Calculating Safe Circuit Loads

Doing the Math With Ohms Law

Power cords in outlet
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The majority of the circuits in your home are either 15-, 20-, or 30-amp circuits. They are located within your electrical circuit panel. But what is an electrical panel? An electrical circuit breaker panel is the main distribution point for electrical circuits in your home. It usually provides between 100 and 200 amps of power to your home, depending on your home’s load demand. Power comes into your home from the utility company, through a service entrance. It flows through an electrical meter, through an electrical disconnect and then to the main breaker in your electrical panel.

First, Locate the Main Breaker

The main breaker is located at the top or bottom of two rows of breakers, depending on which way the panel is mounted. The main breaker is marked with the value of protection (like 100 amps) on the breaker handle. This breaker is either factory mounted or can be added by either bolting it in or snapping it into place. Bow you may ask what the main breaker is and for good reason. 

Your electrical panel is full of circuit breakers, running from top to bottom of the service panel. The odd-numbered breakers are located on the left and the even-numbered circuit breakers are on the right. Atop the many circuit breakers is a larger circuit breaker that is used to turn the entire circuit breaker panel on or off. It is known as the main breaker. It plays probably the most important function in the whole circuit breaker panel. It is a means of disconnect for the entire panel. But you may ask what makes it different from the rest of the circuit breakers within the panel.

So, what is the main breaker anyway? You'd think that it is something special having a name like that. Actually, the main breaker is the breaker that the feeder wire connects to. This breaker is a two-pole breaker that is connected to 240 volts to power your home. The main breaker acts as the disconnecting means to the entire power load of your breaker box. With it off, there is no power being fed to the buss bar that feeds the branch circuit breakers. These dish out power to everything you power up in your home, but to a limit of the breaker size and this is called the electrical load.

Now, Calculate Safe Circuit Loads

But how much should you actually load each of these circuits to maintain a safe circuit load?

Electrical circuit load capacity is the total amount of power that your home actually will use. In order to decide how big of an electrical service is needed in your home, one has to do a little math homework. Older homes often only had a 60-amp electrical service, connected to a fuse panel. I actually lived in one that only had 120-volts coming into it. That's right, no 240-volt service! Now homes have 100- or 200-amp electrical services.

Calculating how much power both you and your electrical appliances use is necessary to calculate this number. As technology continues to advance, it seems we add more and more electrical loads to our home. I'm often asked how to figure this load and foresee the additional electrical load additions in the future. It really is pretty simple if you know what to look for and how to add up the loads.

To determine this figure, you must implement Ohm's law. By simply multiplying the amperage times the voltage, you’ll get the total wattage capacity for the circuit. However, what we’re looking for here is the total safe amperage capacity to load the circuit.

So to determine this, we simply multiply the total wattage times 80%. To demonstrate what these figures look like, look at the chart below for these common circuit amperages used in homes. The figures are based on 120-volt circuits.

Typical Home Branch Circuit Sizes

15-amp Circuit

  • Total Wattage Capacity: 15 amps x 120 volts = 1,800 watts
  • Safe Wattage Capacity: 1,800 watts x 80% = 1,440 watts
  • Safe Amperage Capacity: 15 amps x 80% = 12 amps

20-amp Circuit

  • Total Wattage Capacity: 20 amps x 120 volts = 2,400 watts
  • Safe Wattage Capacity: 2,400 watts x 80% = 1,920 watts
  • Safe Amperage Capacity: 20 amps x 80% = 16 amps

30-amp Circuit

  • Total Wattage Capacity: 30 amps x 120 volts = 3,600 watts 
  • Safe Wattage Capacity: 3,600 watts x 80% = 2,880 watts
  • Safe Amperage Capacity: 30 amps x 80% = 24 amps