300.20(B) Stainless Steel Box

[W@ttson]

Hello,

300.20 states the following:
300.20 Induced Currents in Ferrous Metal Enclosures or
Ferrous Metal Raceways.
(A) Conductors Grouped Together. Where conductors carry‐
ing alternating current are installed in ferrous metal enclosures
or ferrous metal raceways, they shall be arranged so as to avoid
heating the surrounding ferrous metal by induction. To accom‐
plish this, all phase conductors and, where used, the grounded
conductor and all equipment grounding conductors shall be
grouped together.


I know that for aluminum boxes, 300.20 does not apply since they are not ferrous. What about 304 stainless steel or 316 stainless steel? I know that some stainless steel is magnetic. And some stainless steel that is not magnetic because it is in the austenitic family might become magnetic based on the temperature it is worked.

By looking at the letter of the code, it specifically says "ferrous", meaning contains Iron. So the fact that the 316 stainless steel may or may not be magnetic is a mute point since SS is ferrous because it contains iron??? Or is the code saying "magnetic" in actuality?

 

[infinity]

I agree with you that the point may be moot because the NEC wording is lacking in detail where they only state that if it's ferrous (meaning as you've stated in contains iron) that 300.20 would apply. It might be better if it stated that it would need to also have magnetic properties as well as being ferrous.

 

[W@ttson]

I agree with you that the point may be moot because the NEC wording is lacking in detail where they only state that if it's ferrous (meaning as you've stated in contains iron) that 300.20 would apply. It might be better if it stated that it would need to also have magnetic properties as well as being ferrous.

Thank you for confirming/clarifying my suspicions.

 

[gadfly56]

You can set up eddy currents in any conductive metal. The freshman physics lab at Stevens had a demonstration set up with a bunch of permanent magnets forming a "U"-channel and a thin aluminum "hammer" (think slice of Thor's Mjolnir) suspended from uprights at one end of the channel. You'd flip the hammer to make it pass through the upright magnet arms, and the eddy currents would create a back magnetic force, slowing the hammer. If you tried to push it through faster, it resisted even more.