Electrical Panel Capacity Calculator

How This Calculator Works (NEC 220.87)

Electricians have two ways to answer "can this panel take more load?" The standard way is a paper worksheet (NEC 220.82/220.83) that adds up square footage and nameplate ratings with fixed factors. It's deliberately conservative — it routinely demands a service upgrade that your actual usage doesn't justify.

The second way is the one this tool automates: NEC 220.87, "Determining Existing Loads." Instead of estimating what your house might draw, it uses what your house actually drew: the maximum demand recorded over the past year. The rule is simple:

1. Take the maximum demand your meter recorded over 12 months (in kW).
2. Multiply it by 125% — that's the code's safety cushion.
3. Add the new load you want to install (EV chargers count at 125% of their charging current, because they're continuous loads — NEC 625.42).
4. If the total is at or under your service rating, the new load fits — no service upgrade required by load calculation.

The catch has always been step 1: where do you get a year of recorded demand? Electricians rent recording meters for 30 days (the code's alternative path) at real cost. But if your utility has a smart meter — and most now do — the answer is already sitting in your online account as a downloadable interval-usage file. This tool reads that file, finds the true peak, and does the code math. The file never leaves your browser: parsing and math run entirely on your device.

Where to Get Your Utility Data File

Log into your utility's website and look for "Download my data," "Green Button," "Energy usage," or "Usage history." You want the most granular export offered (15-minute if available) covering the last 12 months. Quick pointers for major utilities:

Why You Can't Just Add Up Breakers

A 200A panel with 400A worth of breakers in it is normal and code-legal. Breakers protect wires; they are not a load inventory. Panels are intentionally "over-breakered" because loads don't all run at once — that's the whole concept of demand diversity. So counting breaker handles tells you nothing about spare capacity.

A clamp meter doesn't settle it either: it's a snapshot of one moment. The code cares about the worst 15-minute average over a whole year — the August afternoon when the AC, dryer, and oven overlapped. That moment is unobservable in a site visit, which is exactly why the recorded-data method exists — and why your utility's interval file is the most honest answer available.

Worked Example: 200A Panel + 48A EV Charger

Real numbers from a typical suburban all-electric house:

1. The 12-month interval file shows a maximum demand of 9.6 kW (one brutal July evening, 6:15–6:30 pm).
2. Apply NEC 220.87: 9.6 kW × 1.25 = 12.0 kW → at 240V that's 12,000 ÷ 240 = 50 A of "existing load."
3. Service capacity: 200 A. Headroom: 200 − 50 = 150 A.
4. A 48A EV charger is a continuous load: 48 × 1.25 = 60 A needed. 60 ≤ 150 — it fits easily, with room left for a heat pump and an induction range besides.

Now the same house on a 100 A service: headroom is 100 − 50 = 50 A. The 48A charger needs 60 A — doesn't fit. But a 32A charger (40 A continuous) does, and so does a 24A charger with margin. That's the practical power of this method: it converts "you need a panel upgrade" into "here's the charger size that works today."

Hourly vs 15-Minute Data — the Honest Caveats

This tool labels your result quality because not all interval data is equal:

• 15-minute data matches the demand basis the code method was written around. Results are labeled accordingly.
• Hourly data averages away short peaks. A house that briefly pulled 14 kW inside an hour that averaged 9 kW will show 9. Results from hourly files are labeled an estimate, and your inspector may want 15-minute data or a 30-day recording instead. Empirical comparisons of real residential meters (discussed at length on the Mike Holt electrician forums) found true 15-minute peaks typically run about 1.3× the highest hourly average, and under 1.7× in nearly all homes — so a quick sanity check is to mentally multiply your hourly peak by 1.3–1.7 and see if the verdict still holds. If your margin is comfortable, hourly data is usually persuasive; if it's tight, get better data before spending money.
• Less than 12 months of coverage means the peak season may be missing — a January-to-June file from a house with central AC hasn't seen the real peak yet. The tool flags this too.

Whether an AHJ accepts smart-meter interval history for 220.87 varies by jurisdiction — most do, some want the utility's own demand report on letterhead, a few insist on a 30-day recording. The printable worksheet exists precisely so your electrician can attach the derivation to a permit application and sign it.

What If It Doesn't Fit?

In order of cost:

1. Smaller charger. Most EVs recover a full daily commute overnight on a 24A (5.8 kW) charger. Charger amperage is a settings choice, not a lifestyle sentence.
2. Load management (EVEMS). A power-sharing charger or load-shed device watches whole-house draw and throttles the EV when the house is busy. Code explicitly permits this (NEC 625.42 load management), and it's often the cheapest path to a 48A charger on a small service.
3. Shift the peak. Your recorded peak is often one avoidable overlap (dryer + oven + AC). A year of slightly different habits lowers next year's recorded demand — this is a long game, but real.
4. Service upgrade. $2,000–$5,000+ depending on utility and region. Sometimes it genuinely is the right answer — especially if you're also planning a heat pump and induction range. Get the 220.87 worksheet in front of the electrician first, so you're paying for capacity you actually need.

Off-grid and hybrid readers: if you're adding solar-plus-storage instead of utility capacity, start with our off-grid system calculator and load calculator — the same honest-numbers philosophy, pointed at batteries instead of the grid.

Frequently Asked Questions

Next Steps

• Verdict says it fits? Get quotes citing the 220.87 worksheet — you may skip the upgrade line-item entirely.
• Going hybrid or off-grid instead: size a solar + battery system.
• Wiring the circuit: our wire sizing guide and breaker panel guide cover conductor sizing fundamentals.