RENOVATION : Need Circuits? Start With the Service Panel

Two things determine the expansion capacities of your home’s electrical system. Most important is the amp rating of your service entrance. The other is the amount of circuit space left in your service panel.

To determine the rating of your service panel, open the panel cover and look for the amperage designation near the main disconnect. You should see the amp rating near the panel’s brand name.

Your ability to install new circuits will depend on the number of breaker or fuse spaces left in the panel. A home’s wiring will seldom need all the breaker slots provided in a 100-amp or greater panel. Each vacant breaker slot will hold one new 15- or 20-amp circuit. If you have two vacant slots, your panel will accommodate a new 240-volt circuit.

If you want to expand the service of an older 60-amp system, you’ll have to determine how much expansion your system will handle. The first thing to do is to make a map of your home’s existing circuitry. Tracing circuits sounds complicated, but it need not be. All you’ll need is a floor plan drawing of your home and a voltage tester or lamp.

Start by drawing the perimeter of your home on graph paper. Using a simple one-eighth-inch to 1-foot scale (each little square on the graph paper equals 1 square foot of living space). Draw in approximate locations of all interior walls and indicate the location of each outlet box and each permanent light fixture.

Begin by removing the fuse or tripping the breaker on the first circuit in the panel. Mark that breaker or fuse No. 1. Then take your voltage tester or lamp and plug it into all outlets near the circuit you’ve just interrupted. In addition, determine which light fixtures do not function. On the map next to each outlet and light fixture that doesn’t work, mark the number 1.

Now restore power to the No. 1 circuit on the panel and remove the fuse or trip the breaker to the next circuit in the panel, and mark that breaker or fuse No. 2. Repeat the process for all circuits, being sure to number each circuit in the panel. When you are finished, every outlet and fixture on your map should have a number. Tape the map to the inside of the panel cover for future reference.

To determine total circuit amperage, each permanent light and outlet should be rated at 1.5 amps. This means that a 15-amp fuse or breaker (on No. 14 wire) will support 10 outlets. A 20-amp fuse or breaker (on No. 12 wire) will support 13 outlets. If your mapping identifies a circuit that can support additional outlets, you can tap into that circuit at any location to add new outlets.

If your map identifies two circuits with so few outlets that both circuits could be protected by a single fuse or breaker, you can combine those two circuits in the panel, thus freeing one whole circuit for expansion.