How to Calculate Panelboard Loads
If you know the electrical load you're placing on a switch panelboard, you can subtract it from the amperes rating on the panel's nameplate to learn how close you are to the safe limit. In order to calculate a panel's load, you'll need to note the load of each individual appliance connected to the board. How you put together that data afterward depends on the nature of the building and on the types of appliances you're using -- some appliances do not contribute the entirety of their load to the panel calculation. Many forms exist to help you make the right calculations; however, you will first need to make careful measurements of the environment and take a detailed inventory of your appliances.
Measure the square footage of the entire dwelling or office served by the panelboard, making sure to get its outside dimensions as well.
Collect information on every appliance and lighting system you plan to connect (or already have connected) to the panelboard. Obtain the volt/ampere load for all washers, dryers, air conditioners, heaters and small appliances. Most appliances will have this information printed on a panel; if yours do not, contact the manufacturer or search online for the name and model number to find the volt/ampere load.
Print out a load-calculation form (see Resources). This will provide formulas for collecting the loads of the various appliances and the area of the environment into a general load figure for the panelboard. These formulas vary depending on the type of dwelling or office you're calculating for.
Fill out the form and perform the math it indicates to calculate your panelboard's load.