Smoke Detector Wiring 101

Smoke alarm mounted to a wood ceiling.

Not all municipalities have building codes about smoke alarm installation and location. The general rule, though, is that they should be placed at critical locations on every floor in the house. There are a few different types of smoke detectors available, but they can generally be classified as either AC smoke alarms (also known as interconnected or hardwired smoke detectors), or battery operated units. Each has its particular advantages for specific installations or usage.

In an older home, for instance, it’s not always possible to wire new cabling throughout most of the rooms in the house to set up a complete fire alarm system. In these cases, a battery-operated system not requiring the use of terminal boxes or wiring may be more appropriate, and will still remain in operation the event of a power outage, providing that the battery is good and regularly maintained.

But battery-powered units have the set-back to need immediate attendance when battery replacement is required, which should be once or twice a year according to U.S. Fire Administration (USFA) unless it's a unit equipped with a 10-year non-replaceable lithium-powered battery that will last the life of the alarm.

Some battery-operated alarms come with a wireless capability to communicate with other units through radio frequency transmission, which is sometimes preferred for ease and cost of installation. The weakness of these systems is obviously that they rely solely on batteries for power, while hardwired systems are physically connected to an energy source, allowing them to reliably alert all units. Hardwired systems often include a battery backup component.

smoke detectors with wiring diagram

Most municipality codes prevents the use of battery-operated units in new construction, requiring hardwired alarms in all new residences (Figure 2). With the unit separated as in Figure 3, the wires from the alarm are shown connected inside the junction box in the ceiling.

The hardwired systems are a bit more difficult and expensive to install, requiring terminal boxes and wiring through walls and ceilings, from the main electric panel and interconnecting between each of the alarms in the house. Some units are a bit easier to work with and maintain, as they come with a harness with a quick disconnect to plug into the back of the unit, as in Figure 4 here:

disassembled smoke detector with wiring

Before making any decisions on make, model, or type of alarms to install, it is best to contact your local fire department or city hall to find out about code requirements and restrictions in your area. In most cases, a minimum of one unit per floor level of the house is required, including the basement.

More reliable hard-wired units also have battery back-up for added safety, in case of power outages. A life-saving advantage of the interconnected systems installed in all the critical areas of the house is that each individual unit can triggers a general alarm throughout the entire home, no matter where the fire originated from, to warn all its residents of imminent danger.

Also worth mentioning is that the hard-wired system can be connected directly to alert the fire station in case of fire. If a human error was to cause someone to forget replacing a battery, the wireless system would become useless while the hard-wired would still be operating.

Interconnecting Multiple Smoke Detectors

A hardwired smoke alarm installation involves wiring one of the smoke detectors (closest to the voltage source) to a 120 VAC breaker in the main electric panel or tapping from a 120 V electrical box, wiring it using a 14/2 cable with a black (live), a white (neutral), and a ground wire, as shown by the diagram in Figure 5.

smoke detector wiring diagram

Hard-wiring also involves interconnecting all the other smoke detectors together with the first smoke detector using a 14/3 cable. The 14/3 cable is composed of a black, a white, a ground wire, but also an extra wire which can be red or yellow, the red being more commonly available.

As illustrated in Figure 5, the black wire is always the hot wire, the white is neutral, and the bare ground wire is always connected to all the other grounds and to the electrical box inside each one, making everything safe and grounded within the circuit.

There are three wires present inside the alarm—one is black, one is white and the third is red (or sometimes yellow). Some models might also have a green grounding wire. The line voltage is brought in through the 14/2 Romex coming from the main panel or a terminal box close by.

The black wire carrying 120 V is connected to every black wire from all the 14/3 Romex in the box but also to the black wire from the alarm while all the white wires are connected in the same manner including the white from the detector, thus providing the return path back to the main panel.

As soon as the breaker is switched on, it will supply 120 volts to all the alarms and all the black wires connected together, with a return path through the white wires also all connected together.

That leaves one free wire inside the detector, a red (or sometimes yellow) wire which purpose is to send a signal to all the other detectors or receive a signal from any of the other detectors connected to it.

That way, if an alarm detects smoke in the basement while the occupants of the dwelling are in a room on the top floor and out of hearing range, as soon as the alarm is triggered in the basement, the signal is simultaneously transmitted to the yellow wire triggering every other alarm on that circuit throughout the house.

Smoke Alarm Care and Upkeep

The smoke alarms must be installed and kept in a clean environment to work properly, as dust and grime can accumulate inside and alter how they operate.

The units must be dusted and vacuumed regularly.

On some models, the batteries must be replaced at least once every six months or once a year, depending on the model, so it is best to consult the installation manual to verify which one applies.

On some units supplied with lithium-powered battery, the battery will last the life of the smoke alarm and cannot be replaced. Those units must be replaced every 10 years.

Any smoke alarm must be replaced every 10 years since that is their life expectancy.

The units must be kept free of paint, stickers, decorations, or anything else that could alter their performance.

Smoke Detector Wiring FAQ

Can I use the existing wire harness to connect the new smoke detector?

Most modern smoke detectors are made to be very DIY-friendly so that anyone can easily switch out and upgrade these devices fairly easily. Typically, newer smoke detectors are made with a special plug, or wiring harness, that easily connects to the wiring in the wall or ceiling.

In most cases, you can upgrade or replace a smoke detector using the existing wiring if you stick to the same brand of detector. Devices made by the same company are likely to use a similar wiring harness, so you won't have to do any extra work to swap out smoke detectors.

Do hard wired smoke detectors detect carbon monoxide?

Smoke detectors are specifically designed to detect smoke. Carbon monoxide is a different type of gas and it does not create smoke, so a standard smoke detector will not recognize it.

However, there are some smoke detectors that have a carbon monoxide alarm also included. If you have one of these models, it will alert you to carbon monoxide with a different series of beeps than what you will hear when smoke is present.

You can also purchase a separate carbon monoxide detector that functions in a similar way to a smoke detector and sounds an alarm when harmful carbon monoxide is present.

How do you connect smoke detector wires?

Smoke detectors typically have two, three, or four wires, depending on the design. These wires will have different colors, which can be confusing.

Always connect the same color wires together. White wires inside the smoke detector should connect to white wires on the wiring harness, red to red, and so on.

Unless the smoke detector has specific instructions for you to do otherwise, only connect wires of like colors to each other. The different colors serve as a visual guide to make it easier to install smoke detectors.

Can you splice smoke detector wires?

In cases where no smoke detector has been installed before, it may be necessary to splice wires in order to create the proper connections for the detector. However, this should only be done by an expert because there are specific electrical codes that must be followed and only an approved connector can be used with a splice like this.

In other words, you should never attempt to splice smoke detector wires unless you are a professional electrician. It is dangerous not just while you are doing the work but it can create a later fire hazard if the splice is not done perfectly.

What is the color code for a smoke detector?

Most models of smoke detectors are made with two lights, a red and a green. The steady green light is there to indicate that the smoke detector is receiving power.

A flashing red light, which will flash on and off about once every minute, indicates that the smoke detector is working and sensing smoke.

Some models of smoke detectors also have a third light in yellow to indicate caution or potential danger.