The requirement for bi-level switching is not in ASHRAE/IES 90.1. What was the thinking behind including it in most building space types? What typical savings can be achieved through bi-level switching? Or was the thinking more of requiring a basic infrastructure so that advanced controls are more attractive for installation and can be used to generate higher savings? 

Bi-level switching was included so that the Interim Lighting Rules would result in a 50% reduction below ASHRAE 90.1-2001 for a typical building application and to require additional contracts for retrofits.

Bi-level switching is projected to typically reduce lighting power input by 10-15% on an annual basis.

The reduced lighting power input can reduce HVAC load for most building types.

Bi-level switching would also provide some controls for retrofits (See ASHRAE 90.1-2001 about "lighting alterations" controls requirements).

Bi-level switching strategies include split-ballasting to enable control of 1/2 of the lamps or fixtures, bi-level ballasts, multiple ballasts (as in the case of compact fluorescent hi-bays), dimming ballasts, manual switches or dimmers or photosensors, and other strategies.

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