Amperage Calculator UK
Estimate electrical current in amps from watts, voltage, phase type, power factor, and efficiency. This calculator is designed for common UK assumptions such as 230V single phase and 400V three phase systems.
Calculate current draw
Enter the equipment load before conversion to amps.
Typical UK domestic supply is 230V at 50Hz.
Use 1.00 for resistive loads and a lower value for motors or reactive loads.
If input power is electrical already, leave at 100%.
Results
0.00 A
Enter your load details and click calculate to estimate current draw for UK electrical systems.
- Single phase formula Current = Watts / (Voltage × Power Factor × Efficiency)
- Three phase formula Current = Watts / (1.732 × Voltage × Power Factor × Efficiency)
- Important Final circuit design, cable sizing, and protective device selection must follow BS 7671 and competent electrical advice.
Expert guide to using an amperage calculator in the UK
An amperage calculator helps you convert electrical load into current, usually measured in amps or amperes. In practical terms, this tells you how much current a circuit, appliance, or item of equipment will draw from the electrical supply. In the UK, this matters because current determines whether a load is suitable for a standard 13A plug, whether a dedicated circuit may be needed, and whether the cable size and protective device rating are likely to be appropriate.
For many people, watts are more familiar than amps. Kettles, showers, heaters, EV chargers, compressors, and machinery are often sold with a power rating in watts or kilowatts. Electricians and designers, however, need current because current directly affects heat in conductors, voltage drop, breaker selection, fuse ratings, and the safe loading of final circuits. That is why an amperage calculator is such a useful bridge between product labels and real world installation decisions.
In the UK, the nominal low voltage public supply for single phase installations is generally 230V at 50Hz. Three phase systems are commonly 400V between phases. Those figures are central to current calculations. A 3kW appliance at 230V single phase draws far more current than the same power distributed on a 400V three phase system. The purpose of this page is to make that relationship easy to understand, while also explaining the technical details that matter in British electrical practice.
Why amperage matters in British installations
Current draw is not just a theoretical number. It affects day to day safety and compliance. If the current is too high for the plug top fuse, flexible cord, socket outlet, MCB, RCBO, or fixed wiring, overheating and nuisance tripping can follow. High currents can also increase voltage drop, especially on long runs. This is why electricians estimate or calculate amps early in the design process, then confirm that cable capacity and protective measures are suitable.
- Domestic users often need amp estimates when checking whether an appliance can run from a standard 13A socket.
- Landlords and property managers use current calculations when reviewing fixed appliances, electric showers, ovens, and heating systems.
- Commercial users need them for motors, HVAC plant, catering equipment, and workshop machinery.
- Renewables and transport sectors use amp calculations for battery systems, inverters, and EV charging equipment.
The basic formulas explained
The simplest electrical relationship is power divided by voltage. For direct current and purely resistive loads, amps equal watts divided by volts. In AC systems, especially where motors or electronic equipment are involved, power factor becomes important. Power factor reflects how effectively apparent power is converted into real power. If the power factor is below 1, the current needed to deliver a given amount of real power rises.
Key formulas:
Single phase AC: I = P / (V × PF × Efficiency)
Three phase AC: I = P / (1.732 × V × PF × Efficiency)
DC: I = P / V
Efficiency matters when the power figure you have is output power rather than input power. For example, if a motor delivers 2.2kW mechanically but runs at 90% efficiency, it will draw more than 2.2kW electrically. That extra input must be considered when estimating current.
Typical UK values and what they mean
Most homes in the UK operate on a single phase 230V supply. Many common appliances such as kettles, toasters, irons, fan heaters, and washing machines are therefore assessed against a 230V reference. Large commercial premises and many industrial sites may have access to 400V three phase. A three phase supply allows a given power to be delivered at lower current per line conductor, which is one reason it is preferred for larger motors and heavier loads.
| UK electrical reference data | Typical value | Why it matters |
|---|---|---|
| Nominal single phase supply voltage | 230V | Used for most domestic current calculations |
| Nominal frequency | 50Hz | Relevant for AC equipment ratings and motor performance |
| Nominal three phase voltage | 400V line to line | Used for commercial and industrial load calculations |
| Standard BS 1363 plug fuse rating | 13A maximum | Important benchmark for portable domestic appliances |
| Common ring final protective device | 32A | Often used for general socket circuits in homes |
These are not design values for every situation, but they are real and widely used reference points. For example, a 3kW resistive load on 230V single phase draws about 13.04A, which is right on the edge of what a standard 13A fused plug arrangement is associated with. That is why many high power fixed appliances are hard wired rather than plugged into ordinary sockets.
Examples of appliance currents in the UK
One of the best ways to understand amperage is to compare everyday loads. The table below uses the simple single phase formula at 230V and assumes a power factor of 1 for mainly resistive appliances. Actual measured current can differ slightly, but these figures are a strong planning guide.
| Appliance or load | Typical power | Approximate current at 230V |
|---|---|---|
| LED lighting circuit load | 100W | 0.43A |
| Desktop computer and monitor | 250W | 1.09A |
| Microwave oven | 1200W | 5.22A |
| Kettle | 3000W | 13.04A |
| Portable fan heater | 2000W | 8.70A |
| Electric shower | 8500W | 36.96A |
| EV charger single phase | 7400W | 32.17A |
These figures show why current is such a practical metric. A kettle and an EV charger are both just electrical loads, but one is on the edge of the 13A plug standard while the other clearly requires a dedicated higher current circuit. The amperage calculator helps you spot these differences immediately.
How to use this calculator properly
- Enter the equipment power rating in watts or kilowatts.
- Select whether the supply is single phase AC, three phase AC, or DC.
- Enter the voltage. For most UK domestic applications, 230V is the usual starting point. For many three phase systems, use 400V line to line unless the equipment nameplate states otherwise.
- Set the power factor. Resistive heaters are often close to 1. Motors and some electronic loads may be lower.
- Set efficiency if the power you entered is output power rather than electrical input power.
- Click calculate to get the estimated current, plus comparison data in the chart.
Always check the equipment nameplate where possible. Manufacturer data overrides generic assumptions. If a motor states a full load current, that value is normally more reliable than a broad estimate based on assumed power factor and efficiency.
Single phase versus three phase in UK practice
Single phase is the normal arrangement in homes and small premises. It is adequate for lighting, sockets, cooking appliances, water heating, and many smaller fixed loads. Three phase is common in industrial units, workshops, larger offices, agricultural settings, and some blocks with heavier shared services. The core advantage of three phase is lower current per conductor for the same power transfer, which supports more efficient distribution of larger loads.
For instance, a 9kW load on single phase at 230V and power factor 1 draws roughly 39.13A. The same 9kW on three phase at 400V draws only about 12.99A per line. That difference has major implications for cable size, switchgear, and voltage drop.
Common mistakes people make
- Using 240V or 250V assumptions without checking the current UK nominal value and actual equipment rating context.
- Ignoring power factor on inductive or motor loads, which leads to underestimating current.
- Assuming a 13A plug is suitable simply because the appliance power sounds modest.
- Confusing input power with output power, especially for motors, pumps, and compressors.
- Forgetting startup current. Some motors and compressors draw a much higher inrush current than their steady running current.
How amperage links to cable sizing and protective devices
Current calculation is the start of the process, not the end. Once current is known, the next questions are whether the cable can carry it safely, whether the protective device is correctly rated, whether voltage drop is acceptable, and whether installation factors such as grouping, insulation, ambient temperature, and installation method change the cable capacity. In UK work, those checks are typically carried out in line with BS 7671 requirements.
A cable that is perfectly adequate at 10A may be unsuitable at 32A. A breaker that protects one circuit arrangement might cause nuisance tripping on another. Long cable runs may experience notable voltage drop even when the base current appears ordinary. The amperage calculator therefore helps identify the likely load category quickly, but formal design still requires a broader assessment.
Power factor and efficiency in more depth
Resistive loads such as immersion heaters and many kettles are straightforward because power factor is near unity and almost all supplied electrical energy becomes heat. Motors are different. They use magnetic fields, so the relationship between voltage and current includes a reactive element. A lower power factor means more current flows for the same useful power. Efficiency also matters because some of the electrical input is lost as heat and mechanical losses.
Take a 2.2kW motor on 230V single phase with power factor 0.85 and efficiency 90%. The electrical input power required is higher than 2.2kW output. As a result, the running current may be materially above what a simple watts divided by volts estimate suggests. In real design, this difference can influence cable choice, breaker curves, and startup performance.
Trusted references and authoritative reading
For broader electrical safety and UK context, review these authoritative sources:
- UK Health and Safety Executive: Electricity at work guidance
- UK Government: Electrical safety guidance for landlords
- Penn State Extension: Electricity basics
When to seek a qualified electrician
You should seek competent advice when the load is fixed, high power, outdoors, in a bathroom, associated with EV charging, connected to outbuildings, or part of a commercial or industrial installation. The same applies where circuits are modified, consumer units are altered, or protective devices and cable sizes are being selected. A current estimate is useful, but it does not replace inspection, testing, certification, and proper design.
Final takeaway
An amperage calculator for the UK helps translate appliance power into practical current values that matter for sockets, fuses, breakers, cables, and installation planning. Start with the correct supply type, use the right voltage, and include power factor and efficiency where relevant. For resistive domestic loads the calculation is often simple, but for motors, HVAC equipment, and three phase systems, accurate assumptions make a significant difference. Use this tool to get a fast estimate, then confirm details against equipment data and applicable electrical standards before installation work proceeds.