250.64(E) Magnetic Discussion

[sparkyguy545]

OK, It's been WAY too long since I had a class in magnetics.
I understand the basic reason for bonding both ends of a ferrous conduit when used for GEC is to eliminate the choke effect that is widely described in every NEC handbook However, 250.64E clearly states that the requirements apply to ferrous conduit only. Therefore, NO bonding would be required if the GEC was installed in rigid aluminum conduit.

So, what is unique about this effect and why does it apply with ferrous conduit only? Electrically, and magnetically, wouldn't rigid aluminum conduit functionally create the equivalent of a single-turn, shorted, air-core transformer secondary and therefore ultimately have the same "choke" issue? Obviously, magnetic fields can induce current in aluminum conductors and windings. Many (if not most) xfmrs now are manufactured with aluminum windings. Clearly, it must be due to the fact that ferrous conduit reacts to magnetic field and aluminum does not. However, since aluminum windings can still have a current induced into them from magnetic fields, I don't clearly see why the phenominia doesn't also occur with aluminum conduit.

 

[cadpoint]

This one term has bit me here before " ferrous ". and one needs to remember what involved with the term.

This one application per the article lets one not bond an end as required if nonferrous but other applications as used; as called out by equipment usage, require bonding!

The electron of a material is still there to give and can be wrapped into a transformer and because it's a trannie will be bonded.

 

[hurk27]

OK, It's been WAY too long since I had a class in magnetics.
I understand the basic reason for bonding both ends of a ferrous conduit when used for GEC is to eliminate the choke effect that is widely described in every NEC handbook However, 250.64E clearly states that the requirements apply to ferrous conduit only. Therefore, NO bonding would be required if the GEC was installed in rigid aluminum conduit.

So, what is unique about this effect and why does it apply with ferrous conduit only? Electrically, and magnetically, wouldn't rigid aluminum conduit functionally create the equivalent of a single-turn, shorted, air-core transformer secondary and therefore ultimately have the same "choke" issue? Obviously, magnetic fields can induce current in aluminum conductors and windings. Many (if not most) xfmrs now are manufactured with aluminum windings. Clearly, it must be due to the fact that ferrous conduit reacts to magnetic field and aluminum does not. However, since aluminum windings can still have a current induced into them from magnetic fields, I don't clearly see why the phenomena doesn't also occur with aluminum conduit.

Have you ever seen a transformer with an aluminum core?
The wire in the conduit is not the problem, it is what the conduit is made of Iron, as in a transformer if you have a aluminum core and shorted the secondaries what would happen? nothing since the core would not build up any magnetic flux, same with a conduit, the wire in the conduit can be aluminum and still build up magnetic flux in the conduit, this would be just like a transformer with aluminum windings, so its not the circuit wiring material that is the problem, it is the core (conduit) if ferrous will allow a magnetic flux build up and cause a very high current to flow on it.