I think it fair to say that it is common knowledge amongst sparks to direct all live conductors and associated protective conductor through a common aperture in a ferromagnetic enclosure. So I want to direct attention only to domestic installations where load currents will be relatively light so that I can understand better the consequences of not doing so in that particular environment. I refer to regulation 521.5.1 and item 4.17 on the condition report inspection schedule
There is also a lot of confusion out there about what is an eddy current loss - electric current and ohmic, and what is a magnetic hysteresis loss.
The problem with a unbalanced currents in conduit has two parts - magnetic, which would still be a problem if we sliced the tube into short cylinders, but would be removed it we slit the tube length-ways, and closed the gap with braze metal or even soft solder, and electric effects, where a voltage appears between the ends of the conduit, that is then shorted by other metalwork - often other conduits on the same boxes or plates, and where even if the tube was all brass we would still see the problem, and the solution is to cut the current path into more than once isolated cylinder.
(this is where folk can come unstuck with AWA singles and hot gland plates)
In a thin wall box, only the magnetic effects are seen, and unless the hole is a tight fit about the cable, and the cable is heavily loaded, the magnetic path is going to be long enough not to be present much additional loss. Making the hole bigger with a slot, (or to dogbone the holes for flow and return current)
or using a non-magnetic enclosure are the fixes. Again, slots can be filled with soft solder or braze, or at the cheap end car body filler or at the really cheap end, left open and covered with gaffer tape..
Even without any attempt to enlarge the magnetic path, a 100A tail through a 20mm hole typical galvanized back box will not get anything very warm, so in nearly all domestic with mainly 32A circuits and less it will be a non-issue. The problem is the magnetic losses of steels are very variable, and the sort of stuff used for back boxes is not optimised for that so it is hard to put safe upper limit, and folk like to be cautious.
(If you have an arc welder and some spare socket boxes you can try this for a bench demonstration wrapping the tails through the box holes a couple of times to get N times the current - In my experience, the only thing that gets far too hot is the welder itself and the cable in the normal ohmic sort of way - I have a cheap Chinese-made welder rated for about 10 minutes an hour at full power...)
It is up there with not going swimming after lunch as being far less of an issue than the old folk told you.
There is also a lot of confusion out there about what is an eddy current loss - electric current and ohmic, and what is a magnetic hysteresis loss.
The problem with a unbalanced currents in conduit has two parts - magnetic, which would still be a problem if we sliced the tube into short cylinders, but would be removed it we slit the tube length-ways, and closed the gap with braze metal or even soft solder, and electric effects, where a voltage appears between the ends of the conduit, that is then shorted by other metalwork - often other conduits on the same boxes or plates, and where even if the tube was all brass we would still see the problem, and the solution is to cut the current path into more than once isolated cylinder.
(this is where folk can come unstuck with AWA singles and hot gland plates)
In a thin wall box, only the magnetic effects are seen, and unless the hole is a tight fit about the cable, and the cable is heavily loaded, the magnetic path is going to be long enough not to be present much additional loss. Making the hole bigger with a slot, (or to dogbone the holes for flow and return current)
or using a non-magnetic enclosure are the fixes. Again, slots can be filled with soft solder or braze, or at the cheap end car body filler or at the really cheap end, left open and covered with gaffer tape..
Even without any attempt to enlarge the magnetic path, a 100A tail through a 20mm hole typical galvanized back box will not get anything very warm, so in nearly all domestic with mainly 32A circuits and less it will be a non-issue. The problem is the magnetic losses of steels are very variable, and the sort of stuff used for back boxes is not optimised for that so it is hard to put safe upper limit, and folk like to be cautious.
(If you have an arc welder and some spare socket boxes you can try this for a bench demonstration wrapping the tails through the box holes a couple of times to get N times the current - In my experience, the only thing that gets far too hot is the welder itself and the cable in the normal ohmic sort of way - I have a cheap Chinese-made welder rated for about 10 minutes an hour at full power...)
It is up there with not going swimming after lunch as being far less of an issue than the old folk told you.
