Ring or radial?

[Theo]

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.

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:

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.

1. 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. An alternative would be to use a variation of 1) where the ring takes a long loop down to each socket

The natural way to do this would seem to be a ring with spurs, but if spurs aren't recommended then a ring or radial with loops would seem a sensible option. In that case, would it be sensible to have a long run of cable at ceiling level, with a pair of junction boxes for each socket downlead, ie:

-----+ +---- | | | | socket

where + is a junction box?

Also, Part P wise, am I right in thinking that extending a radial is OK but converting a radial to a ring is notifiable?

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)?

Thanks Theo

[John Rumm]

Approx how many sockets in total?

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)

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

Have you considered dado trunking?

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

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.

Tis probably what I would do, or, drop down to the first socket on the wall, then run horizontally between them, and back up to the ceiling at the last one.

No. Adding a whole new circuit would be notifiable (if anyone actually cared!) Extending one is ok, and arguably still ok if the extension takes it back to the CU.

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

[Fredxx]

<snip>

Are you certain, not that I would worry, but isn't any change of (or new) cable route notifiable. If you don't tell anyone ............

Or have the rules changed?

It's more down to what would happen to the cable if the clips melted. Some form of retention over a doorway would be appropriate to stop it sagging and blocking an exit.

[John Rumm]

Yes

If the work were on a circuit in a special location, then changes other that like for like swaps would be notifiable - and hence in that situation an extension to a circuit could be notifiable. However in the general case it has never been so.

(This was one of the original criticisms of the system - that is made extensions to existing circuits easier to do than add new ones, even in cases where a new circuit would be a better technical solution)

There have been various changes to part P over the years (the most recent ones being the most significant by removing many previously notifiable activities from the list, and providing for the first time a route to third party certification). However I can't immediately think of changes on this bit. The current wording in the 2013 version[1] is:

"12.?(6A) A person intending to carry out building work in relation to which Part P of Schedule I imposes a requirement is required to give a building notice or deposit full plans where the work consists of? (a) the installation of a new circuit; (b) the replacement of a consumer unit; or (c) any addition or alteration to existing circuits in a special location."

[1] (and kitchens are no longer special locations)

I think we are talking at crossed purposes.

Adequate cable support is required in all cases. However enclosing it in conduit for example is not necessarily required (and if the conduit is plastic, would not alone meet the requirements for retaining cables in the event of a fire).

So for example - surface run cables in a garage where all that is needed is somewhere to plug in an inspection lamp or battery charger, are not going to require additional mechanical protection. However in a garage used as a workshop where materials are handled and moved about, or say welding is done, then some mechanical protection would be advised to protect the cables from impact damage or contact with hot sparks / metal splatter.

[tabbypurr]

Any of those options is compliant. Rings have a slight advantage in safety & reliability over radials, and of course use thinner wire for a given rating, though more of it.

Metal cable clips: Just enough are required to stop the wiring collapsing onto someone in a fire, most clips can be plastic.

NT

[Brian Gaff]

Also, brick built but is it detached from the house or part of it? Brian

[Theo]

'Lots'

Possibly on surface power/data trunking in one corner.

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

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.

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

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

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)

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...

Thanks for your help (and everyone else too) Theo

[John Rumm]

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.

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

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.

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.

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

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).

[Graham Nye]

What's the feed to the CU? Is this a stand-alone installation with its own supply company feed and meter? There would be no point me wondering about a 32 A ring in my garage as it only has a 16 A feed from the house.

[ARW]

Unless there is some existing certification to say that it was an exiting radial who would know...

Or care as long as the new work is done properly?

[ARW]

Such a simple answer.

Well done sir.

[Theo]

It's an 80A MCB on the CU in the garage - I'm not sure how it's hooked into the main CU, but it isn't a separate feed. I'll see if I can find out what the MCB on the other end is.

Theo

[Robin]

An 80A MCB? I may well be missing something but I'm left wondering:

a. are the sockets in the garage connected direct to that MCB or to another - lower rated! - one?

b. does the supply definitely come from the house into the garage or does it possibly go /from/ the garage /to/ the house?

[Graham Nye]

That's kind of you. I was vaguely wondering if this is a garage next to (or part of) a house or a separated garage with its own supply, but that's too wide ranging a question. All that's necessary is to establish that the feed to the CU can support the rating of the desired new circuit.

[Graham Nye]

80 A is an unusually high rating for a domestic MCB. Are you sure it is a MCB? Could it be either a main switch or a RCB with an 80 A rating?

If you're not sure, take a clear picture of it, share it somewhere (e.g. imgur.com) and post a link here.

Checking the source end of the feed would definitely be useful.

What protective device(s), if any, are on the radial circuit feeding your existing pair of double sockets?

[ARW]

80A RCD not MCB at a guess.

[Robin]

Ah, yes; it's so obvious when it's someone who knows what they're talking about :(