From 2011 NEC: Article 240 "(3) 3-Phase and 2-Phase Systems. For line-to-line loads in 4-wire, 3-phase systems or 5-wire, 2-phase systems, individual single-pole circuit breakers rated 120/240 volts ac with identified handle ties shall be permitted as the protection for each ungrounded conductor, if the systems have a grounded neutral point and the voltage to ground does not exceed 120 volts."
There are other mentions within '11 I turned up via a search thru the PDF version. Most of them are specific to an installation. (DC; Bulk storage, etc)
On the job I'm working on, I think separate neutrals may be better. Circuits are coming out of an "ancient" 3 phase panel. A (3) phase breaker if it could be found, would cost a bundle. The junction boxes are not too far away. I was trying to conserve space in the conduit but the extra wire will be a lot cheaper. Thanks..
So, before you knock yourself out, pick up a phone. You might find that you can negotiate your price. It's not as if this stuff just jumps off the shelf.
Lastly, if you come across decent gear that is being replaced, these are the folks that buy such gear. It beats throwing the stuff away. The chance that a small EC is going to find a home for such gear is nil.
Forgot to mention above that these circuits are for florescent lighting only . I'm looking at an old 2005 code and it's referring saying if they " are on the same yoke"(outlet). ( don't have my 2011 with me now.
"210.4 Multiwire Branch Circuits. (A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment. FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor. (B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates."
The PRIMARY reason why dedicated neutrals make so much sense is because of what happens during maintenance of the system.
To safely work on a circuit, every hot that's mated to its neutral has to be shut off, too.
In a 480Y277 system (ultra-common) that's a very big foot-print of dark territory. This is especially true for high-bays in a warehouse.
In contrast, it's as common as dust for grocery stores to use 3-hots (A,B,C) on ONE (oversized?) neutral for strip fluorescents. It's not so bad if a single row of lamps goes dark. BTW, these circuits are never loaded more than 10-11 amps on a 20 amp breaker at 277VAC to ground.
Hence, they're half-way to an oversized neutral via design restraint.
For those who've never wired large structures:
Voltage drop is a serious issue. CWA driven lamps will fail to fire up should you be so foolish as to wire up a circuit up towards NEC minimums. Such 'site lighting' runs must use 208VAC or better... only loading the conductors rather lightly. Any attempt to break this iron law will have you out -- time and again -- coming back on warranty calls as the fixtures misbehave.
Like motor loads, CWA's pull heavy to start. Their constant wattages mean extra heavy amps when the voltage drops. This can make them 'self-snuffing.' (The only solution is more wire.) This was alluded to in a prior post INRE EM fixtures.