Per Mode and Per Phase Ratings Defined

When describing Surge Protective Devices (SPDs), manufacturers typically specify surge capacity ratings as either “Per Mode” or “Per Phase.” Users can sometimes be confused by these terms. This paper describes how these terms apply to SPD ratings and demonstrates their application to a four-wire Wye system, which is among the most common wiring configurations. The principles explained herein can be applied to wiring systems using other voltages and configurations.


Per Mode ratings are based on the capacity of any single mode to shunt overvoltage transients. In contrast, Per Phase ratings are based on an SPD’s total capacity to shunt overvoltage transients away from a phase conductor. ASCO reports “Per Phase” ratings to indicate the total capacity of its SPDs. Nevertheless, users should understand how “Per Mode” and “Per Phase” ratings are derived so that they can accurately compare SPDs from different manufacturers.