Electrical Load per Room Calculator
Calculates the simultaneous load and current for each room, then suggests an MCB rating and cable size to BS 7671.
A bedroom with a 1,500 W split air conditioning unit, a 100 W television and 80 W of LED lighting has 1,680 W installed. With a diversity factor of 0.8, the simultaneous load is 1,344 W, or 6.5 A at 230 V with a power factor of 0.9. For that load, a 10 A MCB on 1.5 mm² cable is the usual choice, subject to the design checks in BS 7671. If the same bedroom also has a 300 W computer, the simultaneous load rises to 1,584 W, about 7.7 A, which is still within the 10 A rating, though with less margin.
This calculator sets out that working for each room. It takes the installed power of the lighting, the socket outlet appliances and any fixed heating or cooling, applies a diversity factor, and converts the result to current. From that current it proposes a standard MCB rating and the minimum cable size that can carry it. The approach follows BS 7671, the national standard for electrical installations in buildings, and uses the standard MCB ratings set out in BS EN 60898-1.
A second example shows how the same method applies to a different room. A room in a 2-bed flat in Manchester with an air conditioning unit (1,800 W), a television (100 W), a computer (80 W) and LED lighting (60 W) has 2,040 W installed. At a diversity factor of 0.8, the current is about 7.9 A. Adding a 2,000 W convector heater to the same room raises the current to about 15.6 A, which is close to the limit of a 16 A circuit and calls for a review before the heater is connected.
The calculator is a planning and checking tool. It does not replace the design of an installation by a competent person, and it does not cover cable grouping, ambient temperature, cable length or voltage drop, all of which affect the final cable size. Electrical work in dwellings in England and Wales is subject to Part P of the Building Regulations, and anyone who works on electrical systems has duties under the Electricity at Work Regulations 1989.
The explanation below uses metric units; the calculator works in either system.
The result updates automatically.
Selecting the type fills in typical values — adjust as needed.
Total of the light fittings installed. LED: 5–15 W/fitting; halogen: 35–50 W/fitting.
TV (~100 W), computer (~80 W), games console (~150 W), chargers, etc.
Air conditioning (~1,000–3,500 W), convector heater (~1,000–2,000 W), electric towel rail (~200–600 W).
Electric cooker (~6,000–7,000 W), oven (~2,000–3,500 W), washing machine (~2,000 W).
The percentage of appliances running at the same time. 80% is the typical value for a home.
Recommended breaker
Minimum rating for a current of 7.3 A
Simultaneous load
80% of 1,880 W installed
- Total installed load
- 1,880 W
- Lighting
- 80 W
- Appliances / sockets
- 300 W
- Heating / AC
- 1,500 W
- Other appliances
- 0 W
- Circuit current
- 7.3 A
- Minimum cable size
- 1,5 mm²
How it works
Formula
How the calculation works
How to work out the electrical load per room
Sizing an electrical circuit involves three variables: the installed power, the diversity factor and the supply voltage. Getting any one of them wrong produces one of two faults. An undersized circuit trips the MCB during normal use, which is a nuisance and a sign that the design is being pushed too far. An oversized protective device allows more current than the cable can safely carry, so the cable can overheat before anything trips. The calculator avoids both faults by working from the load that is realistically expected in each room, not from the total of every appliance in the house.
The supply in the United Kingdom is single-phase at a nominal 230 V and a frequency of 50 Hz. Domestic loads are a mix of resistive equipment (heaters, lamps, kettles) and motor-driven equipment (compressors, pumps, washing machine motors), so the overall power factor is below unity. The calculator uses 0.9 as a common working value for simplified sizing. It is an assumption rather than a measurement, and a designer who has the data for the actual equipment can use a different figure.
The formula and the diversity factor
The current in a single-phase circuit follows from the active power: I = P / (V × cos φ). The power used for sizing is not the sum of everything connected to the circuit. It is the power of the equipment that can run at the same time, and the diversity factor expresses that relationship. The calculator uses:
I = (P_inst × fs) / (V × 0.9)
where P_inst is the total installed power in watts, fs is the diversity factor as a decimal, V is the supply voltage and 0.9 is the power factor.
As an example, a room with 2,000 W installed and a diversity factor of 0.8 has a simultaneous load of 1,600 W. The resulting current, 1,600 divided by (230 × 0.9), is about 7.7 A. That figure, not the 2,000 W total, decides the MCB.
The diversity factor is a judgement about how a room is used. BS 7671 deals with maximum demand and diversity in Appendix 1, and it allows the designer to apply diversity where loads are unlikely to run together. A value of 0.8 is a practical working figure for general domestic use, but it does not suit every room. A kitchen with several large appliances may run them together, and a room with a single fixed heater gains little from diversity.
Fixed appliances such as cookers, electric showers and immersion heaters can run at full power for long periods. For these, the full rating is normally counted, which is equivalent to a diversity factor of 1.0. The calculator leaves this choice to the user through the diversity input, so the decision stays with the person who knows the appliance.
Standard MCB ratings and how to choose one
Miniature circuit breakers for domestic use follow BS EN 60898-1 and come in the standard ratings of 6, 10, 16, 20, 25, 32, 40, 50 and 63 A. The device chosen is the first rating at or above the design current. A circuit with a design current of 9.5 A needs a 10 A device. A design current of 17.3 A needs a 20 A device, because a 16 A device would trip under the expected load.
Two MCB types matter for domestic work. Type B devices suit resistive loads such as lighting and heaters, where the starting current is close to the running current. Type C devices tolerate the short surge at switch-on of motor-driven equipment such as compressors, pumps and washing machines. The type is chosen for the circuit as a whole, and the classification is a guide that the designer applies with the installation details. Residual current protection is a separate requirement under BS 7671, and it is decided alongside the MCB rating rather than instead of it.
Cable size: the rule that is often overlooked
The cable does not depend independently on the MCB; the two must work together. The current-carrying capacity of the cable, usually written Iz, must be at least equal to the rating of the protective device, In. BS 7671 gives the current-carrying capacity tables for different cable sizes, installation methods and conditions, and these are set out in its appendices. Fitting a 25 A device to a 1.5 mm² cable defeats the protection: the MCB will not trip before the cable reaches a temperature that degrades its insulation.
In UK dwellings, the cable sizes most often seen are 1.0 or 1.5 mm² for lighting, 2.5 mm² for socket outlets, and 4 mm² or 6 mm² for cookers, showers and larger fixed loads. These are common choices and not a substitute for the tables, because grouping, insulation type and the length of the run all change the answer. Long runs also need a voltage drop check, which this calculator does not perform.
Common mistakes in domestic installations
Connecting an air conditioning unit to a socket outlet is a common error. A 3.5 kW split unit draws about 15.2 A at unity power factor and about 16.9 A at a power factor of 0.9, which exceeds the 13 A rating of a standard plug and socket outlet to BS 1363. Such a unit needs its own circuit, protected and cabled for its rating.
Using extension leads as permanent wiring is another frequent problem. Extension leads are intended for temporary use. Running a heater or other high-load appliance through one for long periods heats the lead and its plug connections, and that heat degrades the insulation gradually. The problem may not show at first, but the risk grows with time.
Ignoring starting current causes nuisance tripping. Motor-driven equipment such as air conditioning compressors, refrigerators and washing machines draws several times its running current for a short moment at switch-on. A Type B device may trip on this surge even when the running current is well within the rating, which is why the device type matters as well as its rating.
Adding loads without rechecking the circuit is the last common mistake. Each new appliance adds to the installed power, and the total is what matters. The circuit should be recalculated before a new heater, an extra socket run or a second air conditioning unit is connected.
When to use a registered electrician
This calculator estimates loads and checks whether an existing installation is within reasonable limits. Any real change to the electrical installation, such as replacing a consumer unit, adding circuits or changing cable sizes, is work for a competent person. In England and Wales, much of this work in dwellings is notifiable under Part P of the Building Regulations. That means it must be carried out by a registered competent person, who certifies it, or notified to the local authority building control. Scotland and Northern Ireland have separate building regulation regimes, and the local building control office can confirm which rules apply.
A periodic inspection report is the usual way to assess the condition of an existing installation, and an Electrical Installation Certificate records new or altered work. Both follow the forms and methods set out in BS 7671. Replacing a like-for-like socket outlet or light switch is simpler work, but it still requires the supply to be isolated and the work to be done safely. Work on the consumer unit or on new circuits should be left to a qualified installer.
Standards and public sources
The main references are BS 7671 for the design and verification of electrical installations, BS EN 60898-1 for the miniature circuit breakers used in domestic installations, and BS 1363 for the 13 A plug and socket outlet system used across the United Kingdom. Part P of the Building Regulations sets the legal requirements for electrical work in dwellings in England and Wales. Electrical installations are also subject to the Electricity at Work Regulations 1989, which place duties on those who work on or control electrical systems.
Frequently asked questions
What is the maximum load on a standard UK socket outlet?
A standard socket outlet to BS 1363 is rated at 13 A, so a single appliance can draw up to about 2,990 W at 230 V (13 A × 230 V), and less in practice when several appliances share the circuit. Appliances above this level, such as electric showers, cookers and immersion heaters, cannot be run from a standard plug and socket and need a dedicated circuit, with a protective device and cable sized to the appliance rating under BS 7671. The circuit design, not the outlet alone, sets the limit for the whole run.
Which MCB should I use for a bedroom with air conditioning?
It depends on the total simultaneous load. A bedroom with a 1,500 W air conditioning unit, a 100 W television and 80 W of lighting has 1,680 W installed. With a diversity factor of 0.8 and a power factor of 0.9, the design current is about 6.5 A, and a 10 A device on 1.5 mm² cable is the usual choice. If a 2,000 W convector heater is also connected to the same circuit, the design current rises to about 14.2 A, which needs a 16 A device and 2.5 mm² cable. A fixed heater is usually better on a circuit of its own, so the existing circuit keeps its margin.
What is the diversity factor and how does it affect sizing?
The diversity factor is the share of installed equipment that is expected to run at the same time. A room with 3,000 W installed and a factor of 0.8 is designed for 2,400 W running together, which lowers the current and so allows a smaller MCB and cable. Diversity is a judgement about use, so it should be applied with care. Fixed loads that can run at full power for long periods, such as cookers, electric showers and heaters, are normally counted at their full rating, which is a factor of 1.0. BS 7671 deals with maximum demand and diversity in Appendix 1.
What cable size do I need for a lighting or socket circuit?
Cable size depends on the device protecting the circuit, the installation method and the length of the run, so the answer must come from the tables in BS 7671. As a common starting point, lighting circuits are often wired in 1.5 mm² cable protected by a 6 A MCB, while socket outlet circuits are often wired in 2.5 mm². The cable's current-carrying capacity must be at least equal to the protective device rating. Fitting a higher-rated MCB to a smaller cable removes the protection, because the device will not trip before the cable overheats. Long runs also need a voltage drop check, which this calculator does not include.
When is a qualified electrician needed in the UK?
Any change to a fixed electrical installation, such as replacing a consumer unit, adding circuits or changing cable sizes, should be carried out by a competent person. In England and Wales, much of this work in dwellings is notifiable under Part P of the Building Regulations, which means it must be carried out by a registered competent person or notified to the local authority building control. Scotland and Northern Ireland have their own building regulation arrangements. Replacing a like-for-like socket outlet or light switch is simpler work, but the supply must still be isolated and the work done safely. A periodic inspection report confirms the condition of an existing installation.
Can I add more appliances to an existing circuit?
Yes, provided the new total does not exceed the design current of the circuit or the rating of its protective device, and the cable can still carry the load. Recalculate the circuit with the new appliance included, because the installed power is what matters, not just the appliance's own rating. Problems usually appear with motor-driven equipment, where the starting current can trip a Type B MCB even when the running current is within the rating. A circuit close to its limit should be reviewed before a heater or another air conditioning unit is added, and a fixed heater is usually better on a circuit of its own.
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By RivoCalc · Reviewed under our editorial methodology · Updated 12 May 2026
Results are estimates for planning and checking. Real loads, cable routing, installation method, ambient temperature and the equipment actually connected all vary, so the figures do not replace a design by a competent person. Electrical work must follow BS 7671 and, in England and Wales, Part P of the Building Regulations.