Electrical capacity isn’t something you need to think about on a day-to-day basis, but it becomes important when planning upgrades, looking for new appliances, and installing kitchen or laundry essentials such as dishwashers and dryers.
Calculating how much power your home needs involves an electrical load calculation, knowing your home’s electrical capacity, and making sure the load is using no more than 80 percent of it.
What Is Electrical Capacity?
Electrical capacity is the maximum amount of electricity a circuit, panel, or generator can handle before it short circuits, trips—or in the worst-case scenario, causes an electrical fire. It’s measured in amperage (or amps).
You can estimate your home’s electrical capacity based on its age.
- Homes built before 1950 with knob-and-tube wiring and screw-in fuses typically have a capacity of 30 amps.
- Homes built between the 1950s and 1960s tend to have a capacity of 60 amps.
- Homes built from 1960-1980 (or upgraded older homes) will likely have a capacity of 100 amps.
- Homes from 1980 onwards have a minimum capacity of 200 amps.
- Newer homes have a capacity of 400 amps, with larger homes of 10,000 square feet and above seeing a higher capacity of 800 amps.
The main source of power in your home is an electrical panel: a metal box typically found in the basement, utility room, or garage. The panel brings in electricity from external wires outside and distributes it to the branch circuits in the outlets throughout your home. It also contains a circuit breaker, which is a switch that can be turned on or off manually, and will automatically shut the system down in the event of an overload.
Plugging in everyday phone and laptop chargers, table lamps, small appliances, and fans won’t make a big impact on your home’s electrical capacity, but knowing the limit becomes important when installing or upgrading larger appliances such as laundry machines or heaters.
Electrical Capacity vs. Load
Electrical capacity is the amount of electricity your home can support, while electrical load is the amount actually being used.
Calculating how much power your home needs involves looking at the amperage load all of your appliances and fixtures consume, and making sure that load never exceeds 80 percent of the electrical capacity. If the load is regularly above 80 percent of the capacity, it’s time to consider upgrading your electrical panel.
How to Calculate Electrical Capacity
The first step to figuring out electrical capacity is to calculate the conversions between watts, volts, and amps—since you’ll see all three of these terms used to describe electricity used. They can be converted in a couple of different ways:
- Volts x Amps = Watts
- Amps = Watts/Volts
Using these formulas will help you to understand all of the electricity in your home on the same measuring system, since appliances list their load in watts, while electrical capacity is often listed in amps, and outlets in the typical US home are 120 volts; while the overall system is 240 volts.
For example, a 20-amp, 120-volt branch circuit has a total capacity of 2,400 watts (20 amps x 120 volts), and with the 80 percent rule in mind, can support light fixtures and plug-in appliances adding up to 1,920 watts of power without the risk of circuit overload.
The same formula can be used to determine the capacity of the house’s overall electrical service. Because a home’s main service is 240 volts, the math looks like this:
- 240 volts x 100 amps = 24,000 watts
- 80 percent of 24,000 watts = 19,200 watts
In other words, a home with a 100-amp system can support up to 19,200 watts of power load at any given time.
How to Calculate Electrical Load
After you know the capacity of individual circuits and of the home’s full electrical service, you can then compare this with the load, which you can calculate simply by adding up the wattage ratings of all the various fixtures and appliances that will be drawing power at the same time.
You can calculate the overall amperage needed to power your home with these steps:
- Add together the wattage capacities and ratings of all lighting branch circuits, outlets, and permanent appliances.
- Subtract 10,000.
- Multiply by 0.4.
- Add 10,000.
- Divide the total by 240.
A typical 1,500 square-foot home has an electrical panel with a 200-amp capacity, or 38,400 watts (calculated by multiplying 200 amps x 240 volts, and multiplying the total by 0.8 for the safety margin).
Example Electrical Loads
| Total Capacity for a 1500 square foot home (at 80% use) |
38,400W |
| Lighting Circuit (at 3W per square foot) | 4,500W |
| Heat Pump | 5,500W |
| Air Conditioner | 3,000W |
| Microwave | 1,000W |
| Dishwasher | 1,800W |
| Stove (per burner) | 1,500W |
| Oven | 4,000W |
| Laundry Circuit (washer and dyer) | 3,500W |
| Outlets (each) | 2,400W |
While it’s always best to be extra-cautious with electrical loads, remember you also don’t run every electrical appliance and fixture in your house at the same time.
For example, you wouldn’t always be running your washing machine, dryer, microwave, and dishwasher, and would only ever use the heater or air conditioner, not both. When using the table and adding up your total electrical load, you only need to factor in one of those appliances at a time.
Where to Find the Wattage of Your Appliances
The electrical load each of your appliances needs to run should always be clearly marked in watts on the back or bottom of the appliance, or on its metal nameplate. If the wattage isn’t listed, the amps and volts might be instead, and you can multiply the amps by the volts to calculate the watts.
When to Call a Professional
While an electrical load calculation is something that can be done yourself using the different formulas, it’s best to call a professional if you’re unsure of your home’s age or electrical capacity. They will be able to examine your electrical panel and share the amperage capacity for you to keep in mind moving forward.
If you’re considering upgrading your home’s electrical panel or oversizing the electrical service to prepare for future electric car purchases, high-powered hobbies such as pottery or welding, or a tankless water heater installation, you’ll need to bring in the pros, too.
Read the full article here
