Load Calculations & Service Sizing for Dwellings and Commercial Buildings

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Load Calculations & Service Sizing for Dwellings and Commercial Buildings. Don't let the plain title fool you. Here's the part that actually matters on the job: Every service, feeder, and panel is sized from a calculated load built step by step under Article 220 — general lighting, fixed loads, demand factors, the larger of heating or cooling, and 125 percent for continuous loads — and the arithmetic, not habit, picks the ampere rating. Learn it well and you will size a service that passes plan review, carries the real load, and never needs the embarrassing upgrade a year after occupancy.

The intro course named the articles. This lesson works the numbers. Editions and amendments vary by jurisdiction and some table values changed in recent cycles, so confirm figures against the edition your inspector enforces.

Where the numbers come from

Article 220 builds a load in layers. Part II sets branch-circuit loads: volt-amperes per square foot for general lighting by occupancy, fixed values for dwelling small-appliance and laundry circuits, 180 volt-amperes per receptacle strap on commercial work, and nameplate ratings for appliances, motors, and HVAC. Part III applies demand factors, which recognize that a building never runs everything at once. Square footage is measured from the outside dimensions, excluding open porches, garages, and unfinished spaces not adaptable for future use. For air conditioning, the number that matters is the minimum circuit ampacity on the unit label, not the compressor's rated load amps.

A dwelling by the standard method

Take a 2,400 square foot house with a 12 kW range, 5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher, 0.9 kW disposal, 1.2 kW microwave, 5 kVA of air conditioning, and 10 kW of electric heat.

• General lighting at 3 VA per square foot: 7,200 VA. Add two small-appliance circuits at 1,500 VA each and one laundry circuit at 1,500 VA: 11,700 VA.
• Demand: first 3,000 VA at 100 percent, remainder at 35 percent: 3,000 + 3,045 = 6,045 VA.
• Range: the range table allows 8,000 VA for one range not over 12 kW.
• Dryer: 5,000 VA or nameplate, whichever is larger.
• Fixed appliances: water heater, dishwasher, disposal, and microwave total 7,800 VA; with four or more, apply 75 percent: 5,850 VA.
• Heating or cooling: the larger, so 10,000 VA of heat. Total 34,895 VA, divided by 240 volts, is about 145 amps. The next standard rating is 150, and most contractors install 200 for the EV charger and heat pump that are coming anyway. A one-family dwelling service can never be smaller than 100 amps.

The optional method and when it helps

The optional dwelling calculation is simpler and usually smaller. Add every general load at nameplate, including range, dryer, and water heater; take the first 10 kVA at 100 percent and the remainder at 40 percent; then add heating or cooling at the percentage the article assigns. For the same house, general loads total 36,500 VA, so 10,000 + 10,600 = 20,600 VA. Central electric heat with fewer than four separately controlled units is taken at 65 percent, or 6,500 VA, which beats 5,000 VA of air conditioning at 100 percent. The total is 27,100 VA, about 113 amps. The optional method also underlies the existing dwelling calculation used when you add load to an older house, where a year of actual utility demand data can replace the formula.

Commercial loads, continuous loads, motors and kitchens

Commercial work changes the inputs. General lighting comes from the occupancy table, which was reworked in recent editions to reflect LED lighting, so an office figure from an older edition may be two or three times the current one. Receptacles are 180 VA each, with 100 percent demand on the first 10 kVA and 50 percent on the remainder. Continuous loads, running three hours or more, are multiplied by 125 percent when sizing the overcurrent device and conductors; most commercial lighting, refrigeration, and EV charging is continuous. Motor loads follow Article 430: the largest motor at 125 percent plus full-load current of the rest, using table values rather than nameplate. Commercial kitchen equipment has its own demand table that lets six or more pieces be taken at 65 percent, which is often what keeps a restaurant on a 400-amp service.

From calculated load to service, feeder and panel

Volt-amperes divided by system voltage gives amperes; for three-phase, divide by line voltage times 1.732. Then match equipment: service conductors that carry the load, a disconnect and overcurrent device at or above it, and a bus rated for it. Round up to the next standard rating, keep margin for growth, and check that the service equipment's short-circuit current rating meets the utility's available fault current. The neutral can be reduced on a feeder because it carries only the maximum unbalance, but nonlinear loads such as LED drivers push neutral current up, which is why many engineers run a full-size neutral anyway.

Going Deeper (Intermediate)

Feeder calculations use the same demand factors as the service, but only for the loads downstream of that feeder. A garage subpanel with a 50-amp EV charger, a compressor circuit, and lighting is sized for its continuous loads at 125 percent plus the rest, not the whole house. Keep the calculation sheet with the permit; the inspector may ask for it, and the owner will need it the next time a load is added.

Advanced / Pro-Level

Pros maintain a calculation for every panel on the job and update it as changes arrive, because a tenant improvement that adds a server room can quietly push a feeder past its calculated load. They request the utility's available fault current in writing, run a short-circuit calculation to the main gear, and specify series-rated or fully rated equipment accordingly. They know that generator and energy storage sizing follows Articles 700, 701, 702, and 706 with their own rules.

Practice Challenge

A homeowner wants a 48-amp continuous EV charger on an existing 100-amp service in a 1,600 square foot all-electric house with a 10 kW heat strip. Can it be done without a service upgrade? (Answer: run the existing-dwelling optional calculation first. The charger alone is 48 amps at 125 percent, or 60 of the 100 available, and with electric heat the total will almost certainly exceed 100 amps. The options are a 200-amp service upgrade or a listed energy management or load-sharing device that limits charging when house load is high, which recent editions allow to be counted in the calculation.)

In Practice

An electrician who works the calculation for every service sizes 150 where 150 is right and 400 where 400 is right, and can show the inspector why. One who sizes by rule of thumb installs a 200-amp service on a restaurant and spends the first month of operation resetting the main.

Common Mistakes to Avoid

• Adding heating and air conditioning together instead of taking the larger
• Using rated load amps instead of minimum circuit ampacity for HVAC equipment
• Forgetting the 125 percent on continuous loads such as commercial lighting and EV chargers
• Pulling general lighting values from an older edition's occupancy table
• Using the optional method for a building that does not meet its conditions

Takeaway: Every service, feeder, and panel is sized from a calculated load built step by step under Article 220 — general lighting, fixed loads, demand factors, the larger of heating or cooling, and 125 percent for continuous loads — and the arithmetic, not habit, picks the ampere rating.

Educational content only — not electrical design or engineering, and not a substitute for formal training, an apprenticeship, or work under a licensed electrician. Follow the NEC edition and amendments adopted in your jurisdiction, NFPA 70E, OSHA, the project specifications, and the manufacturer's instructions.