Installing a Circuit Breaker in an Existing Panel
When adding a new circuit or replacing a faulty breaker, here is what you need to know to work safely in the panel.
Synopsis: Even when the main breaker is turned off, live conductors are still feeding into the box. Follow our steps to keep this installation safe and simple. Learn the right order to work in, the right tools to use, and the right mentality you’ll need when working with electricity. We include a detailed diagram to help you identify every wire and component beneath the panel, and we highlight five types of breakers to help you choose which one is right for your job. This article will guide you step-by-step through the installation, from the moment you shutoff the breaker, to the moment you label your new, working circuit.
If you’ve made it this far, chances are the hard work is done. You’ve built an addition, remodeled your kitchen, or set up a home office, and you’re ready to hook up new electrical circuits to the juice. Not so fast. While this part of installing a new circuit is the easiest, it’s also the most dangerous. Even when the main breaker is turned off, live conductors still are coming into the box. Any time I work in an electrical panel, I treat it as if it were live. Safety is my No. 1 concern.
When in doubt, test the draw
Most likely, a modern house with 200-amp electrical service and room for a new breaker is capable of carrying a larger load. But never assume your service can handle more draw. If you have a smaller or crowded panel, you should test the panel with a voltage meter to determine the available capacity.
Turn on every electric device that you can, and read the amperage being drawn on each main feeder. The incoming feeder on the left side of the panel feeds phase A. The one on the right feeds phase B. The total draw on the panel should not exceed 80% of its capacity.
For example, the draw on a 200-amp service should not exceed 160 amps. If the draw on the panel is more than 80% of the panel’s capacity, you should install a larger service.
If you have more than one space for a new breaker and if there is at least one space available on each phase, you should mount the breaker so that it draws power from the phase with the lower tested load. The phases feed alternating rows of breakers. The A phase feeds breakers one and two, the top row. The B phase feeds the second row, breakers three and four, and so on down the panel.
Install the breaker first
Snapping the circuit breaker into place is the easiest part of the job, but it’s important to take some safety precautions before you begin the installation. Remove any metal jewelry, and use insulated tools. Wear UV-rated safety glasses to protect your eyes in case any arcing occurs. And remember that the best way to avoid a short is to wire carefully.
Turn off the main breaker, and remove the panel cover. This will de-energize the hot buses, but remember that the feeders on the incoming side of the main breaker are still hot. The only way to de-energize the panel completely is to remove the meter, an impractical solution. The trick is to keep the power on the conductors and not where it can cause damage.
Make sure that the new breaker is turned off, and slide the hooks on the back of the breaker onto the mounting strip. The breaker should give some resistance before snapping into place. If it doesn’t feel as if it’s going to snap in, then start over.
From Fine Homebuilding #165
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