An electrical load calculation sizes your service by adding up your home's loads and applying the NEC Article 220 demand factors — because you never run every appliance at once, the code lets you size to demand, not to the nameplate total. There are two methods: the Standard method (each demand table applied separately) and the simpler Optional method (220.82) — 100% of the first 10 kVA of general load plus 40% of the rest, then the larger of heating or cooling. Divide the result by 240 V, round up to a standard breaker size, and never go below the 100 A one-family minimum. A typical 2,000 ft² all-electric home lands around 100–150 A.
Want the number for your house? The electrical load calculator runs a full NEC Article 220 calculation both ways and returns the minimum service amps and wire size — free, no signup. This guide explains what a load calculation is, why demand factors matter, and how the two methods differ.
📐 What a load calculation actually does
A load calculation answers one question: how big does the electrical service — the main breaker and the conductors feeding your panel — need to be? The tempting shortcut is to add up every appliance's nameplate rating, but that badly oversizes the service, because a house never runs the range, dryer, water heater, air conditioner, and every light simultaneously. The NEC accounts for that with demand factors — percentages that reduce the connected load to a realistic calculated load.
That's why a home whose appliances add up to nearly 30,000 VA can be served correctly by a 100 A service. The calculated load — not the nameplate total — is divided by 240 V to get amps, and that sets the service size.
🧮 What goes into the calculation
Every dwelling load calculation is built from the same pieces:
- General lighting & receptacles — 3 VA per square foot of living area (2 VA/ft² under the 2026 NEC), which covers all the general-use outlets and lighting.
- Small-appliance & laundry circuits — at least two 20 A kitchen circuits and one laundry circuit, each counted at 1,500 VA.
- The range — a single electric range of 12 kW or less counts as just 8,000 VA (Table 220.55), one of the biggest diversity credits in the code.
- The dryer — 5,000 VA or its nameplate, whichever is larger (Table 220.54).
- Fixed appliances — water heater, dishwasher, disposal, and the like; four or more get a 75% demand factor.
- Heating or cooling — whichever is larger, never both (they never run together).
The calculator takes each of these as a simple input and applies the right factor automatically, so you don't have to memorize the tables.
🔀 Standard vs Optional method
The NEC gives two ways to do the calculation, and both are code-legal — they usually land within a single service size of each other.
The Standard method (Article 220, Part III) applies each demand factor separately: the general-lighting total is reduced by Table 220.45 (100% of the first 3,000 VA, 35% of the next chunk), the range by Table 220.55, the dryer by Table 220.54, four-or-more fixed appliances by 75%, and then 25% of the largest motor is added. It's the traditional long-form calculation you'll see on a formal worksheet.
The Optional method (220.82) is the shortcut most contractors use for a whole house on a single 120/240 V service of 100 A or more. Total every general load at nameplate, then take 100% of the first 10 kVA and 40% of everything above it, and add the single largest heating-or-cooling figure. It's faster and, for a typical modern home, usually the one that governs.
Which should you use? For a straightforward single-family home, run the Optional method — it's simpler and reflects real diversity well. Use the Standard method when you want the detailed line-by-line worksheet, or when a specific load makes the long form more favorable. The calculator does both, so you can compare.
🔌 From calculated load to service size
Once you have the calculated load in VA, three steps turn it into a service:
- Divide by 240 V to convert VA to amps — that's the current the service must carry.
- Round up to the next standard size from NEC 240.6(A): 100, 125, 150, 175, 200 A, and so on. (There is no standard 320 A — a “400 A-class” meter uses a 400 A device.)
- Apply the 100 A floor. A one-family dwelling service can never be rated below 100 A (230.79(C)), even if the math comes out lower.
The conductor then comes from Table 310.12, the dwelling “83% rule”: a 100 A service uses 4 AWG copper (or 2 AWG aluminum), and a 200 A service uses 2/0 AWG copper (or 4/0 aluminum). That allowance applies only to a standard 120/240 V dwelling service with no ampacity adjustments — hot attics, conduit fill, or long runs push you to the full Table 310.16 instead.
🏠 Upgrading an existing service
Adding a heat pump, an electric range, or an EV charger raises a different question: can my existing panel handle it? NEC 220.83 answers that with its own factors — 100% of the first 8 kVA of load plus 40% of the rest, with any new air conditioning or electric heat added at 100%. If the total exceeds your existing service rating, you need a service upgrade before the new load goes in.
The calculator's existing-dwelling mode runs exactly this check. Note that EV-charger treatment is genuinely unsettled under the 2020 and 2023 codes — the 2026 NEC settles it at 100% with no demand factor — so confirm the approach with your inspector.
🧰 Service size vs branch circuits
A load calculation sizes the service or feeder — the main breaker and the conductors feeding the whole panel. It does not size the individual circuits inside the panel. Each of those is its own calculation: the branch-circuit breaker and wire are sized to that specific load and its distance.
So this is the first of a few tools. Once the service is sized, use the wire size calculator and breaker size calculator for each circuit, and check long runs with the voltage drop calculator.
✅ The bottom line
Size your service to the calculated load, not the nameplate total — demand factors exist because a house never runs everything at once. Run the Optional method for a typical home, divide by 240 V, round up to a standard size, and never drop below 100 A. Use the existing-dwelling method before adding a big new load, and remember the service calc is separate from sizing the branch circuits inside the panel.
This is a screening and planning calculation, not a stamped design: adopted NEC editions and local amendments vary, and many jurisdictions set their own minimum service for new construction. Run your numbers through the load calculator, then have a licensed electrician confirm the final design with your AHJ.
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