What is the NEC Article 220 load calculation and what does it include?

NEC (and CEC) Article 220 is the standardized method for calculating how much electrical demand a dwelling will draw, and therefore how large the panel and service must be. For ADUs it's the foundational document — panel size, service-upgrade decisions, and utility load letters all derive from it. California building departments require it as a submittal, and leaving it out is the most common cause of electrical plan-check corrections.

What gets added up

The calculation totals every category of load in the unit:

• General lighting and general-use receptacles: 3 VA per square foot of living area.
• Small-appliance circuits: 1,500 VA each, minimum two for the kitchen.
• Laundry circuit: 1,500 VA.
• Fixed appliances at nameplate rating: electric range/cooktop, water heater, dishwasher, disposal, microwave, and similar.
• HVAC: heat pump or air-conditioning load at rated amperage (with the larger of heating vs. cooling generally counted).
• EV charger: included at full rated load — typically 40–50A for Level 2.

Demand factors — why the panel can be smaller than the sum

If you simply added every load at full value, panels would be wildly oversized, because no household runs everything at once. Article 220 applies demand factors — code-specified percentages reflecting that loads don't all peak simultaneously. For example, general lighting and receptacle loads are discounted on a tiered basis, and certain appliance groups get their own demand allowances. The calculation method (standard vs. optional) also affects the result. The final figure, in VA, is divided by the service voltage to yield the minimum amperage.

A simplified illustration

An all-electric 700 SF ADU might total general lighting (2,100 VA), two small-appliance circuits (3,000 VA), laundry (1,500 VA), an induction range, a heat pump water heater, mini-split HVAC, and a Level 2 EV charger. After applying Article 220 demand factors, the calculated demand commonly lands in the range that justifies a 125A subpanel — but only the actual nameplate ratings and the chosen calculation method confirm it.

Because the method involves specific code tables and judgment about which loads dominate, it must be performed by a licensed electrician or MEP engineer — not estimated from square footage. Every one of our electrical plan sets includes a documented Article 220 calculation supporting the panel size, ready for plan check and your utility load letter. Requirements vary by jurisdiction — confirm with your local building department.