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How Many Electrical Appliances can you plug in a Power Socket?

If you plug too many things into a socket, won't it be overloaded?

Picture the scene: A 13 amp power socket with a 4-way power adaptor plugged into it, and FOUR 4-way power sockets plugged into that, and lots of computers and other appliances plugged into all of the sockets, with a few more adaptors included to get even more things plugged in. Now the question is: Is this overloaded? Odd as it may seem, some people think it is. Even some people who should understand electrical safety seem to think that the "power overload" question is purely a matter of how many things are plugged in. So, if there are what seems visually to be an unreasonable number of plugs connected up, they feel it's UNSAFE and must be overloading the power socket!

Fortunately there is a scientific way of looking at this which helps to dispel any hazy notions of safety or danger based on appearance. By understanding the explanation you can know when a power socket is actually overloaded and when it isn't.

Imagine a bridge across a river, and there's a guard been put in charge of making sure the bridge isn't overloaded, and the guard is given instructions "No more than ten vehicles shall be on this bridge at any one time, to prevent overload". This ruling might initially seem OK, based on the weight of cars, but on the day of the bicycle race the guard might have a problem with the "letter of the law" being entirely against commonsense! Also, two ten-ton trucks, should they be allowed? The fact is that the rule was badly worded, and a better ruling would be to consider the total weight of the vehicles, not simply the number.

A similar thing works with power sockets. A British 13 amp electrical socket is designed to work on loads of up to 13 amps. As the voltage is 240 volts, that works out at 3120 watts. (240v x 13a = 3120w). That's the available capacity, so for most practical purposes you can plug in anything so long as the total power of all the appliances plugged in at once isn't more than 3120 watts.

(more about volts, amps, and watts at the page of electricity explained)

This is easy if you have an idea of how much power each different appliance uses, but not so easy if you're just looking at a bank of power plugs connected up. However, on close inspection, most appliances have a power rating (measured in watts) on a specification label, somewhere on them. After a while you can get an eye for these things, like an instinct for the power rating of different items. Anything that produces heat directly is going to be quite a high power rating for its size. Kettles, heaters, clothes airers, tumble-dryers, etc are all high power appliances. With motors it's a matter of how many horsepower they are. Lights use less power than you might suppose, especially economy bulbs. Even old-style 100 watt bulbs are such that you could plug 31 table lamps into one 13A socket without overloading it (Provided you turn them on one at a time).

If you're looking at your computer system wondering how you're going to find out the power rating of every item, don't worry, because the computer and monitor are probably only a few hundred watts, and all of those things with a low voltage power supply are typically less than 5 watts each. The whole lot probably uses less than an electric iron or two.

The same sort of idea applies to a 1960s style hi-fi system consisting of separates, where there's an amplifier and a deck and a cassette and this and that and the other, and when it's all added up, it's probably less than the vacuum cleaner.

This page is about power, and questions of overload, and it's a different matter from how much each appliance costs to run. Although the costs of running electrical appliances are proportional to their power, they also relate to how long the appliance is in use (see power and energy). If there was such a thing as an itemised electricity bill you'd see that the fridge (100 watts typically) costs more than the kettle (2500-3000 watts), because the kettle is only powered for a few minutes, whereas the fridge is powered 24 hours per day. This is also why it's important to get a power-economical fridge like the ones they go on about at Iceland. For more about saving money on electricity bills, see the econometer and the choice of energy providers

If you had an Econometer, you could measure the power used by each appliance!

Fuses: These are good at avoiding fires and dangerous overloads through short-circuiting. But don't expect them to save you from plugging too many things in. In particular, unfused adaptor units can be used to circumvent the loading limits. The correct way to avoid power overloading is to know what power each item uses and to add them up, so you know that at no point is the system overloaded.

Power, or energy? These things are quite different, scientifically. Power is the amount of energy used per second. It's easy to understand this by a financial allegory. Energy is like cash, money on the table. (eg. How much money have you got?). Power is more like cost per day. (eg. How much money do you get paid per week?). (see power and energy)

Power can be considered to be the flow of energy. Because power is voltage multiplied by current, then if the voltage remains the same, the current is proportional to the power. It's the current that overloads things. That's also why power lines are such a high voltage, to keep the current low, while conveying the same amount of power.

What if you've got 110 volts rather than 240 volts? The same logic applies to the questions of overload, and the devices mentioned use the same amount of power. However, if you want to run something of the same power off 110 volts, it's going to need a higher current. So, it's more critical in terms of adding up the power rating of each appliance.

In countries where the mains voltage is 110 volts rather than 240 volts, it's usual for any high-power appliances such as heaters and air-conditioners to have their own special sockets, which are often 240 volts or thereabouts. This is also why they have unusual sockets, so you can't accidentally plug a 110 volt machine into 240 volts and destroy it.

To sum up the gist of what I'm saying on this page: You can have as many sockets as you like connected up, and it won't be overloaded, unless the total power rating of all of the items is more than the supply is rated at. Getting this right is done by knowledge and good sense.

Other helpful pages: Electricity Explained, How to get the right power supply, economy lightbulbs, and energy providers , the myth of the startup power of fluorescent lamps, what to do in a power cut, and why there is no such thing as Economy7 Gas