Transfer switch vs interlock is the key decision when you connect a generator or portable battery to your house. Both do the same essential job: they make it impossible to feed power back onto the utility lines, which can kill line workers and damage equipment. They differ in cost, convenience and how many circuits you can use.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
Transfer Switch vs Interlock: Quick Comparison
| Interlock kit | Manual transfer switch | Automatic transfer switch (ATS) | |
|---|---|---|---|
| How it works | Mechanical plate on the main panel; main and generator breakers can’t both be on | Separate switch panel for selected circuits | Senses outage and switches automatically |
| Circuits available | Any in the panel (you manage load) | Only pre-wired circuits | Pre-wired or whole house |
| Convenience | Manual | Manual | Automatic |
| Typical use | Portable generator via inlet box | Portable generator | Standby generator, home battery |
| Relative cost | Lowest | Medium | Highest |
All require a permit in most areas and installation per the NEC and the manufacturer’s listing.
Interlock Kits
An interlock is a sliding or hinged plate that physically blocks the main breaker and a dedicated “generator backfeed” breaker from both being on. It must be listed for your specific panel. You flip off the main, slide the plate, and turn on the generator breaker and the circuits you want.
Pros: low cost, access to any circuit. Cons: manual, and you must avoid overloading the generator.
Transfer Switches
A transfer switch moves selected circuits (or the whole home) between utility and backup.
- Manual: you flip switches during an outage
- Automatic: standard with standby generators and many home battery gateways
Never Use a “Suicide Cord”
A male-to-male cord plugged into an outlet is illegal and deadly. It backfeeds the grid, can energize the plug pins, and bypasses all protection.
What Code Requires
NEC Article 702 (optional standby systems) requires transfer equipment that prevents inadvertent interconnection with the utility. Interlocks qualify when listed for the panel. Your AHJ may also require permits, an inspection and a properly bonded or switched neutral.
Batteries and Power Stations
Home batteries usually include a gateway or backup switch that acts as an automatic transfer switch. Portable power stations can connect through an inlet and interlock, or a transfer switch, just like a generator.
Worked Example: Sizing the Generator Inlet and Breaker
The generator’s output decides the inlet and backfeed breaker size. For a portable generator with a 30 A, 240 V outlet (NEMA L14-30):
| Item | Math | Result |
|---|---|---|
| Max power at the outlet | 30 A × 240 V | 7,200 W |
| Inlet | Match the outlet | 30 A, L14-30 inlet |
| Backfeed breaker | Match the inlet rating | 30 A double-pole |
| Wire | Sized for 30 A per NEC tables | typically 10 AWG copper |
For a 50 A generator outlet (NEMA 14-50 or CS6365), the same logic gives a 50 A inlet, a 50 A breaker and larger wire. Wire size depends on conductor type, insulation and run length. See wire gauge chart and what size fuse do I need.
Worked Example: Managing Load on an Interlock
With an interlock, every circuit in the panel is available, so you have to stay under the generator’s rating. For a 7,200 W generator, plan for about 80% continuous, or 5,760 W:
| Load | Running watts (example) | Running total |
|---|---|---|
| Refrigerator | 150 | 150 |
| Furnace blower | 500 | 650 |
| Well pump (240 V) | 1,000 | 1,650 |
| Lights and Wi-Fi | 300 | 1,950 |
| Microwave (when used) | 1,200 | 3,150 |
| Window AC | 1,000 | 4,150 |
Leave headroom for motor starting surges. Turn off large loads like the electric range, dryer and water heater breakers before switching over. See surge watts vs running watts.
Step-by-Step: Using an Interlock in an Outage
- Turn off all branch breakers, or at least the large loads.
- Turn off the main breaker.
- Slide the interlock plate to allow the generator breaker on.
- Connect the generator cord to the inlet, then start the generator outdoors, well away from doors, windows and vents.
- Turn on the generator backfeed breaker.
- Turn on circuits one at a time, starting with the largest motor loads.
To return to utility power, reverse the steps: turn off the generator breaker, slide the plate, turn the main back on, then shut down the generator.
Neutral Switching and Bonding
Most portable generators have a bonded neutral (neutral connected to the frame). When feeding a house through a transfer device that doesn’t switch the neutral, the house’s main panel already has the neutral-to-ground bond, so the generator may need a floating neutral, or the transfer equipment must switch the neutral, to avoid parallel ground paths. The right setup depends on the generator and the transfer equipment. Your electrician and AHJ decide. See grounding vs bonding and neutral vs ground wire.
Interlock vs Transfer Switch for Batteries
| Backup source | Common connection |
|---|---|
| Portable generator | Interlock + inlet, or manual transfer switch |
| Standby generator | Automatic transfer switch |
| Home battery (Powerwall, Enphase, FranklinWH) | Maker’s gateway or backup switch (automatic) |
| Portable power station with 240 V output | Interlock + inlet, or manual transfer switch, if the maker allows it |
Home battery gateways handle switching automatically. See critical loads panel explained and home battery backup for power outages.
Choosing Between Them
- Low budget, portable generator, willing to manage loads: interlock kit listed for your panel.
- Want only certain circuits, simple operation: manual transfer switch.
- Standby generator or hands-off operation: automatic transfer switch.
- Home battery: use the maker’s gateway.
For runtime and fuel planning, see generator vs battery backup for home.
FAQ
Is an interlock kit legal?
Yes, when listed for your panel and installed under permit. Local rules vary.
Can I install a transfer switch myself?
Some areas allow homeowner permits. Many require a licensed electrician.
Which is better for a portable generator?
An interlock with an inlet box is the most common, lowest-cost option.
What size interlock kit do I need?
Interlock kits are specific to the panel brand and model, not to generator size. Buy one listed for your panel, and size the backfeed breaker to the generator inlet.
Do I need a permit for a generator interlock?
In most areas, yes. Panel work typically needs a permit and inspection. Check with your local building department.
Can I use an interlock with a portable power station?
If the power station has a 240 V or 120 V generator-style output and its maker allows connection to home wiring, yes, through an inlet and interlock or a transfer switch. Follow the maker’s instructions.
Is an automatic transfer switch required for a standby generator?
Standby generators are normally installed with an automatic transfer switch, and NEC Article 702 requires transfer equipment for any optional standby source.
What happens if I forget to turn off the main breaker?
With a listed interlock, you can’t turn on the generator breaker while the main is on. That physical block is the whole point of the device.
Sources: NFPA 70 (NEC) 2023 Article 702; U.S. Department of Energy and CPSC portable generator safety guidance; NFPA 70 (NEC) Articles 250 and 702; generator and inlet manufacturer instructions. Accessed Oct 2026.
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