"Stefek Zaba" wrote in message
.. .
snip
He had some 32A 'commando' style sockets fitted in at least a couple
of
the colleges (Goodricke and Vanbrugh, IIRC) shortly after spreading
the
load across a couple of sockets in separate rooms.

Which were on different phases...


Ouch, Oops !... :~(

Owain wrote:

So although, as Andy Wade showed, the "diversity" of a 5kW hob may only
be 13.6A, if the cable is protected by a 16A or 20A MCB, the design
current of the circuit then becomes the rating of the protective device.

Only if the designer so decides. The definition of design current (Ib)
is the "current intended to be carried by the circuit in normal service."

I can think of two cases where you might be able to use a smaller cable
size by setting Ib In (where In is the device rating). The first is
where the circuit is voltage-drop limited, so a lower value of Ib allows
a greater circuit length for a given cable size (Zs permitting).

The second case is where a grouping factor (Cg) applies and you've
elected to take advantage of the "not liable to simultaneous overload"
relaxation. In this case the required cable rating (before application
of any Ca & Ci factors) is given by the greater of the two expressions

It = Ib/Cg, and

It = sqrt(In^2 + 0.48*Ib^2*(i - Cg^2)/Cg^2).

This can give an It value of less than In/Cg so might just save a step
on cable sizing. [See Section 6 of Appendix 4 to BS 7671 for full details.]

--
Andy

In message ,
"dennis@home" wrote:

"Martin Angove" wrote in message
...
In message ,
"dennis@home" wrote:
snipped
Hmmm!
Two 13A plugs and a relay or two and it wouldn't have been a bodge.


Two 13A plugs, and a whole cabinet full of DIN-rail mounted connectors,
contactors and a transformer. Transformer was for LV remote switching of
the contactors, one contactor per hot air gun, four installed, but
contactors for two more. Even I was impressed.

So why weren't the guns split across two mains feeds then?
Then it would run without overloading and wouldn't need any special work to
fit it.

Just make sure you use a couple of connectors that have to be removed to get
in so no idiot leaves one plugged in while working on it.


Several reasons:

Firstly, your last point, which *is* a bodge when there is a better way.

Secondly the fact that there were no socket outlets near the intended
point of use, so we were going to have to wire something up, also this
would be a large point load on any existing sockets circuit and so would
probably be treated to its own circuit anyway.

Thirdly there was a not-full-utilised distribution board a mere couple
of yards away, with a nice knockout on the side perfectly sized for a
32A connector.

Fourthly, if it had been installed somewhere else I didn't want someone
plugging it into two phases.

Fifthly the guns were 2kW each and intended to be used at full power, so
it wouldn't have been a couple of sockets, it'd have been four or six.

Shame it seems that it never worked once they built a proper (rip-stop)
balloon for it.

Erm... come to think of it (look, this was two and a half years ago and
my memory's dodgy at the best of times) I don't think it *was* wired for
32A single phase. I think I wired it for 20A three phase. That would
have made more sense (two guns per phase) but without going digging into
my (unsorted) photo collection (I did take one of the innards) I can't
be sure.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... I am Popeye of Borg. Prepare to be askimilgrated.