[PW] ?How is a 277 Volt Lighting System Wired in Series to Turn On a Large Group of Fluorescent Light Fixtures Wired in Series?

Peter Zilahy Ingerman, PhD pzi at ingerman.org
Wed Sep 30 14:12:45 PDT 2015

 From a PE/EE friend ...

"A 277 V lighting system is derived from the three phases of a 480 V 
(between phase conductors) electrical system.  Every lamp ballast is fed 
from one of the phases with respect to neutral, hence 277 V.

The three phase conductors may run all together through a raceway, or 
they may be routed in three different directions.  Either way, each 
ballast is connected between one phase conductor and the (common) 
neutral conductor.  Typically every third ballast in an installation is 
fed from the same phase.

The ballast acts as a transformer whose primary is 277 V and whose 
secondary is the (higher) voltage needed to strike an arc in the lamp.  
All lamp ballasts fed from any given phase and the neutral are in 
parallel, not in series.

The extra wires on the ballasts are transformer taps such that the same 
ballast can be installed for different voltage systems by selecting the 
appropriate tap.  The labeling on the ballast will show its wiring 

Do not mess with 277 V wiring unless you know what you are doing. Be 
sure the frames of all ballasts are connected to the equipment ground 
conductor; this is a different wire than the neutral conductor, and 
people sometimes confuse them.. "

On 2015-09-30 15:29, Sam Shipley wrote:
> I have a rather lengthy technical electronics question to submit for a patron.  Please see below:
>       "How is a 277 volt lighting system wired in series to turn on a large group of fluorescent light fixtures wired in series?  One switch turns on around 50 fixtures at once.   I want to understand this from the breaker to the last fixture.
>       When changing out the ballast in a fixture, there are some wires that are not needed, that the electrician has removed and capped.  This process is similar to a 120 volt system, but has more wires in the 277 volt system that (I think) remain hot.
>        I have tried to find the information by searching the internet, using the keywords Fulham Ballast Co. Inc.   I also search the "How Things Work" web site."
> Thanks for any help you can provide.
> Sam Shipley
> Reference Librarian
> Dodge City Public Library
> Dodge City, Kansas
> sams at dcpl.info<mailto:sams at dcpl.info>
> (620) 225-0248, x201
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