Slotting Enclosures

tom baker · Jul 28, 2017

NEC allows slotting between individual holes to minimize inductive heating....

300.20 Induced Currents in Ferrous Metal Enclosures or
Ferrous Metal Raceways.(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

Has anyone ever done this?
Is it a listing issue with the enclosure to cut slots
And the slots make the enclosure "open", could be a safety issue?

The other option is
(2) passing all the conductors thru an insulating wall

texie · Jul 28, 2017

tom baker said:
NEC allows slotting between individual holes to minimize inductive heating....

300.20 Induced Currents in Ferrous Metal Enclosures or
Ferrous Metal Raceways.(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

Has anyone ever done this?
Is it a listing issue with the enclosure to cut slots
And the slots make the enclosure "open", could be a safety issue?

The other option is
(2) passing all the conductors thru an insulating wall

For the few times I've needed to do this I prefer to use a square or rectangle aluminum plate bolted on the 4 corners over a cutout in the enclosure. Then use a KO punch in the plate as needed. To me it is easier and you don't end up with structural integrity issues.

jap · Jul 28, 2017

texie said:
For the few times I've needed to do this I prefer to use a square or rectangle aluminum plate bolted on the 4 corners over a cutout in the enclosure. Then use a KO punch in the plate as needed. To me it is easier and you don't end up with structural integrity issues.

Man,
That's good thinking.

JAP>

GoldDigger · Jul 28, 2017

You could also use brass or stainless for the plate if appropriate. But for really high current you might need a non ferrous cable clamp too.

Sent from my XT1585 using Tapatalk

infinity · Jul 28, 2017

tom baker said:
NEC allows slotting between individual holes to minimize inductive heating....

300.20 Induced Currents in Ferrous Metal Enclosures or
Ferrous Metal Raceways.(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

Has anyone ever done this?
Is it a listing issue with the enclosure to cut slots
And the slots make the enclosure "open", could be a safety issue?

The other option is
(2) passing all the conductors thru an insulating wall

Do you have an example of where you would use these slots?

gadfly56 · Jul 28, 2017

infinity said:
Do you have an example of where you would use these slots?

If you ran only 2 legs of a 3-phase system into an enclosure? Don't know why you would, but anything that can happen, will happen.

infinity · Jul 28, 2017

gadfly56 said:
If you ran only 2 legs of a 3-phase system into an enclosure? Don't know why you would, but anything that can happen, will happen.

I'm trying to envision where this would be applicable I'm sure that Tom has a particular installation in mind. Several non-ferrous raceways might be one application but then you would still need non-ferrous locknuts.

kwired · Jul 28, 2017

gadfly56 said:
If you ran only 2 legs of a 3-phase system into an enclosure? Don't know why you would, but anything that can happen, will happen.

two legs of a three phase system would still be carrying equal current but with opposing magnetic fields. We do this all the time with say a 208 volt single phase branch circuit.

When you would need the slots is if you ran one circuit conductor through one hole in the enclosure and the other circuit conductor through another hole. You then would have inductive heating effects on the surrounding ferrous metal.

Cut a slot between the two holes and you effectively make them one hole and both conductors are now in that hole.

About the only time you would run into needing to do this is with single conductor cables of some sort.

GoldDigger · Jul 29, 2017

kwired said:
two legs of a three phase system would still be carrying equal current but with opposing magnetic fields. We do this all the time with say a 208 volt single phase branch circuit.

When you would need the slots is if you ran one circuit conductor through one hole in the enclosure and the other circuit conductor through another hole. You then would have inductive heating effects on the surrounding ferrous metal.

Cut a slot between the two holes and you effectively make them one hole and both conductors are now in that hole.

About the only time you would run into needing to do this is with single conductor cables of some sort.

When you run two out of three phases the fields for line-neutral loads are 120 degrees out of phase, and so their magnetic fields do not cancel. But the field from the neutral will add to cancel.
It is correct but not necessarily obvious that line-line load currents will cancel even though the voltages are not 180 degrees out of phase.

Sent from my XT1585 using Tapatalk

tom baker · Jul 29, 2017

infinity said:
Do you have an example of where you would use these slots?

conductors in parallel where they are run phase a in one raceway, phase b in one raceway...

kwired · Jul 29, 2017

GoldDigger said:
When you run two out of three phases the fields for line-neutral loads are 120 degrees out of phase, and so their magnetic fields do not cancel. But the field from the neutral will add to cancel.
It is correct but not necessarily obvious that line-line load currents will cancel even though the voltages are not 180 degrees out of phase.

Sent from my XT1585 using Tapatalk

120 degree phase difference is when referencing to the neutral.

If you are not referencing the neutral you still have 180 angle between the two points involved don't you?

Only reference to neutral that would involve current flow on a 208 volt two wire load would be during ground fault conditions.

Slotting Enclosures

tom baker

First Chief Moderator & NEC Expert

texie

Senior Member

jap

Senior Member

GoldDigger

Moderator

infinity

Moderator

gadfly56

Senior Member

infinity

Moderator

kwired

Electron manager

GoldDigger

Moderator

tom baker

First Chief Moderator & NEC Expert

kwired

Electron manager