Branch-Circuit and Feeder Load Calculations

The general lighting load is figured per square foot, then you add the known loads and apply demand factors because not everything runs at once. Do it in order and the number comes out honest.

Demand factors are where beginners get nervous, but they are just the Code admitting that your dryer and your range are rarely both maxed at the same second.

When it applies

Article 220 is the arithmetic behind every service and feeder. Before you pick a panel or size a conductor, you add up the loads. Lighting gets figured by floor area at a set watts-per-square-foot, then you layer in the fixed appliances, the small-appliance and laundry circuits, and apply demand factors so you are not sizing everything as if it all runs at once.

Common mistakes

• Using connected load without applying the demand factors, which oversizes the service and wastes the customer's money.
• Forgetting to include the largest motor at its required extra percentage.
• Leaving out fixed loads like the range or the HVAC because they were added late in the plan.

In the field

I run the numbers twice on a service upgrade. Once quick to quote it, and once careful before I pull the permit, because the calc is what defends the whole install.