Are Furnaces Hardwired? What You Need to Know

Using a furnace is a great way to heat up residential or commercial buildings during harsh winters. But if you look at one closely, you probably won’t find an electrical plug. Are all furnaces hardwired, or is there a way to plug them into an outlet?

Furnaces are mostly hardwired into your building’s electrical system for safety reasons. However, the arrangement causes trouble if urban power goes out. You can connect the furnace to a generator to solve the issue, but you need to install a transfer switch, an interlock kit, or a plug set.

The rest of this article will explain why furnaces are hardwired, and you’ll learn how to implement different workarounds without risking your safety. 

Why Are Furnaces Hardwired?

According to NEC’s section 400.7 code requirement, you shouldn’t use flexible cords and cables for furnaces. That’s why these appliances usually leave the factory hardwired without a flex cord or an attachment plug. 

The NEC is against plugged-in furnaces for a good reason: they consume so much electricity that they need a dedicated circuit. When you have a plug-in furnace, you’re likely to plug it into a shared outlet with other appliances. As a result, the circuit gets overloaded, damaging the furnace and the other plugged-in electrical devices or even resulting in a fire. 

So, converting a hardwired furnace into a plugged-in constitutes an NEC code violation. However, if you decide to do it, only plug the furnace into a dedicated outlet to stay safe. 

The hardwiring won’t cause you any trouble as long as the main electrical supply works. To get the furnace up and running, you pay a professional electrician to hook it up to your junction box, and it’ll work permanently. 

However, sometimes extreme weather conditions such as ice storms may cause power outages that last a few hours to a few days. In these cases, you need a quick way to warm up your house.

How to Attach a Hardwired Device to a Generator

A generator is the best solution to a power failure, but the question is, how can you attach a hardwired device to a generator? Here are some solutions:

Change the Hardwired Furnace to a Plug-In

The easiest way to deliver backup power to your furnace is to replace the breaker box with a receptacle and a plug. 

To do this, you need a plug set, including a receptacle and a cord with a male plug. You’ll also need a generator cord or a 14 gauge 15 amp extension cord attached to a female plug.

Once you have these items, continue with the following steps:

Install the Plug

1. Shut off the furnace and disconnect it from the circuit breaker panel so you won’t get shocked.
2. Detach the furnace wirings. If you have a junction box or on/off switch, use a screwdriver to pull off its plate and access the device wiring from there. If a junction box isn’t included, go to the furnace housing to unhook the wiring.
3. Loosen the caps at the end of the neutral (white), hot (black), and ground (green) wires.
4. Before attaching the cord, remove one of the knock-outs on the furnace wall or switch wall. Pass the replacement cord through the hole. (If this is a permanent solution, also use a clamp to cover the hole and fixate the wire.)
5. Attach the furnace/junction box wires to the cord’s pigtail ends. Make sure you match them by color. 
6. Close the switch box.

Watch this video on how to run temporary power to a furnace:

Set Up the Receptacle

1. Buy a single outlet receptacle because furnaces need a dedicated circuit to meet their watt requirements. A two-way outlet with an extra device attached may overload the circuit and cause a fire.
2. Mount the receptacle in place of the breaker box so that the furnace won’t have a direct electrical flow.
3. Go to the panel and turn on the power. Before you plug in your furnace, test the new outlet to make sure it works. 

Attach an Extension Cord to the Generator

1. Install an extension cord on the generator and cover its end with a female plug.
2. When the power goes out, unplug the furnace from the main supply. 
3. Plug it into the extension cord running out of the generator.

Use a Power Transfer System

A safer, more convenient way to stay warm during an outage is to install a transfer switch between the furnace and main breaker. This way, all your house circuits are powered. So, you won’t have to convert the hardwired furnace into a plug-in. As you can imagine, this approach is much safer. 

Preferably, ask an electrician to install your transfer switch. However, if you’re in an emergency and have experience with wires, buy a reliable generator-to-furnace transfer switch, like the Reliance Control TF151W Transfer Switch, and go through these steps to install it:

1. Open up the switch. Ensure the black wire is connected to the gold screw, the white one to the silver comet, and the green wire to the green screw. 
2. Turn off the main breaker.
3. Install the switch on the wall and open up the furnace to get to the wiring.
4. Disconnect the black wire from the furnace breaker, and replace it with the transfer switch’s red wire.
5. Attach the white wire to the neutral bus and the green one to the green bus on the panel.
6. Connect the two remaining black wires and secure them with a cap.

Watch this video on how to install your transfer switch:

If you’re looking for a more affordable solution, follow the instructions in this video to connect a manual transfer switch:

Use an Interlock Kit

An interlock attaches directly to your panel, allowing your generator to safely back-feed a panel and power the entire house.

You don’t need to be a professional to install an interlock. A bit of experience with power drills and screwdrivers will suffice. Here’s what you should do:

1. Remove the panel cover.
2. Mark it according to the interlock bracket’s mounting holes and drill four holes.
3. Fix the bracket in position using a few screws.
4. You can’t switch on the generator power with this bracket while the main breaker is off and vice versa.

To install a generator interlock kit, watch the video below:

Avoiding the Suicide Cord

Some electricians hardwire furnaces to the back of an existing receptacle. So, when the power goes out, some people come up with the brilliant idea of feeding the furnace by attaching the generator to this outlet. They use an electrical cable with prongs on both ends and plug one end to the generator and the other to the home’s dead outlet.

Unfortunately, this is common, especially among those who don’t want to pay for a transfer switch or don’t like calling an electrician for a temporary issue. 

Although the trick may work fine as a temporary solution, it could be life-threatening. First, if you forget to disconnect the house circuit breaker switch, it can electrocute the power company lineman, and no one’s to blame except you. 

Second, you may receive an electrical shock since one end of the male plug will be exposed while you’re trying to attach the other end. Even if you’re careful enough not to get anyone killed, it’s a code violation and may also cause a major fire.

Final Thoughts

Furnaces are powerful heating devices with a voracious appetite for electricity. So, to avoid fires and other safety issues, manufacturers never include an electrical plug. 

However, a hardwired furnace only works as long as the building’s urban power is working. During an outage, you need a way to connect the furnace to a generator. To do this, you have three choices: 

• Convert the furnace.
• Use a power transfer system.
• Install an interlock kit.

Whichever option you choose, never get reckless with electricity. It’s best to have a professional electrician do the work and avoid temporary fixes. 

Alanna Greene

Alanna is an avid traveler who lives in Michigan. In addition to writing for Temperature Master, he also sells crafts on Etsy and takes long walks through the forests near her home.