Planning Paths for New Circuits

Electric cable often does not run in a straight line. Instead, it zigzags from one outlet to another in a circuit. Sharp turns and long trips do not bother electricity. You can snake cable up and down walls, along or across joists, and around obstructions without impeding the flow of electrons.

Saving money by planning - Even though electricity isn’t affected by bends and detours, cable is priced by the foot, and costs for extra feet can add up fast—in labor as well as materials. So to be economical, keep your runs as short and direct as possible. In new work, that’s not too difficult. In this illustration of a well-planned new home, most of the cables travel directly to their destinations. The plan saved money by installing the dryer outlet near the service panel, minimizing the amount of heavier, more costly cable needed for its 240-volt circuit. The plan also saved the electrician time because there was no need for many bends in the conduit to carry the wire through exposed locations. For example, the cable for the 240-volt receptacle for the electric stove takes as direct a path as possible along the basement sill plate.

Drawing up your plan - In planning an electrical layout, especially if you’ll be running more than one circuit, draw a floor plan of your home to scale, then mark the routes cable will travel. To estimate how much cable you’ll need, measure the distances involved, and add 10 percent for bends, unexpected detours, and waste. Be sure to add in another 6 to 8 inches to make connections each time cable enters or leaves a junction or outlet box.

Working in finished spaces - If you plan to fish through finished walls or ceilings, be prepared to use more cable than you would have to if the framing were exposed. You’ll also have some detective work ahead of you. Because cutting holes in walls and patching afterward takes so much time and effort, saving cable is a low priority when wiring in finished space. Search out the path that involves the least damage to your walls and the greatest ease in running the cable. Your first task is to determine exactly what’s in the space through which you want to run cable. If it’s an exterior wall, for instance, there will probably be insulation, which makes fishing more difficult. In addition, many older homes have fire blocking spanning the studs about halfway up the wall. If faced with these barriers, you will have to notch the wall surface at those points.

Planning Major Circuits

Adding a new circuit to your home’s service panel is an advanced project for which you may want to call in a professional —especially if your service panel is already crowded. Begin by seeing if you can add to an existing circuit. Failing that, make sure the service can be expanded. Look for an amperage rating on the main fuse, main circuit breaker, or disconnect switch. Older 60-amp service can’t be easily upgraded; call in an electrician. Newer 100-amp service may have enough reserve to handle a new circuit or two, and 150- or 200-amp service usually has plenty of capacity.

I. Try adding to a circuit. Different circuits have different capacities. If your need for extra capacity is modest, a few extra receptacles for a bedroom, for example, see if you can add to a general-purpose or small appliance circuit. (Never add on to a heavy-duty, single-use circuit.) Figure the total circuit load by totaling the demand of the appliances and fixtures . Then check the chart at right to see if the demand is within safe capacity. The safe capacity of a circuit, as prescribed by the National Electric Code, is 20 percent less than maximum capacity.

2. Estimate capacity needed. If you can’t add to an existing circuit, check the chart at right for the capacity your new circuit is likely to require. Rooms like living rooms and bedrooms that have about 10 light or receptacle outlets require only 15-amp capacity. Ideally, you should have one general-purpose circuit for every 500 square feet of living space. Some local codes require that lighting and receptacles be on separate general-purpose circuits. The kitchen is appliance-intensive and needs at least two 20-amp circuits. A bathroom gets by on one 15-amp circuit protected by a ground-fault circuit interrupter. Circuits for the garage, laundry room, and workshop need 15- to 20-amp capacity.

Circuit Need Selector

Location: living and dining rooms, bedrooms, hallways, finished basements

Circuits: A 15-amp general-purpose circuit for each 500 square feet. Separate circuits for lights and receptacles may be required by code. For a room air-conditioner, install a small appliance circuit.


At least two 20-amp small-appliance circuits and a 15-amp lighting circuit. An electric range needs a 240-volt circuit. A microwave oven may need its own circuit.


A 15-amp general-purpose circuit with GFCI protection.


A 1 5- or 20-amp general-purpose circuit (depending on tools and machinery, if any), with GFCI protection.


A 20-amp small appliance circuit for the washer and a gas dryer. An electric dryer needs a 240-volt circuit.


A 20-amp GFCI circuit. For larger shops run two 20-amp circuits or a separate circuit for lighting.


One 20-amp GFCI circuit.

3. Check the total service capacity. Now that you know what additional circuit capacity you’ll need, can your service capacity handle it? If you add up the amperage ratings for all the circuit breakers or fuses, plus the circuits you want to add, you may discover that the total equals or even exceeds the amperage rating of your service panel. Does that mean you can’t add new circuits? Not likely. Few if any of the circuits ever work at full amperage capacity. And some of your electrical fixtures and appliances never run at the same time—a furnace blower and an air-conditioner, for example. That’s why codes allow you to de-rate your service capacity. De-rating is a standardized reduction used when computing service capacity. The chart shows a de-rating calculation for a 2,000-square-foot house with 100-amp service, five small-appliance circuits, and two heavy-duty circuits.

In assigning wattage values, don’t count each general-purpose circuit. Instead, use 3 watts per square foot of house area. Small-appliance circuits rate at 1,500 watts each. Use the full wattage rating for heavy-duty circuits. If two items never run simultaneously, ignore the one that draws less. Rate only the first 10,000 watts of the total at full value, then calculate 40 percent of the remainder. Divide the total by 240 volts. The answer, 64.2 amps, shows that the system could accommodate more circuits.

4. Check for room in the box. Once you’ve decided the type and capacity of your new circuit, see if you have room for it in your service panel. If your panel has circuit breakers, you might find a blank space or two. (Unbroken knockouts on the panel indicate space for a breaker underneath.) If not, you may be able to double up two circuits by replacing an existing breaker with a tandem device, also called a skinny. In a fuse box you might find an unused terminal and socket that could be used. More likely, you’ll have to add a secondary fuse box called a subpanel.

Adding Major Circuits

Caution! Remember that the main breaker or main fuse in a service panel does not de-energize the main power cables coming into the box. If you cannot shut off power to the service panel, work carefully or call the power company to disconnect your service temporarily.

1. To install new circuits, shut off power to the box. Before working on the box itself, work backward from the new electrical installation. Mount boxes, connect them with cable, and run wiring back to the service panel in what the pros call a “home run.” Next comes the serious business of adding and tying into a new circuit. First, look for your home’s main disconnect switch. You may have a switch outside the house near the meter (as shown) or inside near the service panel. If you have such a remote disconnect, flip the switch or pull the fuse. Then open the service panel and test it. If your main disconnect is part of the service panel, seek advice from an electrician, an electrical inspector, or your utility company. To make sure there is no power coming into the service panel, you may have to ask the utility company to have the meter pulled and reconnected later.

Outdoor Shutoff Switches:

■ If your indoor service panel is located more than 5 feet from the meter, an outdoor shutoff switch is required.

■ If you have an outdoor shutoff switch, it’s a good idea to keep it secured with a padlock. Otherwise, anyone passing through your backyard can easily turn off the power to your entire house.

2. Test to make sure power is off. Check that power is off, that main breakers are off or the main fuses are removed. Stand on a board, rubber mat, or other insulator. Being careful not to touch electrical or plumbing fittings, remove the cover plate. Test the terminals of the main power lines with the probes of a voltmeter. If you’ve got a reading, there is power to that point.

3. Hook up the new breakers. Punch out the center from a convenient knockout on the circuit panel box. If needed, pry out one or more of the knockout’s concentric rings to make a hole the size of the cable connector you’ll be using. Strip the cable sheathing, allowing for enough wire to reach the neutral bus bar as well as the blank space where you’ll install the new breaker. Connect the cable to the box. Inside the service panel, run the white and ground wires (bare copper or green) to the neutral bus bar. (No ground wire is used if you are using conduit or BX.) For a 120-volt circuit, attach the black or red wire to the terminal of a single-pole breaker. To finish the job, simply clip the breaker onto one of the hot bus bars.

Some 240-volt appliances, such as water heaters, do not require a third neutral wire. For this type, connect two hot wires to a two-pole breaker (twice as wide as a single-pole breaker), and attach the ground wire to the neutral bar. Most 240-volt circuits run a white neutral wire and are sometimes called 120/240-volt circuits. Connect the two hot wires to the two-pole breaker and the white wire to the neutral bar. GFCI breakers install somewhat differently.

4. Add a subpanel. If you have a crowded fuse box or are adding several circuits far from the main service panel, connect them to a remote subpanel. Then make just one home run back to the main panel. You will need a new panel with breakers or fuses, as well as three-wire cable with wire sized to handle the total amperage that the entire subpanel will draw. Connect the cable at the main service panel with terminal lugs.

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