Fluorescent Lighting Harmonics

[ATJ]

Hello,
Our company is in the process of installing lighting circuitry for an office building. The electrical engineer has circuited these lights with a seperate neutral for every lighting circuit. The fluorescent lights are 120 volt, 3-lamp T-8, electronic ballasts, approximately 80 fixtures. Many of these fixtures are dual switched and have two seperate circuits feeding them. This is the first time I have ever seen this type installation.
Will this wiring method reduce the amount of harmonics in this system?

Thank you, Andy

 

[mpparent]

The answer is no. The separate neutral is used so that it can accomodate the possible harmonics injected into the system by electronic ballasts. I've heard that 10% THD ballasts provide more harmonics back on the system than the 20% variety. Any thoughts on this from the Tip Masters in our group?

Mike

 

[alehman]

I haven't heard anything about the 10% vs. 20% question. I can see how it might be possible for a 10% THD rated ballast to have a larger amount of one specific harmonic component than the 20%, but otherwise I would say someone would have to be exaggerating their claims for this to be true.

As for separate neutrals, they don't reduce harmonics. The concern is that 3rd harmonic current adds in common neutrals. If there is enough 3rd harmonic current, the neutral can overheat. Running separate neutrals (or some people use oversized common neutrals) eliminates this possible problem.

It is my opinion that running separate neutrals is an unnecessary cost for typical fluorescent lighting. I am not aware of any reported cases of excessive harmonic current in lighting branch circuit neutrals. Separate neutrals also increase voltage drop in your circuits.

 

[jbartos]

Suggestion: It depends how much is the lighting circuit loaded in VAs. If the lighting load is low, about a half of the conductor rated ampacity, then there is a plenty of room for the harmonic content.

 

[busbar]

It may be time well spent discussing harmonic performance with a few fixture/ballast reps. It seems like there has been significant advances in the last decade, considering that losses from harmonics in discharge lighting has been made a priority at the regulatory level.

It would probably be worthwhile writing a performance spec, and specifying field verification before project turnover.

 

[ATJ]

Thanks for all your helpfull replys

 

[jbartos]

Suggestion: Visit

http://www.gelighting.com/na/downloads/ballast.pdf

for fluorescent fixtures ballast parameters. THD varies from below 10% to below 32% (which is high).