How to Install Hardwired Smoke Detectors

Installing a Smoke Detector

Lee Wallender 

Smoke detectors are required by electrical code nearly everywhere, and for good reason: Research demonstrates that smoke detectors save lives and property by alerting occupants to both early and smoldering fires. Fires often begin so quietly that occupants have no idea that their residence is on fire until it is too late. Smoke detectors remain awake all day and all night, continually sensing even the faintest evidence of smoke and fire.

Smoke detectors work only when they have a continuous electrical charge. When batteries die, smoke detectors are useless. Studies have shown that about 25-percent of all failed smoke detectors were caused by dead batteries.

What Is a Hardwired Smoke Detector?

Hardwired is a term that means that an electrical cable runs directly into an electrical device. The device does not plug into an electrical receptacle.

From the outside, hardwired smoke detectors look much like battery-powered smoke detectors and are located in the same areas within the home. The difference is that an electrical cable runs unseen behind the ceiling or wall, directly into the back of the smoke detector. The electrical cable provides power to the smoke detector all the time, except in the event of a power failure. Should the power fail, the on-board battery takes over and continues to power the smoke detector.

Codes and Regulations

Consult your local permitting agency for codes and regulations regarding hardwired smoke detectors. In many communities, hardwired smoke detectors are increasingly being required in new-construction and home remodeling work. Even if code does not require you to install hardwired smoke detectors in your home, you may wish to consider doing so since they are more reliable and thus safer than battery-only smoke detectors.

When to Install Hardwired Smoke Detectors

Because all work related to hardwired smoke detectors is located indoors, this project is not season-sensitive; they can be installed at any time during the year.

Safety Considerations

Make sure that all electrical circuits running to the smoke detectors are flipped off. Be sure to check all connections with a voltage detector before working on them. Wear eye protection at all times.

Project Metrics

  • Working Time: 4 hours (for two smoke detectors in two adjoining rooms)
  • Total Time: 5 hours
  • Skill Level: Expert
  • Material Cost: $50 to $100

What You'll Need

Equipment/Tools

Materials

  • Hardwired smoke detectors
  • Electrical boxes
  • Wire nuts
  • 12/2 NM cable
  • 12/3 NM cable

Instructions

Mark Locations for the Smoke Detector Boxes

Find the best locations for the smoke detectors. Ceilings are generally the best place for smoke detectors since smoke rises. If installing the detector on a wall, install it within 12 inches of the ceiling. Use the stud finder to locate the ceiling joists or wall studs. Hold the electrical box backward and use the perimeter of its face as a template. Draw around the perimeter with a pencil. Be sure to locate the boxes directly next to joists or studs.

Cut the Drywall for the Box

With the drywall saw, cut out the drywall for the two boxes.

Install the Cable to the First Box

From the electrical service panel (otherwise known as the circuit panel or fuse box), run 12/2 NM cable, or otherwise as indicated by the device's instructions, to the area of the first box.

Install the Cable from the First Box to the Second Box

From the first box, run 12/3 NM cable, or otherwise as indicated by the device's instructions, from the first box to the second box.

Connect the Wires

In the first box, using wire nuts where applicable:

  • Connect the two black wires, adding the black wire from the second smoke detector
  • Connect all white wires
  • Connect all bare copper ground wires and pigtail them to the box
  • Connect the red wire to the interconnect wire from the smoke detector, usually marked as yellow

Mount the Bracket to the Box

Put the smoke detector mounting bracket on the box, aligning it with the two screw holes. Feed the wires through the bracket. Screw the bracket into place.

Plug in the Smoke Detector

Attach the smoke detector via the three-prong connection in the back of the detector.

Install the Backup Smoke Detector Battery

Insert the battery into the smoke detector, aligning positive and negative terminals in the correct configuration. Replace the cover.

Attach the Smoke Detector to the Base

All hardwired smoke detectors attach to their bases differently. Consult the instructions. Generally, though, you will need to slide notches on the detector into slots on the base, then twist.

Repeat for the Second Smoke Detector

Repeat the process for the second, connected smoke detector with the wire connection step. You will be connecting fewer wires if this is an end-run smoke detector.

Attach the Cable to the Breaker

At the service panel, remove the front panel. Attach the electric cable feeding the smoke detectors to an appropriately sized circuit breaker. Generally, this will be a 20-amp circuit breaker.

Turn on the Circuit Breaker and Test

Replace the front panel, then flip on the circuit breaker for the smoke detectors. Test each circuit breaker by pressing its test button.

Since hardwired smoke detectors have an interconnect feature, make sure that other smoke detectors sound when one smoke detector is tested.

Tip

Make sure that you always wear eye protection when turning on the circuit breaker for the first time after wiring it to the smoke detectors.

When to Call a Professional

Hardwiring smoke detectors is an advanced project that has a direct impact on your safety. If you feel at all uncomfortable about your ability to install these devices, call a licensed electrician for assistance.