Resources / How to Size a Static Transfer Switch
How to size a static transfer switch
Sizing an STS takes three steps: find the load current, add margin, and choose a continuous rating at or above it. The feeder breaker and wiring then follow your electrical code and the operating manual.
Start with the load current
For a single-phase load, current is volt-amperes divided by voltage. For three-phase, divide by the line-to-line voltage times 1.732. If you have kilowatts, divide by the power factor first to get kVA. A 10 kW single-phase load at 208 V and unity power factor draws 48 A, which is a 50 A switch. An 18 kW rack on 208/120 V three-phase draws 50 A per phase.
Make sure the rating is continuous
Every TwinSource rating is 100% continuous: a 30 A switch carries 30 A with no derating, so compare ratings on that basis. The SCRs also carry their rating across the 0.8 to 1.0 power factor range of electronic loads, where relay ratings can fall by as much as half.
Protect the feeder
Every TwinSource rating is 100% continuous: a 30 A switch carries 30 A without derating. Size the feeder breaker and its conductors to your electrical code and the installation requirements in the operating manual for the model.
Leave some margin
Allow for the small load that arrives unplanned next year, and for temperature: the closer a unit runs to nameplate, the warmer it runs. Within a product family the enclosure and electronics are often the same across ratings, so the larger rating buys more protection per dollar. If a transformer or large motor sits downstream, tell us. Inrush during an out-of-phase transfer is a sizing question of its own.
Single phase vs three phaseWhere to startOpen the configurator
Questions engineers ask
Do I size to the breaker or to the load?
To the load, with margin. Then size the feeder breaker and its conductors to your electrical code and the operating manual for the model.
Do I need to derate for power factor?
Not for the switch. Convert kW to kVA using your power factor so the current is right, then pick a rating at or above it.
What if my load falls between two ratings?
Go up. Ratings are 100% continuous, so the next size gives you headroom and a cooler-running unit.
Put it to work
Narrow the range to your voltage, current and mounting, or send us your one-line diagram and talk to an engineer.