Determining the safe gross electrical load for a subpanel requires several calculations. The load of the circuit is the total load that you will be applying to the subpanel. You'll need to know the square footage of the area you are supplying power to as well as the type of electrical devices and appliances that will be served by the subpanel. Load calculations for sizing a subpanel will also help you find the correct amperage for the subpanel circuit breaker and determine the cable size for the feeder cable supplying power to the subpanel.
NEC Load Calculations
All subpanel, breaker, and feeder calculations must be made in accordance with the local electrical code. Most codes follow the National Electrical Code (NEC) and use the NEC's "Long Form" for load calculations. Essentially, this is a form that helps you tally the wattage, or electrical usage, of the devices and appliances in the area served by the subpanel. Once you find the total wattage, you divide by 240 (volts) to find the minimum amperage required for the subpanel and its breaker and feeder cable.
Subpanel Coverage Area
The first load calculation involves adding up the total area of the portion of your home (or other building) that the subpanel will supply power to. This is an easy way to determine the load of the general lighting and receptacle circuits for the area. Measure the length and width of each room and multiply them together to find the square footage of the room. Add up the square footage of all of the rooms to find the total square footage. Multiply the total square footage by 3 (watts) to complete the calculation.
Calculating appliance loads varies by the types of rooms you're supplying power to. For example, if the subpanel will supply a remodeled kitchen, you'll need to account for a minimum of two small appliance circuits, at 1,500 watts each. Another category of appliances are those "fastened in place," such as a dishwasher, water heater, food disposer, or attic fan.
Wattage for large appliances, such as ranges, clothes dryers, and electric heaters or air conditioners, typically are tallied at a minimum specified wattage (such as 5,000 watts for a dryer) or at the appliance's nameplate rating, whichever is larger.
Once all appliance wattages are totaled, multiply by 1 if there are fewer than four fixed appliances; multiply by 0.75 if there are four or more appliances. This does not include the small appliance circuits, which are circuits feeding receptacles for plugging in portable appliances.
Finally, you may have to add 25 percent of the largest motor load to the wattage total (with some exceptions). This is additional wattage to meet the extra load large motors need when starting up.
Calculating Subpanel Wattage
To calculate the required wattage rating needed to supply the subpanel, multiply the total wattage (from the square footage and appliance calculations) times 1.25 to get the adjusted load. This safety adjustment is required by the National Electrical Code and provides a buffer for voltage drop on the feeder circuit. Voltage drop is a loss of voltage that occurs when electricity travels over long runs of wire or cable.
Subpanel Circuit Breaker Sizing
The circuit that feeds the subpanel must be protected by the appropriate size of the circuit breaker to prevent overheating of the feeder wiring. To calculate the breaker size, simply divide the adjusted wattage by 240 volts to find the rated amperage needed for your subpanel. Often, the result is not a common circuit breaker size. and you can simply round up to the next higher size of the breaker. For example, if the load calculation comes out to 48 amps, you should use a 50-amp breaker to protect the circuit. Feeder circuits supplying subpanels are 240-volt and require a double-pole circuit breaker.
Subpanel Wiring Sizes
The wiring feeding the subpanel must match or exceed the rated circuit breaker size, not the calculated load of the subpanel. That means that if the breaker is rated for 50 amps, the feeder cable wiring should be rated for 50 amps or more. However, if there is a long distance run for the feed, the next larger wire size should be used, to account for voltage drop. Determine wiring size using a wiring size chart that lists the wire types and wire sizes based on the application. Use the chart for initial estimation only. Again, all system design must conform to the local electrical code.