The best way for a bit of electircal work?

[GMM]

I have a room which was a bathroom, which I shall be re-purposing to storage / freezer etc. It may become a laundry room depending on what else happens. It shares a wall with the garage, which has no power at present. Although a bathroom, it has been fitted with an electric shower, which has a 32A supply on its own mcb. I haven't looked but assume this is fed by 6mm T+E. The house was re-wired in the relatively recent past and the CU has RCD protection on all the power circuits but not the lights

It would make sense to use this shower supply to base the power circuit for the room on and, opportunistically, get some mains into the garage. It seems to me there are a couple of options for this: I could make it a simple radial in 6mm T+E or I could generate a ring in 2.5mm T+E, either of which would allow the full supply of 32A. (4mm would be a bit marginal and should have a smaller mcb)

The radial would be a simpler design but 6mm is a bit of a pain to work with, in terms of wiring sockets, so the ring sounds like an easier job. Someone might differ though, and tell me that 6mm isn't so bad to work with.

If I make it a ring, should I put a small CU at the start or is it acceptable practise to simply join both ends of the ring to the 6mm feed in a JB or similar?

Within this (not that it's ever been a concern to me before) comes the question of where a final circuit starts for Part P purposes. Since it's not a special location (which seems to be about to change anyway to just bathrooms), then it shouldn't be a Part P issue as (from my point of view) it's simply an addition to an existing circuit. On the other hand, using a 'garage' CU as the start of the ring could well define it as a new final circuit. Does anyone have any wisdom on this?

[ARW]

Why do you think the 4mm would be marginal? 32A 4mm radials are allowed.

[GMM]

My understanding was that 4mm was on the edge for 32A so should be de-rated to 20A. You, however, are the local expert, so who am I to quibble? Running it as a radial in 4mm would certainly seem the best bet to me, if OK.

[ARW]

It is on the edge depending how it is installed.

keep it away from insulation:-)

[GMM]

Just plastered in should be fine then?

I somehow doubt it will ever draw anything like 32A anyway and certainly not for any significant period but you never know what might happen down the line and I wouldn't want to install anything unsafe, regardless of Part P and all that pony.

Not because I'm so worried about burning the house down but because I might have to go back and fix it, and nobody wants to have to do a job twice....

[John Rumm]

Quite a dinky shower then.... (7kW ish?)

You could - rather like having a submain feeding a ring. Its slightly unusual, but allowable.

Depends on the installation method.

(you can also make 4mm into a ring if require0

6mm in normal accessories would be a pain. You can take spurs from the radial in the same way you would on a ring. The same limitations would also apply with respect to unfused spurs (i.e. only one double or single socket per spur)

A JB would be acceptable since you are not changing overcurrent protection, and the fault current protection at the origin of the circuit would protect all the cables.

I would argue its an extension (and re-purposing) of an existing circuit.

[GMM]

To be honest, I haven't even looked at the shower, except tp decide it's going. It doesn't look like anything you would want to stand under! The old boy who had the house before was a bit disabled, which was the reason for a ground floor bathroom.

I must have misunderstood the situation with spurs, or branches, on a radial but had assumed the reason for limiting unfused spurs on ring circuits was because they would be made in 2.5mm, so needed fault current protection to stop them drawing too much current, while radials could branch multiple times. Not that it makes any difference to my design, which will be essentially a linear series of sockets (hence why I started thinking radial in the first place).

I'm glad your interpretation of the Part P situation agrees with mine. Of course, that doesn't mean we're right, but who will ever know anyway? Well, unless I were to make a c*ck-up of the whole installation and burn the house down.

That said, I have been around the house already tightening screws in all the sockets. Despite being professionally installed etc, I have found about half to be loose by a turn or so, some to the point where one or more connection falls out when the socket is removed from the box. I get the impression that the copper compresses over time (and I always re-visit accessories after a little while to allow them to settle) but there must be millions of loose (potentially high-resistance) connections around the country.

[A.Lee]

All electrical work in a domestic setting has to comply with Part P of the Building Regs. Some is notifiable, most not. Yours doesnt seem to be a special location, you are not changing the characteristics of the circuit (if you comply to BS7671 with the additions), so you shouldnt need to have to notify it.

[ARW]

Unfused spur.

When using a radial circuit with 4mm cable and a 32A MCB an unfused spur is is just the same thing as an unfused spur on a ring. It's just a 2.5mm T&E unfused spur to a single or double socket from the radial/ring ciruit.

It allows you to use a cable that has a lower CCC (current carrying capacity) than the MCB from the radial circuit to power one single or one double socket.

You will still need fault current protection. The fact that you are only supplying one single or one double socket from the 2.5T&E provides the overload protection

dennise will now have a fit.

[GMM]

I'm amazed to get this far without someone having a fit (!)

I appreciate your point entirely, if the spur is in 2.5 but I had made the assumption that a spur or branch in 4mm would be different as it has the same CCC as the radial it spurs from. Academic point in this instance as there will be no need to spur anyway but it's good to know what's right (and/or good practise) for future reference.

Cheers

[John Rumm]

Spurs and branches are different. Typically a radial will branch - i.e. split or daisy chain at any point it likes, and continue in the same cable. However you can also take a spur in a reduced size of cable. Then it works the same as the situation in a ring. The protective device at the head end of the circuit will provide adequate fault protection for a single length of 2.5mm^2 T&E, but not overload protection. The limitation of a one socket however provides the overload protection.

Even then its doubtful!

Yup - often not made off well enough in the first place, and then they relax a little over time.

[John Rumm]

Hence why the need to be careful with the language. A 4mm^2 circuit can branch anywhere in 4mm^2, and its just part of the circuit - not a spur. A spur (unfused) is a specific case where you use a reduced cable size to feed a single outlet.

Its not often needed - but sometimes you see systems with a larger sized cable used as a backbone, then all the sockets fed from smaller spurs from it.

[Mike Tomlinson]

En el artículo , GMM escribió:

I'd be tempted to do this, put in a small sub-consumer unit at the end of the 6mm and have two 20A radials in 2.5mm, one for the utility room and another for the garage. Perhaps a 4-way one (RCD and two 20A breakers).

No idea if this is compliant, someone will be along soon to give chapter and verse. The idea of the two 20A radials is to provide discrimination from the 32A breaker in the main consumer unit.

[dennis]

don't forget the lights.

[ARW]

Which would add how much to the diversity calculations?

[ARW]

Perfectly allowable and would work with the diversity calculations. And there is still the option that the 32A MCB at the house end could be changed for a 40A MCB.

Two RCDs would not be a good idea (the feed is already RCD protected)

[dennis]

Unless you can find a one position RCD as you will want a breaker or two for the lights.

[ARW]

For once I will be polite to you Den.

What is your definition of a 4 way CU?

[Bob Eager]

I am not sure he knows.

[GMM]

(Pssst....the lights are already there on a lighting circuit ;-)...might change the fitting though )