How much power can be used through a spur


Postby D I M (did it myself) » Sat May 22, 2010 9:14 am

Can any tell me if i can power a PC, Printer, Modem, VDU, Paper shreader any other Office equipment of of a Long extention then a 8 way power trailing socket. All this is coming off from a spur from the mains! Will it over load the spur?
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Postby ericmark » Sat May 22, 2010 9:08 pm

In theory assuming correctly wired a spur can draw a max of 26A (One double socket) but considered as being just 20A as no one expects the max to be drawn from both sockets.

Any single socket will supply 13A after that the fuse should blow so any extension lead will have a max of 13A as supplied from one plug.

Having said this some extension leads are rated below 13A and you would need to check rating.

Because all extension leads have some resistance plugging one lead into another can result in the built in safety devices (Fuse in plug) not working. For a 13A fuse to blow in the required time the resistance needs to be 2.42 ohms. For the main MCB (B32) to open it needs 1.44 ohms so assuming the installation has be built correctly then that leaves just under 1 ohm for your cable. With 1.5mm flex having a resistance of around 0.029 ohms per meter that gives around the 34 meter mark max for an extension lead however since there are also leads on each bit of equipment really 15 meters is about the max. Use thinner cable 1mm and of course the distance is a lot less.

As to overloading it's just a case of maths
13A x 230V = 2990W so the whole needs to add up to less than 2990W.
Most the items you list are around the 25W to 500W mark so even if 6 x 500W items were used the 13A fuse would hold.

So likely there would be no problem. However with questions like you ask it is very easy to give an answer and be unaware of some fact.

So except where the lead length is very short as with many 4,6, 8 way adaptors you should avoid plugging and extension into an extension and rounding up you should not exceed 3KW = 3000W with any 13A socket.
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