What a load calculation answers
A load calculation answers one question: will your electrical service safely carry everything connected to it, including what you want to add? It is the step that decides whether you can add an EV charger to your existing panel, whether a basement suite needs a bigger service, or whether a new restaurant kitchen fits the building's supply.
It is not guesswork, and it is not simply adding up every appliance. The Canadian Electrical Code sets out a method, and electricians and designers follow it. This guide explains the idea so you can follow the conversation.
Watts, volts and amps
Every appliance has a power rating in watts (W) or kilowatts (kW) on its nameplate. Your service is rated in amps (A). To compare them, divide watts by volts. Most homes have 240 V available, with 120 V circuits for smaller loads.
- A 1,500 W space heater on 120 V draws about 12.5 A, most of a 15 A circuit.
- A 7,200 W EV charger on 240 V draws 30 A, continuously, for hours.
- A 100 A service at 240 V can supply about 24,000 W at its absolute limit, though you never plan to run it there.
How the house calculation is built
For a single house, the code method starts with an allowance based on the living area, which covers general lighting and receptacles. It then adds specific amounts for:
- Electric space heating, if you have it.
- Air conditioning or a heat pump.
- The range or cooktop and oven.
- Other large loads such as a dryer, water heater, hot tub or EV charger, some counted in full and some at a reduced percentage.
The result is a calculated load in watts, which is converted to amps and compared with your service. If it fits, you are fine. If it does not, you need to reduce or manage the load, or upgrade. Exact allowances and percentages come from the code edition in force, so leave the arithmetic to your electrician.
Have the approximate floor area of each level, including the basement, ready when you call. It is the first number an electrician will ask for.
Connected load vs real demand
If you added up the nameplate of every appliance in your house, the total would be far more than your service. That is normal. You never run the oven, dryer, kettle, hair dryer, heaters and every light all at the same moment. Code calculations account for this with demand factors.
The exception is continuous loads, which run for three hours or more at a time. EV chargers, electric heat and some commercial equipment fall into this group. They are counted in full and, in most cases, equipment carrying them is limited to 80 percent of its rating. That is why EV chargers often tip a borderline service over the line.
What your electrician will ask
- The approximate floor area of each level, including the basement.
- How the house is heated, and how water is heated.
- Whether you have or plan air conditioning or a heat pump.
- The nameplate ratings of large appliances: range, dryer, hot tub, EV charger.
- Whether there is, or will be, a secondary suite.
Photos of appliance nameplates and the main panel make the conversation quick. Many calculations can be roughed out from these details before anyone visits, with a site check to confirm.
Load calculations for businesses
Commercial and industrial calculations follow the same idea with different rules for each occupancy type, plus real equipment schedules for motors, kitchens, HVAC and process loads. For an existing building, actual measured demand from utility records or a load recording can sometimes be used. Whether that is allowed for your project, and over what period, is a question for the designer and the permit authority.
For tenant improvements, a load calculation tells the landlord and the tenant whether the existing panel and building service can handle the fit-out before anyone signs off on equipment. See tenant improvements.
When you need one
- Before adding an EV charger, heat pump, hot tub, air conditioner or electric heat.
- Before developing a basement suite or adding a garage subpanel.
- When applying for a permit for a service upgrade.
- Before a commercial fit-out or adding production equipment.
Alberta Electrical prepares load calculations as part of design and permit work. Often the answer is good news: there is more room than you thought, or EV load management can avoid an upgrade. When it is not, you will know exactly why. See also 100 A vs 200 A service.
Common questions
Can I do my own load calculation?
You can estimate using the watts-to-amps maths, which is useful for planning. The calculation that goes with a permit should be done by a licensed electrician or designer using the current code method.
Why does an EV charger count so heavily?
It is a continuous load, running for hours at a time, so it is counted in full rather than reduced like occasional loads.
Does gas heat change the calculation?
Yes. Homes with gas furnaces and water heaters have much lower calculated loads than all-electric homes, which is why many Edmonton houses manage on 100 A.
What if the calculation says my service is too small?
You can reduce or manage the new load, for example with EV load management, or upgrade the service. Your electrician will explain the options.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.