What is an electrical load calculation and why is it required?

An electrical load calculation — performed under NEC (and CEC) Article 220 — is the standardized method for determining 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, the decision about whether your main service needs an upgrade, and your utility load letter all derive from it. California building departments require it as a submittal, and leaving it out is the single most common cause of electrical plan-check corrections.

What the calculation totals

The Article 220 calculation adds up every category of demand 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 (generally the larger of heating vs. cooling).
• 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 chosen method (standard vs. optional) also affects the result. The final figure, in VA, is divided by the service voltage to yield the minimum amperage.

Why it's required — and why you can't copy one

1. It justifies the panel schedule amperage and main breaker size.
2. It feeds the single-line/riser diagram feeder and conductor sizing.
3. It supports the utility load letter when you request a meter or service change.
4. It documents whether the existing main service can carry the house plus the ADU, or whether a panel upgrade is required.

Because the method involves specific code tables and judgment about which loads dominate, the calculation must be performed by a licensed electrician or MEP engineer — not estimated from square footage.

A neighbor's calc or a generic template won't do: the result depends on your exact square footage, your specific appliance nameplate ratings, your HVAC equipment, and your existing main-service loads, so it has to be run for your project. 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.