Load calculations (Article 220): sizing a dwelling service, and what an EV charger or a heat pump adds
In one paragraph
How to size a dwelling service the way the code and the inspector expect: the standard method of Article 220 Part III, the optional method of 220.82 that most residential contractors use, the two ways the code lets you prove an existing house can take a new load (220.83 and the 220.87 utility-data method), and the 2023 edition's rule that an EV charger counts at 7,200 volt-amperes or its nameplate, whichever is larger. A worked 1,800-square-foot house runs the numbers with and without a 40-amp EV circuit and shows why a 100-amp service that was fine for thirty years is not fine the day the car arrives. Ends with the California readiness rules that decide how big the panel has to be on new work.
The rule
A dwelling service or feeder is sized by Article 220 of the 2025 California Electrical Code: the standard method (Parts II and III) or the optional method of 220.82, which takes general loads at 100 percent of the first 10 kVA and 40 percent of the remainder plus the largest heating or cooling load; an existing dwelling's ability to take new load may be shown by 220.83 or by 220.87 using the maximum demand over the past year times 125 percent; and electric vehicle supply equipment is counted at 7,200 volt-amperes or its nameplate rating, whichever is greater (220.57). On new single-family work, California's Energy Code readiness sections, incorporated into the CEC at 408.2(B), 422.3, 440.3 and 706.10, set the minimum breaker spaces and, for battery-ready homes over 125 amps, a 225-amp busbar.
Why it matters
The load calculation is the one piece of engineering every C-10 does alone, and it is what decides whether the customer needs a $600 circuit or a $6,000 service upgrade. Doing it on paper, by the code's method, and handing the customer the sheet is what makes the expensive answer believable and the cheap answer defensible when the inspector asks.