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.
