On 28/08/2019 10:09, Theo wrote:

John Rumm wrote:
On 27/08/2019 22:53, Theo wrote:

I'm speccing up installing sockets in a brick garage. It currently has a CU
with an adjacent pair of double sockets on a short radial. I want to put some
sockets at various points around the far sides.

Approx how many sockets in total?

'Lots'

Possibly on surface power/data trunking in one corner.

What do you anticipate running from them?

(i.e. do you need the extra power available from a 32A ring rather than
a 16 or 20A radial)

Nothing particularly power hungry - soldering irons, wall warts, AA battery
chargers kind of thing - lots of sockets but only taking a few watts each.
A bit of future proofing might be useful in case someone (possibly
me) wants to use more power tools in there. I suppose one way to do that
would be to spec a 16A radial and then leave the option of closing the far
end to make a 32A ring.

By the sounds of it, a fairly small load overall.

If running radials for general purpose socket circuits, there is little
point in speccing 16A since the cables you would typically install will
be fine with 20A as well in most cases, and that gives you a bit more
wiggle room in case you do need to run a 3kW kettle or a fan heater plus
other stuff.

(I'll have to check the CU - any Part P implications if changing the MCB for
a different current?)

There are no implications that I can think of...

(and for the pedants, yes you could run a 32A radial in 4mm^2, and spur
each socket in 2.5mm^2(

Because of the construction of the garage, it would be easiest to run the
wiring at head height, and run down trunking to the places where sockets are
wanted. There are roughly two options:

Have you considered dado trunking?

For above work surfaces, yes. But I don't want to disrupt other uses of the
walls (eg shelving) by running wiring at waist level.

1. A radial around three sides, with the sockets on essentially long loops
down from the ceiling. This is just an extension of the existing radial.

You can do that - or the backbone and branch style with spurs from the
radial.

How should I calculate the wire sizes on that setup? Assume all the current
is taken from the branch furthest away?

The cable will need to be able to take the full nominal circuit design
load "as installed" (i.e. including any de-rating factors). So for a 20A
radial, the normal choice would be 2.5mm^2 T&E unless there is some
reason (e.g. high ambient temp, or running through insulation etc) to
de-rate it below 20A.

2. A ring main around at ceiling level. The simplest way would be for each
socket to be spurred off, but I understand spurs aren't well received.

In a conventionally wired ring its usually better to start with few
spurs, however its equally valid to take a design decision to use spurs
exclusively if it makes more sense.

Why are spurs generally disliked? Because the spur itself only has one
route for the current?

Partly that, also on a ring circuit the B32 MCB normally provides both
overload and fault protection. In the case of a spur, it can still
provide fault protection, but can no longer do overload protection
(since the single length of 2.5mm^2 cable is good for 27A at best - less
than the MCB rating), so you have to rely on the restriction of one
single or double socket per unfused spur.

Having lots of spurs then makes it difficult to further extend the
circuit, since what may be a natural place to make a connection turns
out to already be a spur, and hence not available unless you also
convert it to a fuse spur. That also has disadvantages. Also more spurs
means more connection points with three or more sets of cable connected
to one point, which is more difficult to wire, and more complex to test
and maintain.

So for new ring circuits is preferable to start with few or no spurs,
and that gives the best scope to allow it to grow and adapt according to
need.

Finally, is it acceptable to cable-clip T&E cables (bearing in mind the new
rule to use metal clips occasionally) in an outbuilding, or is additional
protection (trunking) needed (even at ceiling level)?

Depends on what you are doing in there, and what the likely risk of
mechanical damage is.

No heavy industry. I assume placement matters too (ie protection from
solder splashes at bench level might be useful, but not much point at
ceiling level)

Manual soldering etc is unlikely to pose a problem - but then again that
is the place dado trunking might be good anyway.

I'm sort of leaning towards on running a 16/20A radial around the ceiling,
with spurred pendants in trunking. Anyone who wants to bump up the current
capacity can convert it to a ring and double up the pendants (easy given
it's all surface wiring), but on the KISS principle that will do for the
time being...

If going that route, then a lap of 4mm^2 at ceiling level, looping
through 30A JBs at each socket location would be easiest. Then install
2.5mm^2 drops to each socket. You are then free to protect it at
whatever current limit you want (up to 32A), and future additions would
be easy - add new JB and drop wire.

(you could also in theory add a "sub" ring to the backbone - say drop
down to some dado trunking, run through half a dozen sockets, and then
back up at the other end).


--
Cheers,

John.

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