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Home / Code Guides / Electrical Service Size

Electrical Service Sizing — 100A Minimum, Load Calcs & the Outdoor Disconnect

Whether a house needs a 100, 150, or 200-amp service is not a vibe — it is an Article 220 arithmetic problem. The code sets a floor, a method, and (since 2020) a new box on the outside wall.

RequirementModel Code StandardSection
Minimum service100 amperes minimum for a one-family dwellingNEC 230.79(C)
Standard load calcGeneral lighting at 3 VA/sq ft plus small-appliance and laundry circuits, with demand factors applied per Part III of Article 220NEC 220.41–220.55
Optional methodFor 100A+ services: 100% of the first 10 kVA plus 40% of the remainder, plus the larger of heating or cooling — usually the friendlier numberNEC 220.82
Emergency disconnectOne- and two-family services require an outside emergency disconnect, readily accessible and markedNEC 230.85
Workspace30 in. wide, 36 in. deep, 78 in. high clear working space at the panel — no exceptions for closets that “almost” fitNEC 110.26
Conductor sizingService conductors sized to the calculated load using the special dwelling services table or 310.16NEC 310.12

Worth knowing

  • Heat pump conversions, EV chargers, and induction ranges are pushing 100A houses to the line — run the 220.82 calc before assuming an upgrade is needed; load management (NEC 750) can often avoid it.
  • Panels in clothes closets and bathrooms are prohibited; the workspace rule kills most “hide it behind the door” plans.
  • The outside emergency disconnect (2020+) mainly bites on service changes: a like-for-like panel swap inside can trigger adding the outdoor disconnect, depending on state amendments and AHJ policy.
  • Utilities size their transformer and drop separately — a 200A panel does not mean 200A of utility capacity was ever provisioned.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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