cam locks. Sorry if I messed up the quote thing still new at it.
Click to expand...
Cutting slots between the cam locks would be fine.
You can figure out the consequences for yourself as follows:

1. Draw a plan view of the penetrations/terminations.
2. Mark which ones are current carrying (or mark them all).
3. See whether you can draw a closed loop (not necessarily circular) around any one conductor OR any set of conductors less than the full number without having to cross a slot.
4. Figure that a ferrous plate bridging a slot counts as allowing you to draw the loop without crossing the slot.

In this case, it will be a bit harder to cut the lateral slots, but the results will be better.

BTW, the reason that I mention combinations of conductors being circled is the following:

Draw four conductors in a line, labelled A, B, C and D.
Cut a slot between A and B and another between C and D (two slots total). You do not have a continuous path circling any one of the conductors, but you do have a path circling A and B and another one circling C and D. Cut another slot between B and C and you are now OK. The only circle now encloses all four wires, and that is OK since the net current will be zero.
 
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