Transfer Switch for a Portable Generator
How to connect a portable generator to your house safely — manual transfer switch versus interlock kit, what amp size you need for your generator, what installation costs, and the code rules that keep the connection safe for you and for the crews restoring your street.
A portable generator only becomes useful backup once it can feed the circuits that matter — the furnace, the well pump, the hardwired things you cannot reach with a cord. That connection is a transfer switch or an interlock kit, and it exists for one non-negotiable reason: it makes it impossible for generator power to travel back up the utility line, where it can electrocute a lineworker restoring your street. Running a cord from the generator into a dryer outlet does the opposite. This page covers which of the two you want, what size you need, and what it costs.
Transfer switch or interlock kit?
| Option | Installed cost | How it works | Who it suits |
|---|---|---|---|
| Manual transfer switch | $400–$1,500 installed | A separate sub-panel beside your main panel with its own switches for 6–12 chosen circuits. | Most homes. You pick the circuits in advance, and switching over is unambiguous — each circuit has its own labelled position. |
| Interlock kit | $150–$500 installed | A sliding metal plate on your existing panel that makes it physically impossible to have the main breaker and the generator breaker on at once. | Homes whose panel has a listed kit available. Cheaper, and gives you access to every circuit — with the discipline of managing the load yourself. |
Both are code-compliant when installed and permitted properly. The interlock is cheaper and reaches every circuit in the panel; the transfer switch is more expensive and limits you to preselected circuits, which is precisely what makes it hard to overload the generator by accident. If your panel has a listed interlock kit available and you are comfortable watching your own load, the interlock is usually the better value.
What size transfer switch do you need?
Size the switch and the inlet to the generator’s output, not to your main panel. At 240 volts, amps are watts divided by 240 — so a 10,000 watt generator draws about 42 amps and wants a 50-amp switch. Confirm against the generator’s nameplate and the cord you intend to use; the cord has to be rated for the same amperage.
| Generator output | Current at 240 V | Switch & inlet |
|---|---|---|
| 3,000–4,000 W | ≈ 13–17 A | 30 A switch + 30 A inlet (L14-30) |
| 5,000–7,500 W | ≈ 21–31 A | 30 A switch + 30 A inlet (L14-30) |
| 8,000–10,000 W | ≈ 33–42 A | 50 A switch + 50 A inlet (CS6375 / 14-50) |
| 11,000–13,000 W | ≈ 46–54 A | 50 A switch + 50 A inlet (CS6375 / 14-50) |
Not sure which generator size you are matching? Run your appliances through the generator sizing calculator to get the running and starting watts first, then come back and size the switch to that.
What the installation involves
- A licensed electrician and a permit. The work lands on your main service panel, so in most US jurisdictions it is licensed work with an inspection. That is also what keeps your homeowner’s insurance and the generator warranty intact.
- An exterior inlet box. A weatherproof inlet on an outside wall, positioned so the generator can sit well clear of doors, windows and vents while the cord still reaches.
- Circuit selection. With a transfer switch you decide in advance which 6–12 circuits get backed up. Furnace, fridge, well or sump pump, a few lights and outlets is the usual list.
- Half a day, typically. Most of the cost is labor and the wiring run between panel and inlet — which is why the distance between them moves the quote more than the hardware does.
Portable switch or a standby system?
What you get here is the manual version: you start the generator, plug it in, and throw the switch. A permanently installed whole-house standby uses an automatic transfer switch instead — it senses the outage and changes over on its own, which matters if the house is ever empty during a storm. That convenience is the bulk of the price difference: roughly $400–$1,500 here against $3,000–$5,000 for a standby installation. The portable versus standby guide works through which one fits your situation.
Costs are national planning estimates and vary with panel type, the distance from panel to inlet, and local permit fees — get itemized quotes. Transfer switch and interlock installation is licensed electrical work under the NEC and local code; connecting a generator to house wiring any other way risks backfeeding the utility line. Run the generator outdoors only, well away from doors, windows and vents, because the exhaust contains carbon monoxide.
Frequently asked questions
Can I use a transfer switch with a portable generator?
Yes — a manual transfer switch is the standard, code-compliant way to run house circuits from a portable generator. The electrician fits the switch (or an interlock kit) next to your main panel and an inlet box on an outside wall; you plug the generator into the inlet with a single heavy cord, flip the switch, and the selected circuits run from the generator instead of the utility. It is also the only method that reliably prevents backfeed onto the utility line, which can kill a lineworker.
Can you install a generator transfer switch yourself?
In most US jurisdictions, no — the work happens at the main service panel and is treated as licensed electrical work requiring a permit and an inspection. Doing it yourself can void your homeowner’s insurance and your generator warranty as well as failing inspection at resale. Budget for a licensed electrician; the job is normally half a day, and the permit fee is small compared with the cost of getting the interlock between utility and generator power wrong.
What size transfer switch do I need for a 10,000 watt generator?
A 10,000 watt generator at 240 volts draws about 42 amps, so a 50-amp transfer switch and a matching 50-amp inlet is the standard match — you size the switch to the generator’s output, not to your panel. For generators in the 5,000–7,500 watt class, a 30-amp switch is the usual pairing. Have the electrician confirm against the generator’s actual nameplate rating and the cord you will use.
How much does it cost to install a transfer switch for a portable generator?
Expect roughly $400–$1,500 installed for a manual transfer switch, covering the switch, an exterior inlet box, the wiring run and the permit — the spread depends mostly on how far the panel is from the wall where the inlet goes. An interlock kit is the cheaper route where your panel supports one, often $150–$500 installed, because it uses your existing breakers instead of a separate switch. Both are a fraction of the $3,000–$5,000 an automatic transfer switch and standby installation costs.