100-amp vs 200-amp service: do you actually need the upgrade?

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What the numbers mean

Service amperage times 240 volts is the raw capacity: 100A × 240V = 24,000 W; 200A × 240V = 48,000 W. The 125% continuous-load rule means any load running three hours or more is sized at 125% of its rating, which is the same as saying such a load should not exceed 80% of the breaker feeding it. That is a rule about CONTINUOUS loads and their circuits — it is not a blanket reduction of the whole service to 80%. A load calculation adds continuous and non-continuous loads by their own rules, and it is the calculated demand, not a flat 80% figure, that is measured against 24,000 W or 48,000 W.

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:

1. General lighting and receptacles: 3 VA per square foot of living space. A 2,400 sq ft colonial = 7,200 VA.
2. Small-appliance and laundry circuits: 1,500 VA each; a typical kitchen has two plus one laundry = 4,500 VA.
3. Fixed appliances at nameplate: range, oven, dryer, water heater, dishwasher, disposal, microwave, EV charger, hot tub — add them all up.
4. Apply demand: 100% of the first 10,000 VA, 40% of everything after that. Then add the heating or air-conditioning load under NEC 220.82(C), which is not a flat 100% — it sets out several selections and you take the largest that applies. Central air or a heat pump counts at 100%; electric space heating counts at 65% where fewer than four separately controlled units are involved, and at 40% where there are four or more. Divide by 240 to get amps.

Worked example, using the optional method set out immediately above — not the standard method, which handles lighting differently. 39,700 VA of connected load. The first 10,000 VA counts at 100%, the remaining 29,700 VA at 40% — so 10,000 + 11,880 = 21,880 VA of demand, or about 91 A at 240 V. That fits a 100A service with roughly nine amps to spare, which is exactly why so many 1960s–1990s Fairfield County houses pass today and fail the moment an EV charger or heat pump is added.

What pushes a house over 100A

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.

Published 2026-09-10 · Fairfield County, CT · Every figure above is sourced in the list above, with a date where the source publishes one.