What Is a Subpanel Breaker Box and When Is One Necessary?

Knightly Electrical Services
Jul 9, 2025
4 min read

Subpanels provide a number of benefits and may be required for code compliance.

From time to time, homeowners interested in having electrical upgrades or repairs completed will hear that they need a new subpanel breaker box installed. But while some homeowners may nod along and smile, many don’t really understand exactly what a subpanel breaker box is or why one is necessary.

Don’t worry; we’ll explain everything you need to know about subpanel breaker boxes, including their purpose, the benefits they provide, and the reasons they’re sometimes helpful (or even necessary), below.

What Is a Subpanel Breaker Box?

A subpanel may have between four and 20 or more individual breakers.

A subpanel breaker box, which is often simply called a “subpanel,” is a secondary electrical panel that is connected to your home’s main panel. In a nutshell, it draws power from the main panel and then distributes it to a specific area. For example, you may have a subpanel that provides power to your basement or garage. They’re also common in detached buildings.

They’re called subpanel breaker boxes because they contain circuit breakers, just like your main panel does. Subpanels often have far fewer breakers than main panels (some may have as few as four), but large subpanels, such as those supplying large garages or workshops, may have 20 breakers or more. These brakers help to provide the same kind of safety that the breakers in your main panel do, and they also make it easy to disconnect the power to the circuits connected to the subpanel when necessary.

It's important to note that subpanels do not have their own electric meter. They rely on power that’s already flown through your meter enroute to your main breaker panel.

Why Are Subpanel Breaker Boxes Helpful (or Even Necessary)?

Subpanel installation is definitely NOT a DIY project.

There are several reasons electricians often recommend having a subpanel installed, including:

· They help extend power to places that are located a significant distance from the main panel. This is simply more efficient than running individual circuits across long distances.

· They help keep your home’s wiring simple. Instead of needing to run multiple long wires for each circuit coming off the main panel, you can run one circuit to the subpanel and then distribute the power using shorter circuits to the surrounding area.

· They can provide more circuit spaces. Sometimes, you may run out of space in your main panel to add new circuits and accommodate new outlets or other devices. But by adding a new subpanel, you’ll have the capacity to add several (potentially as many as 20 or more) new circuits.

· They provide localized control. It’s often necessary to be able to turn the power to a circuit off from the breaker box. And because the main panel may not be nearby, a subpanel can provide this kind of convenient access.

· Sometimes, they’re simply required to comply with electrical codes. In some cases, electrical codes dictate that you have to install a subpanel. For example, if you’re installing a basement kitchen, hot tub, or some kinds of EV chargers, a dedicated subpanel is required.

There are other reasons your electrician may recommend installing a subpanel, but these are the most common reasons they’re installed in homes.

Subpanel Breaker Boxes: FAQ

There are a variety of reasons you may need a subpanel installed.

Still have questions about subpanel breaker boxes? Not quite clear on what they do yet? We’ll try to help by answering a few of the most common questions homeowners have below!

Why would you need a subpanel?

There are several reasons you may need a subpanel. For example, you may not have space in your home’s main panel to add more circuits, or you may need to run power to a detached building. Subpanels are very helpful in these cases.

What is the difference between a breaker box and a subpanel?

Although subpanels often have circuit breakers built into them, the term “breaker box” is usually reserved for a home’s main electrical panel.

Do I need an electrician to install a subpanel?

Unequivocally. Installing a subpanel is a complicated and potentially dangerous job that requires the kind of knowledge and skill a licensed electrician has. Additionally, there are a number of important code requirements you’ll need to follow to install them properly, and this is generally not something that the average homeowner is capable of accomplishing.

DIY subpanel installation may not only result in a lack of code compliance, but it may also put your home and family at risk of electrical fires.

How do I know if I need a subpanel?

The best way to know if you need a subpanel or not is to ask your licensed electrician. However, they’re often required when you’re having a large appliance installed in a new place (such as a basement kitchen, hot tub, or electric vehicle charger), or anytime you’re running power to a detached building.

You may also need a subpanel installed if you don’t have room for new circuits in your main electrical panel.

How much does a subpanel breaker box cost?

The costs associated with having a subpanel breaker box installed will vary from one home and situation to the next, as well as with the electrician you hire to complete the work. But broadly speaking, subpanels usually cost between $500 and $2,000 to install.

Knightly Electrical Services: Your Trusted Local Electricians!

Ryan, happily working on a customer's electric panel.

Whether you need a new subpanel installed, or you’re simply curious about what would be involved in your next home improvement project, the licensed electricians at Knightly Electrical Services are ready to help!

We provide a full range of electrical services, from electrical panel installation to surge protector installation to fluorescent fixture repairs.