What an electrical load calculation actually answers
Before you can size a supply, a main breaker or a meter, you need to know how much current a building will really draw. That is the job of an electrical load calculation: it turns a list of appliances and their ratings into a realistic design current.
Two kinds of project need this number. A new build has to apply for a supply capacity from the utility. A renovation needs to know whether the existing service can take an added EV charger, heat pump or induction hob without an upgrade.
To follow this guide you need three things: a schedule of loads with their power ratings, the system voltage (230 V single-phase or 400 V three-phase), and a rough idea of which loads run at the same time.
In day-to-day practice the calculation is also a check on common sense. If the diversified demand comes out far above what similar buildings use, the input assumptions are probably too pessimistic; if it comes out far below, a load has almost certainly been missed.
Utilities, insurers and inspection bodies all ask for the design current, so it is worth keeping a clear record of the loads you used and the factors you applied.
Connected load versus demand load
The connected load (also called installed load) is the simple sum of every load's rated power. Add up the rating of every light, socket circuit, boiler and motor in the building and you get a large number that almost never occurs in practice.
The demand load, or maximum demand, is the current the installation is expected to draw at its busiest moment. It is always lower than the connected load, because no home switches on every appliance at once.
Maximum demand is normally measured over a short window of 15 to 30 minutes, because a brief inrush current does not heat a cable the way a sustained load does. That is why a motor's starting current is not simply added to the demand total.
Designers size the supply and the main protective device from the demand load, not the connected load. Using the connected load would oversize everything; ignoring diversity altogether would be safe but wasteful.
Demand factor and diversity factor
Two simple ratios connect the two numbers.
- Demand factor = maximum demand ÷ connected load. It shows how much of the installed load a single installation actually uses.
- Diversity factor = sum of the individual maximum demands ÷ maximum demand of the whole installation. It is always greater than one and reflects the fact that different loads peak at different times.
As a simple illustration, if a house has a connected load of 24 kW and a maximum demand of 12 kW, the demand factor is 0.5. If the individual maximum demands of its largest loads add up to about 20 kW, the diversity factor is 20 ÷ 12 ≈ 1.7.
Standards such as IEC 60364 and national wiring regulations give guidance tables rather than one fixed number. A block of identical dwellings, for example, can use a smaller per-dwelling demand than a single house, because the chance of all of them peaking together is low.
Diversity is applied after you have grouped similar loads. It is a statistical allowance, not a safety margin, so it should never be used to hide a load that genuinely runs for hours.
Step by step: calculating the design load
- List every load and note its rated power in watts or kilowatts, plus its voltage.
- Convert power to current. Single-phase: I = P ÷ (U × cos φ). Three-phase: I = P ÷ (√3 × U × cos φ).
- Group the loads by circuit so you can apply the right diversity to each group.
- Apply diversity to lighting, socket outlets and other intermittent loads using the factors from your regulation.
- Add continuous loads — heating, EV charging, motors — at or near full value, because they run for long periods.
- Sum the diversified currents to get the design current of the installation.
- Round up to the next standard supply size and choose a main device that protects it.
Single-phase versus three-phase
For a resistive load the power factor is close to 1, so the current is simply P ÷ U. For motors and electronics the power factor drops, and the current rises for the same power, so using the right cos φ matters.
On a three-phase supply, spread the total power as evenly as you can between the phases before you add the currents. Unbalanced phases can overload one line while the others still have spare capacity.
Worked example: a three-bedroom house
Take a single-phase 230 V house with the following installed load.
| Group | Connected power | Diversity applied | Demand |
|---|---|---|---|
| Lighting | 1.2 kW | 66% | 0.8 kW |
| Sockets (general) | 6.0 kW | 40% | 2.4 kW |
| Kitchen (oven, hob) | 7.0 kW | 70% | 4.9 kW |
| Water heater | 3.0 kW | 100% | 3.0 kW |
| EV charger | 7.4 kW | 100% | 7.4 kW |
Connected load is 24.6 kW; the diversified demand is 18.5 kW. Assuming a power factor of 0.95, the design current is 18,500 ÷ (230 × 0.95) ≈ 85 A.
That is well above a typical 63 A single-phase service, so the practical answer is a three-phase supply or a load-management scheme that limits the EV charger while other loads are peaking.
The figures here are illustrative and use rounded factors. In a real project, replace them with the values from the regulation that applies to you, and add any loads this example omits, such as a pool pump, a workshop machine or a second heating system.
Choosing the supply and the main device
Once you have the design current, pick the next standard supply rating above it. Common single-phase services are 40 A, 50 A, 63 A and 80 A; three-phase services are usually 3 × 25 A, 3 × 35 A and 3 × 50 A and above.
The main breaker or fuse must carry the design current continuously and still protect the meter tails. Check that its rating is not below the design current and not so far above the cable rating that the tails are left unprotected.
If the design current sits just above a standard service size, a small change in assumptions can avoid a costly upgrade. Rechecking one heavy load is often far cheaper than paying for a larger supply for years.
Common mistakes
- Using the connected load as the design load and oversizing the whole service.
- Diversifying continuous loads such as EV chargers, which should stay near full value.
- Applying residential diversity factors to commercial kitchens or workshops where many loads really do run together.
- Ignoring power factor, especially with motors, and understating the current.
- Adding a large new load during a renovation without rechecking the supply.
- Mixing single-phase and three-phase totals without converting the currents correctly.
- Forgetting the standing loads — heating controls, pumps, alarms — that run continuously in the background.
- Leaving no margin for a realistic future load such as a heat pump or a second EV.
Keeping the calculation consistent with the design
A load calculation only stays correct if it is linked to the drawing. When you add a circuit or change an appliance, the schedule and the supply have to change with it.
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FAQ
What is the difference between connected load and demand load?
Connected load is the sum of every appliance rating. Demand load is the current the installation actually draws at its busiest moment, after diversity is applied.
Which diversity factor should I use for a house?
There is no single figure. Residential guidance tables in IEC 60364 and national regulations give factors by load type, so confirm the values for your country and load group.
Can I add an EV charger without upgrading the supply?
Only after a load calculation. If the diversified demand plus the charger exceeds the existing service, you need a larger supply or a load-management device.
Do I need a three-phase supply for a large house?
Not automatically. It depends on the design current. When the single-phase demand exceeds standard service sizes, three-phase is usually the practical choice.
Does the calculation differ for an apartment?
The method is the same, but a single apartment usually has fewer heavy loads and often relies on a shared diversity figure for the whole building.