Homeowner’s Guide to an AC Fuse Box

Updated: Jul. 18, 2023

Fuse boxes were the precursors to modern circuit breaker panels and are largely obsolete. But some large appliances are still protected by fuses even today.

Back in the 1950s, as the post-war building boom was in full swing, electric service panels with circuit breakers began to replace fuse boxes in North American homes. The changeover surged in the 1960s and was largely completed in the 1970s. But even today, some holdouts remain.

Fuses and circuit breakers perform the same function, shutting off the flow of electricity under three possible overcurrent conditions: overload, short circuit or ground fault.

Because they are resettable, circuit breakers are far more convenient. When a breaker trips, you just flip a switch to turn the power back on. And if it trips again, you know right away there’s a problem that needs diagnosis and repair.

When a fuse blows, you have to replace it. If it blows again, you need an electrician to determine what’s wrong.

Although fuses are mostly gone from service panels, they still perform important functions around the house. One is protecting large appliances like air conditioners, heat pumps and furnaces from current surges that could damage them. These require manual disconnects. If the manufacturer requires fuses, they’re usually included in the boxes that house the switches.

Many homes have these fusible disconnect switches. If your HVAC system stops working, one of the first things you should do is check the fuse.

What Is an AC Fuse Box?

The fuse box, the precursor to the modern circuit breaker panel, performs the same role as the control center for all the electrical circuits in the home. It features lugs for connecting the service wires that carry alternating current (AC) from the service provider. It has several spaces for connecting wires to power circuits in the building. A removable fuse protects each space.

Old fuse-box plug fuses are enclosed in round containers and have threaded bases that let you screw them into place. The fusible link is a strip of metal with a low melting point, visible behind a small glass window. When the current exceeds the fuse rating, the strip melts and stops electricity from flowing, and you can see the broken strip through the glass.

Fuses for protecting HVAC equipment are typically cylindrical cartridges that snap into fuse holders inside the manual disconnect box. They’re opaque, so when one blows you may not be able to tell by looking at it. You can test for a blown fuse with a multimeter, or do as some people do and simply replace it.

Where Is the AC Fuse Box?

You pretty much have to live in a home that predates 1950 to have a fuse box instead of a breaker panel. If so, it’s usually in the basement or a utility closet.

It could be anywhere. People — and the electrical codes — weren’t as fussy about placement back in the day as they are now. An old house I rented about seven years ago had a 60-amp fuse box on an interior wall right next to the front door.

If you’re looking for the fusible manual disconnect for your HVAC system, code mandates you must see it from the unit it controls, and it can’t be more than 50 feet (15 meters) away. The fusible manual disconnect for an outside air conditioning unit is usually on the wall of the house next to the unit itself. For a furnace, it’s usually on the furnace room wall or directly mounted to the side of the furnace.

What’s Inside a Fuse Box?

When you open the door of an old-school fuse box, you’ll see a row of glass fuses. There are rarely more than six of them, and each controls a circuit.

Old houses didn’t have as much electrical demand as modern ones. The amperage rating for each fuse is marked on it, and you can remove one by unscrewing it like a light bulb.

It actually works exactly like a light bulb: When you screw it in tightly, a nib on the tip of the threads contacts the hot bus. Electricity flows through the fusible link and onto the metal screw-shell threads to power the circuit.

Next to or above the row of fuses are two Bakelite or plastic fuse pullouts with metal handles for easy removal. The main disconnect cartridge fuses are behind these pullouts. The main cartridge fuses are cylindrical and held in place by metal fuse holders. When one blows, simply pull it out and replace it with an identical one.

How to Tell if a Fuse Has Blown

When a plug fuse blows, you can tell by looking through the glass to see the fusible link. It will be broken, and the glass may also be burned or discolored.

You can’t see the fusible link in an opaque cylindrical cartridge fuse. But when it blows, the fuse is usually — but not always — discolored.

If you suspect one of these has blown, remove the fuse and conduct a continuity test with a multimeter or continuity tester. Set the meter to measure ohms of resistance, touch one of the meter leads to one end of the fuse and the other lead to the other end.

If the meter reads zero resistance or it beeps, the fuse is good. If the meter reads infinite resistance and doesn’t beep (some meters display OL for Open Line), the fuse has blown.