What's the maximum number of computers I can safely power on a 13-amp supply?

Due to a lack of power in a new office building, I am having to daisy chain strip plugs off a single socket.

Roughly speaking, how many amps does a desktop computer with screen consume?

How many might be safe to attach?

Chris, I notice you're in the UK like I am - what I'm going to say isn't of any help to you now but may be in the future - get the building's facilities management people to look at this, don't just guess yourself.

The reason is to do with professional liability under UK H&S law, if you don't show due diligence and there's a fire or similar then you personally can be taken to court and potentially go to prison over this. I know it sounds dramatic but I know someone who did something very similar indeed, another person saw them doing this, when there was an accident all fingers pointed to this mate of mine and he's currently on gardening leave awaiting a legal decision. It's just not worth the effort, just call the building people and ask them, get it in writing and then you're clear.

Hope you understand my intention here :)

Speaking very broadly your average Desktop PC should pull anything up to around an Amp on 230V (normal voltage in the UK).

Very roughly speaking Core2Duo boxes pull around 0.3A to 0.5A and Quad Cores 0.7A to 0.9A.

When they're running during normal use this is called Ambient power draw.

I'd be very careful going past 7A of ambient draw on a single 13A feed though because when computers power up they use more power (how much more depends on numbers of drives installed, fans and the overall spec etc) so you have what's called Rush power draw.

You're likely to have monitors and maybe USB drives adding to that so proceed with caution.

For £10 you can buy a surge protector (which should go straight into the wall socket and then your PCs should feed straight off that without any daisy chaining) and for about £15 you can buy an ammeter. Spend some time adding each PC to your ammeter (measuring the block of plugs directly from the wall socket's supply) and see how much each PC increments the power draw during boot and ambient use to be sure.

If you plan a little in advance then it might not mean replacing four PCs at once rather than one PC which had a faulty PSU in the first place.

In answer to "how many can you put on one 13A plug ?", it's easier to ask how valuable are the PCs and how much time does it take to replace them. With a surge protector and some measurements beforehand I'd be tempted to say four at most (so you don't blow fuses or trip breakers when you boot two up at once) but obviously it's all depending on spec and power draw. Your mileage may vary ! It's not wise to overload any power feeds because of the risk of fire and as a result it's entirely down to your judgement if you don't consult an electrician.

Also remember different countries have different ways of delivering current (AC/DC).

Proceed with caution ! (I think I've added enough disclaimers now :-))