Generator Interlock vs Transfer Switch

Updated 2026-11-12 · Published by Charland & Co. · Based on published specs; we have not tested the products discussed.

Important: read before you use this guide

This page is general information only. It is not professional, electrical or safety advice, and every figure is an estimate.
Always follow your equipment maker’s manual and your local electrical and building codes.
Hire a licensed electrician for any wiring or any connection to a home, cabin or RV electrical system. Never plug a generator into a wall outlet.
Fuel generators are for outdoor use only. Their exhaust contains carbon monoxide (CO), which can kill within minutes. The U.S. Consumer Product Safety Commission (CPSC) says to keep them at least 20 feet from doors, windows and vents.
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Quick answer: For a USA planning example, a 120 V, 20 A circuit class is 120 x 20 = 2,400 W, and a 240 V, 30 A class is 240 x 30 = 7,200 W. Those are planning class sizes, not a wiring instruction. A licensed electrician and a permit plan this job. Do not backfeed a panel.

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All numbers here are estimates for planning only - not professional, electrical, or safety advice, and not an install guide. This page compares two classes at a planning level only. It does not tell you how to install, wire, or connect an interlock or a transfer switch. A licensed electrician should handle wiring, a transfer switch, an interlock, or a panel in the USA, and a permit may be required under local code. Do not backfeed a panel, a dryer outlet, or any other inlet. Fuel generators stay outdoors, never in a house, a garage, or near windows, doors, or vents, because of carbon monoxide. Store fuel in an approved container, away from ignition. A swollen lithium pack, if a power station is also in the plan, is a fire risk: do not use it. Follow the generator, switch, and panel maker manuals. For planning questions about this page, see our contact page. See weekend home-backup sizing, power station vs generator, and generator surge sizing.

Fuel-burning generators make carbon monoxide (CO), which you can't see or smell and which can kill in minutes. Run them outdoors only, at least 20 ft (6 m) from any doors, windows and vents, with the exhaust pointed away from the home. Never run one inside a home, garage, basement, shed, tent or RV, even with the door or windows open. Install battery-operated or battery-backup CO alarms on every level of your home and outside sleeping areas, and test them monthly. If a CO alarm sounds, or anyone feels dizzy, sick, confused or weak, get to fresh air right away, then call 911. Battery power stations make no CO and can be used indoors, but don't charge or store them where they get hot or wet, and follow the maker's manual.

Two classes, not a wiring lesson

Never plug a generator or power station into a wall outlet or dryer outlet, and never connect one directly to your electrical panel. This is called backfeeding. It can kill utility workers and neighbours, and it can start a fire. The only safe way to power house circuits from a generator is an approved transfer switch (or, where local code allows, a listed interlock) installed by a licensed electrician, under a permit where one is required. In the US, the generator connection must meet NEC Article 702 and your local code. In Canada, it must meet the Canadian Electrical Code (CSA C22.1) and provincial rules. In Quebec, the work must be done by a licensed electrical contractor (RBQ licence, CMEQ member), and Hydro-Québec must be notified. Quebec does not accept a panel interlock as a transfer switch, so use an approved transfer switch there. Check your utility's rules before you buy anything. This site does not give wiring instructions.

A generator interlock is a mechanical or listed kit class that is meant, when a licensed electrician installs it on a panel the maker allows, to keep the utility main and a generator backfeed breaker from being on at the same time. A transfer switch is a listed switchgear class that moves selected loads, or a whole service, from utility to generator, again only when a licensed electrician installs it as code and the manuals allow. Both classes exist to reduce the chance of backfeed, which can energize utility lines and kill people. Neither class is a DIY weekend project on this site.

This page does not name panel brands, kit models, or breaker types to buy. It does not give a parts list, a torque spec, or a sequence of connections. If you need hardware, a licensed electrician will choose a listed combination for your panel and your generator, pull any required permit, and do the work. Your job on this page is to understand the class difference, write a planning load size, and know that backfeed is never an acceptable shortcut.

A portable power station that feeds a few plug-in loads is a different class again. It does not replace a transfer switch or an interlock for a house panel. See the station vs generator guide if you are still choosing between a pack and a fuel generator.

Class table

Shop the class first, not a brand name. The example numbers below are planning assumptions, not product specs, test results, prices, or a license to install anything.

Class	Best for	Watch out for	Example planning numbers
Generator interlock (listed, electrician-installed)	A planning class for selected panel loads when an electrician and the panel maker allow that kit	Not DIY; permit may be required; not a whole-service automatic switch. Not accepted in Quebec; use an approved transfer switch there	120 x 20 = 2,400 W as one labelled circuit class
Transfer switch (listed, electrician-installed)	A planning class for a selected-load or whole-service transfer, including automatic types when listed	Not DIY; permit may be required; sizing and load selection are the electrician's job	240 x 30 = 7,200 W as one labelled class size
Plug-in power station, no panel tie-in	A few corded loads, no backfeed	Will not run a house panel	See the live station vs generator guide

The 2,400 W and 7,200 W figures are voltage times amps, written as planning class sizes so you can talk about scale. They are not a wiring instruction, not a breaker substitute, and not a claim that your house uses those numbers. A licensed electrician will read the actual panel, the generator nameplate, and local code.

Planning math, not install math

For a USA planning example, a 120 V, 20 A circuit class is 120 x 20 = 2,400 W. A 240 V, 30 A class is 240 x 30 = 7,200 W. If you are only thinking about one 15 A, 120 V circuit, 120 x 15 = 1,800 W. If two such 15 A classes were added as a rough talk-through, 1,800 + 1,800 = 3,600 W. None of those sums is a permit, a wire size, or a breaker schedule. They are conversation numbers for a meeting with a licensed electrician.

Generator running watts still have to cover the loads you hope to transfer, including motor starting watts on a well pump, a fridge, or an air conditioner. That surge method lives on the surge sizing page. Do not copy a surge example onto this page as if it selected an interlock or a switch. The electrician will size the listed equipment.

A fuel generator in this plan stays outdoors, away from windows, doors, and vents, with fuel in an approved container. Carbon monoxide does not go away because an interlock or a transfer switch is in the future plan. A station on the same property still has lithium fire rules: do not use a swollen pack, and follow the station maker manual.

How to pick, which means how to hire

Write the loads you care about in a weekend or outage window, in watts, using the home-backup and surge hubs. Write whether you want selected circuits or a broader transfer, as a preference, not as a design. Then stop and hire a licensed electrician. Ask that person whether an interlock class or a transfer-switch class is even listed for your panel and your generator, and whether a permit is required. Follow the generator, switch, and panel maker manuals that the electrician uses.

Do not buy a kit from a photo and install it. Do not backfeed a dryer outlet, a range outlet, or any other inlet. Do not run a generator indoors or in a garage to 'test' a hookup. If a salesperson claims a kit is a simple homeowner job, that claim does not override code or this site's lock: a licensed electrician does the work.

If a portable power station already covers your real loads, you may not need a panel class at all. That is a valid outcome. Use the home-backup and station vs generator pages to test that, then come back only if a building tie-in is still in view.

Worked planning example

A USA homeowner wants to talk through scale before calling an electrician. One 120 V, 20 A comfort circuit is 120 x 20 = 2,400 W as a planning class. A 240 V well-pump class at 30 A would be 240 x 30 = 7,200 W, which is a much larger conversation, and that pump hookup is itself a licensed-electrician job. Adding those two talk-through numbers, 2,400 + 7,200 = 9,600 W, is only a way to see that a small portable generator may be the wrong class; it is not a design and not a switch rating.

The homeowner then writes a shorter list: a fridge, lights, and a router, sized on the home-backup page, and brings that list to a licensed electrician, along with the generator nameplate. The electrician decides whether an interlock class, a transfer-switch class, or no panel tie-in is the listed, permitted path. This page ends there. There is no next wiring step here.

If the same homeowner is tempted to backfeed a receptacle 'just for a weekend,' the answer is no. Do not backfeed a panel.

Common mistakes

The first mistake is treating this page as install instructions. It is not. The second is backfeeding a dryer or range outlet. Do not do that. The third is installing any kit without a licensed electrician and, where required, a permit. The fourth is sizing a switch or an interlock from a blog watt number instead of a listed combination. The fifth is running a fuel generator in a garage or near a window while you think about hookups; carbon monoxide still applies. Use an approved fuel container, outdoors, away from openings.

The sixth is using a swollen lithium pack on a side plan. Do not use it. The seventh is ignoring the generator and panel maker manuals that the electrician will follow.

Who this is not for

This page is not for a homeowner who wants a step-by-step install, a parts shopping list, or a way around a permit. It is not a power-station runtime page. If you only need plug-in loads, stay on the station and home-backup hubs. If you need surge math for a generator, use the surge page, then hire a licensed electrician for any building connection.

FAQ

What is the difference between an interlock and a transfer switch?

They are two listed classes for keeping utility and generator power from backfeeding, when a licensed electrician installs the equipment the panel and generator manuals allow. This page does not install either one.

Can I install an interlock or a transfer switch myself?

No. Use a licensed electrician. A permit may be required under local code. This page has no install steps.

What does 120 x 20 = 2,400 W mean here?

It is a labelled planning class size, voltage times amps, so you can talk about scale. It is not a wiring instruction and not your breaker schedule.

Can I backfeed a dryer outlet instead?

No. Do not backfeed a panel or any outlet. That can energize utility lines and kill people.

Can I run the generator in the garage while we plan the hookup?

No. Never run a fuel generator indoors, in a garage, or near windows, doors, or vents, because of carbon monoxide. Keep fuel in an approved container.

What if a battery pack in the same plan is swollen?

Do not use it. That is a lithium fire risk. Follow the station maker manual.

Next step

Write the loads you care about, then hire a licensed electrician to choose a listed interlock or transfer-switch class, or to tell you that a panel tie-in is the wrong class. Do not backfeed. A permit may be required. Keep fuel generators outdoors, away from windows, with fuel in an approved container. For a planning question on this page, see our contact page.