When it comes to designing a circuit breaker panel, you must understand how the appliances "pull" current. If an appliance attached to a 120V energy source requires 1200 watts to run, it will automatically pull 10 amps of current through the line. As current moves through the wire, the electrical resistance of the wire's conductive material will convert a portion of this electricity into heat. Because the subordinate, or "branch," circuits in a house are wired in parallel with the main power line, the amperage pull experienced by the main line is equal to the sum of these branch circuits. If the total branch circuit pull exceeds the maximum capacity of the main line, it will trip the main circuit breaker, cutting off all electricity to the entire building. To determine whether another branch circuit can be added to an existing electrical panel, you must first calculate the remaining amount of unused amperage in the main line.
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Find the main circuit breaker. Located on either the very top or very bottom of the columns of branch circuit breakers, it consists of a pair of linked switches known as a "double pole breaker."
Add together the numbers written on each of its two switches. This sum is the total amperage capacity.
Write down the number printed on each of the branch circuit breakers. For example, if the first breaker said "15 A," the second said "15 A," the third said "20 A" and the fourth said "30 A," you would write down 15, 15, 20 and 30.
Add together the amperage from all the branch circuit breakers. This sum represents the higher range of current which the panel can pull from the main line. However, this number is actually 25% higher than the average amount of current pulled by the panel; the overestimate is a safety measure, required by law.
Multiply the sum from Step 4 by 0.8 to determine the actual amps in the circuit breaker.
If you are trying to determine how much unused amperage the breaker has, you should subtract the answer from Step 4 from the total amperage capacity (Step 2). Don't use the answer from Step 5 for this particular calculation.
A Chicago-based copywriter, Andy Pasquesi has extensive experience writing for automotive (BMW, MINI Cooper, Harley-Davidson), financial services (Ivy Funds, William Blair, T. Rowe Price, CME Group), healthcare (Abbott) and consumer goods (Sony, Motorola, Knoll) clients. He holds a Bachelor of Arts in English from Harvard University but does not care for the Oxford comma.