# How to Calculate Your Home's Electrical Load

## Brush up on some basic electrical knowledge about fuse boxes and service panels with these tips on how to use energy safely and efficiently.

A basic understanding of your home's electrical load is important knowledge for homeowners. After all, there will almost certainly come a time when you'll need to shut off or restore power. If you understand the fundamentals of how your home's electrical system works, you won't need to call an electrician for every small problem. Although fuse boxes and services panels can seem intimidating, the systems are fairly straightforward once you know the basics. We'll walk you through a few key tasks you might encounter when working with electrical loads.

## How to Calculate Your Home's Electrical Load

Different homes need different amp services. You may be wondering, "Do I need 200 amp service?" With a bit of math, you can figure out what kind of electrical load your home requires. A 60-amp service, for example, is probably inadequate for a modern home, while a 100-amp service is good for a home of less than 3,000 square feet that does not have central air-conditioning or electric heat. A home larger than 2,000 square feet that has central air-conditioning or electric heat probably needs a 200-amp service. Use the equation listed in the graphic above to deduce exactly how many amps your home needs.

Installing a new circuit is not difficult, but before you begin, make sure your service panel can handle the extra load. A service panel with too many circuits is dangerous.

Fuse boxes rarely have space for new circuits, so if you have a fuse box and need new service, replace the fuse box with a new service panel or install a subpanel. If you see an available slot in a breaker box, either an open space or a knockout that can be removed, chances are you can simply install a new breaker there and run cable to it. If there is no open space, local electrical codes might allow you to replace a single breaker with a tandem breaker, which supplies power to two circuits.

It is critically important that you do not overload your service panel. But how do you tell the difference between 100 amp and 200 amp service? A panel's total amperage is printed near or on the main circuit breaker, which controls all the circuits in the panel. Most breaker boxes are 100, 150, or 200 amps. Add the amperages of all the individual breakers in the box. The total might be more than twice the total amperage of the box. For example, a 100-amp service panel could have circuit breakers that add up to more than 200 amps. This is normal.

Take your total amperages and the name of the service panel manufacturer with you to meet with the inspector to ask about adding another circuit. This is also helpful to know when computing your home's power needs.

## Electrical Load Box Sizes

The size of your electrical load box will determine your home electrical capacity. Here are three types of systems you might encounter:

### Small Fuse Box

A small, 60-amp fuse box might be found in an older home that has not had its wiring upgraded. It can supply power to only one 240-volt appliance, such as an oven or a clothes dryer. Since most homes have more than one such appliance, this type of service panel is probably inadequate for a home of 1,200 square feet or more. Consider upgrading to a larger size if you plan to add more circuits to your electrical system.

### Medium-Sized Service Panel

Most homes require an electrical service of at least 100 amps. This is also the minimum panel amperage required by the National Electrical Code (NEC). A 100-amp service panel will typically provide enough power for a medium-sized home that includes several 240-volt appliances and central air-conditioning. If you plan to complete a major renovation or home addition, you might need to upgrade your electrical service for more power.

### Large-Capacity Service Panel

Many newer homes and some older large homes have a 150- or 200-amp service panel. This type of service might be required in a home that uses a lot of large electrical appliances and includes heating and air conditioning equipment. Upgrading to a larger service panel can give you more flexibility to use more circuits without worrying about overloading the system. A 400-amp service might be needed for larger homes with equipment that demands a lot of energy, such as a backyard hot tub or a home theater system.

## How to Inspect a Service Panel

A service panel should be located where adults can get to it easily but children can't. Any exposed cables leading to it should be firmly attached to the wall and clamped tightly to knockout holes in the panel. If there are any open holes, cover them with a knockout plug (\$2, The Home Depot).

If a 14-gauge wire is connected to a 20-amp circuit breaker or fuse, replace the breaker or fuse with one that is 15 amps to prevent the wire from overheating. In most cases, a 20-amp fuse or breaker should connect to a 12-gauge wire; a 30-amp fuse or breaker should connect to a 10-gauge wire.

Wires should run in a fairly orderly way around the perimeter of the panel. If you find a hopeless tangle, call in an electrician for an evaluation. You should also call in a pro if you find melted or nicked wire insulation, any signs of fire, or extensive rust.

In an older home, there's a good chance that new wiring has been added to a service panel. It might have been done by a pro, but it could be amateur work too, so check all the connections to ensure safety.