[Elecraft] [OT] NEC wire size requirements

[Jim Brown]

There are several issues with shared neutrals, one of which is that 
currents in single phase systems may not balance well. In the large 
sound systems I designed, I always specified a home run of all three 
conductors from each duplex that I distributed around a stage or worship 
area to the panel. I specified 20A outlets and #12 conductors. The 
intent was to allow portable gear at any of these locations to grab a 
lot of power.

The far greater issue with "shared neutrals" is related to 3-phase 
power, where, thanks to the harmonics present in distorted load 
currents, triplen harmonics (harmonics divisible by 3)  add in the 
neutral rather than cancel. This can result in neutral currents 
significantly larger than the current in one phase, even when loads are 
perfectly balanced. This is what overheated the wiring that started "The 
Towering Inferno," and it was the burning insulation that propagated the 
flames. That' why building codes were changed. :)

Few of us (none?) have 3-phase power in our homes, but most of us are 
fed from one phase of 3-phase systems in the alley or on the street. 
Those harmonics add in the "ground" for the same reasons that they add 
in the neutral, and because most of us are fed by "high-leg Delta" 
systems, we tend to see a lot of harmonic current on our grounds. THIS 
is why we hear "buzz" (triplen harmonics of 60 Hz) in our systems due to 
improper bonding rather than "hum" (pure 60 Hz).

There's an extensive discussion of this in several places on my website. 
The most detailed is in the "White Paper" for audio/video professionals, 
but it's also in the slide show for my talk on Power and Grounding for 
Hams.  k9yc.com/publish.htm

73, Jim K9YC

On Thu,8/11/2016 2:18 PM, Matt Zilmer wrote:
> Was this a "shared neutral" type of problem?  I thought NEC ruled that 
> out in post-1979 wiring plans.