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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.

Common questions

What does the code say about minimum service for Electrical Service Size?

100 amperes minimum for a one-family dwelling. (NEC 230.79(C))

What does the code say about standard load calc for Electrical Service Size?

General lighting at 3 VA/sq ft plus small-appliance and laundry circuits, with demand factors applied per Part III of Article 220. (NEC 220.41–220.55)

What does the code say about optional method for Electrical Service Size?

For 100A+ services: 100% of the first 10 kVA plus 40% of the remainder, plus the larger of heating or cooling — usually the friendlier number. (NEC 220.82)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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