100-amp vs 200-amp service: do you actually need the upgrade?
What the numbers mean
Service amperage times 240 volts is the raw capacity: 100A × 240V = 24,000 W; 200A × 240V = 48,000 W. Practical continuous capacity is 80% of that — about 19,200 W and 38,400 W — because continuous loads are sized at 125% under the NEC. Those are the two ceilings every load calculation is measured against.
The NEC 220.82 calculation, without the jargon
Electricians size a dwelling service with the "optional method" in NEC §220.82. It is four lines:
- General lighting and receptacles: 3 VA per square foot of living space. A 2,400 sq ft colonial = 7,200 VA.
- Small-appliance and laundry circuits: 1,500 VA each; a typical kitchen has two plus one laundry = 4,500 VA.
- Fixed appliances at nameplate: range, oven, dryer, water heater, dishwasher, disposal, microwave, EV charger, hot tub — add them all up.
- Apply demand: 100% of the first 10,000 VA, 40% of everything after that, then add the largest of heating or cooling at 100%. Divide by 240 to get amps.
Worked example from an exam prep source: 39,700 VA of connected load becomes 24,825 VA of demand, or 103 A — just over a 100A service, which is why so many 1960s–1990s Fairfield County houses sit right at the line before anyone adds a charger.
What pushes a house over 100A
| Addition | Typical circuit | Load in the calc |
|---|---|---|
| Level 2 EV charger, 48A | 60A, 240V | 11,500 VA continuous (NEC 625.42) |
| Level 2 EV charger, 32A | 40A, 240V | 7,700 VA continuous |
| Cold-climate heat pump (3–4 ton) with backup strips | 30–60A | 8,000–15,000 VA (largest of heat/cool at 100%) |
| Induction range | 40–50A | 8,000–12,000 VA nameplate |
| Heat pump water heater | 30A | 4,500 VA |
| Hot tub | 50A | 10,000–12,000 VA |
| Central AC, 3 ton | 30A | ~4,500 VA |
Two of these on top of a normal all-electric kitchen and dryer is where the 100A math stops working. One of them, in a gas-heated house, often fits.
Which code applies in Connecticut right now
Connecticut enforces the 2022 State Building Code, which adopts the 2020 NEC (effective October 1, 2022). The 2026 code, adopting the 2023 NEC, was scheduled for July 1, 2026 and is delayed pending regulatory review as of the state's adoption page. Your electrician and town inspector work to whichever is in force on the permit date; the load calculation is the same under both.
Three honest outcomes
- Stay at 100A, add a subpanel. The calculation clears with margin; you're out of breaker spaces, not capacity. $400–$1,750 nationally, $1,000–$2,000 in CT.
- Stay at 100A with load management. A charger with dynamic load control, or a smart splitter on the dryer circuit, keeps a single EV under the ceiling. Reasonable stopgap; check the utility incentive rules if you're enrolling in managed charging.
- Go to 200A. Heat pump plus EV, or any plan that electrifies the house over the next decade. This is the Eversource standard residential service and the one every future contractor assumes. What it costs →
Ask the electrician to show the calculation. It's a one-page form; if they won't produce it, they're guessing.
Ready for a real number?
Planning ranges are not quotes. A licensed Connecticut electrician prices your panel after seeing it.
Request a free estimate →- NEC §220.82 optional dwelling calculation, explained (ExpertCE) — https://expertce.com/learn-articles/dwelling-service-calculation-optional-method-nec-220-82/ — page dated 2025-11-28
- NEC §230.79(C) minimum 100A service, worked example (Dakota Prep) — https://www.dakotaprep.com/nec-exam-guides/guide-to-service-size-calculation-questions-on-nec-electrical-exams — page dated 2026-09-08
- What a 200-amp service can carry (Home Inspection Insider) — https://homeinspectioninsider.com/how-many-watts-can-a-200-amp-house-handle/ — page dated 2024-02-04
- NEC §625.42 EV loads are continuous (text) — https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=1534 — no date on page
- Eversource CT Electric Service FAQ — standard residential service is 200A — https://www.eversource.com/residential/about/doing-business-with-us/builders-contractors/electric-service-faq — no date on page
- Wilton CT — 2022 CT State Building Code (2020 NEC) effective Oct 1, 2022 — https://www.wiltonct.gov/building-department/pages/ct-state-building-code-information — no date on page
- CT DAS — code adoption process, 2026 code (2023 NEC) pending — https://portal.ct.gov/das/office-of-state-building-inspector/building-and-fire-code-adoption-process — no date on page