Advice of maximum 13amp loads on power sockets pls

  joelmb 11:18 18 Mar 2005

Hi. Can anyone tell me what the maximum load / electric devices I can put on one 13 amp double power point? I currently have 1 13 amp power plank with 4 sockets on it, and I'm using two adaptors on this plank, totalling 7 computer devices (pc, tft's, printer etc) which I fear may be too much? So was thinking maybe two planks runnig from the mains socket might be safer but this is why I'd like to know what the maximum on the socket itself is and if this would be ok? i know this is slighlty off topic but nonetheless important safety wise!

  €dstowe 11:21 18 Mar 2005

Remember that one 13A socket can power your electric kettle or heat the large amount of water in a washing machine.

A (modern) computer setup has a long way to go before it reaches anything like that.

  John B 11:26 18 Mar 2005

Add together the total ratings (in Watts) of all the appliances and divide by the voltage to get total current in amps.

  joelmb 11:49 18 Mar 2005

thanks, but i hated p=va calculations in physics al those years ago ;)

i was kind of just hoping for advice on whats safe i.e will it be ok to run two separate 4 point power planks from the mains power socket, running a total of 4 or 5 pc appliances?

  Salinger 11:54 18 Mar 2005

With a 220v supply you should be able to run appliances totalling about 2,800 watts.

  SEASHANTY 14:59 18 Mar 2005

You are nowhere near the limits. Say that a PC base unit uses less than 600 watts. A CRT monitor around 75 watts. A printer perhaps 80 watts. You could likely run two separate systems from one 13A supply socket with various adapters. This website indicates the individual PC components approx wattage click here
Remember also that you have a PSU unit on your PC
which will state its max output. My guess is that yours is less than 550 watts (and this doesn't mean to say that the PC is using this amount of power - most PSU's regulate their output to the load encountered.

  skeletal 19:01 18 Mar 2005

As another guide I am running extension leads off extension leads; in total I have the following sometimes all running together:





Cable modem


External hard drive

PDA cradle


Force feedback steering wheel

And I don’t even think about it. BUT you ask, why do fires start when people have too many things on an extension lead? Well, as has been previously mentioned, it’s the total amount of power being consumed, not the number of devices, so two electric fires is bad, dozens of small computer things is OK.

A couple of interesting facts: 13A plugs are not really designed to carry 13A, and a 13A fuse won’t normally blow until about twice that current passes, i.e. 26A. Now, let’s consider a 13A plug to socket resistance of 0.1ohm (a low value); connect two electric fires to this plug via a multi-way socket. Say this gives a 25A load, at 0.1ohm will give a power dissipation IN THE PLUG of 25 squared times 0.1 (to use the physics forgotten all those years ago!). This gives 62.5 watts in the plug. My 15W soldering iron rises to a temperature of about 300 deg C. Thus, this situation could lead to a fire.

Even a single electric fire could give about 17W in the plug; and this is why I clean a plug’s pins with emery paper if it is going to be used with a big load. If you don’t believe me, feel how hot an old plug gets if it is connected to a big load (fires, tumble driers etc). I once did this and actually got burnt!

And one last point, you can have a large (ish) power dissipation in things and still be OK if there is enough ventilation to take the heat away (hence fans on processors), so all your loads on the extension lead could start to become dodgy if everything is piled together under loads of old cloths for example (my leads are all open to moderate ventilation). This is why you are always told to unreel long extension leads; these may have a total lead resistance of an ohm (ish), run a moderate load of 5A (squared to give 25, times 1 this time) gives about 25W in the whole lead. Spread out this is not problem at all, wound up and all that heat can’t get away; result one melted cable reel (I have seen such a melted reel, all the cable was welded together!).

A bit off topic, but hey, I may have prevented a fire!!


  SEASHANTY 23:20 18 Mar 2005

I think the main point in unwinding leads off the reel on high current use appliances (such as power dills) is the fact that the AC on looped wiring gives an enhanced transformer effect which could cause a high current buildup.

  Dirty Dick 23:30 18 Mar 2005

Don't underestimate the fire risk from electricity. Just because there is no flame doesn't mean there's no risk. Electric wires don't even need to touch anything for a spark to jump and a fire start.

Look out for the following signs:

Hot plugs and sockets, fuses that blow for no reason, flickering lights and scorch marks on sockets or plugs
Check electrical leads and plugs for wear and tear and faulty wiring. Frayed leads or exposed internal wires are a fire risk
Don't overload sockets - use one plug in each socket
Throw away and replace damaged cables. Never run cables under mats or carpets where you can't see wear and tear
Use a "bar-type" fused adaptor on a lead rather than a "block-type" adaptor
Don't allow the total amps of all plugs in the adaptor to add up to more than 13 amps (or 3000 watts of power)
Don't plug adaptors into adaptors - use one adaptor in each socket
Wire plugs carefully. Keep the outer covering of the power lead securely inside the plug. Make sure the internal wires are firmly in place and that the right colour wire is in the right place

Damaged or ill-used electrical wiring can cause the following problems:
•Overload. This is caused by misuse of a circuit.If appliances are placed on a circuit which drawmore current than the circuit is designed for, the I2 dependence of power dissipation results in arapid heat build-up in the wiring. If a fuse or circuit breaker does not blow, a fire will result. Inl ess severe overloads, the condition may persist and break down the wiring insulation

  skeletal 17:13 19 Mar 2005

SEASHANTY I have heard that theory before, and although there will be a tiny transformer effect, it will be tiny. The reason is that a typical extension reel has both the positive and negative conductors next to each other (in a typical cable)so the EM field from the "inward" current will cancel the EM field from the "outward" current; there is very little left over for the transformer effect (which has current carriers all in the same "direction" next to one another).

Virtually all problems are due to some resistance consuming power (dirty plugs, cable not being rated highly enough etc) and there not being enough ventilation to remove the heat.

Another example: immersion heaters and power showers are often fed from cable in the loft; these must not be covered in loft insulation.


  bremner 17:26 19 Mar 2005

The reason for unwinding extension leads is one of heat generation not high current build up.

Think of a light filament or an electric fire. In both the wire is coiled this produces heat.

If you leave your extension lead coiled the effect is the same.

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