When installing a standby generator system, the transfer switch is the critical link between utility power and backup power. Sizing it correctly ensures safe, reliable operation and code compliance.
Understanding Transfer Switch Types
Automatic Transfer Switches (ATS) detect power loss and automatically switch to generator power within seconds. They're ideal for whole-home backup and critical commercial applications.
Manual Transfer Switches (MTS) require someone to physically flip the switch. They're more affordable and suitable for occasional use or non-critical loads.
Load Calculation Basics
The transfer switch must handle your total connected load. Follow these steps:
- List all circuits you want on backup power
- Add up the breaker amperage for each circuit
- Apply a 80% derating factor for continuous loads
- Select a transfer switch rated at or above your calculated load
Common Sizing Mistakes
Undersizing: Choosing a transfer switch smaller than your generator's output limits your backup capacity and can cause nuisance tripping.
Ignoring starting currents: Motors draw 3-7x their running current on startup. AC compressors, well pumps, and refrigerators need this headroom.
Our Recommendations
For most residential applications with a 22-24kW generator, we recommend a 200-amp automatic transfer switch. PES Supply stocks Generac, Eaton, and Siemens transfer switches with same-day shipping.