Transfer Switch Size Calculator
Screen ATS / MTS amp frames from generator kW, service amps, or selected circuits—instant planning for portable and standby backup.
Calculator
Quick: generator kW → amp frame. Advanced: whole-house service match or selected-circuit amps + continuous factor.
About this calculator
Screens transfer switch amp frames for generator backup. It is not a panel schedule, utility interconnect study, or OEM ATS selector. Decide interlock vs switch in interlock kit vs transfer switch; plan UPS bridge timing in ATS transfer time. Browse the power calculator hub.
Calculation Results
Current screen: 48.8 A
7.5 kW @ 240 V 1φ PF 0.80 → 39.1 A × an explicitly selected 1.25 factor = 48.8 A. This result does not select a transfer-switch frame or hardware class.
Assumptions: constant PF and voltage; no altitude/temperature derating; no selective coordination. Treat as planning only.
Continue Your Calculation
Confirm generator running and starting watts before you buy the switch hardware.
Planning disclaimer
Not a substitute for NEC 702 design, manufacturer ATS pairing, or AHJ approval. Hire a licensed electrician. Backfeeding a panel without a listed transfer method is dangerous and typically illegal.
Transfer-switch current examples
These rows demonstrate current arithmetic only. Open a row to load the calculator; do not treat the result as a product rating.
| Scenario | Design cue | Current screen |
|---|---|---|
| ≈ 39 A × 1.25 | 48.8 A | |
| 24 × 1.25 = 30 | 30 A | |
| Entered reference only | 100 A reference | |
| Entered reference only | 200 A reference | |
| ≈ 115 A × 1.25 | 143.2 A |
Formula & explanation
Single-phase: I = (kW × 1000) ÷ (V × PF)
Three-phase: I = (kW × 1000) ÷ (√3 × V × PF)
Optional current factor: Iscreen = I × factor. Select 1.25 only when it follows the rule or design basis you have adopted; it is not universal.
Whole-house mode: the service/main value is displayed as a reference only. It does not establish transfer-equipment rating, suitability or listing.
PF is required here when converting kW→A. For Wh suitcase stations use power station sizing instead.
Example: 7.5 kW, 240 V, 1φ, PF 0.8 → I = 7500 ÷ (240 × 0.8) = 39.1 A; with an explicitly selected 1.25 factor, the current screen is 48.8 A.
Chain: generator size → this page → interlock vs transfer → generator + UPS → UPS runtime.
Selection checks beyond current
| Check | Why it matters | Verify with |
|---|---|---|
| Switching architecture | Interlock, manual transfer and automatic transfer solve different operating needs. | System design and listed equipment instructions |
| Poles and neutral | Grounding arrangement and separately derived system rules can change switching requirements. | Adopted code, AHJ and source configuration |
| Fault and equipment ratings | Current arithmetic does not establish WCR, SCCR, service-entrance suitability or source compatibility. | Available-fault-current study, labels and manufacturer data |
Frequently Asked Questions
What size transfer switch do I need for my generator?
This calculator converts kW to current and can apply an explicit factor. The resulting current is only one constraint; select equipment using the adopted code, load calculation, switching configuration, fault ratings, listing and manufacturer pairing.
How do I size an automatic transfer switch?
Start with the applicable load calculation and service/source architecture, then verify poles, neutral switching, service-entrance suitability, WCR/SCCR, listing and generator-controller compatibility. The whole-house input here is a reference, not an automatic rating decision.
Is a 200 A switch OK on a 100 A service?
A larger switch alone does not increase service capacity. Match the protected feeder and listing—see the formula note above.
Interlock kit or transfer switch?
Compare on interlock kit vs transfer switch, then return here for amp screening.