ASHRAE 90.1-2007

Section: 9.4.2 - Tandem Wiring

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Luminaires designed for use with one or three linear fluorescent lamps greater than 30 watts each shall use two-lamp tandem-wired ballasts in place of single-lamp ballasts when two or more luminaires are in the same space and on the same control device.

Exceptions:

1. Recessed luminaires more than 10 ft apart measured center to center.
2. Surface-mounted or pendant luminaires that are not continuous.
3. Luminaires using single-lamp high-frequency electronic ballasts.
4. Luminaires using three-lamp high-frequency electronic or three-lamp electromagnetic ballasts.
5. Luminaires on emergency circuits.
6. Luminaires with no available pair.