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#71586 - 11/01/06 05:54 PM T5 fluorescent fixture circuits
poorboy Offline
Registered: 06/02/05
Posts: 124
Loc: Central Maine
3 rows of 9-T5 fixtures. Each row on its own circuit. I am sharing a neutral (3 phase 208 system). One of my co-workers said he has been told these need dedicated neutrals. Any truth to this?

[This message has been edited by poorboy (edited 11-01-2006).]
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#71587 - 11/01/06 06:12 PM Re: T5 fluorescent fixture circuits
kencr Offline
Registered: 07/25/04
Posts: 35
Loc: Southern, NJ USA
I NEVER share neturals when running circuits for electronic ballasts ...

The Boss made it clear .. not too .. Rite or wrong ?? I am not sure
#71588 - 11/01/06 06:39 PM Re: T5 fluorescent fixture circuits
renosteinke Offline
Cat Servant
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
Look inside the fixture. There is likely a lable warning you of the risks when you share neutrals.

Can you do it, and will it work? Sure- I have a town full of grocery stores that succeed, doing just that.

Using dedicated neutrals is an easy way to prevent a problem from coming up. If you share the neutral, and there are problems, it will be costly to fix, and may be difficult to identify the problem.

As they say, 'prevention is worth a pound of cure.'
#71589 - 11/02/06 12:19 AM Re: T5 fluorescent fixture circuits
Trumpy Offline

Registered: 07/05/02
Posts: 8211
Loc: SI,New Zealand
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 grin
#71590 - 11/02/06 10:16 PM Re: T5 fluorescent fixture circuits
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
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.

Sample Circuitry:

System = 208Y/120V 3Ø 4 Wire.

3Ø 4 Wire, 20 amp Lighting Circuit - using #12 THHN cu.

"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.

Good luck!

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#71591 - 11/03/06 02:28 AM Re: T5 fluorescent fixture circuits
iwire Offline
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
I share neutrals on every thing.

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
#71592 - 11/03/06 04:00 AM Re: T5 fluorescent fixture circuits
electure Offline

Registered: 12/24/00
Posts: 4259
Loc: Fullerton, CA USA
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" )
#71593 - 11/03/06 05:05 AM Re: T5 fluorescent fixture circuits
HotLine1 Offline

Registered: 04/03/02
Posts: 6776
Loc: Brick, NJ USA
Again a great, detailed explanation.

I also have to go with Bob.

If it's in the spec, that's how it goes.

#71594 - 11/03/06 07:20 PM Re: T5 fluorescent fixture circuits
mxslick Offline
Registered: 10/24/04
Posts: 803
Loc: Atomic City, ID USA
Here we go again

I ain't sayin nothin!!!
Stupid should be painful.
#71595 - 11/03/06 08:25 PM Re: T5 fluorescent fixture circuits
Eddy Current Offline
Registered: 09/26/06
Posts: 111
Loc: Ontario,Canada
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.
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