Tandem wiring is where the ballast in one fixture lights a lamp in the adjoining fixture. This method allows the more energy efficient two lamp ballasts to be used with a roomful of three lamp fixtures.
From the 2003 IECC: "805.3 Tandem wiring. The following luminaires located within the same area shall be tandem wired: 1. Flourescent luminaires equipped with one, three or oddnumbered lamp configurations, that are recess-mounted within 10 feet (3048mm)center-to-center of each other. 2. Flourescent luminaires equipped with one, three or any other odd-numbered lamp configuration, that are pendant- or surface-mounted within 1 foot (305 mm) edge-to-edge of each other. Exceptions: 1. Where electronic high-frequency ballasts are used. 2. Luminaires on emergency circuits. 3. Luminaires with no available pair in the same area."
It is a fact that electronic ballasts use much less energy than magnetic ballasts. If you read the section above, it is clear that only odd number of lamp fixtures will get the tandem treatment. The intent is to limit the number of lamps lit by ballasts that only energize one lamp to a minimum. This is to save energy.
Check out Table 805.5.2. It limits the allowed wattage of lighting for spaces. Look at offices, they are limited to 1 watt per square foot, hardly seems like enough.
See 2002 NEC, 410.77(C) Wired Luminaire (Fixture) Sections.
Wired luminaire (fixture) sections are paired, with a ballast(s) supplying a lamp or lamps in both.
For interconnection between paired units, it shall be permissible to use metric designator 12 (trade size ) flexible metal conduit in lengths not exceeding 7.5 m (25 ft), in conformance with Article 348.
Luminaire (fixture) wire operating at line voltage, supplying only the ballast(s) of one of the paired luminaires (fixtures), shall be permitted in the same raceway as the lamp supply wires of the paired luminaires (fixtures).
Look at offices, they are limited to 1 watt per square foot, hardly seems like enough.
It's NOT enough. I have Seasonal Affective Disorder ("SAD"), and I've been effectively disabled because I physically can't work at a permanent position in the most of the buildings in Silicon Valley. After about two weeks of working inside one of those caves, I reach such a high level of exhaustion that I can't function.
I've compensated by working as a consultant, which allows me to set things up so that 70-80% of my work is done at home ("at my business offices" ), where I'm able to get natural light.
There's been talk the last ten or fifteen years about the sudden, mysterious appearance of an epidemic of "Cronic Fatigue Syndrome," which they haven't been able to find a cause for. My own suspicion is that they just haven't figured out yet that it's result of forcing everybody to work in near-dark conditions. (I hear this is a popular "California" aliment. Could that be because they put in place the PC "California Energy Code" a few years back? You can't even have decent windows--they all have to be heavily tinted to reduce the heat load.)
It's really depressing to go to work on the darkest, dingiest day of the year, and realize as you walk in the front door that it's much darker still inside the building you're supposed to be trying to work inside of.
[This message has been edited by SolarPowered (edited 12-08-2004).]