How do you work out maximum demand for a domestic installation?
You determine it, and you are permitted to reduce it for diversity. Regulation 311.1 requires the maximum demand of an installation to be determined, and says diversity may be taken into account in determining it. BS 7671 gives no figures. The numbers everybody quotes are in Table A2 of the IET On-Site Guide, which describes its own values as guidance only.
What Regulation 311.1 actually says
Chapter 31 of BS 7671 is headed Purposes, Supplies and Structure, and its first regulation is headed 311 MAXIMUM DEMAND AND DIVERSITY. The whole of 311.1 runs to two sentences:
For economic and reliable design of an installation within thermal limits and admissible voltage drop, the maximum demand shall be determined. In determining the maximum demand of an installation or part thereof, diversity may be taken into account.
Two verbs, and they are not the same verb. The maximum demand shall be determined. Diversity may be taken into account. Determining the demand is a requirement. Applying diversity is a permission granted to whoever is doing the design, and there is not a single number anywhere in the regulation.
Regulation 313.1 sits alongside it. It lists the characteristics of the supply that shall be determined by calculation, measurement, enquiry or inspection, and item (e) is “the suitability for the requirements of the installation, including the maximum demand”. Calculation, measurement, enquiry or inspection. Four permitted routes, and copying the figure off the previous certificate is not one of them.
What diversity is defined as
Part 2 of BS 7671 defines it in one line:
Diversity. Means of determining maximum demand for an installation or part thereof, taking into account usage patterns.
Usage patterns. Not ratings, not duty cycles, not thermostats. The word is doing a specific job: diversity is the recognition that the shower, the oven, the immersion heater and the tumble dryer in one house are unlikely to be drawing full load at the same moment. It is a statement about how a collection of loads behaves, over time, together.
A single appliance is a different question
“Allow for diversity” gets offered as the answer to questions about one appliance on one circuit, and that is where the word starts doing work it was not defined to do. An oven cycling on its thermostat is one load with a duty cycle. Whether a circuit can carry it is a question about that load and that cable, not about usage patterns across an installation.
The On-Site Guide is careful about which level each allowance belongs at, and it is worth being equally careful. It says the current demand of a final circuit is determined by adding the current demands of all points of utilization and equipment in the circuit and, where appropriate, making an allowance for diversity. So diversity is not banned at final circuit level. But the allowances in Table A2 are described as applying to the total current demand of all the equipment supplied by the installation, which is a different calculation at a different level.
The honest position: applying diversity to a group of loads is what the definition describes. Applying it to a single fixed appliance that runs at its rated power when it runs needs a reason, and “diversity” on its own is not that reason.
Where the numbers come from, and what the guide says about them
The tables everybody works from are Appendix A of the IET On-Site Guide, “Maximum demand and diversity”, referenced to Regulation 311. Table A1 gives the current demand to be assumed for points of utilization and current-using equipment. Table A2 gives the allowances for diversity.
Appendix A opens by qualifying itself heavily, and these are the sentences worth knowing before quoting anything out of the table:
The information and values given in this appendix are intended only for guidance, because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for special knowledge and experience. The recommendations in Table A2 have not been updated for some time, and do not necessarily align with modern loads and usages. The values given in Table A2, therefore, may be increased or decreased as decided by the installation designer concerned.
Read that middle sentence again. The guide itself says its diversity allowances have not been updated for some time and do not necessarily align with modern loads and usages. That was written before every third house acquired a car charger, a heat pump or a hot tub.
The same paragraph also says no guidance is given for blocks of residential dwellings, large hotels, or industrial and large commercial premises, and that such installations should be assessed case by case. A lot of people apply Table A2 to a block of flats. The guide says it does not cover that.
The method, and the mistake it warns against
Appendix A sets out one method, one alternative, and one thing not to do.
Final circuit. Add the current demands of all points of utilization and equipment in the circuit, using Table A1, and make an allowance for diversity where appropriate.
Installation. Apply the Table A2 allowances to the total current demand of all the equipment supplied by the installation.
The mistake. The guide states that the current demand of the installation should not be assessed by adding the current demands of the individual final circuits obtained as outlined above. Working out each circuit, diversifying each one, then adding the results together is the method it specifically tells you not to use.
The alternative. Adding the diversified demands of the individual circuits and then applying a further allowance for diversity is permitted, but in that method the Table A2 allowances should not be used, and the values chosen are the responsibility of the designer. Other methods are not precluded either, where the designer specifies them. Whichever route is taken, voltage drop still has to be checked once the design currents are settled.
Standard circuit arrangements do not get diversified
One line in Appendix A settles a surprising number of arguments:
The current demand for any final circuit that is a standard circuit arrangement complying with Appendix H is the rated current of the overcurrent protective device of that circuit.
A ring final circuit built as a standard arrangement and protected by a 32 A device has a current demand of 32 A for this purpose. Not the sum of what is plugged into it, not a diversified estimate of what the kitchen might draw. The device rating.
What goes in the box on the certificate
Section F of the Electrical Installation Certificate and Section D of the Electrical Installation Condition Report both carry a Maximum demand (load) field, alongside the supply characteristics and earthing arrangements. Neither form prescribes how the figure was arrived at, and neither asks you to show the working.
That combination, a mandatory box with no prescribed method behind it, is why the figure so often turns out to be the rating of the main switch, or the number that was on the last certificate. Regulation 311.1 requires the demand to be determined. Regulation 313.1 lists calculation, measurement, enquiry or inspection as the ways of establishing the supply’s suitability for it.
Common questions
Does BS 7671 give diversity figures?
No. Regulation 311.1 requires the maximum demand to be determined and permits diversity to be taken into account, and stops there. The figures are in Appendix A of the IET On-Site Guide, which is guidance rather than the standard.
Are the On-Site Guide allowances mandatory?
No. Appendix A says its information and values are intended only for guidance, and that the Table A2 values may be increased or decreased as decided by the installation designer concerned.
Does diversity apply to a single oven?
Diversity is defined in Part 2 as a means of determining maximum demand taking into account usage patterns, which is a statement about a group of loads rather than one appliance. The On-Site Guide does allow an allowance for diversity at final circuit level where appropriate, so it is not a flat no. It does mean the allowance has to be justified by how the loads on that circuit are actually used, rather than asserted.
What is the current demand of a ring final circuit?
For a standard circuit arrangement complying with Appendix H of the On-Site Guide, it is the rated current of the overcurrent protective device of that circuit.
Can the installation demand be worked out by adding up each circuit?
Not by adding the diversified demands obtained from the final circuit method. Appendix A says the current demand of the installation should not be assessed that way. There is a separate alternative method, and it does not use Table A2.
Does Table A2 cover blocks of flats?
No. Appendix A states that no guidance is given for blocks of residential dwellings, large hotels or industrial and large commercial premises, and that such installations should be assessed case by case.
What should go in the maximum demand box on an EICR?
A figure that was determined, by calculation, measurement, enquiry or inspection. The form does not prescribe a method and does not ask for the working, which is exactly why the figure is worth being able to defend.
Recording it
Maximum demand is one field on a form that has several hundred of them, and it sits on every EIC and EICR you issue. iCertifi: Electrical Certs produces both, on an iPhone or iPad, with the supply characteristics carried across so the box is filled in on site rather than from memory in the van afterwards. iCertifi on the App Store.
Regulation 311.1, the Part 2 definition of diversity and Regulation 313.1 were read from BS 7671:2018+A4:2026 at printed pages 57, 37 and 62. The Appendix A wording was read from the IET On-Site Guide. No value is quoted from Table A1 or Table A2, because table cells are not reproduced from scanned text.