Catchup time.

The Hell of the strange shifts has ended. Tomorrow is catch-up-on-sleep time and then I am on a new shift pattern. Short shifts for four days and long ones for two. This suits me since the short ones will leave me with enough energy to write and build models. I’ll only come home tired two days a week.

Those days are Friday and Saturday which would deter most young folk from my job. They are the socialising days for most but at my age, I can do the Smoky-Drinky any time I don’t have to be up early next day. Most of my friends are crippled and/or unemployed (we freaks stick together) so there are no specific days that must be observed.

This week was another covering-holidays thing for the guy who cleans Local Gadget Shop and I am cured of any temptation to buy electrical goods there. All week, the fire alarm has been faulty and sending out a piercing beep every few seconds. After two hours in there it’s like having nails driven into your eyes. I was sober all week (the horror!) so turned up at 8:30 or earlier each day, all bright and almost awake. When I left I felt like I had downed a bottle of whisky the night before.

Nobody in Local Gadget Shop knew how to fix it and nobody seemed bothered about getting it fixed.

They have heaters because they think it’s cold (I worked in a T-shirt in there, it was bloody tropical). Not just heaters, these are the industrial fan heaters that look like jet engines. 2.8 kilowatts each. There were two plugged into a double wall socket. I pulled one out to plug in the vacuum cleaner. Nothing happened so I asked the manager if he had tripped the circuit breaker again. It had popped earlier in the week too.

His response? ‘Oh, I guess that socket can’t take two heaters’, and off he went to reset the breaker.

I was aghast. Calculating in my head, I worked out that he had a current drain of approximately 23.25 amps on a 13 amp outlet, in a shop with a faulty fire alarm. And he is the manager of an electrical goods  shop.

When you have a double socket in the wall, you do not have 13 amps per socket. There are two sockets but only one cable coming out of the back of it and that cable is rated 15 amp. You use and fuse at 13 amp so the cables don’t burn. A twin socket is 13 amps total load.

The equation is P=VI where P is power in Watts (remember a kilowatt is 1000 Watts), V is voltage (for UK assume a non-fluctuating 240 volts, it’s not real but it’ll do) and I is current flow in Amps.

So to work out the current drain, simple algebra turns the equation to I=P/V. Whereby you have those two heaters at 2.8 kW, total 5.6 kW or 5600 Watts as P and 240V as V. The answer you get is approximate because the 240V fluctuates and transmission is not perfect but it’s already way beyond mains rating anyway.

Where did I learn all this complex electrickery? In school. I knew this before I left school. The manager of Local Gadget Shop does not know this. The man in charge of selling and installing electrical equipment and who is qualified to do so, does not know this. The EU does not want me to install my own electrical stuff, they want me to pay an idiot to balls it up.

Scary, isn’t it?


10 thoughts on “Catchup time.

  1. A double socket on a properly-installed ring main can supply more than 13A in total – there’ll be a 30 or 32A MCB at the distribution board. But that’s for the entire ring main; so three big heaters would be just over the maximum nominal current drain: the MCB would hold for a while (Stewie it’s a wile not a while). But your shop sounds seriously sloppy with all its electrics!


    • I sit corrected. I’m only working to A-level physics as far as things electrical go.

      Aside from the two heaters at the front, there was one more at the back and two upstairs. So I suppose they tripped the circuits on the days when they turned all of them on.


  2. These experts obviously stick to the “if it’s not smoking it’s fine” dictum. I’d bet that they’ve got certificates to back it up.


  3. Reminds me of one of my wife’s yoga days where they were warned in advance the central heating would not be available. They ALL took fan heaters and they ALL plugged them in 🙂 Needless to say it was a cold session anyway, while a sparks was found to sort the fuse board.


  4. A ring main has two “15Amp” cables, completing the “ring”, so it can have a fuse/breaker rated at 30Amp. Each plug will only allow 13Amp, as the plug fuse has that as a max. Plug too many loads in any ring, and the breaker trips. No problem. Huge numbers of stacked adaptors are NOT a problem. The breaker or plug fuses will calmly intervene if you overdo the current. There’s a good safety margin.

    Fires aren’t caused by overloads, but by poor connections, and loose plug fuseholders.

    There’s only one practical rule. If the plug gets warm, there IS a poor connection, (Not an overload). It WILL get worse, and ultimately start a fire. Tighten the screws in plug and/or socket, and nip the fuseholder to tighten it. Never use rubber plugs. They appear to survive being trodden on, but the fuseholder gets distorted.

    Sorry to seem pedantic, but this is very useful knowledge. 🙂

    Incidentally, “15Amp” cables won’t melt til they get to about 100Amp. (Compare the thickness to fuse wire, they’re both just copper.) But the insulation could be degraded by the copper getting too warm, and the voltage may also drop unacceptably.


  5. Off the top of my head 2.5mm cable has a rating of about 22amps (depends on whether it’s surface mounted, or buried in the wall/conduit). So (assuming it IS a ring main) even the normal 32 amp trip is not going to overload it. However, that is a continous rating, and it will take more than that to actually trip. A sudden large load should see it “pop” almost instantly, but steady loads like your example might take some time, even at 40 amps. There are different classes of MCB denoted by a letter. “Motor” rated (C) will take a higher surge current than A or B types, to allow for the start up time of things like compressors and pumps.


  6. Oh, and thanks to EU “harmonisation” UK mains supplies are supposed to be at 230 volts now. This depends on whether your local utility has replaced the substation equipment or not. We still have 240 ish, but friends are nearer 230. This will have an effect on the calculations you’ve done – particularly if the heaters were originally designed for 240, and are now running on 230.

    But, as you’ve said, the lack of knowledge out there is still scary….


      • Good point. If you were to use a volt meter, the actual voltage is not exactly whatever is expected. It usually is a bit less than whatever, seldom over the expected. Household juice here is supposed to be 120, but the meter reads anywhere from 114 to 121, and varies as you watch.


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