To figure out the safe amount of electricity a subpanel can handle, a few things need to be considered.
You need to know the total amount of electricity that the subpanel will be used for, which depends on the size of the space and what electrical devices and appliances will be connected to it.
This information will also help determine the right amperage for the circuit breaker and the size of the cable that will provide power to the subpanel.
NEC Load Calculations
When working out the subpanel, breaker, and feeder wire, it's essential to follow the local electrical code.
Most codes use the National Electrical Code (NEC), specifically the "Long Form" for load calculations.
This form helps calculate the total electrical usage of the devices and appliances in the space covered by the subpanel.
Divide the total wattage by 240 volts to find the minimum amperage needed for the subpanel, breaker, and feeder wire.
Subpanel Coverage Area
To calculate the initial load for the subpanel, you need to measure the size of the area that the subpanel will power, which includes the lighting and receptacle circuits.
To do this, measure the length and width of each room and multiply them to get the square footage of each room.
Add up the square footage of all the rooms to find the total square footage. To complete the calculation, multiply the total square footage by 3 watts.
Appliance Wattage
Calculating appliance loads depends on the type of rooms that the subpanel will power.
For example, if it powers a kitchen, you need at least two small appliance circuits, each rated at 1,500 watts.
Other appliances like dishwashers, water heaters, garbage disposals, and attic fans are considered "fixed in place."
Large appliances such as stoves, dryers, heaters, and air conditioners are rated at a minimum specified wattage or the appliance's nameplate rating, whichever is greater.
To calculate the total wattage, add up all the appliance wattages.
Multiply by 1 if there are less than four fixed appliances and by 0.75 if there are four or more appliances. Small appliance circuits aren't included.
You may need to add 25% of the maximum motor load to the total wattage to handle the higher load required by big motors when starting up (with some exceptions).
Calculating Subpanel Wattage
To find the required wattage rating for the subpanel, multiply the total wattage (from the square footage and appliance calculations) by 1.25 to get the adjusted load.
This safety modification is required by the National Electrical Code and acts as a buffer for voltage loss on the feeder circuit.
Voltage drop happens when electricity flows over long distances via wire or cable, causing a loss in voltage.
Subpanel Circuit Breaker Sizing
To prevent overheating of the feeder wiring, you need to protect the circuit that feeds the subpanel with the appropriate size circuit breaker.
To determine the rated amperage required for your subpanel, divide the modified wattage by 240 volts.
The circuit breaker size needed may not always be standard, so you can round up to the next larger breaker size.
For example, if the load calculation results in 48 amps, you should use a 50-amp breaker to protect the circuit. The subpanel feeder circuits are 240-volt and require a double-pole circuit breaker.
Subpanel Wiring Sizes
The wire that supplies the subpanel must be rated for at least the size of the circuit breaker, not the estimated load of the subpanel.
For example, if the breaker is rated for 50 amps, the feeder cable wiring should be rated for 50 amps or more.
However, if the feed has to travel a long distance, use the next bigger wire size to account for voltage loss.
To determine the wiring size, refer to a wiring size chart that specifies wire types and sizes based on the application. Use the chart for rough estimates only.
Once again, make sure that all system designs adhere to the local electrical code.
0 comments