BS7671 Maximum Demand Calculator
Calculate estimated maximum demand and design current using BS 7671 diversity principles. Ideal for UK electrical installations, consumer unit sizing, cable selection and supply capacity assessment.
BS7671 Maximum Demand Calculator
How to Use BS7671 Maximum Demand Calculator
Calculating the estimated maximum demand of an UK electrical installation in accordance with BS 7671 and the IET On-Site Guide is essential for sizing main incomers, consumer units, and distribution boards. Follow these steps to perform the load diversity calculation:
- 1Select installation type. Choose from domestic dwelling, flat, office, retail shop, small commercial unit, or mixed-use system to load the corresponding default diversity factor.
- 2Enter connected load. Input the total connected load in Kilowatts (kW), representing the sum of all electrical equipment and circuits.
- 3Verify voltage. Confirm the supply operating voltage (default is 230 V for single phase, but adjustable up to 690 V).
- 4Select phase system. Toggle between a Single Phase or Three Phase configuration depending on the installation supply.
- 5Review diversity factor. Check the pre-loaded diversity factor or enter a custom value based on project-specific conditions or IET guidelines.
- 6Calculate results. Click the Calculate button to view the estimated maximum demand and design current.
- 7Use design current. Apply the computed design current (Ib) for selecting appropriate cable sizes and circuit protective devices.
How to Calculate BS7671 Maximum Demand
Assessing maximum demand involves applying diversity factors to the total connected load of an electrical installation. This ensures that electrical systems are designed for realistic peak conditions rather than an improbable scenario where every appliance runs simultaneously. Here are the core formulas used in the calculation:
Formula 1 — Maximum Demand (kW)
The estimated maximum demand is calculated by multiplying the total connected load of the installation by the diversity factor appropriate for the premises type.
Formula 2 — Single Phase Current (A)
For standard UK single-phase installations (typically 230 V), the design current is calculated by converting the maximum demand to Watts and dividing by the line voltage.
Formula 3 — Three Phase Current (A)
For three-phase systems (typically 400 V line-to-line), the design current accounts for the square root of three factor in balanced three-phase systems.
Step-by-Step Worked Example
To illustrate how these formulas apply in practice, consider a standard UK domestic dwelling with the following parameters:
- Installation Type: Domestic Dwelling
- Total Connected Load: 18 kW
- Assumed Diversity Factor: 60% (0.60)
- Supply Voltage: 230 V (Single Phase)
Step 1 — Calculate Maximum Demand (kW)
Apply the domestic diversity factor of 0.60 to the total connected load:
Step 2 — Calculate Single Phase Design Current (A)
Convert the maximum demand of 10.8 kW to Watts and divide by the single-phase supply voltage (230 V):
Step 3 — Determine Recommended Supply Size
Select the next standard UK supply size greater than or equal to the calculated design current of 46.96 A. Since 46.96 A is less than 60 A, the recommended supply size is:
This assessment ensures that a standard 60 A cut-out fuse is adequate for the property without risk of nuisance tripping under normal peak operations.
BS7671 Maximum Demand Calculator Chart
The following table provides typical maximum demand results for various installation types with a constant reference connected load of 20 kW. This illustrates how different building usage profiles affect the estimated maximum demand under BS 7671 diversity guidelines.
| Installation Type | Typical Diversity Factor | Connected Load (kW) | Estimated Demand (kW) |
|---|---|---|---|
| Domestic | 0.60 | 20 kW | 12.0 kW |
| Flat / Apartment | 0.65 | 20 kW | 13.0 kW |
| Office | 0.80 | 20 kW | 16.0 kW |
| Retail Shop | 0.75 | 20 kW | 15.0 kW |
| Commercial Unit | 0.85 | 20 kW | 17.0 kW |
| Mixed Use | 0.80 | 20 kW | 16.0 kW |
Note: Values are illustrative examples. Actual diversity assessment should follow BS 7671 and IET guidance for the specific installation.
BS7671 Maximum Demand Calculator Frequently Asked Questions
Maximum demand is the estimated peak electrical power required by an installation. BS 7671 requires an assessment of maximum demand to size the incoming supply, main switchgear, distribution boards, and cables. It takes into account the likelihood that not all connected loads will operate at full capacity simultaneously.
Diversity is applied by multiplying the total connected load by a diversity factor (typically between 0.1 and 0.9) or by applying specific percentage rules from the IET On-Site Guide for different circuit types, reflecting that loads operate intermittently.
No, connected load is the sum of the maximum power ratings of all installed electrical equipment. Maximum demand is almost always lower than connected load because it accounts for diversity, meaning not all appliances run at the same time.
BS 7671 does not mandate fixed diversity factors; instead, it allows designers to use engineering judgment. However, the IET On-Site Guide Appendix A provides standard guidance and tables of diversity factors for domestic and commercial installations.
Electricians calculate design current (Ib) by dividing the estimated maximum demand (in Watts) by the operating voltage (230V for single phase, or ®3 × 400V for three-phase systems), accounting for the power factor where applicable.
Yes, diversity is widely used in commercial installations like offices, retail shops, and hotels. However, diversity factors in commercial settings are generally higher than in domestic ones due to more continuous, simultaneous loads like lighting and HVAC.
Determining maximum demand is critical to prevent over-sizing cables and switchgear, which increases installation costs unnecessarily, while ensuring that the consumer unit and supply capacity (DNO fuse) are not overloaded.
Connected load is the total capacity of all connected equipment. Maximum demand is the actual peak load drawn from the supply at any one time after applying diversity factors based on usage patterns and installation type.
Yes, this calculator can help assess whether an existing installation's supply and main switchgear are adequate for the current connected loads during an Electrical Installation Condition Report (EICR) assessment.
Yes, the Distribution Network Operator (DNO) requires an accurate maximum demand calculation to approve new connections, supply upgrades, or low-carbon technology installations like EV chargers and heat pumps.