Hi Don, since you asked, I'll first link a couple of threads where I had given my answers to that question:
https://www.electrical-contractor.net/ubb/Forum2/HTML/002445.html https://www.electrical-contractor.net/ubb/Forum2/HTML/001963.html And now for some cut'n'paste from those threads to summarize:
1: The impedance of the neutrals at feeders and panel buss are (electrically) significantly lower than the #12 or #10 branch circuit wiring thus allowing any noise or harmonics to be "absorbed" (Not exactly the right word I'm wanting, but you get the idea);
2: ANY electronic load of ANY kind is far more sensitive to power quality issues than incandescant lighting or pure resistive loads like heating devices;
3: Electronic dimming controls of any make or style are VERY noisy (just take a portable AM radio near any of those things and you'll see what I mean.);
4: Most of these systems are extremely expensive and sophisticated in their control schemes and do not take kindly to noise issues on incoming power (not to mention the risks of a lifted neutral frying everything)
Finally, if the manufacturer says don't do it, then don't do it! There are reasons for such instructions.
(The above was in reference to Crestron multi-zone lighting dimmers)
and:
....but a few years back I did a refit on a screening room where the audio racks were serviced by 5 20amp breakers (on a 3-phase panel) with ONE #10 neutral for all 5 circuits!!! The client was wondering why the sound system had a massive hum and why he'd been replacing burned-out amplifiers every few months.
There were NO loose/bad connections, which was my first thought.
After I had the EC pull in the proper number of neutrals and wire the rack recepts correctly, the system was quiet and no more burning amps.
It has been proven many times in my line of work that a shared neutral causes noise issues. Many installs I've had to rework have been cured of noise by pulling dedicated neutrals for each circuit. I don't always understand why that's so, but it has proven itself many times.
(edit) To (re)quote iwire in one thread:
quote:Electrically sharing a neutral on a branch circuit is not any different than what happens at the panel where all the branch circuit neutrals share a single conductor back to the source.
My response was:
For non-power-sensitive loads, I agree. For audio gear, no. It's the same idea as using isolated grounds, IMHO.
But in summary, I can state that first off, I'm not an EE, and thus don't have the affirmative background in electrical theory to back up my position, BUT, I do have the 20+ years of experience in cinema and high-end audio where my position has been proven correct repeatedly. So my best guess at why the elimination of shared neutrals has cleared up many problems I have encountered is probably best explained by Item #1 in my cut'n'paste.
And as I had mentioned in one of the posts in those threads, statistically the percentage of trouble calls I have attended caused by shared neutral issues is quite low, well under 10%.
I think we can all agree that electricity doesn't always behave in the manner we wnat or expect.
Sorry for the long post....
see edit above for quote credit...
[This message has been edited by mxslick (edited 06-06-2006).]