Transfer Switches and Interlock Kits

A transfer switch is the piece of equipment that makes it safe to power your house wiring from something other than the utility. Everything else in this kind of installation is detail; this is the part that matters.

The problem it solves

If your house can be fed by two sources, the utility and a generator, then the one thing that must never happen is both being connected at once.

If a generator’s power reaches the utility line, it goes through your service transformer backwards. That transformer steps the voltage up, meaning your modest generator can put thousands of volts onto the line outside your house. A utility worker restoring service, who has every reason to believe that line is dead, can be killed by it.

It can also destroy your generator and start a fire when utility power returns out of phase with it.

A transfer switch prevents this mechanically, not electronically and not by asking you to remember. It is physically impossible for both sources to be connected simultaneously.

The thing you must never do

The dangerous shortcut is a double-ended extension cord, male plug on both ends, running from a generator into a wall outlet. It is widely known as a suicide cord.

It kills people in three ways. It backfeeds the utility line, with the consequences above. Whichever end is not yet plugged in has live exposed prongs. And it bypasses your panel’s overcurrent protection entirely, so the house wiring is fed through a circuit with nothing protecting it.

There is no version of this that is acceptable, no matter how briefly, how carefully, or how sure you are that the utility is not working nearby.

Interlock kit or transfer switch

Two ways to achieve the interlock, and the right answer depends mostly on budget and how many circuits you need.

An interlock kit is a metal plate fitted to your existing panel. It slides between the main breaker and a dedicated generator breaker so that only one can be switched on at a time. The mechanical impossibility is the safety mechanism.

It is much the cheaper option, and it gives access to every circuit in the panel, so you can choose what to run at the time based on what your generator can handle. The tradeoffs are that it is manual, it must be a kit approved for your specific panel make and model (a generic plate is not acceptable and will fail inspection), and it relies on you managing the load sensibly.

A transfer switch is a separate device, typically feeding a subpanel containing selected circuits. It is tidier, unambiguous about what is backed up, and available in automatic versions that sense an outage and switch over unattended.

It costs more, and the set of backed-up circuits is fixed at installation, so getting that selection right matters.

Automatic versions are worth it only if power needs restoring when nobody is home, which usually means a sump pump, medical equipment, or a business. They typically pair with a permanently installed standby generator and cost several times a manual setup.

Choosing circuits

Every circuit you back up increases the generator size and the cost. Be ruthless.

Usually worth it: furnace or boiler controls and blower, well pump, sump pump, refrigerator, a few lighting circuits, a few outlet circuits, and whatever your internet equipment is on.

Usually not: electric water heater, electric range, electric dryer, air conditioning, and anything else with a heating element or a large compressor. These dominate the generator size for something you can live without for a day.

The heating question is the one people fight hardest. A gas or oil furnace needs only its blower and controls backed up, which is a modest load. An electric furnace or heat pump is an entirely different proposition and often the reason a project becomes unaffordable.

Sizing the source

Add up the running watts of the circuits you have chosen, then add the largest motor’s starting surge on top of that total. A well pump can surge to several times its running draw.

Two things people underestimate: motor starting surge, which sizes the generator more often than the running load does, and the duty cycle of thermostatic loads, which means the total is rarely all running at once.

For battery systems rather than generators, the same figures apply plus the question of how many hours of storage you want, which is the expensive part.

Code, permits, and who does the work

NEC Article 702 covers optional standby systems, which is what this is. The requirements include the interlock itself, proper grounding and bonding, correct conductor sizing, and labeling at the panel.

A permit and inspection are required in essentially every jurisdiction. This is not a formality to be worked around. An unpermitted alteration to your service equipment is a problem when you sell the house, potentially a problem with your insurer after a fire, and the inspection exists to catch exactly the errors that make this dangerous.

In most jurisdictions the panel work requires a licensed electrician. Some allow a homeowner to work on their own dwelling under permit. Check locally rather than assuming, and be honest with yourself about whether working inside a live panel is something you should be doing.

One detail worth knowing because it is easy to get wrong: the neutral-to-ground bonding must exist at exactly one point in the system. Many portable generators are bonded internally, which conflicts with the bond at your service panel and can defeat protective devices. Whether your generator needs to be a bonded or floating neutral depends on the switch type. This is the sort of thing inspections catch, and the sort of thing an unpermitted install gets wrong silently.

What good looks like

• A panel-specific interlock kit or a listed transfer switch, installed under permit and inspected.
• A proper inlet box on the outside of the house, so the generator connects with one cord to a fixed, weatherproof point.
• Clear labeling at the panel: which breaker is the generator, what the switching procedure is, and in what order.
• A written procedure somewhere near the panel, because you will be doing this in the dark, in bad weather, probably having not done it for a year.
• The generator outdoors, well away from windows and vents, with working CO alarms inside the house.
• Tested twice a year, under load, before you need it.