NEC Article 220: Load Calculations
for Service and Feeder Sizing

General lighting load (220.12 / 220.41)

The starting point for any residential load calculation is NEC 220.41, which assigns 3 VA per square foot to cover general lighting and general-use receptacles throughout a dwelling unit. This single figure does a lot of work: it accounts for every convenience outlet and standard lighting circuit without requiring you to count individual devices on the plans.

Floor area is measured from the outside dimensions of the building. Open porches, unattached garages, unfinished spaces not adaptable for future use, and unused or unfinished areas in a basement are excluded. If a space is finished and conditioned, it counts — even if the owner hasn't furnished it yet. Getting the area right is step one, because every subsequent calculation multiplies from it.

Beyond the general lighting figure, NEC 220.52 requires adding two 1,500 VA small-appliance branch circuits serving kitchen and dining areas, plus a separate 1,500 VA laundry circuit. These three additions — totaling 4,500 VA — go into the general load subtotal before demand factors are applied, regardless of the home's size.

• 3 VA per square foot for general lighting and general-use receptacles (NEC 220.41)
• Floor area from outside dimensions; exclude open porches, garages, unfinished unused spaces
• Add two 1,500 VA small-appliance circuits + one 1,500 VA laundry circuit per 220.52

Demand factors (220.42 / Table 220.45)

Raw VA from the square-footage formula overstates what the service will actually see, because not every light and outlet in a house runs at full power simultaneously. NEC 220.42 and Table 220.45 address this with a tiered demand factor schedule. The first 3,000 VA of general lighting load is taken at 100% — this baseline is always fully counted. The portion of general lighting load from 3,001 VA through 120,000 VA is reduced to 35%. Any amount beyond 120,000 VA drops further to 25%.

In practice, nearly every single-family home falls entirely within the 35% tier after the first 3,000 VA, so the combined effect is a substantial reduction in the calculated service load. A 2,000-square-foot home generates 6,000 VA of basic general load plus 4,500 VA from the small-appliance and laundry circuits, totaling 10,500 VA. After the Table 220.45 demand schedule, the general lighting contribution to the service load drops considerably below that raw figure.

Appliance loads, electric dryers, and electric ranges each carry their own separate demand factor tables. Dryer loads are derated per Table 220.54, and range/cooking equipment uses Table 220.55, which applies column factors based on the number of units and their individual kW ratings. Each of these applies independently and is summed into the total calculated load before converting to amps.

Standard vs optional method

Article 220 gives you two paths to a residential service size. The standard method, covered in Part III (220.40 through 220.55), requires you to itemize every load category — general lighting, small appliance, laundry, dryers, ranges, fixed appliances, HVAC — and apply the applicable demand factor to each individually. It's methodical and works for any dwelling, but it involves more line items and more table lookups.

The optional method for a single dwelling unit, found at 220.82, is a shortcut that most jurisdictions accept. You add up all general loads (general lighting at 3 VA/ft², the 1,500 VA circuits, all other appliances rated 1 kW or more, and the nameplate of all fastened-in-place equipment) and apply a single blended factor: 100% on the first 10 kVA, then 40% on everything above that. To this reduced general load you add whichever is larger — the total nameplate of all air conditioning equipment or the total nameplate of all electric space heating equipment — at 100%.

For most typical homes the optional method produces a smaller calculated load and therefore a smaller required service. It's also faster to complete because it eliminates the individual appliance-by-appliance demand scheduling. The trade-off is that it's only available for individual dwelling units; it cannot be used for multifamily feeders or commercial services, which still require the standard Part III or Part IV approach.

Service and feeder minimums

Once you have a total calculated load in VA, converting to a minimum service amperage is arithmetic: divide by 240 V (the nominal voltage for a standard single-phase residential service) and round up to the next standard overcurrent device size. NEC 230.79(C) establishes 100 amps as the absolute floor for a one-family dwelling, regardless of what the calculation yields. In practice, most new residential construction is designed to 200 amps, and homes with electric vehicle charging, heat pumps, or induction cooking are increasingly being specified at 320 or 400 amps.

Feeder sizing follows the same arithmetic but applies to the branch circuit panel or subpanel downstream. The conductor must be sized to carry the calculated load per Article 220, then checked for ampacity at the appropriate temperature rating using Table 310.16. If the calculated load includes any motor loads — well pumps, HVAC compressors, elevator equipment — the largest single motor is taken at 125% of its full-load current per 220.50 and 430.24 before the total is summed.

• Minimum dwelling service: 100 A per NEC 230.79(C); 200 A is standard for new construction
• Minimum service amps = total calculated VA ÷ 240 V, rounded up to a standard size
• Largest motor load taken at 125% per 220.50 and 430.24

Why estimators care about load calcs

Load calculations are not just an AHJ checkbox — they set hard quantities in the electrical takeoff. The calculated service amperage determines panel size, meter socket rating, service entrance conductor gauge, conduit diameter, and grounding electrode conductor size. Every one of those is a line item with a material cost and an associated labor unit. Underestimating the service size early means either a change order later or a value-engineering scramble before bid day.

The difference between a 200A and a 400A service is not a rounding error. It typically means stepping from 4/0 aluminum service entrance cable to 350 kcmil or 500 kcmil conductors, moving from 2-inch to 3-inch conduit, and doubling the panel bus. Those three changes alone can swing the electrical cost of a residential project by several thousand dollars. Knowing which service size the load calc supports — before the plans are issued for bid — lets an estimator price the work correctly rather than guessing conservatively.

On commercial work the stakes are higher still. Commercial load calculations use NEC 220 Part IV, which adds connected load scheduling, the 125% continuous-load adjustment per 215.2, and demand factor analysis for each panel and feeder in the distribution system. A large office building or retail center might have dozens of panels, and each feeder must be individually calculated and sized. An AI takeoff tool that reads the one-line diagram and panel schedules from the PDF drawings can extract those loads systematically — reducing the chance of a missed panel or a transcription error from the schedule to the takeoff sheet.

Questions estimators actually ask