Residential Load Calculations Explained (NEC Article 220)
A load calculation is how you prove a service or circuit is big enough — without guessing. Here's the standard-method walkthrough: what to add, where the demand factors come in, and why 'add up every nameplate' gives you the wrong (and oversized) answer.
A load calculation answers a simple question with real consequences: is this service (or feeder) big enough? Do it right and you size correctly and pass inspection. Do it by gut — or by summing every nameplate — and you'll oversize the service, waste the customer's money, or worse, undersize a feeder. Here's the standard method, plainly.
Why not just add everything up
The instinct is to total every appliance's wattage. Don't — you'd be sizing for a scenario that never happens (oven, dryer, every light, and the AC all maxed simultaneously). Article 220 builds in demand factors: allowances that reflect how loads actually diversify in real use. That's the whole point of the calculation.
The standard method, step by step
- General lighting and receptacle load. 3 VA per square foot of living area. This covers general lighting and general-use receptacles.
- Small-appliance and laundry circuits. Add 1,500 VA for each required small-appliance circuit (at least two in the kitchen) and 1,500 VA for the laundry circuit.
- Apply the demand factor to that subtotal: the first 3,000 VA at 100%, the remainder at 35%. This is where the big reduction happens — you're not really running all those lights and receptacles at once.
- Add fixed appliances (water heater, dishwasher, disposal, etc.) — with a 75% demand factor allowed when there are four or more.
- Add the range per Table 220.55 (which is well under nameplate) and the dryer per 220.54 (5,000 VA or nameplate, larger).
- Add HVAC — the larger of heating or AC (they don't run together), at 100%.
- Total the VA, divide by the voltage (240 V) to get amps, and size the service to the next standard rating above it.
Verify, then trust
Our residential load calculator runs the standard method — enter the square footage, appliances, range, dryer, and HVAC, and it applies the demand factors and totals the service size. It's a fast way to check a service upgrade or a plan review. But know the steps yourself: the value of a load calc is being able to defend each line to an inspector, and the service upgrade decision often rides on it.
Bottom line
A load calculation is diversity made into arithmetic: real loads, real demand factors, honest total. Follow Article 220's steps — general load with its demand factor, then the appliance, range, dryer, and HVAC rules — and you'll size services right every time.
Frequently asked questions
Why can't I just add up every appliance's wattage?
Because you never run everything at once. If you summed every nameplate, you'd size a 400-amp service for a house that comfortably lives on 150. Article 220 uses demand factors — allowances that reflect real diversity of use — so the calculation reflects actual maximum demand, not an impossible everything-on scenario. The general lighting load, for instance, gets a big demand factor after the first 3,000 VA.
What's the difference between the standard and optional methods?
The standard method (220 Part III) adds general lighting and receptacle load, small-appliance and laundry circuits, then applies demand factors, and adds fixed appliances, dryer, range, and HVAC with their own rules. The optional method (220.82) is a simpler single-formula approach allowed for most dwellings and often yields a smaller result. Both are valid; many electricians run the optional method for services and the standard method when they need the detail.
How do I handle the range and dryer?
The range uses Table 220.55, which gives a demand load well below the nameplate for typical ranges (a single 12 kW range calculates at 8 kW, for example). The dryer uses 220.54 — 5,000 VA or the nameplate, whichever is larger, with demand factors for multiple units. These special rules are exactly why nameplate-summing overshoots.
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