Bar shows demand as a share of the main fuse. A design estimate only: a qualified electrician must assess maximum demand to BS 7671 before adding circuits.
Formula and breakdown
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This maximum demand calculator estimates the peak current a UK home is likely to draw, after diversity, and compares it with a 60 A, 80 A or 100 A main supply fuse. It is for homeowners planning an EV charger, heat pump or electric shower, and for students and electricians who want the On-Site Guide style method laid out with every allowance visible and editable.
Quick answer: A maximum demand calculator adds each group of loads in amps after a diversity allowance, because not everything runs at once. Example allowances: 66% of lighting, the largest socket circuit plus 40% of the others, cookers at 10 A plus 30% of the rest plus 5 A, and EV chargers and heat pumps at 100%.
What Is a Maximum Demand Calculator?
A maximum demand calculator is a tool that estimates the highest current an installation will draw in normal use, so the supply, main fuse, meter tails and main switch can be checked as adequate. In plain words, the IET describes maximum demand as the sum of the connected loads multiplied by a factor for diversity: each load is converted to amps, reduced by an allowance for how likely it is to run at full power together with the others, and then added up.
Diversity is the difference between what is connected and what is used at the same time. A home might have 150 A of appliances wired in, yet rarely draws more than a fraction of that, because the oven, kettle, shower and heaters seldom all run at full power together. The allowances turn that experience into numbers a designer can defend.
How Do You Use the Maximum Demand Calculator?
- Check the voltage (230 V in the UK) and select the rating of the main fuse in your cut-out; your electrician or network operator can confirm it.
- Enter the total lighting load in watts and the lighting allowance.
- Enter the number and rating of socket circuits; the first counts in full and each further one at the percentage you set.
- Enter the cooker rating in kW and say whether its control unit has a socket.
- Add electric showers, an immersion heater, an EV charger, a heat pump and any other fixed loads in kW.
- Read the total in amps and kVA, how it compares with the main fuse, and the breakdown by group.
How Is Maximum Demand Calculated in a UK House?
Maximum demand in a UK house is found by converting each load to amps (watts divided by 230 V), applying a diversity allowance to each group, and adding the results. The method in the IET On-Site Guide is guidance, not a rule; the designer may use measured data or their own knowledge of how the home is used instead.
The IET’s answers on electrical diversity and EV charging confirm several of the allowances this maximum demand calculator uses: 10 A plus 30% plus 5 A for a cooker with a socket on its control unit (from On-Site Guide Table A1), no diversity on a heat pump circuit, none on an instantaneous shower when sizing its cable, and EV chargers matched to the supply with no diversity. For socket-outlets, the same answers apply 100% to the first and 40% to each further one. Other figures, such as the 66% lighting allowance used in a worked answer published by Voltimum, are shown as editable example values.
| Load group | Allowance used by default | Basis |
|---|---|---|
| Lighting | 66% of connected load | Example value from a published worked answer; editable |
| Socket circuits | Largest at 100%, others at 40% | IET guidance quoted in its diversity answers |
| Cooker | 10 A + 30% of the remainder + 5 A if a socket is fitted | IET On-Site Guide Table A1, as cited by the IET |
| Electric showers | 100% | Conservative example value; editable |
| Immersion heater | 100% | Conservative; thermostat controlled loads can run for hours |
| EV charger | 100% | IET: no diversity applied |
| Heat pump | 100% | IET: no diversity on the heat pump circuit |
| Other fixed loads | 100% | Example value; editable |
To convert a single appliance before entering it, the kW to amps calculator gives the current at 230 V, and the watts to amps calculator handles smaller loads.
Is My 60 A, 80 A or 100 A Supply Big Enough?
Your supply is big enough when the estimated maximum demand is at or below the rating of the main fuse, with some margin for loads you plan to add. A demand of 46.8 A fits a 60 A fuse with 13 A spare; a demand of 114 A does not fit even a 100 A fuse.
Results from a maximum demand calculator using the On-Site Guide method often land above the fuse rating for homes that have never blown a main fuse. An electrician’s question in the IET webinar says exactly this: the guidance “almost always” produces a figure well over a 60 A supply. That is because the allowances are deliberately cautious. When the maximum demand calculator shows a result over the fuse, the next steps are a measured demand survey over one or two weeks, load management for new loads such as EV chargers, or a supply upgrade arranged with the network operator.
Do EV Chargers and Heat Pumps Count Without Diversity?
Yes. The IET says an EV charger should be matched to the available supply with no diversity applied, and no diversity should be applied to a heat pump circuit. A typical home charger is 32 A single-phase, which the IET describes as 7.4 kW.
These loads run for hours at a time, often in the evening alongside cooking and heating, so treating them as fully on is realistic. The IET’s answers also say that if the total load including the charge point exceeds 60 A, permission must be sought from the distribution network operator (DNO), and that installations whose demand exceeds 13.8 kVA require notification to the DNO before the equipment is installed. Northern Ireland’s network operator, NIE Networks, likewise asks installers for a maximum demand assessment before connecting EV chargers and heat pumps. Our home EV charger calculator covers charging time and cost once the supply is confirmed.
What Can I Do If Maximum Demand Is Over the Main Fuse?
If the estimate is over the main fuse, the safest first step is to have an electrician check it with real data rather than adding the load anyway. A maximum demand calculator works from ratings and allowances; a clamp-on logger measures what the house actually draws.
- Measure: a two-week logging survey on the meter tails shows the real peak, and NIE Networks notes that installers can use remote monitoring for this.
- Manage: load-limiting EV chargers reduce charging current when the house is busy; the IET notes that BS 7671 does not preclude this.
- Upgrade: ask the DNO about a larger fuse or supply. The IET understands that DNOs have committed to upgrade dwellings to 100 A per phase.
New circuits for any of these loads are notifiable under Part P in England. The Approved Document P guidance covers when work must be notified, and the circuit itself should be sized with a cable size calculator and checked by the electrician to BS 7671.
Worked Example: Maximum Demand for a House With an Electric Shower
Example: a house in England has 1,000 W of lighting, two 32 A socket circuits, a 10 kW electric cooker with a socket on its control unit and a 9.5 kW electric shower. The main fuse is 100 A. Allowances are the defaults in the maximum demand calculator.
- Lighting: 1,000 / 230 = 4.35 A x 66% = 2.87 A.
- Sockets: 32 A + (32 A x 40%) = 44.80 A.
- Cooker: 10,000 / 230 = 43.48 A. 10 A + 30% of 33.48 A + 5 A = 25.04 A.
- Shower: 9,500 / 230 = 41.30 A at 100%.
- Total: 2.87 + 44.80 + 25.04 + 41.30 = 114.0 A, or 26.2 kVA, which is 14 A over the 100 A fuse. Connected load without diversity would be 158.1 A.
Adding a 7.4 kW EV charger would put another 32.17 A on top, at 100%. On paper this house already exceeds its fuse, so an electrician would most likely measure the real demand or specify a load-managed charger rather than rely on the estimate alone.
Frequently Asked Questions
What is maximum demand in electrical terms?
Maximum demand is the highest current, or power, an installation is expected to draw at any one time in normal use. It is lower than the total connected load because appliances rarely all run at full power together, and that difference is called diversity.
How do you calculate maximum demand with diversity?
Convert each load to amps, apply an allowance to each group (for example 40% for each socket circuit after the first, or 10 A plus 30% of the rest for a cooker), then add the groups. EV chargers and heat pumps are counted in full.
What is the diversity allowance for a cooker?
The IET On-Site Guide allowance for a household cooker is the first 10 A of its rated current, plus 30% of the remainder, plus 5 A if the cooker control unit has a socket. A 10 kW cooker at 230 V gives 25.04 A with the socket.
Is diversity applied to an EV charger?
No. IET guidance says the designer should match EV charging loads to the available supply with no diversity applied. A 7.4 kW home charger therefore adds about 32 A to the maximum demand, unless it is load-managed to reduce current when the house is busy.
Can I install an EV charger on a 60 A supply?
Sometimes, but it needs checking first. IET answers say that if the total load including the charger exceeds 60 A, the DNO’s permission is needed. A measured demand survey or a load-limiting charger often makes it possible. An electrician must assess it.
Is this maximum demand calculator good enough for a certificate?
No. It is a planning estimate. Maximum demand must be assessed by the designer of the installation, usually a qualified electrician working to BS 7671, who may use measured data. New circuits in homes in England are notifiable under Part P.
Checked October 2026 by the Solaxyra Editorial Team. Sources: IET: electrical diversity and load curtailment, EV charging webinar answers, Voltimum Q and A: what is maximum demand (worked answer), NIE Networks: low carbon technologies FAQ, Approved Document P (gov.uk), Approved Document P 2013 PDF (notifiable work).