September 5, 2008
Updating Electrical Circuits
Q. I read recently your column on updating a home electrical system, and I have a question or two about electrical circuits. First, is there a rule of thumb as to when to use a particular size breaker switch (15 amps, 20 amps or 30 amps)? Also, what type of wire should be used with what type of breaker switches? What goes with 12-gauge wire, for example?
For my last question: If you put high hats in the ceiling, is there a limit to how many you can have on a single circuit? -Frank Spano, Malverne
A. All good questions. Let's begin with breaker switches, which are safety devices located in the service panel. The breakers turn electricity to a circuit on or off and help divide the household current into several circuits. Most of these circuits carry 120 volts. For larger applications, such as electric ovens, clothes dryers and air-conditioning units, there will be a few 240-volt circuits, too. If the outlet on a circuit draws too many amps, the circuit overloads and the breaker is automatically "tripped" or turned off. This is how the breaker switch protects the electrical system.
One 120-volt circuit may supply power to a series of lights or receptacles. Usually, dishwashers and refrigerators have their own dedicated circuits (one 120-volt circuit for each appliance).
To determine if a circuit has enough power or capacity, you need to do a little math. Circuit capacity, measured in watts, is determined by multiplying volts times amps. So if you have a 120-volt circuit controlled by a 15-amp breaker, you have circuit capacity of 1,800 watts (120 x 15 = 1,800). With a 120-volt circuit controlled by a 20-amp breaker, the capacity is 2,400 watts. To work the other way or determine the size of breaker you need for a circuit, use this equation: watts divided by volts = amps.
There are rules for wire usage, too. Those gauges are important. For standard 120-volt circuits, 14-gauge wire should be used on a 15-amp circuit, 12-gauge wire should be used on a 20-amp circuit, and 10-gauge wire is used on a 30- amp circuit. In most cases, if a circuit is going to be used for heavier appliances or power tools, a 20- or 30-amp circuit will be used. For lighting and smaller appliances, a 15- amp circuit is appropriate. For 240- volt circuits, a special four-strand wire should be used.
To find how many high hats you can use on a 120-volt circuit, you'll need to know the number of watts for each high hat. (Twelve high hats at 60 watts each would be using 6 amps; 720 total watts divided by 120 volts=6 amps.) Circuit capacity should not be pushed right to the limit, so most electricians try to leave a little cushion when calculating capacity. The rule of thumb is to allow 100 watts for each amp. That means a 15-amp breaker would be limited to 1,500 watts (a little less than the 1,800 maximum).
In addition to matching the correct wire gauge and breaker switches, remember to use the proper 120-volt receptacles. Grounded receptacles should be labeled by the manufacturer with the correct amperage and voltage.
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