Garage power supply - Recap [Longish]

[Roger Mills]

I participated in (but didn't initiate) the thread back in June - from which I learned quite a lot - but have a few more questions.

My detached double garage is currently being built, and is about 1.1 metres from the house. I intend to use a garage consumer unit - probably

which I intend to connect to a spare way in my domestic CU. This currently has a 6mm^2 T&E cable connected to it, which was originally intended for an electric shower - but we used stored water and a shower pump instead - so this way and cable (with a suitable MCB) can be re-deployed.

The 6mm^2 cable ends (with a couple of feet spare) just above the ceiling of an en-suite bathroom - more or less vertically above the point where the house end of the garage connection needs to be. I intend to join some 6mm^2 SWA cable to this cable, and take it through the wall and down the outside of the house, and across a passage door frame into the garage.

Although the garage consumer unit can handle 40 amps, this is more than I need (and the sum of the individual MCBs is a lot less than this anyway) so I intend to use a 32 amp Type-B MCB at the house end of the connection - which seems more appropriate than a higher value if relying on the 2.5mm^2 earth within the existing T&E cable.

My questions concern earthing arrangements, and physically joining the cables together.

From my description of my incoming mains back in June, Andy Wade thought that I have a TN-S system, and David Hansen thought it was more likely TN-C-S. In case it helps, I've posted a photo of the main fuse and meter connection at

Just to remind you, I estimate that I have about 10 metres of T&E, and will need about 6 metres of SWA.

The options for earthing appear to be as follows:

• Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join it onto the end of the T&E's 2.5mm^2 protective earth conductor - thus exporting the earth from the domestic CU

• Use 2-core SWA, and use the steel armour as an earth, again joined to the T&E's earth conductor - again exporting the earth. [There will be no external connections to be subjected to 'weather', so corrosion shouldn't be a problem]

• Use 2-core SWA and an earth spike at the garage end - thus providing the garage with its own earth - independent of the house supply

• Some combination of the above. [Is there a down side to exporting the earth *and* having an earth spike?]

I would welcome constructive comments on the relative merits of each of the above options - plus identification of any viable options I may have missed.

Now to joining the cables . .

The most convenient solution would be to have the joint inside a deep (metal) socket box high on the wall of the en-suite bathroom - sunk into the blockwork, and covered by a flush blanking plate. The T&E would come down from above, and be chased into the plaster and the SWA would go straight out through the wall from the back of the box - with its gland fitted to a knock-out in the box. Does anyone see any problems with this? How should I physically join the cables? Is a large chocolate block connector (inside the box) ok - or is there anything purpose-made which would be better? Do I need to earth the SWA's armour even if it isn't being used as the protective earth?

Your expert advice will be much appreciated.

Oh, and in case anyone mentions Part P - I think that what I am doing is probably covered by Part P. AIUI, extending an existing radial circuit isn't, but when you start taking it outside, it is. The building itself is exempt from building regs, being a detached garage of less than 30 M^2 floor area, and meeting the other exemption conditions. My inclination therefore is to forget about Part P and hope that it gets lost in the noise when the property is eventually sold (hopefully far into the future). Nevertheless, I want to ensure that I end up with a 'safe' electrical installation - hence the questions.

[John Rumm]

It looks rather like the RCD covers both ways on that. Depending on how you plan to use the garage you may wish to opt for some arrangement that will not lose the lights at the same time you lop through a power cord with a still spinning circular saw! (i.e. switched two way CU with a RCBO for the sockets, or perhaps the one pictured with a non maintained emergency light)

On the same theme as above you may need to bear in mind is descrimination of the protective devices. You want to try and ensure that a trip on the power circuit in the garrage only takes out the local MCB and not the upstream one in the main CU as well hence losing your lights.

That is a different cable head end from any I have seen in use in this area, so can't give you a definitive answer. However the connection of the (undersized!) earth connector to the side of the main cutout like that would suggest TN-C-S.

Where does that other earth wire go that drops down from the CU? What is that little box at the bottom of the frame?

This is slightly more reliable that the next option in that there is less chance of the earth becoming poor due to corrosian etc at the gland

- but as you later say these are internal connections and a new garage is unlikly to be damp so there is not much in it. If doing this I would tend to use one core *and* the screen.

One slight advantage with this one is the cable will be a little thinner and easier to handle.

If the house end is not already TT then the arguments for a separate TT supply in an outbuilding usually come down two one of two reasons - either it is a long distance (obviuosly not the case here), or, when there will be potential difficulty extending the house's equipotetial zone into the outbuilding (of most concern with a PME house).

If coming from above, could you not have an adaptable metal box in the loft space instead?

You could use crimps, but since it will remain accessable screw connectors are ok. If you opt for using the screen as a CPC I would be reluctant to have the gland connection burried in a wall behind a metal box.

It is good practice since it offers better protection to the cable (especially since you don't currently appear to have a RCD at the head end). If you were using TT in the garage then you can earth it at the head end, but leave the other end disconnected (would require a insualted CU in the garage)

As usual it is worth what you paid! ;-)

[David Hansen]

On Thu, 24 Aug 2006 00:44:13 +0100 someone who may be John Rumm wrote this:-

Agreed.

Agreed. An excellent way to introduce corrosion into the CPC. In addition, although I don't share the panic about using steel as the CPC I think it must be easily accessible for inspection. Provided the extra lengths are acceptable I too would look at the loft as the place to make the joint.

[Andy Wade]

Yes, for TT a main switch and two RCBOs would be a good option, or main switch with one MCB (lights) and one RCBO (power) for TN. I'd be inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B and 16 A Type B might tend to trip on motor starting currents, or the inrush current of a 230 - 115 V tool transformer. (Alternatively keep the 16 A circuit for sockets only and provide separate motor circuits for any machines.) You can buy all the MK consumer unit parts separately from a wholesaler, including TLC if you want to buy on-line.

That would probably require the feed to be via a separate switch-fuse rather than an MCB in the CU - certainly a better arrangement, but does involve teeing in to the meter tails with a service connector block.

Looks like TN-C-S to me, with old earthing and bonding conductors which are undersized by current standards. These ought to be upgraded if you intend to use the exported PME earth.

Looks a bit like an old GPO telephone wiring junction box (a 'block terminal' something-or-other)?

Using a core as CPC gives you a 6 mm^2 conductor with a resistance of about 3.7 milliohm/m. Using the armour gives you a CPC equivalent to

9.8 mm^2 copper, but with a resistance of around 7 milliohm/m. I'd use the 2-core unless the CPC resistance (R2) is a problem in the design.

The weakest link in this design though is the 2.5 mm^2 CPC in the T&E cable. Note that it would not be acceptable to use this if there are any metal services rising in the garage that would need main bonding - this because the CPC of the submain also acts as the main bonding conductor, so would need to be a minimum of 10 mm^2 Cu equivalent.

As I usually say here, the TT/TN decision depends on the use of the outbuilding and the practicality of creating an equipotential zone there. At one extreme is a dry workshop building where there will be little use of electrical equipment outdoors (TN OK here) and at the other extreme is the metal greenhouse (TT essential). Any use extensive of portable Class 1 (earthed) equipment outdoors again signals the need for TT earthing.

No, and there is an advantage to doing that - but understand that it's still TN-C-S with its attendant risks. You are adding another earth to the M (multiple) in PME, so you'll help reduce the touch voltage in the event of a broken supply neutral. You do need a damn good earth electrode to do much good though, and it should be connected to the main earth terminal in the house, not at the garage end via a weedy 2.5 mm^2 CPC.

That's completely impractical because you've go no access to the back of the box to tighten the gland nut. It would also be extremely fiddly to do this with a single-gang accessory box. Better as John said to connect the SWA in the loft, alternatively take the T&E through the wall to a box on the outside, and run the SWA straight down from there.

[...]

Yes, definitely, to provide protection in case the cable is damaged.

[Roger Mills]

Many thanks for your response.

The local MCBs are 16A for power and 6A for lighting - so, under overload conditions, should trip prior to the 32A one at the house end. Of course, if I cut through a wire and connect live to earth before live to neutral, the RCD will trip and I'll lose the lights as well. I guess I have to live with that.

The other earth wire goes into an adjoining cupboard which houses the gas meter, and is clamped onto a gas pipe. The little box at the bottom is a junction box for telephone wiring.

I *could* join it in the loft space - but that bit of loft is pretty inaccessible, so I was trying to avoid it. What does an adaptable metal box look like?

I guess I'd need a half-way decent ratchet crimping tool to use crimps - which I don't have. I take your point about burying the gland in the wall.

If you're selective about what advice you accept, much of the advice given here is actually worth a lot more than we pay for it!

[John Rumm]

Hager do a MCB sized HRC fuse carrier, which can solve that problem in many cases.... although having just typed that, I remembered that Roger's CU ain't a DIN rail jobbie anyway, so this is of little practical use to him! ;-)

[John Rumm]

By a slight change to the choice of CU though that is easy enough to avoid.

So no main bond to the water service then?

Thought as much, but thought I had better check!

Box looks like:

>You could use crimps, but since it will remain accessable screw

In which case why not chase down the bathroom wall as described, but rather than installing a metal box in the bathroom, simply drill throught the wall at that point (keeping within 6" of the bathroom ceiling). You can stick some capping over the T&E to facilitate future replacement if you wish.

Now install your cable and plaster over the chase leaving no visible accessory in the bathroom. Now visit the other side of the wall where hopefully you have a bit of T&E dangling, and poke it into the back of one of the aformentiond boxes (a IP65 one would probably be a good choice here). Screw it to the wall, and you can then stick a nice neat SWA gland into the knockout at the base of the box. Choose the right box ad you may even get one complete with junction terminals. That means your joint is accessable and serviceable, no extra clutter to tile round the bathroom, and no groveling in the loft!

Yup, second that many times over, thanks guys!

[Andy Wade]

Oops, quite right, I forgot that option.

Doh! - his picture tells all, and I guess the new circuit is going in the empty fuseway on the right (currently 15 A). In this case the obvious thing to do is to use a 30 A BS 1361 fuse and not another plug-in MCB.

[Christian McArdle]

Don't be so sure. You want the house end to blow as slowly as possible to get maximum possible discrimination with the garage CU. I would consider a Type C MCB, or even an HRC fuse. If 40A is allowed as well, then use that, too. I haven't done the calculations so can't tell you what you will get away with. This way, any fault is more likely to be picked up at the local MCB in the garage.

I'd just do this given you are only 1.1m from the house. This is only possible if the garage has no main equipotential bonding requirements, or the main equipotential bonding can actually be achieved.

Remember to use the armour as earth as well, both to get the ELI even lower and because the armour must be earthed anyway. The reason to have the additional copper conductor is that copper doesn't rust at the termination and is more reliable over the decades.

No problem.

Chocolate block or crimp is fine.

Yes. Either earthed by using a metal box, or using a decent earth ring, not the s**te one that you get with the gland.

Christian.

[Roger Mills]

Yes, the circuit is going at the extreme right - which currently has a blue base for a 16A MCB.

I may well still have a 30A fuse - not sure about the BS1361 bit though - is that something special? [The CU started off with fuses, and was converted to MCBs!]

What is the rationale for using a fuse rather than an MCB? Is it that it takes longer to blow, making it more likely that the one at the garage end will trip instead? If so, it makes sense, but it flies in the face of the advice given in the earlier thread which said that I needed a Type B (and

*not* Type C) MCB on account of only having a 2.5mm^2 earth wire for most of the length.

[Roger Mills]

Is there an idiot's guide somewhere, defining the calculations which I need to do?

As mentioned in my post in answer to Andy Wade, I had previously been advised (back in June) to use a Type B MCB, and to go for 32A rather than

40A in the light of my rather weedy 2.5mm^2 earth wire - so I'm getting a bit confused!

It's 1.1 metres as the crow flies - but about 6 metres of SWA down the wall, across the gap, and into the garage. Does this make any difference - or are we only looking at the likely difference in earth potential between 2 adjacent buildings? There will be no 'services' (water, gas, etc) in the garage, and no equipotential bonding is required AIUI.

[Roger Mills]

Could you elaborate on that please?

Yes there *is* - but the water comes in elsewhere. The earth wire goes up through the ceiling from the CU, and drops down into the kitchen where the mains stop-tap is located.

or

Thanks for the links. I now see what you mean.

That's certainly a possibility. It means that I would be making the connection while perched 16' up a ladder, but I could probably terminate the SWA in the box before fixing the box to the wall. How would I seal round where the T&E would come in?

[Andy Wade]

The BS 1361 fuse is a cartridge fuse, like this

opposed to the rewireable one that you probably still have (which is to BS 3036).

In effect, yes. The problem with cascaded MCBs is that if the fault current exceeds a certain figure both MCBs are on the 'instant' magnetic tripping part of their characteristics, and they'll both trip together - i.e. there's no discrimination.

OK - I've probably confused you there. We're looking at different aspects of the design in isolation - discrimination only in this sub-thread - and have lost sight of the whole picture. Let's look at some other aspects. I've found the June thread and assume that 10 m of T&E plus 5 m of SWA still stands. I'll also assume that the 30 A BS

1361 cartridge fuse is to be used.

- Voltage drop: 15 m (total) of 6 mm^2 will have a voltage drop of under

3.3 V at 30 A; that's 1.4% of Uo, so OK there.

- Disconnection time: 10 m of 6/2.5 T&E will contribute about 0.13 ohm to the earth fault loop impedance (Zs) and 5 m of 6 mm^2 2-core SWA will contribute a further 0.06 ohm; taking the cautious assumption that the supply is in fact TN-S we add 0.8 ohm for the external (supplier's side) part of Zs, so Zs (total) is 0.8 + 0.13 + 0.06, which is 1 ohm, as near as damn. The earth fault current it thus 230 A which will take out the

30 A fuse in somewhere between 0.2 and 0.3 seconds - so the 5 s disconnection time requirement is met.

- CPC sizing: assuming 0.3 s disconnection then for the T&E section S(min) = sqrt(230^2 * 0.3) / k, with k = 115 (from Table 54C). This gives a min CPC size of just over 1 mm^2, so the available 2.5 mm^2 CPC is OK. It's hardly necessary to do this calculation for the SWA section, but for completeness the relevant k value is 51 (Table 54D, assuming 70 deg.) so S(min) is just under 2.5 mm^2 (steel). The armour CSA in 2-core 6 mm^2 cable to BS 5467 is 22 mm^2, so definitely no problems there.

Unless I've missed anything, we can conclude that the submain design complies with BS 7671.

[Roger Mills]

Hi Andy,

Many thanks for your very detailed reply - complete with calculations. A few more questions, I'm afraid . . .

I've looked at my old ones, and they *are* BS 1361 cartridges - so I already a couple.

So what are we aiming at here - how high can it be?

Sounds like we're *well* in.

What would be the effect on the calculations if it were a 40A or 45A BS1361 fuse rather than 30A?

Others have suggested going out through the wall with the T&E, and joining it to the SWA using an IP65 box mounted on the outside of the wall. That might be easier in some ways, but I'm not sure how best to seal the T&E cable entry into the box. Any suggestions? Do I drill the back of the box, and align the hole with where the cable comes through the wall, and seal round the cable with silicone sealant? Or what?

Someone suggested using a different type of CU in the garage - so that if I were to do something with a power tool which caused the RCD to trip, it wouldn't also take out the lights. Is that what is meant by a 'split load' CU? Any suggestions as to exactly what I would need (make, model, etc.)?

Many thanks.

[Andy Wade]

4% (9.2 V) maximum between the meter and all points of utilisation.

That's where you want to be.

The first question here is why would you want a 40+ A feed when your stated load in the garage is only about 20 A? Gut feel says that for

40+ A the submain would need to be in at least 10 mm^2 and that 6 is likely to be marginal at best.

Anyway, the current to blow the 45 A fuse in 5 s is 240 A. We calculated 230 A earth fault current (at 70 deg. conductor temperature) so calculations fail at the first hurdle in that case, unless you can get positive confirmation that the supply is TN-C-S (the DNO should be able to tell you). Even it did just meet 5 s disconnection, the required CPC would be 6 mm^2, so that T&E cable would be at serious risk. (BS 7671 doesn't give any data for a 40 A '1361 fuse, so I can't comment on that.)

An option to overcome the Zs issue would be to fit a 100 mA Type S (time-delayed) RCD at the house end of the feed.

To use 6 mm^2 T&E on a 45 A circuit the installation conditions would have to be 'clipped direct' at all points - no grouping and no thermal insulation allowable. This cable goes through the loft, so that might not be practicable in any case.

By now you should see why (in June) I said stick to a 30/32 A circuit.

You'll never seal it completely against porous brick. Drill a drain hole in the bottom of the box.

Incidentally, there's a restriction on wiring in a bathroom that you need to consider. Wiring which is in Zones 0, 1 or 2 and is not in metal conduit or is not done in a metal-clad cable such as SWA is only allowed to supply equipment in those bathroom Zones. This applies even if the wiring is on the surface or in the normal safe zones - see Reg.

601-07-01. However it doesn't apply if the cable is buried >50 mm deep in the wall. So if your old shower cable is over the bath (Zones 1 & 2) you'll have to bury it more than 50 mm in the wall in order to be able divert it for another purpose. The Zones finish at 3 m above the floor, so there's a get-out if you have high ceilings. For the region alongside a bath (where a shower pull-switch might previously have been located) Zone 2 finishes at 2.25 m above the floor and 601-07-01 then ceases to apply.

Yes, it would be split load. This was covered earlier in the thread and I made some suggestions then. Again:

Yes, for TT a main switch and two RCBOs would be a good option, or main switch with one MCB (lights) and one RCBO (power) for TN. I'd be inclined to make the 'power' RCBO a 20 A one - RCBOs only come as Type B and 16 A Type B might tend to trip on motor starting currents, or the inrush current of a 230 - 115 V tool transformer. (Alternatively keep the 16 A circuit for sockets only and provide separate motor circuits for any machines.) You can buy all the MK consumer unit parts separately from a wholesaler, including TLC if you want to buy on-line.

[John Rumm]

There are a few options. I would be inclined to mount the box just above the exit point of the T&E and come up into a hole in the bottom of the box either with one of the connical grommets/blanking plugs that you get with some of these, or, as you suggest silicone. The very short bit of T&E ought to be either painted or covered by a short bit of mini trunking to protect from the UV.

If you come throught the back of the box, then silicone is probably the best option.

You could use a split load unit - this has a switch on the incomer, and then a RCD positioned mid way down the bus bar. Hence you can choose which circuits are RCD protected by placing them on the protected "side" of the CU. However for a garrage this might be overkill since a split load CU will probably be a dozen ways wide.

If you are exporting the earth to the garage, then you could simply use a small CU (say 4 way)[1] with a normal incomer switch, and then fit a type B/C MCB for the lights, and a RCBO for the power.

[1] I say 4 way to allow for a little expansion, and also give you a better choice of RCBOs - many of which take up two ways in width (there are single width ones that are instead double height, but not all CUs will take those)

[Roger Mills]

Hi Andy,

Many thanks for your latest answers.

I think someone else suggested 40A or 45A to provide greater discrimination between the protection devices - but 30 A is adequate, and I've already got some fuses, so I'll stick with that.

It's not a bathroom per se in that it doesn't contain a bath. It's an en-suite facility which has a toilet and bidet one end, and a wash basin and shower cubicle the other end. Does that make any difference? The relevant bit looks like

pull switch on the ceiling is for isolating the shower pump. It used to be fed by the 6mm cable but is now fed by a fused spur off a ring main. You can see the 6mm cable lying across the small hole I have made in the ceiling. It would need to be chased into the wall, and then go out through the wall just to the left of the fan. [It needs to come down to this level to get below the fascia on the outside]. I don't know what zone that would be. Please advise! What is the reason for not having any cables which are on their way elsewhere - sounds a bit OTT?!

Thanks. John Rumm has also made some useful suggestions. Many thanks to you both!

[Andy Wade]

No, the same zoning applies.

the wall in the RH corner, if poss, as that's more than 600 from the shower cubicle, i.e. is in Zone 3. Otherwise, is the visible part of the shower enclosure on the left fixed or movable, and how high is the fan, above floor level?

Zone 2, I suspect, up to a height of 2.25 m, then Zone 3 above that.

I guess the thinking is that it's not obvious that it would be there, as there's no electrical equipment in the room to give a clue to the presence of a cable. The principle is to keep all non-essential cables out of the the wet areas, thus avoiding danger when you decide to drill through the wall while standing in the shower... It is a bit of a pain in this case, although it looks as if chasing in to 50+ mm deep might not be too difficult, if that is necessary.

I presume a new direct SWA run from the consumer unit to the garage is out of the question.

[Roger Mills]

There is a fixed panel alongside the vanity unit, which is 500 wide and about 1860 high, then a folding door - which is open in the photo. Does the fixed panel alter the zoning definitions?

The centre of the fan is about 2100 above the floor. The existing cable isn't long enough to reach the RH corner - it's just long enough to come straight down and through the wall, with enough to make the joint on the outside. I am 6'2" tall (sorry for the mixed units!) and I can't reach to where the cable would be, whilst standing in the shower cubicle - so would have great difficulty drilling through the cable whilst having a shower!

I *could* chase it in to a depth of 50mm, or I suppose I could join it to SWA above the ceiling, and bring the SWA down and through the wall. I might even be able to join the cables below the ceiling in a universal box, and then poke the box up through the hole - but I doubt whether 6mm SWA is flexible enough to allow that.

It's highly undesirable, involving interfering with the decorations in several rooms, moving a lot of furniture and and taking up a lot of floor coverings and floor-boards. To be avoided if at all possible!

[Andy Wade]

It would if it was 600 wide and went up to the ceiling...

6 SWA isn't /that/ unflexible - buy your length and play around to get the feel of it. Why not join the cables up in the loft in a metal adaptable box fixed to the woodwork somewhere (not poked up thro' a hole and left flapping around)? Then you can bring the SWA out through the en-suite wall wherever you like, because it's a protected cable. Drill through the wall at an angle, sloping downhill and round off the lower outside edge of the hole, so the SWA can be dressed against the wall at its exit point.

That was a rhetorical question ;-)