[Shop-talk] Electrical Outlet wiring question

[Randall]

> the two hot leads will be protected to 20A but a common 
> return could be carrying 40A if you plug in 2 20A loads, 
> twice the rating for 12 gauge.

This could be true, only if you have loads with extremely high harmonic
content.  Without harmonics, the two phases cancel each other and, if both
hot leads are carrying 20A, there is no current in the neutral at all.

Earlier versions of the NEC required the neutral to be oversize, due to the
possibility of high harmonic loads; but I believe this has been removed in
the latest version.  Harmonic content is now required to be less than 5%
(which is a giant PITA when designing a switching power supply).

-- Randall