A load calculation estimates the most power a building, feeder or panel will draw at one time, so that the service, transformers, conductors and protective devices can be sized to carry it. It is not the sum of every nameplate in the building. It applies demand factors and code rules to the connected load, or uses measured demand from the existing installation, to arrive at a realistic maximum.
Why the number matters
Almost every electrical decision in a building depends on knowing the load. A load calculation answers questions such as:
- Is the existing service big enough for a new tenant, a heat pump retrofit or EV chargers?
- What size should a new service, transformer or feeder be?
- How much spare capacity is left on a panel or switchboard?
- Will the utility need to upgrade its transformer or service drop?
An undersized service trips breakers, overheats equipment and blocks future changes. An oversized one costs more to build, can raise utility charges and makes protective devices harder to coordinate. The aim is a defensible number with a sensible margin, not the biggest possible service.
Connected load vs demand load
The connected load is the total of every load that could be switched on: lighting, receptacles, heating, cooling, motors, kitchen equipment and so on. In practice those loads never all run at full output at the same moment. Lights in unoccupied rooms are off, receptacles are lightly used, and heating and cooling rarely peak together.
The demand load is the realistic maximum. The Canadian Electrical Code sets out how to get from one to the other in Section 8, which covers circuit loading and demand factors. Its rules differ by occupancy: single dwellings, apartment buildings, schools, hospitals and other occupancies each have their own method, often based on floor area, the type of load and demand factors that reduce the total as the number of units or loads grows. Check the edition adopted in your province and any local amendments.
Measured demand from an existing building
For an existing building, measured data is often more accurate than a calculation from scratch. Utility bills for commercial accounts usually show the peak demand in each billing period, and the code permits measured maximum demand to be used in place of a calculated value in some circumstances, generally where a full year of records is available so that both the heating and cooling seasons are captured. Check the current rule and your inspection authority's expectations before relying on it.
Where billing data is not detailed enough, a power quality analyzer or logging meter can be installed on the service or a feeder for a period of weeks. This shows the peak, when it happens and how the load is shared between phases. The new loads are then added to the measured peak, using the applicable demand factors for those loads.
Continuous loads and equipment ratings
A load that runs at its maximum for three hours or more is treated as continuous. Electric heating, lighting in many commercial buildings and EV charging commonly fall into this category. The code limits continuous loading of most breakers and conductors to a percentage of their rating, commonly 80 percent, unless the equipment is marked for continuous operation at 100 percent. That limit has to be applied when comparing calculated demand with a service or feeder rating, and it is one of the most common reasons a service that looks big enough on paper is not.
A typical case
Consider a two-storey office building on an older service. The owner wants to replace gas rooftop units with heat pumps and add a row of Level 2 chargers in the parking lot. A sensible approach looks like this:
- Gather the records. Twelve months of utility bills showing peak demand, the single-line diagram if one exists, and the nameplate ratings of the service and main switchboard.
- Find the existing peak. In this case the highest demand appears in summer, when the existing cooling runs. If billing data is missing, a logger is installed for several weeks.
- Add the new loads. The heat pumps' electrical load is listed from the equipment data, noting that their winter heating load may exceed the old cooling load. The chargers are added at their rated load unless a load management system will limit them.
- Remove what is leaving. Loads being replaced, such as old cooling equipment, may come off the total if they will not run alongside the new ones.
- Compare and decide. The new calculated demand is compared with the service capacity after the continuous-load limit. If it fits with margin, the work proceeds. If not, the options are load management, a phased approach or a service upgrade.
The key detail in a case like this is the seasonal shift. Moving heating from gas to electricity often moves the building's peak from summer to winter, and a calculation that only looks at last summer's bills will miss it.
Common mistakes
| Mistake | What goes wrong |
|---|---|
| Adding up nameplates with no demand factors | Grossly oversized service and wasted money |
| Using a short logging period | Misses the seasonal peak, especially for electric heating |
| Ignoring continuous-load limits | A service that looks adequate trips or overheats in use |
| Checking only the main service | A feeder, panel or transformer downstream is the real bottleneck |
| Forgetting phase balance | One phase overloaded while the total looks fine |
| Leaving out the utility side | The utility transformer needs an upgrade nobody planned for |
What a load calculation report should include
- The scope: which service, feeders or panels were assessed
- The method used, either calculated from connected load or based on measured demand, and the code rules applied
- The source and period of any measured data
- A table of existing loads, new loads and loads being removed
- The demand factors and continuous-load assumptions applied
- The resulting demand compared with each rating, showing remaining capacity
- Recommendations, including load management or upgrade options where capacity is short
A calculation is often the first step in deciding when to upgrade your electrical service. For EV projects, an EV load management system can sometimes avoid an upgrade altogether.
Measurements on live equipment. Installing logging meters on a service or switchboard means working near exposed energized parts. It must be done by qualified electrical workers following the site's procedures and arc flash precautions.
How ehZee can help
Our service upgrade and load calculation work starts with why you need the number: a new tenant, electrification, chargers or a new building. We review records, arrange measurement where needed, prepare the calculation and explain the options. Where a permit is needed, drawings are sealed by a licensed professional engineer, and installation is done by licensed electrical contractors under permit.
Common questions
Can we just use the main breaker size as the capacity?
Not by itself. The breaker rating is one limit, but the conductors, switchboard, transformer and utility supply all have their own ratings, and continuous-load limits apply. The usable capacity is set by the weakest part of the chain.
How long should we log the load if we have no billing data?
Long enough to capture typical operation, and ideally including the season with the highest load. A few weeks is common, but a short summer log will not show a winter heating peak. Your engineer may adjust the measured values to account for this.
Does the utility need to see the load calculation?
Usually, when you apply for a new or upgraded service. The utility uses it to size its own transformer and service conductors. Requirements vary by utility, so ask early.