Dwelling load calculations

Why it matters

Before a house gets its electrical service, someone has to decide how big that service must be: 100 A, 150 A, 200 A or more. Too small, and the main breaker trips or the service overheats; too big, and the owner pays for copper and equipment they never use. The NEC gives a step-by-step load calculationThe NEC's step-by-step method for working out how large a building's electrical service must be. for this. Permit offices often ask for it, and the journeyman exam almost always includes one.

Awareness level only. This lesson teaches the method on paper. Sizing a real service is done by or under a licensed electrician or engineer, using the edition and amendments your jurisdiction enforces.

Where it lives (and why the edition matters)

In the 2023 edition, branch-circuit, feeder and service load calculations are in Article 220. The 2026 edition reorganised Chapter 1 and moved these calculations into a new Article 120. That is a good example of why you must check which edition is adopted where you work. The method below follows the standard methodThe NEC's main dwelling load calculation method; an optional method also exists and often gives a smaller result. for a single dwelling. There is also an optional method for dwellings that is often simpler and gives a smaller result; your exam may ask for either.

The standard method, step by step

The calculation works in volt-amperes (VA), and the result is divided by the service voltage (240 V for a typical single-phase home) to get amps.

1. General lighting and receptacles. Multiply the dwelling's floor area (outside dimensions, not counting open porches, unfinished spaces not adaptable for future use, or garages) by the unit loadThe VA per square foot of floor area used for the general lighting and receptacle load. in VA per square foot from the code's table.
2. Small-appliance and laundry circuits. Add a fixed VA amount for each required kitchen small-appliance branch circuit (at least two) and for the laundry circuit.
3. Apply the lighting demand factorsPercentages that reduce a load total because not every outlet or appliance is used at once.. Not every light and receptacle is used at once, so the total from steps 1 and 2 is reduced in tiers: the first block of VA counts in full, the next block counts at a lower percentage, and anything above that at a lower percentage still.
4. Fixed appliances. Water heater, dishwasher, disposal and similar. When there are four or more, a demand factor may apply.
5. Cooking equipment. A household range is taken from its own table rather than its nameplate.
6. Dryer. At least a minimum VA or the nameplate, whichever is larger.
7. Heating or air conditioning. Use the larger of the two, because they do not run at the same time (non-coincident loadsLoads that do not run at the same time, such as heating and air conditioning, so only the larger is counted.).
8. Total the VA and divide by 240 V for the minimum service in amps, then select the next standard service size.

Worked example (illustrative values)

The unit load, appliance amounts and demand percentages below are illustrative practice values that follow the shape of the standard method. They are not a copy of any code table: use the tables in your edition for real work.

A 2,000 sq ft house, illustrative values:

Illustrative demand tiers: first 3,000 VA at 100%, the next VA up to 120,000 at 35%, the rest at 25%.

first 3,000 VA × 100%           = 3,000
remaining 7,500 VA × 35%        = 2,625
lighting after demand           = 5,625 VA

Add the other loads (illustrative):

28,125 VA / 240 V ≈ 117 A

The minimum is about 117 A, so a 125 A or (more commonly) 150 A or 200 A service would be chosen, leaving room for future loads such as an EV charger or heat pump.

Why demand factors are tiered

The first few thousand VA of lighting and receptacles in any home are likely to be in use together (lights, fridge on a small-appliance circuit, TV). As the house grows, the chance that every outlet is loaded at once drops. Tiered demand factors capture that. The same idea, with different numbers, applies to cooking equipment and to groups of fixed appliances.

Common mistakes

• Using inside dimensions or including the garage in the floor area.
• Applying the lighting demand factor to the range, dryer or air conditioner.
• Adding both heat and air conditioning instead of the larger one.
• Dividing by 120 V instead of 240 V for a 120/240 V service.
• Forgetting EV chargers, hot tubs and heat pumps, which now often dominate a home's load.
• Copying values or article numbers from a different edition (the 2026 edition moved the calculations to Article 120).

Quick recap

A dwelling load calculation adds general lighting (area × unit load), small-appliance and laundry circuits, applies tiered demand factors to that group, adds fixed appliances, range, dryer and the larger of heat or cooling, and divides by 240 V. Check the article and table values in your adopted edition (NFPA 70; checked on 2026-10-23).

Try it

Open the Do tab. The load calculator is complete except for the tiered lighting demand factor.