db/cu and 521.5.1 Ferromagnetic enclosures: electromagnetic effects

Unless you are throwing hundreds of amps down the CPC, in normal operation,  or the enclosure is a very odd very thick super lossy steel, technically this is a non issue.  you could put a stud-bolt and a pair of wingnuts on each side, such that you had a wire on either side of the box wall, and the bolt carrying the current was itself made of steel, and in intimate contact with the box, and still see a temperature rise dominated by the normal I2R considerations.

There are a few things in the regs that charitably suggest that a blanket rule has been made when the case it applies to is infrequent, or less charitably, the level of understanding of magnetics among the writers of some technical parts is not good. 

From a high frequency fast transient point of view of wiring to an SPD you want to keep the loop area small, so running the CPC beside the L and N it relates to is sensible, but threading a few washers on the wire - equivalent to magnetic-only effects of the wall of a box is not going to add enough inductance to be measurable, as the ‘mu’ of the steel may be high at 50Hz, but it falls smartly at increasing frequency, due to the time it takes to turn a large no of spins around en-masse.  Even if this was not true, saturation effects mean that much like the RCD blinding, with high current  events, the core more or less is not present.

Mike

Unless you are throwing hundreds of amps down the CPC, in normal operation,  or the enclosure is a very odd very thick super lossy steel, technically this is a non issue.  you could put a stud-bolt and a pair of wingnuts on each side, such that you had a wire on either side of the box wall, and the bolt carrying the current was itself made of steel, and in intimate contact with the box, and still see a temperature rise dominated by the normal I2R considerations.

There are a few things in the regs that charitably suggest that a blanket rule has been made when the case it applies to is infrequent, or less charitably, the level of understanding of magnetics among the writers of some technical parts is not good. 

From a high frequency fast transient point of view of wiring to an SPD you want to keep the loop area small, so running the CPC beside the L and N it relates to is sensible, but threading a few washers on the wire - equivalent to magnetic-only effects of the wall of a box is not going to add enough inductance to be measurable, as the ‘mu’ of the steel may be high at 50Hz, but it falls smartly at increasing frequency, due to the time it takes to turn a large no of spins around en-masse.  Even if this was not true, saturation effects mean that much like the RCD blinding, with high current  events, the core more or less is not present.

Mike