Backup Power guide
Interlock Kit vs. Transfer Switch: How to Connect a Portable Generator to Your House
Have a licensed electrician install either one. Consumer Reports recommends a licensed electrician for a transfer switch, and says an electrician can tell you whether an interlock works with your panel and meets local building codes.[1][3] Reliance Controls warns that an improper transfer switch installation can cause electrocution or fire and must be done by a qualified electrician to code.[2] This page explains the options so you can talk them through with an electrician. It isn't installation instructions.
Quick answer
- Both do the same safety job: they keep generator power and utility power from ever being connected at the same time, so power can't flow back out to the lines.[1][4]
- A manual transfer switch is a small separate panel wired to a fixed set of circuits you choose in advance, usually 4 to 10.[2] With fewer circuits on it, it's hard to overload the generator by accident.[4]
- An interlock kit is a sliding plate on your main panel that lets a generator breaker feed the whole panel, so any circuit is available. You have to manage the load yourself.[1][4] Consumer Reports calls it the cheaper option.[1]
- Either way, it's an electrician's job. Reliance Controls says its transfer switches must be installed by a qualified electrician to code, and Consumer Reports says an electrician can tell you whether an interlock works with your panel and meets local codes.[1][2]
Why you need one of them at all
Consumer Reports never recommends connecting a generator to a circuit panel without an interlock or transfer switch, and calls it dangerous and, in many cases, illegal. It gives two reasons. When grid power comes back while the generator is connected, it can overload the wiring and damage electronics. And a generator wired to the panel without a transfer device can send electricity back into the power lines while utility crews work on them.[1]
A transfer switch or interlock also lets you power things that don't plug in, such as overhead lights, a well pump or a water heater, from a single generator cord instead of a tangle of extension cords.[3]
How each one works
Manual transfer switch
A manual transfer switch mounts beside your main panel. The circuits you pick are rewired to run through it.[1][2] On a Reliance Controls switch, each circuit has its own selector, set to LINE (utility) or GEN (generator), and its own 15 or 20 A breaker that protects the circuit on generator power. 240-volt circuits use two adjacent switches joined by a handle tie.[2] Some models have watt meters that show the load on each side of the generator.[2]
Reliance's 4-, 6-, 8- and 10-circuit models set the maximum number of circuits you can move to the generator.[2] Its contractor kits pair a 6- or 10-circuit switch with a 30 A power inlet box and a 20-foot 30 A generator cord, for generators with a 125/250 V, 20 A or 30 A locking outlet, and are cULus 1008 listed.[5]
Interlock kit
An interlock is a metal bracket installed on your existing panel. An outdoor power inlet feeds a breaker in the panel, and the bracket blocks the main breaker so it can't be on while the generator breaker is on. When grid power returns, you slide it back.[1] To switch over, you turn the main breaker off, slide the plate, and turn the generator breaker on. Both can be off, but never both on.[4]
Side-by-side comparison
| Factor | Manual transfer switch | Interlock kit |
|---|---|---|
| What gets installed | A separate switch panel beside the main panel, plus an inlet box[2][5] | A bracket on the existing panel, a generator breaker, and an inlet box[1][4] |
| Circuits you can power | Only the ones wired through it (4 to 10 on Reliance models)[2] | Any circuit in the panel[4] |
| Overload risk | Low; the circuits are chosen and balanced at install[4] | Higher; easy to overload or unbalance the generator[4] |
| Switching over | Flip each circuit's selector from LINE to GEN[2] | Main breaker off, slide the plate, generator breaker on[4] |
| AFCI/GFCI breakers | Their protection is lost on GEN; Reliance says to provide it at the outlets[2] | Circuits stay on their own panel breakers |
| Big 240V appliances | Reliance says not to use its switches for electric water heaters, dryers, ranges or central air[2] | Possible if the generator can carry them; you manage the load |
| Cost | Higher, per Consumer Reports[1] | Significantly less than a transfer switch, per Consumer Reports[1] |
| Fit and code | Installed by a qualified electrician to code[2] | An electrician confirms it fits your panel and local code[1] |
Overload and balancing the load
This is the real trade-off. With a transfer switch, the limited number of circuits makes it unlikely you'll overload the generator, and the electrician spreads them across both 120 V legs of the generator so each carries a fair share.[4] Reliance's instructions say to pick emergency circuits during install, and to put the paired halves of any shared-neutral (multi-wire) circuit on opposite legs with a handle tie.[2]
With an interlock, the generator can feed every circuit in the panel, which makes it easy to overload the generator or load one leg much more than the other.[4] So before you switch over, turn off the breakers for anything you don't plan to run, then turn on only what the generator can carry. Our wattage calculator totals running and starting watts for the things you plan to run.
What size generator you need
Consumer Reports says most transfer switches are designed for 220-volt input, so you'll need a generator of 5,000 watts or more. Smaller generators generally lack a 220V outlet to connect the power cord.[1][3] Look for a 120/240V generator with a locking outlet that matches the inlet the electrician installs, such as the 30 A, 125/250 V outlet the Reliance kits are built around.[5] Our comparisons of inverter generators and dual-fuel generators list which models have 120/240V transfer-switch outlets.
What to ask your electrician
- Is an interlock kit made for my panel, and does my local code accept it?[1]
- If I go with a transfer switch, which circuits should go on it, and how will you balance them across both legs?[2][4]
- Which of my circuits have AFCI or GFCI breakers, and how will that protection work on generator power?[2]
- What inlet and cord match my generator's outlet?[5]
Consumer Reports suggests labeling your breakers with a circuit breaker finder first, so the electrician knows what each one feeds.[3] It also says to plan early, because electricians are hard to book when a big storm is in the forecast.[1]
FAQ
Is an interlock kit as safe as a transfer switch?
Both keep utility and generator power from being connected at the same time.[1][4] The difference is load: an interlock makes every circuit available, so it's easier to overload the generator.[4] Whether one is allowed on your panel is up to your electrician and local code.[1]
Can I plug my generator into a wall outlet instead?
No. Connecting a generator to your panel without an interlock or transfer switch is dangerous and, in many cases, illegal, and it can send power back into the utility lines.[1]
How many circuits does a manual transfer switch handle?
Reliance Controls makes 4-, 6-, 8- and 10-circuit models; a 240-volt circuit takes two adjacent switches.[2]
Do I need a 240V generator for a transfer switch?
Usually. Consumer Reports says most transfer switches are designed for 220-volt input, which means a generator of 5,000 watts or more.[3]
Sources
- Paul Hope and Allen St. John, "Why a Portable Generator Needs a Transfer Switch to Safely Power Your Whole House," Consumer Reports, updated Jan 11, 2024. consumerreports.org/…/portable-generator-needs-a-transfer-switch-a1069571021/
- Reliance Controls, transfer switch installation and operating instructions (Pro/Tran; warnings, components, circuit selection, models). Copy hosted by a dealer. nooutage.com/pdf/RelianceControls_ProTranInstructions.pdf
- Consumer Reports, Generator Buying Guide ("What's a Transfer Switch and Do I Need One?"). consumerreports.org/home-garden/generators/buying-guide/
- Norwall PowerSystems, "Generator Interlock Kit vs Transfer Switch" (a retailer that sells transfer switches). norwall.com/power-expert/transfer-switch-vs-interlock/
- Reliance Controls, transfer switch catalog (transfer switch kits for non-retail environments). api.reliancecontrols.com/…/DAFL6723827471620578.pdf