Motor & switching
Automatic Transfer Switch (ATS) Sizing Calculator
Get the load current, the utilization category, the next standard ATS rating, and the pole configuration from the load and the system. Motor-heavy feeders need the AC-33 category and extra margin — confirm the final rating against the ATS AC-33 table.
How the ATS is sized
The transfer switch must carry the full load continuously and switch it between sources, so it is sized from the load current at the operational voltage:
Pick an ATS whose rated current is at least the load current. The utilization category matters: AC-31 is for resistive loads, AC-33 for feeders with motors — where a switch is tested against high inrush. If motors are more than about 30 % of the feeder current, use AC-33 and confirm the rating from the AC-33 table, which can be lower than the AC-31 rating for the same frame. Neutral switching sets the poles: 3-pole where the neutral is not switched, 4-pole for TT systems and where a switched neutral is required between independent sources.
Frequently asked questions
What size ATS do I need?
At least the feeder load current at the operating voltage, in the correct utilization category. For motor-heavy feeders use AC-33 and select from the AC-33 rating, which accounts for inrush.
3-pole or 4-pole?
3-pole where the neutral is solidly earthed and common and not switched (typical TN). 4-pole for TT systems, and where the neutral must be switched between a utility and a separately derived generator.
AC-31 or AC-33?
AC-31 for resistive and non-inductive loads; AC-33 for feeders with significant motor load. When in doubt on a mixed feeder, choose AC-33.
Specify the ATS
With the rating, category, and poles set, choose a PC- or CB-class transfer switch and its controller.
Estimate for guidance only. Final ATS rating, utilization category, poles, controller, and source coordination must be verified by a qualified person against the load, the earthing arrangement, and the manufacturer’s datasheet.