Standard manual transfer switch sizes
| Switch size | Typical generator range | Circuits covered |
|---|
| 20 A | Up to ~2,400 W (120V only) | A few essential circuits |
| 30 A | Up to ~7,200 W (120/240V) | 6–10 circuits — the most common size |
| 50 A | ~7,200–12,000 W (120/240V) | 10–16 circuits |
Standard automatic transfer switch sizes
| Switch size | Matches this panel size |
|---|
| 60 A | 60 A panel |
| 100 A | 100 A panel |
| 150 A | 150 A panel |
| 200 A | 200 A panel |
| 400 A | 400 A panel |
The switch size follows the generator's outlet, not the appliances you plan to run. A manual transfer switch is rated for the maximum current that can safely pass through it — matching it to your generator's outlet rating (not exceeding it) keeps you from overloading the switch, even if your calculated appliance load is lower. If you haven't sized your generator yet, use the generator sizing calculator first.
The switch is sized to your panel, not to the generator. An automatic transfer switch needs to safely handle everything your panel could ever draw, since it sits between the utility, the generator, and your whole panel — so it's matched to your panel's rating, not your generator's output. Your generator itself is a separate sizing decision (use the generator sizing calculator), and a smaller generator paired with a correctly-sized switch is normal; the switch just won't draw more than the generator can supply.
This is a starting point, not a permit. Transfer switch installation involves your home's main electrical panel and is regulated by local electrical code — it should be installed by a licensed electrician, and in most areas requires a permit and inspection. Use this to understand what size to ask for, not as a substitute for that electrician's assessment.