Open your panel door and read the number stamped on the main breaker handle — usually 100, 150, or 200. That's the best single indicator of your service size, and most of the time it's correct. But it can lie: panels get retrofitted with smaller mains, busbars are rated higher than the breaker installed in them, and the utility side can be smaller than everything in your garage. The reliable method is agreement between two of three checks: the main breaker handle, the service-entrance wire size, and the meter class. This guide walks all three.
Why it matters: your service size gates every big electrical add — EV charger, heat pump, hot tub, tankless water heater. Before buying any of those, size the circuit with the free Wire Size Calculator and check the load against your service. This guide gets you the service number the honest way.
🔢 Method 1: The Main Breaker Handle
The stamped amp rating on the main disconnect handle ("100," "150," "200") usually equals the service size. The National Electrical Code requires the service disconnect to be rated for at least the calculated load, and NEC 230.79(C) sets the floor for houses: "For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire."
Note what the code actually specifies, though: the disconnect rating — not the service size. The two usually match, but they can diverge, and inspectors see every one of these cases:
- Downsized retrofit main — a 200 A-rated panel fitted with a 150 A main breaker. The panel label says 200; the service is 150.
- Busbar rated higher than the installed main — the label on the panel states the maximum busbar rating (NEC 408.36 governs panelboard protection); the breaker actually installed can be smaller.
- Utility-side limits — the transformer or the service drop/lateral can be sized below your panel. The utility's service manual governs its side, not your panel label.
- Meter-main combinations — a 400 A meter-main is commonly split into two 200 A mains; neither handle alone tells the story.
- Service-conductor limits — NEC 230.42 requires the service wires to carry at least the disconnect rating (with the 83% allowance below), so an old conductor set can be the real ceiling.
🔌 Method 2: The Service-Entrance Wire (Most Authoritative)
The fat conductors entering the top of your panel are the ground truth. NEC 310.12(A) lets one-family dwelling service conductors (100–400 A services) be sized at 83% of the service rating — the code's acknowledgment that a whole house never runs everything at once. Working backward from the wire tells you what the service was built to be:
Common Service-Entrance Conductor Sizes (NEC Table 310.12)
| Service rating | Copper | Aluminum |
|---|---|---|
| 100 A | 4 AWG | 2 AWG |
| 200 A | 2/0 AWG | 4/0 AWG |
The math behind the 200 A row: 200 × 0.83 = 166 A minimum ampacity, which 2/0 copper (175 A at 75°C) and 4/0 aluminum (180 A) both clear per Table 310.16. For the full table across every service size, use NFPA's free-access NEC viewer at nfpa.org — the values above are the two sizes that decide most houses.
📟 Method 3: The Meter Class (Weakest — Here's Why)
Look at the faceplate of your electric meter and you'll usually find a marking like CL200. That means Class 200 — a meter rated to carry up to 200 amps continuously. Common residential classes are CL100, CL200, and CL320, defined under ANSI C12.1, the metering standard. A nominal "400 A" residential service is typically metered at 320 A continuous with a CL320 meter.
Here's the catch: the class is the meter's maximum continuous current — not your service ampacity. Utilities stock CL200 meters by the pallet and install them on 100 A and 150 A services all day long. The meter-socket jaw rating is yet another separate specification. So "the meter says 200" proves only that your service is at most in the 200 A class. Use it as a tiebreaker, never as the answer.
🏚️ Older-Home Tells: Fuse Boxes, Split-Bus Panels & Problem Brands
- Fuse box — 30 A and 60 A services were common in the knob-and-tube era (roughly pre-1950s–60s). But don't assume: some fuse panels are 100 A, some are 30 A. Read the main pull-out or the service wire.
- Split-bus panel — no single main breaker; killing the whole house takes up to six throws. Common from the 1950s–60s into the 1970s under the code's old six-disconnect allowance (a rule dating to the 1937 NEC). The 2020 NEC rewrote 230.71 to require a single service disconnect, so these panels mark an installation at least several decades old.
- Outdoor emergency disconnect — if your home has one at the meter, the service work was done under the 2020 NEC or later (230.85, added 2020 and expanded in 2023). A useful dating clue in reverse.
⚡ What Each Service Size Realistically Supports
These are typical, real-world framings — the code answer always comes from an NEC Article 220 load calculation (or the 220.83 / 220.87 optional methods for existing homes):
Typical Capacity by Service Size
| Service | Typical reality |
|---|---|
| 60 A | Minimal older homes; generally can't add an EV charger; upgrade usually required |
| 100 A | Gas-appliance homes can often add a Level 2 EV charger; all-electric homes near capacity usually can't without load management |
| 125–150 A | Moderate headroom; typically room for one large new load |
| 200 A | The modern standard; EV charger plus heat pump typically fits, confirmed by a load calc |
| 320–400 A | All-electric and large homes |
One modern escape hatch: the 2023 NEC recognizes energy management systems (750.30), and load-management devices can put a Level 2 EV charger — a 40–60 A continuous load, sized at 125% — onto a 100 A service by capping total draw below the service rating. Case-by-case, but it turns some "must upgrade" verdicts into "maybe not."
Planning a specific appliance? The Tankless Breaker Calculator checks your service against manufacturer minimums, and the EV Charger Circuit Calculator sizes the charging circuit against what your panel can give it.
🏢 Why a Service Upgrade Always Involves the Utility
The utility owns everything up to the demarcation point — customarily the line side of the meter — and furnishes the meter itself. You own the service-entrance conductors, weatherhead, meter socket (customer-provided in many territories), and the panel. PG&E's service manual (the "Greenbook") puts it plainly: "The customer must furnish, install, and maintain the service entrance wiring and service equipment beyond the point of attachment to PG&E's service wires," while meters "will be furnished by PG&E."
Because the meter must be pulled and the service de-energized — and because the transformer or drop may need upsizing too — there is no such thing as a service upgrade the utility doesn't know about. The exact demarcation varies by company, spelled out in each utility's electric service requirements manual (PG&E, Con Edison, Duke, ComEd, and the rest each publish their own). Call yours early: transformer lead times, not electrician schedules, are often what sets the upgrade timeline.
🚫 Five Misreads That Produce the Wrong Number
- "The panel label says 200, so I have 200 A service." The label is the busbar's maximum. The installed main and the utility side can both be lower.
- "I added up my breakers and got 480 amps!" Branch breakers are supposed to sum past the service size — loads never run all at once (that's load diversity), and there's no code limit on the total. Article 220's calculation is the real capacity math.
- "It's a fuse box, so it's 60 amps." Often — but some are 30, some are 100. Read it, don't assume it.
- "The meter says CL200." That's the meter's continuous rating, not your service ampacity — CL200 meters sit on 100 A services everywhere.
- "The wire is #2, so it's 200 amps." Only if you ignored the copper/aluminum marking. #2 copper serves a 100–125 A service; the 200 A copper answer is 2/0.
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