What Size Breaker Do I Need for a 100-Amp Sub-Panel?

Image by Pok Rie via Pexels

Image by Pok Rie via Pexels

Homeowners install sub-panels either to extend the available circuit space of their main panel or to connect a distant part of their property to the main panel. Whatever the reason, it’s essential that the installer correctly connects the sub-panel via a double-pole breaker. Safety should be the primary concern when working on anything with electricity within the home. It is recommended to always get a professional to do any work with electricity.

Typically, you need to connect a 100-amp sub-panel to the main panel via a correctly-sized breaker of 100 amps. You can still use a 100-amp sub-panel on a lower-amp breaker, but you will not be able to utilize the full 100-amp capacity of the panel. Installing a 100-amp sub-panel on a 200-amp breaker would allow the wires to overheat.

In this article, we will examine the uses of sub-panels and breakers in delivering power around the home. We will also look at the correct way to install the sub-panel while ensuring that safety is always the first consideration.

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Sub-Panels and Their Uses

Your electric company supplies power to your home via the electric meter to your main service panel. This circuit breaker panel protects your wiring, and the breaker design trips if there is an overload.

Should the capacity of this main panel be insufficient for the needs of the home, many homeowners then install a sub-panel. 

An electrical sub-panel feeds off the main panel and is a mini version of that main panel. Electricians refer to these as a service panel or circuit breaker sub-panel. People install sub-panels for a variety of reasons, such as separating electrical areas, creating space for circuits, and running new cables for a remodel.

Separating Electrical Areas

You may want to separate the electrical functions in one area of your property from another. For example, if there is a separate dwelling on the property or an office, it can be useful to have separate circuits. 

Creating More Space for Circuits

If the main panel has run out of spaces to install new circuit breakers, then you may need to install a sub-panel to allow for additional circuits.


When remodeling an area of your home, it can be useful to run new cables to a sub-panel near the new space. It can be challenging to reroute multiple wires, and this can be a solution since you only have to route one main cable needs to the main panel.

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Sub-Panel Capacity 

When adding a sub-panel, it is essential to consider how much capacity you require and how much the main panel can spare. If your main panel does not have extra capacity, then you might need to consider upgrading it before installing a sub-panel. 

For example, when adding a 100-amp sub-panel to power a shed or garage, you should ideally have a 200-amp main panel so that adding the extra power demand does not risk overloading the system. 

In theory, you can add a 100-amp sub-panel to a 100-amp main panel, but the main panel would have to have a significant unused capacity to allow this.

Also, the sub-panel must be correctly-sized for its needs. You will need to consider the extra load that you require to decide what sized sub-panel to install. It’s a good idea to add a small amount of spare capacity for any future needs.

Electrical Circuits

Each breaker on the main panel or a sub-panel connects a circuit of power somewhere in your home. These are generally 120-volt circuits (15-20 amps) that run most household items. In bedrooms and living rooms, there are usually 15-amp circuits that connect lighting and outlets in a specific area. 20-amp circuits typically supply areas like kitchens or garages where there are larger appliances. 

Some large appliances, such as air conditioners or range ovens, may even have a dedicated circuit due to the amount of power they need to pull. This is usually the case if they require more than half the circuit’s capacity. These circuits are generally 240-volt circuits that can pull 20 to 60 amps.

On the main panel, there would be a breaker specifically linking the sub-panel to the main supply. The sub-panel would, therefore, control a branch circuit from the main panel. This branch circuit would then get divided into multiple smaller circuits fed from the sub-panel.

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A breaker is a switch that can be manually shut off or automatically tripped when there is a failure in the system. This is a safety feature that enables the system to protect itself from electrical fires. The breaker’s rating in amps indicates the maximum amount of power of the circuit should provide.

A breaker consists of a fixed contact and a moving contact called electrodes. These contacts are closed when a breaker is functioning normally. If there is a fault, the contacts will automatically open, and you can also open them manually.

When the contacts are separated, an arc is struck between them, and this is known as the arc phenomenon. The current will continue to flow until this arc discharges, and the circuit is therefore interrupted. A circuit breaker, therefore, seeks to disrupt this arc as fast as possible.

On the main panel, there will be circuit breakers for each circuit. If there is a sub-panel, then there will be a large breaker that connects to it. This breaker would supply the feeder circuit that powers the sub-panel and would be a double-pole breaker with double-size levers.

Single-Pole and Double-Pole Breakers

Electricians typically use single-pole breakers for the smaller 15- to 20-amp circuits. These consist of one hot wire and one neutral wire. The use of double-pole breakers is for larger circuits and consists of two hot wires connected by a neutral wire.

Single-pole breakers have narrow switches on the panel and tend to power appliances such as lighting, fans, computers, televisions, etc. If there is a system fault or if it is overloaded, then the single breaker will trip to protect the circuit.

Conversely, double-pole breakers have double-switches. They usually power large appliances such as electric ranges, air conditioners, electric water heaters, or a sub-panel. If there is a short circuit caused by a fault or overloading on the circuit, then both switches will trip (source).

Connecting a Sub-Panel

Connecting a sub-panel can be a solution for various electrical challenges. The size of the sub-panel will depend on what it will power as well as how much excess capacity exists in the main panel. It connects to the main panel via a double-pole breaker and cannot exceed the amperage of the main panel.

Importantly, the breaker feeding the sub-panel must be correctly sized. A 100-amp sub-panel can be connected by a breaker up to 100 amps, depending on its requirements. You could use a smaller breaker, but then that would limit the load that the sub-panel would be able to deliver.

Generally, when installing a 100-amp sub-panel, it would be connected by a 100-amp breaker.

What’s also important is using the correct-size wires to feed the sub-panel (source). When installing a 100-amp sub-panel, you will need to use either 4AWG (American Wire Gauge) copper cable or 2 AWG aluminum, three-conductor cable. 

Aluminum cable is cheaper than copper cable and carries the risk of corrosion. Three-conductor cable comprises four wires — two hot wires, a ground wire, and a neutral. 

The panel itself must be attached to a wall or other secure surface. If it is installed outside, then it must be one that has been approved for exterior use. It will need at least three-feet clearance on all sides and be easy to reach so you can access switches. You cannot install it in a bathroom or a closet.

The sub-panel connects to the main panel via a circuit breaker in the main panel. This is known as a feeder breaker and is a 240-volt circuit breaker. Because it is a double-pole breaker, it will take up two adjacent spaces on the main panel, so you need to have or create capacity for that.

Connecting a sub-panel is best done by a qualified electrician. Many local codes make it illegal to install your own sub-panel, and some don’t allow homeowners to do any of their own electrical work (source). Installing a sub-panel will involve the following steps:

  • Run the feeder cable from the main panel to the sub-panel.
  • Connect the two hot wires to the sub-panel hot bus bars.
  • Connect the neutral wire to sub-panel neutral bus bar.
  • Connect the grounding wire to sub-panel grounding bus bar.
  • Attach the hot feeder wires to the feeder breaker at the main panel.
  • Connect the feeder neutral and ground wires to the main panel. 
  • Connect the feeder breaker into the main panel. 

Once connected, a sub-panel can then be set up with individual circuit breakers for the circuits it is being installed to service. Ultimately, the connections should all be concealed by a cover, and a homeowner should only access the breakers.

Consult An Electrician

An article such as this cannot cover all eventualities and codes that apply in all countries and all local municipalities.

For this reason, as well as for the safety of you, your family, and your home, it is best to consult with a local electrician before any type of electrical work.

When electricity is concerned, a little bit of knowledge can be deadly, and it is not worth taking the risk simply to save a little money.

Final Thoughts

Any type of electrical work should never be considered a DIY job with little basic knowledge of electrical work. Sub-panels offer great convenience to the homeowner. They can provide proximity to circuit breakers, and they can reduce the amount of wiring required to add a new area to your power source. They also enable homeowners to update or increase their power requirements for a relatively low cost. 

You must have them correctly and safely installed, following all the required guidelines and codes. Wherever possible, sub-panels should be installed by a qualified electrician so that breakers and wiring are correctly sized, and the installation is as safe as possible to prevent any avoidable electrical faults.

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