How to work out Amps, Volts and Kilowatts for Electrical Projects at Home

Summary: Amps, volts and kilowatt calculations for appliances. How electricity works. Advice on household fuses, lighting circuits, cooker circuits and switches and related subjects.

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How are amps worked out? How do you know which fuse to put in ? What are kilowatts? What part do volts play? Just a few questions we get asked at DIY Doctor many times a week. Most will be answered in a project we have planned called: How does electric work, but for now we will deal simply with amps, kilowatts and voltage. Click on any links to get through to other relevant projects.

If you are embarking on any electrical projects in the home please see our project on electrical safety (mentioned above) and regulations. In particular please study the project on Part P of the building regulations.

Fuses are placed in a circuit to act as a weak point which will melt if things get too hot. This stops fires happening after a cable overheats or an appliance explodes.

If the current flowing through a cable is too great for either the size of the cable, or the appliance it is feeding, something has to give....The idea is the fuse will go first.....Ergo, its important the right fuse is put in!

A normal house hold fuse is rated at 13 amps. An amp, short for ampere is a unit of electrical current.

An electrical appliance draws power from a power source and when power is drawn a current is generated.

How much electricity it needs to work, is measured in watts. It is very important to know how many watts each piece of electrical equipment in your home draws from the power supply because there is only a finite amount of power available before things start to cook!

Choosing the right fuse

Choosing the right fuse

If you know how many watts an appliance needs, you know which part of the system to get the electricity from and which fuses to apply and which size cables and wires to use to stop the fuses or wires overloading or overheating.

To work out how many watts an appliance needs to draw from the supply you need to know the Wattage of the appliance. A watt is the way of measuring the rate at which an appliance uses the power available to it.

Appliances, even light bulbs, are marked with a Wattage. To stop the numbers getting too big, 1000 Watts equals 1 kiloWatt (Watt being the name of the guy who sorted it all out in the first place).

To find out what size fuse to use to protect an appliance you need to know the wattage and the voltage available.

Most homes in the UK are served by a 230 volt supply. The super speedy fast kettle we have in the DIY Doctor office is rated at 2500 - 3000 Watts.(2.5 -3kW) We must take the larger figure for safety. 3kWatts is then divided by the voltage to give us the current rating of the cable and fuse.

3kWatts divided by 230 = 13.04amps.

3000 Watts is the maximum it is safe to put on a 13amp fuse in a plug. The cable size is determined by looking at our other projects.

This is how amps are worked out. If you know how many amps are available, eg you have a fuse, MCB or RCD in your consumer unit but you are unsure of what you can put on this circuit, you can do the calculation the other way round. Multiply the voltage by the amps and this will give you the maximum number of Watts you can place on the circuit.

If your lighting circuit at home is protected by a typical 5 amp fuse you can multiply this by the voltage to get 1150 Watts.

Now you can work out how many bulbs, and of what size, it is safe to have in your fittings. For low voltage lighting see another project.

A cooker circuit is slightly different from this however. Should you be trying to work out the size of cooker you can buy, a cooker, of 12kW or 12,000 Watts, with all rings blazing and the oven and grill firing out juicy steaks can potentially use a whole shed full of electric and should need a 52amp fuse to protect it.

12,000 divided by 230 volts = 52 amps

The chances of every part of the cooker being on at one time are very small, however a principle called The Diversity Principle is applied to the cooker circuit.

For the diversity principle to be understood you must assume that the first 10 amps of current are always needed by the cooker. Probably only 30% of the remaining, available current will be used at any one time however.

The actual demand will then be 10 amps (used all the time) added to 30% of the remaining 42amps (=13amps) making a total of 23 amps.

If the cooker control unit has a 13 amp socket as well as the cooker switch (see our cooker connection project) another 5 amps must be added, making 28 amps. It would then be appropriate to place the cooker on a 30 or 32 amp circuit.

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