What Size Transfer Switch Do I Need? Amps by Load
A transfer switch is rated in amps, and the starting point is the load you will put through it: amps = running watts ÷ 240 volts. The household essentials used in our generator size calculator (refrigerator, sump pump, furnace blower, microwave, lights, laptop and router) total 3,550 W, which is 14.8 amps. Add central air and it is 29.4 amps. This guide gives the amps for common loads and generator sizes. It does not tell you which rated switch to buy: that depends on the generator's outlet and breaker, your panel and your local code, so an electrician must size and fit it.
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Transfer switch amps by load
Each row is a set of appliances run through the calculator. Running amps are the steady current at 240 V. Peak amps are the current for the moment when the largest motor starts.
| What is on the generator | Running watts | Running amps at 240 V | Peak amps at 240 V | Generator to shop for |
|---|---|---|---|---|
| Refrigerator, lights, laptop and router | 950 W | 4.0 A | 10.2 A | 2,000 W |
| Essentials: add the sump pump, furnace blower and microwave | 3,550 W | 14.8 A | 21.3 A | 5,000 W |
| Essentials plus well pump, freezer and TV | 5,300 W | 22.1 A | 28.5 A | 6,500 W |
| Essentials plus a window air conditioner | 4,750 W | 19.8 A | 29.8 A | 6,500 W |
| Essentials plus central air | 7,050 W | 29.4 A | 58.5 A | 12,000 W |
| Essentials plus central air, electric water heater and electric dryer | 16,450 W | 68.5 A | 97.7 A | 20,000 W |
The calculator shows the same figure for your own list in its row "Running load at 240 V (transfer switch estimate)". Tick your appliances and note the amps.
Generator watts to amps
A switch also has to suit the generator feeding it. This is the current each generator size delivers at its full running watts:
| Generator running watts | Amps at 240 V | Amps at 120 V |
|---|---|---|
| 2,000 W | 8.3 A | 16.7 A |
| 3,000 W | 12.5 A | 25.0 A |
| 3,500 W | 14.6 A | 29.2 A |
| 4,000 W | 16.7 A | 33.3 A |
| 5,000 W | 20.8 A | 41.7 A |
| 6,500 W | 27.1 A | 54.2 A |
| 7,500 W | 31.3 A | 62.5 A |
| 9,000 W | 37.5 A | 75.0 A |
| 11,000 W | 45.8 A | 91.7 A |
| 12,000 W | 50.0 A | 100.0 A |
| 15,000 W | 62.5 A | 125.0 A |
| 20,000 W | 83.3 A | 166.7 A |
| 22,000 W | 91.7 A | 183.3 A |
The amp rating marked on the generator's largest outlet, and on its breaker, is the figure an electrician will ask for. For other wattages use the watts to amps calculator.
What each large appliance adds
The eight largest loads from the appliance wattage chart, as amps at 240 V. Leaving one of them off the generator is the quickest way to bring the total down.
| Appliance | Running watts | Amps at 240 V |
|---|---|---|
| Electric dryer | 5,400 W | 22.5 A |
| Electric water heater | 4,000 W | 16.7 A |
| Central air conditioner, 2 to 3 ton | 3,500 W | 14.6 A |
| Dishwasher | 1,500 W | 6.3 A |
| Space heater | 1,500 W | 6.3 A |
| Window air conditioner, 10,000 BTU | 1,200 W | 5.0 A |
| Washing machine | 1,150 W | 4.8 A |
| Well pump, 1/2 hp | 1,000 W | 4.2 A |
Why you still need an electrician
- The amps here are a load estimate, not a rating. They come from typical appliance watts. The nameplates on your own appliances may be higher or lower.
- The switch must suit the generator and the panel, not only the load.
- It is a safety device. A transfer switch isolates the house from the grid, so that generator power cannot back-feed into the utility lines. Never back-feed a house through a dryer outlet.
- Local code applies to what is allowed and how it is inspected.
Next: what size generator do I need, what size standby generator do I need, or the breaker size calculator for a single circuit.
Frequently asked questions
How do I work out the amps for a transfer switch?
Add the running watts of the circuits you want on the generator and divide by 240. The essentials in this guide total 3,550 W, which is 14.8 amps at 240 V. An electrician then chooses the switch rating for your generator and panel.
How many amps does a 7,500 watt generator put out?
7,500 W ÷ 240 V = 31.3 amps at full output, or 62.5 amps if all of it were drawn at 120 V. Give the electrician that figure and the amp rating marked on the generator's largest outlet.
How many amps is a whole house on a generator?
All 19 appliances in our chart running together are 25,250 W, or 105.2 amps at 240 V. Leave out central air, the electric water heater, the electric dryer and the space heater and it falls to 45.2 amps.
Can I install a transfer switch myself?
It is work inside the electrical panel, so have a licensed electrician size and fit it. The switch isolates the house from the grid so that generator power cannot back-feed into the utility lines.
More guides
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.
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