Re: x10 over three phase

[Nigel Orr <nigel.orr@xxxxxxx>]

At 09:29 05/03/99 -0000, you wrote:
>I've done some homework and my numbers come up thus:
>
>Main fuse (circuit breaker actually):  63A

There's usually a fuse, just a conventional "bit of wire wot gets hot
and
melts" fuse, right beside the meter- probably black, and usually 60 or
100A.  Is the circuit breaker you mention on its own, or built into the
consumer unit?  If the latter, there will usually be another fuse somewhere
else, with an electricity board seal on it, and a rating printed on it.  If
it is separate, and has a 'push to test' switch on it, it's an ELCB or
RCCD, designed to give you some protection against shocks.

>Loads:
>24 sockets (12 doubles) which presumably have to be rated at 13A each.

Nope- sockets are usually on a ring main, rated at 30A total- if you look
in the consumer unit or fusebox, you will see how your house is divided
into 'circuits', and if it's been done well, there will be labels on each
fuse or MCB (miniature circuit breaker), or on the back of the fusebox lid,
telling you what its rating is, and where it supplies power to. Typically
there are a couple of 30A MCB/fuses for ring mains (eg upstairs,
downstairs, kitchen etc), a couple of 5A circuits (lighting), and some 15A
circuits (radial circuit, usually immersion heater or something), and maybe
another 30A or 45A one for a cooker or shower.

>Now, clearly I don't understand how they work out what total load my
>board should take, because 24 sockets at 13A comes to 312 Amps already,
>which is mad.

Diversity is your friend ;-)  Basically, if you don't take into account the
fact that every socket in your house will not be in use at once, the
electrical cables will probably be about the same size as the floor joists
;-)

For example, typical lighting diversity for domestic use is 66%, so your
supply has to be capable of 100W x 66% per light fitting.  Your 9 lights
would need to have 600W of supply available to them (under 3A). Assuming
you have 2 ring final (30 A) circuits, and one radial (45A) circuit, the
diversity is 100% of largest plus 40% of remainder, 45+(60x0.4)=69A.  My
guidance notes on the wiring regulations don't mention showers directly, so
I am assuming you would treat the shower as a radial circuit.  There may be
a more lax alternative available, but I don't know it.

So there's probably a case for you having a single phase 100A supply, and
no reason why you should need 3-phase to your house.

>Oh goody, I'm being ripped off AGAIN!

Maybe, maybe not, but it's worth checking... it sounds like the electrician
either doesn't understand or isn't explaining very well what is needed.

>> I'll be speaking to someone who works on the technical side of
>> the local EB tonight- I'll ask him what he reckons...
>Thanks.  I must say I'm very impressed at all the help I'm getting from

I didn't get the chance to check with him- sorry!  I know some houses have
a 60A main fuse, others have a 100A main fuse, but I'm not sure what the
board are obliged to supply.

Nigel

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