OK, Good on you for using the new T5 fluoro's. Considering that these units use High Frequency ballasts. I've never liked the idea of shared neutrals, in fact, to a degree here in New Zealand they are illegal. The thing that I hate about them is that you never know exactly what else is on that particular neutral. I disconnected a light fitting in a chiller at a meat works I used to work at and found that a lot of stuff was tied to that particular neutral, in so much that the main neutral heading back to the board was warm, considering that that was in a -12C Chiller.
Let's face it, these days if you're not young, you're old - Red Green
If you do not load up each Lighting Circuit to near maximum, you are covering both bases!
It appears that you are taking an approach to try and eliminate a possibly overloaded common grounded conductor, which would experience an overload problem due to excessive Harmonic Distortion ("Triple-N" scenario).
OK, for right now, let's just say the claims of excessive Triple-N Harmonics is a valid consern in this installation (like the Ballasts have THD ratings >33.3%).
If you design your Lighting Circuits so that the _Included LCL Value_ is something like 12 - 13 Amps, then even with a crazy THD of 33.3%, the capacity of the Common Grounded Conductor (the "Neutral") has not been exceeded yet.
"Actual" Lighting Load limit per 20 Amp Circuit: 11 Amps.
"Added" LCL value per Lighting Circuit (125% of the load value, if run for 3 hours or more): 13.75 Amps.
"Highest" LCL load level which might be found on the Common Grounded Conductor: 27.5 Amps.
Table 310-16 lists #12 THHN cu as 30 Amp capacity, so we are below the rating *so far!!!* Derated value of the #12 for 4-6 Current Carrying Conductors in the same raceway is now 24 Amps, so with an LCL added + 33.3% THD on all 3 lines, we may have exceeded the capacity of the Common Grounded Conductor.
*** Real World Scenario ***
You are unlikely to find THD ratings in excess of 20%, so Triple-N values on the Common Grounded Conductor, will (if present) fall below 2x that of the highest Ungrounded Conductor - something in the range of 1.3x might be expected.
Limiting the total load per circuit to something less than the maximum rating of the circuit is good design practice. Be sure to include an LCL adder for circuits running 3 hours or more (180 minutes and longer). Lighting Circuits normally fall into this classification.
If you are concerned about an excessive voltage drop within the Common Grounded Conductor - due to Triple-N currents, or whatever else may be taking place (power factor of loads falls below 70%, and may result in a nearly equal load current flowing L-N on all 3 circuits), then increase the size of the Common Grounded Conductor from #12 THHN cu to #10 THHN cu.
If you have already increased the size of _ALL_ Circuit Conductors due to Voltage Drop, you may not need to increase the Common Grounded Conductor's size additionally - as it was already covered in the first increase. It would not hurt to upsize it, but might as well go ahead and upsize the entire multiwire circuit as well.
As far as "Dedicated Neutrals" go - if the Plans or Project Manual "Forces" you to do this on _ANY CIRUITRY_, and the job was definitely bid that way, you better install Dedicated Neutrals!!!
Many Electricians in the field have been told way too many "Old Sparkies' Tales" (a twist on the term "Old Wives' Tales"), which are simply misquoted technical things, or a "commonly believed falicy" - depending on what level of information is at hand.
The "Dedicated neutrals" thing is a direct misquote, either from someone describing a workable method dealing with harmonic loads (still is an incomplete statement), or was something from a certain project where extremely bored people compiled specifications in project manuals, + text on plans, which specified "Dedicated Neutrals" on any non-linear load circuitry; and this quote became "Default" to that certain Electrician.
My suggestions on these Lighting Circuits would be to attempt to balance the L-C (Line to Common Grounded Conductor) loads as much as possible, as viewed across a certain 4 wire circuit - and to utilize 3Ø 4 Wire Multiwire Circuitry wherever possible.
Limit the maximum load to no more than 12 Amps (at 100%) per circuit. Increase the size of _ALL CONDUCTORS_ if the length from the Panelboard to the first fixture is 100 feet or greater.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
You ever see a panel feeder with a dedicated neutral for each circuit? (Don't laugh, the electrons behave the same on either side of a panel)
It is purely a design issue, about the only area I will not try to run MWBCs (Multi wire branch circuits) are 'mission critical' type loads like in an IT center.
It's not that it would not work fine assuming you don't load the heck out of each circuit, I use dedicated neutrals in those areas as down the road when changes have to be made you can work on one circuit without effecting two others.
Bob Badger Construction & Maintenance Electrician Massachusetts
Here we go again I'm with Bob on this. I run multiwire circuits wherever I can. I will stop this practice when I am supplied with a 3Ø - 6 Wire system to work with. Until that time, running dedicated neutrals and connecting them to a common grounded conductor at a panel, feeder, etc. seems akin to installing gold plated audiophool receptacles in your "sound room" and then connecting them all to the aluminum PoCo service.
(BTW the last 2 syllables of "multiwire" spell out "Iwire" )
Lately every "fit up" I've done has dedicated neutrals in the spec. Makes for alot of extra work when it was previously wired using shared neutrals. Inspectors around here are really picky about colours(no tape)so some circuits are easier to just re-wire.