According to ASHRAE std. 90.1 2010, appendix G, section G3.1.2.9.1/ ASHRAE std. 90.1 2016, appendix G, section G3.1.2.8.1:
“If return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.”

Which means that, for example, a relief fan’s design air quantity will always be less than the supply fan air quantity (assuming minimum outdoor air > 0).

In the case where the economizer is fully open (100%) the air flow through the relief fan will have to be equal to the supply fan air quantity, thus operating above its design flow (PLRfan > 1). How should the fan power (Pfan) be calculated when the fan is operating above its design air quantity?