Interlock kit vs. transfer switch: which one do you need?
Short answer: An interlock kit lets you power any circuit in your existing panel from a generator and costs less; a manual transfer switch powers only the 6–10 circuits wired to it but makes switching foolproof. Either is legal and safe when installed under permit; a cord plugged into an outlet ("backfeeding") is neither.
What both have to do
When the grid is down, your generator must never be connected to the utility lines at the same time. If it is, it energizes the transformer outside your house and sends thousands of volts back up the line toward the crews repairing it. The NEC requires transfer equipment that makes that impossible (Article 702, optional standby systems).
Side by side
| Interlock kit | Manual transfer switch | |
|---|---|---|
| How it works | A sliding plate on your main panel: the main breaker and a generator breaker can never be on together | A small separate panel; each selected circuit has a Line / Generator switch |
| Circuits you can power | Any circuit in the panel, chosen by flipping breakers | Only the 6–10 circuits wired through it |
| Load management | You turn off what you don't need — easy to overload the generator | Built in: only the chosen circuits exist |
| Panel requirements | Kit must be listed for your exact panel model; needs two free breaker spaces at the top | Works with almost any panel; needs wall space next to it |
| Typical parts cost | $50–200 kit + $100–150 inlet box and cord | $300–700 switch + inlet box and cord |
| Typical installed | $600–1,500 | $900–2,500 |
Which one to pick
- Interlock if your panel has a listed kit, the panel is not overfull, and you're comfortable switching breakers by hand in the dark. It is the most flexible: you can run the well pump in the morning and the dryer in the afternoon.
- Transfer switch if others in the house will operate it, if your panel has no listed interlock, or if you want it impossible to overload a small generator.
- Automatic transfer switch only with a standby generator that starts itself.
What you'll also need
- A power inlet box outside (L14-30 for 30 A or CS6365/14-50 for 50 A) and a matching generator cord. Sizing that is covered in 30 A vs 50 A inlets.
- A double-pole breaker for the generator feed, held in place by a retainer (NEC 408.36(D)).
- To know whether your generator has a bonded or floating neutral: most transfer setups that don't switch the neutral need a floating-neutral generator, or a neutral-bonding plug arrangement the electrician specifies.
Work out how big the generator must be
Safety. Never use a "suicide cord" (a double-male plug) to backfeed an outlet. Beyond the risk to line workers, the cord's exposed prongs are live whenever the generator runs.