Interlock Kit vs Transfer Switch: Which Is Right for Your Home Backup?

If you want your generator or portable power station to power circuits in your home — not just appliances plugged into extension cords — you need transfer equipment. The two most common choices for homeowners are a generator interlock kit and a transfer switch.

Both do the same essential job: keep your backup power and the utility grid from ever being connected at the same time. But they work differently, cost differently and suit different households. This guide explains the difference, what the electrical code says, and how to decide.

Safety note: Installing either option is electrical work on your main panel. In most places it needs a licensed electrician, a permit and an inspection. This guide helps you understand your options — it is not installation instructions.

Why you need transfer equipment at all

Plugging a generator into a wall outlet — “backfeeding” — is dangerous. The US Consumer Product Safety Commission says backfeeding “presents an electrocution risk to utility workers and neighbors served by the same utility transformer” and “bypasses some of the built-in household circuit protection devices” (CPSC).

The National Electrical Code (NEC) addresses this in Article 702 (optional standby systems). Section 702.5 says transfer equipment “shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment” (quoted by IAEI Magazine). Both interlock kits and transfer switches can meet that requirement when they are properly listed and installed.

For background on the whole setup — inlets, cords and power stations — see our guide on how to connect a power station to your house safely.

What is a generator interlock kit?

An interlock kit is a mechanical plate or slide fitted to the front (deadfront) of your existing electrical panel. It is positioned so that:

• when the main breaker is ON, the backup-feed breaker cannot be turned on, and
• when the backup-feed breaker is ON, the main breaker cannot be turned on.

You pair it with a power inlet box — a socket mounted on an outside wall that your generator or power station cord plugs into — wired to that dedicated backup-feed breaker.

How the code treats interlocks

According to IAEI Magazine’s UL Question Corner, panelboard interlock kits “are considered ‘transfer equipment’ as permitted in NEC Section 702.5” and “are part of the panelboard and are investigated for compliance with ANSI/UL 67, the Standard for Safety for Panelboards.” Panels designed for a specific kit may be marked “Suitable for use in accordance with Article 702 of the National Electrical Code ANSI/NFPA 70 when provided with interlock kit Cat. No. ____” (IAEI; UL Solutions panelboard guide).

In plain English: the interlock must be the right kit for your exact panel. A municipal guidance sheet from East Hanover Township, New Jersey gives the example that an interlock “specific to the panel it is being installed on, such as a Square D interlock device on a Square D panel” meets the listing and labeling requirement in NEC 110.3(B) (East Hanover Township PDF). Home-made plates or generic “universal” kits not identified for your panel are a red flag for inspectors.

Pros of an interlock kit (general guidance)

• Uses your existing panel, so there’s less new hardware.
• Any circuit can be powered — you choose which breakers to turn on during the outage.
• Often described as a lower-cost route than a separate transfer switch panel (get quotes — labour, inlet and permit costs vary widely).

Cons of an interlock kit (general guidance)

• You manage the load manually. Because any breaker can be switched on, it’s easy to overload your generator or power station.
• Requires a kit listed for your panel; some older or unusual panels have no compatible kit.
• Requires an open breaker slot near the main breaker, in many designs.
• Not accepted by every local inspector — always confirm with your authority having jurisdiction (AHJ).

What is a transfer switch?

A transfer switch is separate equipment, evaluated to a different standard. IAEI notes that “transfer switches are evaluated to ANSI/UL 1008, the Standard for Safety for Transfer Switch Equipment,” and manual (non-automatic) transfer switches are listed in their own product category (IAEI).

Manual transfer switch

Typically a small sub-panel beside your main panel. Your electrician moves a set of essential circuits onto it. Each has a switch to choose LINE (grid) or GEN (backup). You connect your generator or power station, start it, and flip the circuits over.

Automatic transfer switch (ATS)

Detects a grid outage and switches circuits to backup power automatically, then switches back when the grid returns. Standard with standby generators, and increasingly offered by home-battery and power-station brands as smart panels.

Pros of a transfer switch (general guidance)

• Pre-selected circuits make it harder to overload your backup source.
• Simple to operate for anyone in the household.
• Automatic options exist — no one needs to be home.
• Works regardless of whether your main panel has a compatible interlock kit.

Cons of a transfer switch (general guidance)

• Limited to the circuits wired to it. Adding more later means more electrical work.
• More hardware and labour, so it’s often more expensive than an interlock.
• Takes wall space beside your panel.

Interlock kit vs transfer switch: side by side

	Interlock kit + inlet	Manual transfer switch	Automatic transfer switch
How it isolates the grid	Mechanical plate on main panel	Separate switch per circuit	Switches automatically
Standard (per IAEI)	Part of panelboard, UL 67	UL 1008	UL 1008
Circuits available	Any in the panel	Only those wired to it	Only those wired to it (or whole panel, depending on design)
Load management	You, manually	Built in by circuit selection	Built in; some have load shedding
Someone must be home?	Yes	Yes	No
Panel compatibility	Needs a kit identified for your panel	Works with most panels	Works with most panels
Relative cost	Often lowest (get quotes)	Moderate	Highest

Load calculations and sizing

Whichever you choose, make sure your backup source can handle what you connect. The East Hanover guidance notes that the 2008 NEC (702.5(B)) required a load calculation for generators with manual or automatic transfer switches, and that with manual transfer equipment “the user of the optional standby system shall be permitted to select the load connected to the system” (East Hanover Township PDF). NEC editions and local amendments change, so ask your electrician what your AHJ requires today.

General guidance for sizing:

• List running and starting (surge) watts for each essential load.
• Compare with your generator’s or power station’s continuous output and surge rating.
• Check voltage: many portable power stations are 120V only, which means 240V loads (many well pumps, dryers, some AC units) won’t run.

Which should you choose?

Choose an interlock kit if…

• Your panel has a listed interlock kit available and space for the backup breaker.
• You want flexibility to power different circuits at different times.
• You’re comfortable managing loads and keeping an eye on your generator or power station’s output.

Choose a manual transfer switch if…

• You want a simple, foolproof setup that others in the house can use.
• You know exactly which essential circuits matter.
• Your panel has no compatible interlock kit.

Choose an automatic transfer switch or smart panel if…

• You have a standby generator or home battery system.
• You want backup to happen without anyone at home — important for sump pumps, medical devices or frequent travel.

Still deciding what to connect? Compare all home backup power options. Still deciding between fuel and batteries? Read home battery backup vs generator: real costs. Renting? Neither option is for you without landlord approval — see backup power for apartments and renters.

Questions to ask your electrician

1. Is there an interlock kit listed for my exact panel model?
2. Does my local AHJ accept interlocks? What permit is needed?
3. Which circuits would you put on a transfer switch, and why?
4. Will my generator or power station’s output, surge and voltage run those circuits?
5. How will the neutral and grounding be handled for my backup source?
6. What inlet and cord do I need (for example L14-30 or TT-30)?
7. Can you label the panel and give me a written switchover procedure?

The bottom line

An interlock kit is a flexible, often lower-cost way to power any circuit from your main panel — as long as a listed kit exists for your panel and you manage the load. A transfer switch is simpler and harder to misuse, and the automatic version needs no one at home. Both are legitimate transfer equipment under NEC 702.5 when properly listed and installed. Either way, use a licensed electrician, get the permit, and keep a printed copy of our power outage preparation checklist by the panel.

General information only, not professional electrical advice. Requirements vary by location and code edition; your local authority having jurisdiction has the final say.

Frequently asked questions

Is a generator interlock kit legal?

A listed, panel-specific interlock kit is generally accepted as transfer equipment under NEC 702.5 when it is identified for your panel and installed per its instructions. Your local authority having jurisdiction decides, so have a licensed electrician install it with a permit.

Is an interlock kit cheaper than a transfer switch?

Interlock kits are often described as the lower-cost route because they use your existing panel, but total cost depends on the inlet, wiring, permit and labour. Get quotes for both from licensed electricians.

Can I use an interlock kit with a portable power station?

Potentially, if the power station's output, socket and voltage suit the inlet and circuits, and your electrician confirms the setup. Many power stations output 120V only, which limits which circuits can run.

What is the main disadvantage of an interlock kit?

You must manage the load yourself. Because any breaker can be switched on, it is easy to overload a generator or power station. A manual transfer switch limits you to pre-selected circuits.

Do I need a permit for an interlock kit or transfer switch?

Usually, yes. Local rules vary, but transfer equipment is electrical work that typically needs a permit and inspection. For example, East Hanover Township, New Jersey requires an electrical permit for a transfer switch used with a portable generator.