Where in the NEC does it say that a single neutral can be "shared" between
two or more circuits? My question is this, that I've seen where someone has
ran up to seven 120v single phase circuits for HID lighting, yet there was
only two neutral wires, the same gage wire as the hot lines, i.e. 12 AWG,
20amp. I've also seen where a neutral is shared between two circuits for
120v receptacles. It has always been my practice that when running a circuit
off of a single phase breaker to whatever load that I run a separate neutral
wire for that circuit and keep it separated.
You can share neutrals on two or more different phases. You
can't share it on the same phase.
I have never used this technique on more than two phases, for no
other reason than having a 12-4 wire handy is unusual. The logic
is that sharing the neutral on the same phase the unbalanced
load will add to one another, and the same can be said on the
converse.
However, there are new problems with modern electrical systems,
such as places with electronic equipment. What building anymore
doesn't have computers or electronic balasts? Don't ever share a
neutral in such a case even in your own house if a neutral the
circuit might be used for an electronic device. Whole buildings
have had to be upgraded in the past 20 years because of
Harmonics. And harmonics indeed is almost an entirely new
problem of the modern age of computers.
http://www.powerquality.net/problem.htm
Other than this go ahead and share a neutral. However, you will
find out that the wire is a ton cheaper when buying 12-2 than
buying 12-3 or 12-4. That alone makes it prohobitive in such a
situation where every dollar counts like residential wiring.
To go one step further, even though you can share neutrals still
and stay in code, why do it? Electronic devices are coming in
our lives daily by the minute. Do you really want to upgrade in
another 10 years when the NEC no longer allows it and shared
neutrals are causing you problems?
Where your friend shared so many neutrals I would never do such
a thing. I am sorry to say I can't find it in the code, and
therefore cannot say for sure if what he did wasn't up to par.
From what I know he did wrong, as well as the references I found
also indicate he did wrong.
More references:
http://www.squared.com/us/products/circuitb.nsf/07a0210021262d45
862564b5006e4f84/78265ef31153098c85256c4e005921a1/$FILE/0760DB02
03R902.pdf
http://www.kennedyelectric.com/SureTest/Suretest-
SharedNeutrals.htm
http://www.psihq.com/iread/strpgrnd.htm