A Guide to Electrical Load Calculation

The electrical panel, also known as the service panel, is the control center for all the electrical circuitry in a residence.

Today’s panels are bigger than earlier versions. They have to be, because modern homes have so many more appliances, smart controllers and other electrical and electronic devices. With the push to eliminate fossil fuel-powered cars, garden tools, cookware and furnaces, residential electrical demand will only increase. Due to this, there is always a possibility of a power cut or overload.  Here are some effective ways to charge your phone without electricity in that case!

A few years ago, I lived in an old house with an undersized panel woefully unequipped for today’s modern appliances and devices. The house had an electric stove, but I prefer microwave ovens, so I purchased one. Whenever I tried operating both at the same time, the main breaker in the panel tripped, because the combined load exceeded the panel’s capacity.

In a house with all the modern electrical conveniences, matching the capacity of the panel to the projected load is more important than ever. If the panel capacity is too low, something as simple as swapping a gas water heater for an electric one would require an expensive upgrade.

Here’s how to calculate your electric load, so you’ll know if it’s feasible to add a heat pump or a car charger to your system.

Electrical Load vs. Electrical Capacity

Put simply, “electrical load” is the amount of power all the electrical devices in your house would draw if all were on at the same time.

Electrical capacity is the maximum power the service panel can supply at one time. Electrical power is measured in watts, but power is the product of voltage and current. And because residential voltage always stays the same, electricians measure load and capacity in units of current, which are amperes, or amps.

In basic electrical calculations, electricians use the unit of volt-amperes (VA) instead of watts. To keep things simple, let’s just say watts and volt-amperes are synonymous. And because the supply voltage to your home is rated at 240-volt/120-volt, sometimes 120 volts is used in the math, and sometimes 240 volts.

It’s easy to determine the existing capacity of your service panel. Just open the panel door and look at the main breaker, in the center at either at the top or the bottom. The current rating is clearly marked on the breaker. That’s the panel’s capacity.

These days, the minimum capacity the National Electrical Code (NEC) allows for new residential panels is 100 amps, with 200 amps the standard for new construction.

Existing panels in older homes may be rated for 60 amps or even less. That was the rating for the panel in the house I used to live, which explains why the main breaker kept tripping. On the other hand, some larger homes have panels rated for 300 or 400 amps.

How To Calculate Electrical Load

To determine the feasibility of adding a new large appliance to your electrical system without upgrading the panel, you need to calculate the load your current system draws.

Electricians have a simple formula for this. But master electrician John Williamson — the retired chief electrical inspector for the Minnesota Department of Labor and Industry — says this isn’t how the NEC calculates load. However, because the NEC method is too complicated for most homeowners, it’s a good approximation.

1. Using the exterior dimensions of your home, calculate the approximate square footage, including attached garages.  Exclude open porches and unfinished areas not adaptable for future habitable use.
2. Determine the power draw for all your general lighting and receptacle circuits, as well as circuits supplying major appliances.
3. Include 1,500 volt-amperes for each 120-volt 20-amp kitchen circuit, and likewise for a 20-amp laundry circuit.
4. Add the power rating (in volt-amperes) for all appliances fastened in place, permanently connected or on dedicated circuits, like your washer/dryer, dishwasher, electric stove, microwave, refrigerator and water heater. Get the rating by reading the label on the appliance, or by checking the product specs online. If the label specifies current draw, multiply that by the operating voltage (either 120 or 240 volts, depending on the size of the unit) to get the power rating in watts.
   Note: If you’re doing this calculation to determine whether your panel can handle a new appliance like a water heater, add the wattage of that appliance here.
5. Check the labels on your furnace and air conditioning system for their power ratings. Because you won’t ever use these appliances together, choose the larger number and add it to the total.

Williamson shared the following table that shows a sample calculation for a 1,500 square foot home:

Compare Load to Panel Capacity

To prevent overloads and possible overheating, the load on the panel shouldn’t exceed 80 percent of its capacity. So the maximum load you place on a 100-amp panel shouldn’t be more than 80 amps.

Once you’ve calculated 80 percent of the panel’s capacity, compare it to the load. If the load is smaller, you’re good to go. But if the load is larger, you need to upgrade the panel for safety.

Because this is an expensive project that involves installing new wiring, it’s worth first consulting with a licensed electrician for possible ways to reduce your load.