When the generator capacity is already determined, selecting a suitable automatic transfer switch mainly involves converting the generator power (kW) into the required current (Amps) and then choosing an appropriate ATS rating. Since ATS devices are rated in amperes rather than kilowatts, this conversion is a necessary step in all practical applications.
Under standard low-voltage conditions—typically 220V for single-phase systems and 380V or 400V for three-phase systems—the relationship between power and current can be simplified for engineering use.
For most three-phase generator systems, the operating current can be estimated by multiplying the generator power by a factor of approximately 1.8. In field applications, a more conservative and widely used approach is to use a factor of 2, which provides a reasonable safety margin and simplifies selection.
Based on this method, a 100kW generator will correspond to roughly 200A, while a 200kW generator will require approximately 400A. In practice, standard ATS ratings should be selected above the calculated current rather than exactly matching it. For example, a generator with a calculated current of 180–200A is typically paired with a 250A or 315A ATS, rather than a 200A unit. This approach accounts for temperature rise, future load expansion, and operational stability.
It should also be noted that load characteristics significantly influence ATS sizing. Resistive loads such as lighting systems generally follow the basic calculation without major deviation. However, inductive loads—including motors, pumps, and compressors—introduce high inrush currents during startup, often several times the rated current. In such cases, selecting a higher-capacity generator switch (ATS) is strongly recommended to avoid contact stress and premature failure.
The simplified sizing method described above is valid specifically for common 220V/380V systems. When dealing with other voltage levels, such as 480V or 600V systems, a more accurate calculation must be applied using the standard three-phase formula:
Current = kW ÷ (1.732 × Voltage × Power Factor)
This ensures that the selected ATS is properly matched to the actual operating conditions.
In practical terms, a reliable selection rule can be summarized as follows: convert generator power to current using a factor of 2 under 380V conditions, and then select the next higher standard ATS rating. This method balances simplicity with sufficient engineering safety and is widely adopted in real-world projects.
In conclusion, proper sizing of an automatic transfer switch (ATS) depends not only on generator capacity but also on voltage level, load type, and necessary safety margins. A correctly selected ATS will improve system reliability, reduce thermal stress, and ensure stable power transfer under both normal and emergency conditions.
