Load calculations for dwellings

Before an electrician sizes a service, picks a panel or applies for a permit for a new house or a major renovation, someone has to answer one question: how much current will this dwelling draw? Adding up every nameplate in the house would give a huge number, because the range, dryer, heaters, air conditioner, car charger and every light are never all on at full power together. The electrical code therefore provides a calculation method that applies demand factors reflecting how homes are actually used. The result, the calculated load, sets the minimum size of the service and feeder conductors and the main overcurrent device.

This course follows the wiring of a dwelling from the service to the receptacle. This lesson covers load calculations; the section at the end of this lesson maps the rest of the course. It builds on the code navigation, conductor sizing and grounding rules of ELEC-120.

In Canada the method is Rule 8-200 of the Canadian Electrical Code, Part I, for single dwellings (including each unit of a semi-detached house or row house), with Rule 8-202 covering apartment and similar multi-family buildings. Always use the edition of the code adopted in the province or territory where the work is done.

Why demand factors

Demand is the load actually drawn over a period, usually averaged over 15 or 30 minutes by utility meters. Connected load is the sum of every nameplate. The ratio of maximum demand to connected load, the demand factor, is well below 1 in homes: lights are on mostly in the evening, the range runs at full power only briefly, heating and air conditioning do not run together, and thermostats cycle heaters on and off. The code's demand factors are based on long experience of dwelling loads and are deliberately conservative. They are not optional: the code method must be followed even if the designer believes the real load will be lower, and loads that run continuously at full power, such as electric vehicle chargers, are counted at or near 100%.

The Rule 8-200 method

Rule 8-200 builds the calculated load from a basic load based on floor area plus specific additions for major appliances. The steps for a single dwelling are summarized below; check the adopted edition for exact wording.

1. Living area and basic load

The living area is the area of the ground floor (measured to the inside of the exterior walls) plus the areas of any upper floors, plus 75% of the area of a basement with a height of 1.8 m or more. The basic load is 5000 W for the first 90 m² of living area, plus 1000 W for each additional 90 m² or portion of it.

2. Space heating and air conditioning

Electric space heating is added with the demand factors of Section 62: the first 10 kW at 100% and the balance at 75%. Air conditioning is added at 100%. Where the heating and air conditioning are interlocked so they cannot operate at the same time, only the larger of the two is included. Heat pumps are counted according to the code's provisions for combined heating and cooling equipment, including any supplementary heaters.

3. Cooking

An electric range is counted as 6000 W for a single range, plus 40% of the amount by which its rating exceeds 12 kW. Separate cooking units (a cooktop and wall ovens) are treated together as a single range.

4. Other loads over 1500 W

Any other loads rated over 1500 W, such as a water heater, dryer or hot tub, are added at 25% of their rating if an electric range is provided, or at 100% of the combined rating up to 6000 W plus 25% of the remainder if there is no electric range. Tankless water heaters and steamers are added at 100%. Electric vehicle supply equipment is added at 100% of its rating, unless an electric vehicle energy management system limits the load (the code allows the managed value to be used).

5. Minimum service size

The calculated load in watts is divided by 240 V to give amperes. The service conductors and service equipment must be rated at least for this current, and in any case not less than 100 A where the living area is 80 m² or more (60 A for a smaller dwelling).

Heating and cooling details

Space heating usually dominates Canadian calculations. Points to watch: thermostatically controlled heaters use the Section 62 demand factors, but heaters without automatic control are counted at 100%; a heat pump with electric back-up heat must include both the compressor and the back-up elements that can run with it; and air conditioning counts at 100% unless interlocked with the heating. Where a heat pump replaces baseboard heat in a retrofit, the calculation often falls, freeing capacity for an electric vehicle charger.

Additions to existing dwellings

When a heat pump, hot tub or EV charger is added to an existing house, the electrician must show that the existing service can carry the new calculated load. The Rule 8-200 calculation can be repeated with the new load included. Alternatively, Canadian practice allows the use of actual measured maximum demand data (for example, a year of utility demand records) as the basis for adding load to an existing service, where the inspection authority accepts it. Load management devices that shed the EV charger or the hot tub heater when the house load approaches the service rating are a common, economical alternative to a service upgrade.

Multi-unit buildings

For apartment buildings, Rule 8-202 calculates each unit (with its own basic load for its floor area, plus heating, air conditioning and appliance loads) and then applies further demand factors to the total across units, because peak loads in different apartments do not coincide: the first unit at 100%, the next two at 65%, the next two at 40%, the next 15 at 25% and the remainder at 10%, with heating and air conditioning handled separately. House loads (corridor lighting, elevators, parking garage ventilation) are added in full. The result sizes the building's service and the feeders.

How this course is organized

Lesson 1 covers load calculations and Lesson 2 planning the layout and circuits of a dwelling. Lesson 3 covers the service entrance and panelboard, and Lesson 4 installing cable, boxes and devices. Lesson 5 covers branch circuits and receptacles, and Lesson 6 the circuits for major appliances. Lesson 7 covers ground-fault and arc-fault protection, and Lesson 8 heating, cooling and ventilation. Lesson 9 covers inspection and testing, Lesson 10 EV charging, solar, storage and backup power, and Lesson 11 renovations and older homes. Lesson 12 covers alarms, communications and smart homes, and ends with a complete house project.