# How to Determine the Size of Your Main Electrical Service

The main electrical service delivered to your house from the electrical utility company has a total available capacity, measured in amps, or amperes. Most homes have an electrical service of between 100 to 200 amps. Amps is a measurement of the volume of electricity flowing through wires, and this measurement can vary between 30 amps in very old homes that have not been updated to as much as 400 amps in a very large home with electrical appliances and extensive electrical heating systems.

Knowing the size of a house's electrical service can help you know if an update is needed, or if the service is large enough to handle an update, such as a remodeled kitchen or room addition.

### How Electrical Current Reaches Your Home

Electrical service reaches your home from the power utility through two 120-volt service wires that offer a combined 240-volts of power (voltage is a measurement of electricity's pressure, or rate of flow). The main service wires reach your home either through overhead service wires that enter a service mast and pass down through an electrical meter into your home or through underground wires that also pass through an electrical meter. The first stop for the electrical service once it enters your home is the main service panel.

### What the Main Service Panel Does

The main service panel is the distribution center that splits the main electrical service into individual branch circuits that run through your home to power the lights, outlets, and individual appliances. The main service panel is usually a gray metal box located somewhere along the inside surface of an exterior wall. It is often found in a utility area, such as a garage, basement, or furnace room. Where it is located in a finished living space, it is sometimes contained inside a finished cabinet mounted on the wall.

The main service panel includes two hot bus bars that run side-by-side down the panel, and one hot 120-volt service wire is attached to each of these bus bars. A home circuit connected to just one bus bar will deliver 120 volts of power, while a circuit connected to both bus bars will deliver 240 volts of power.

### Fuse Box vs. Circuit Breaker Panel

In most homes, the main service panel uses circuit breakers that control and protect the individual circuits. Circuit breakers are specially designed safety switches that prevent individual branch circuits from drawing more power than the circuit wires can safely handle. Virtually all home built since the early 1960s use circuit breakers as the power distribution method. Older homes also have circuit breaker panels if their electrical service was updated after 1960.

Where an electrical service was installed before the early 1960s and has not been updated, it may use a different style of power distribution—a fuse panel. A fuse panel protects individual circuits with screw-in or cartridge fuses.

The use of fuse panels and circuit breaker panels for residential wiring follows a historical pattern:

• 30-amp fuse panel: Installed before 1950, these service panels provide only 120-volt current. They provide insufficient power for modern usage and generally need to be updated.
• 60-amp fuse panel: Installed from 1950 to about 1965, 60-amp fuse panels provide 240-volts of power, but are still insufficient for most homes. An update is usually needed.
• Circuit breaker panel: Since the early 1960s, homes have generally been wired with circuit breaker panels that provide 240-volt current. Early services may provide 60-amps of power, while large houses built today may have 200 amps or more of power. Homes with 60-amp or 100-amp service often require an electrical service update during major remodeling or expansion projects.

### Tools and Supplies You Will Need

Flashlight (if needed)

### Instructions

#### Inspect the Electrical Meter

In many instances, you can determine the size of the home's electrical service simply by looking at the electrical meter outside the house. Look for the point at which the main service wires from the utility company enter the home. If electrical service is delivered by overhead wires, they will enter a metal service pipe that runs down an exterior wall of your home to the meter. If the meter is a glass dome mounted on a square metal base, the home probably has 100 amps of power, while a newer 150-amp or larger service will have a rectangular base that extends below the glass dome.

If you see a glass dome mounted on a round base that is the same dimension as the dome, or a meter located behind a flat glass window that is flush with the front of an enclosed metal box, you probably have 60 amps of electrical service.

#### Find the Main Service Panel

Now, locate the main service panel—a circuit breaker box or fuse box—inside your home. This panel will usually be on the opposite side of the wall where the exterior electrical meter is located. In most homes, this will be a vertical gray metal (or sometimes black) box mounted on a wall in a utility area. If the main service panel is in a finished living space, it may be enclosed in a finished cabinet of some type.

#### Locate and Read the Main Circuit Breaker or Fuse Block

Make sure the floor around the main service panel is dry, then open the metal door on the service panel. Inside the panel, you will see two rows of individual circuit breakers with small toggle levers. These individual circuit breakers are numbered, and they control individual branch circuits running through your home. The amperage of these breakers will be between 15 and 50 amps, usually.

At the top end of the two rows of branch circuit breakers, there will be a central circuit breaker that controls the power to the entire panel. This is the main circuit breaker, and its amperage rating will something like 60, 100, 150, or 200 amps. In rare instances, the main circuit breaker may be mounted at the bottom of the service panel. The main circuit breaker may be bolted in place, or it may be a snap-in breaker similar to those serving branch circuits.

This main circuit breaker dictates how much power is available to your entire house. It is a double-pole breaker, connected to both 120-volt service wires to power both hot bus bars running down through the panel. Turning this main breaker to the OFF position shuts off power to the entire house and all the branch circuits. The amp rating on this main circuit breaker identifies your electrical service size.

• Note: In rare instances, a home may have two main service panels, such as a 200-amp and a second 100-amp service. This normally occurs when a home has been updated with a major expansion, though it can also occur during new construction where the planned electrical load is quite high. In this instance, the home's total electrical service size is the combined amperage of the two service panels. However, where an electrical service has a subpanel that feeds off the main service panel, the subpanel does not add to the total amount of amperage available.

Although most homes now have circuit breaker panels with a main circuit breaker, if your home has an older electrical service, it may use fuses to control the individual branch circuits. In this instance, there will be a main fuse block with an amp rating that identifies the total service size of your home. This main fuse block has a metal handle, and by pulling the handle outward so the block separates from the panel, you shut off power to the entire house. By reading the amp rating on the cartridge fuses found within the main fuse block, you can identify the size of your electrical service. Most homes served by fuse panels have 60-amp or 30-amp service.

Fuse panels simply do not provide the amount of current demanded by most modern homes and the appliances they contain. Sooner or later, such an electrical service will need to be updated.

### Planning Electrical Service Size

When an electrical contractor computes the necessary size for electrical service during new construction or when updating an electrical system, the process involves computing the likely total demand of all appliances and fixtures, then sizing the electrical service to provide a comfortable margin. The calculations are fairly complex, so most electricians use a convenient calculator tool to properly size the main electrical service.

• 100-service generally provides enough power for a range, water heater, plus general lighting and receptacle outlets. In modern construction, 100-amp service is now installed only where the heating system and most of the heating appliances use gas rather than electricity.
• 200 amps generally provide enough power for an electrical heating system, electrical appliances, plus general lighting and receptacle circuits.
• 400-amp services are used in large homes with very extensive electric heating plus electrical appliances and lighting and receptacle circuits.

In some instances, a home may have two main service panels, such as a 200-amp plus a 100-amp service. This is normally done where a 200-amp service is not sufficient for the expected demand.