So why would this need to be put in the NEC?

Let’s just get into a little electrical theory for a minute.

When alternating current (ac) flows through a conductor, a magnetic field is created around that conductor which is constantly expanding and contracting with the amplitude of the current.

You know that the amplitude in the US is at a frequency of 60 cycles per second or 60 Hz.

Now, since Alternating Current reverses polarity 120 times per second, it stands to reason that the magnetic field also reverses its direction….. 120 times per second.

So why does that matter? Simple, the expanding and collapsing of the magnetic field induces something called eddy currents in the ferrous metal parts that surround the conductors.