Garage consumer unit...are tails needed?

[LittleMissTerious]

Hi

I'm planning to run power to my garage from my house. I will be running armoured cable from the house consumer unit (there is a spare 20A MCB,

2.5mm T&E into a connection box into armoured cable) underground out to the garage. This then enters another connection box, using a gland..hopefully I've got this right so far...

I intend to fit a seperate RCD protected consumer unit in tha garage with 2 MCBs/rings, one for lights, one for a couple of power sockets (to power an outdoor freezer & the occasional power tool!)

Is it ok to come out of the garage connection box (that the armoured cable feeds into) with 2.5mm T&E and straight into the garage consumer unit, or do I have to use tails???

[Owain]

You can go straight into the consumer unit with armoured if you want to.

You do not have to use tails.

Owain

[Jim Alexander]

The garage consumer unit will probably have 20mm cable entries allowing you to gland in directly. Check out pirahna locknuts, available from tlc-direct. These allow a much more reliable earth connection if using the armour for the earth and glanding onto plastic.

Jim A

[LittleMissTerious]

Ah, even better - thanks to both of you! Will check there's a hole/push-out for armoured straight into the consumer unit before I buy it! So I'm ok to rely on the earth through the armoured cable back to the house, I don't need to put in a seperate earth rod for the garage? Wasn't entirely sure if I needed to, is prob only 5 or 6Mtrs from the house... As you can tell, I'm still at the planning stage :0)

[David Hansen]

On 20 Jun 2006 10:52:07 -0700 someone who may be "LittleMissTerious" wrote this:-

Presumably this box is near to where the SWA cable leaves the house and there is some distance between it and the consumer unit. If there is not then, provided the consumer unit is suitable, why not run the SWA all the way to that?

The same at the other end, take the SWA to the unit if possible. Cable joints are best avoided if possible.

Lighting rings again. Where does this idea come from?

Most garages are eminently suitable for sockets on radial circuits and, despite occasional claims to the contrary, I doubt if any domestic lighting circuits are wired up as rings.

[Christian McArdle]

No need for the connection box in the garage, just put the gland straight into the plastic consumer unit.

2.5mm is not enough. Run it in 6mm SWA XLPE. Put it on a 32A MCB. You can use 6mm PVC T&E from the consumer unit and then switch to 6mm SWA when it exits the building.

Good plan. Choose a waterproof version, such as:

Is it ok to come out of the garage connection box (that the armoured

You can do it that way, but terminating the armoured actually on the consumer unit is also fine. Again, use 6mm right up to the incomer on the consumer unit. Only go down to 2.5mm when you get to the garage socket circuit itself.

Christian.

[Christian McArdle]

At 6m from the house, I would personally say that exporting the earth is acceptable. However, don't rely on the armour for this. Buy 3 core cable and run the earth on one of the cores. The 6mm copper core is much more reliable than steel armour which can rust at the terminations and have a poor connection. You still need to earth the armour as well for safety.

Christian.

[Jim Alexander]

Yes, the impedance of the the armour is less than the impedance of 1.5mm copper. I rejected using 3c SWA partly to avoid introducing unharmonious

3ph colours into a domestic installation.

Ha, a different question which you will find lots on if you Google.

Doubt you will be up against any limiting design factors not applicable to your existing circuits at 5 or 6Mtrs unless your 20A breaker is a Type C (probably will be Type B), however I thought my garage was fairly close but ended up using 18m of cable, so check. Just the drop from the garage consumer unit to underground will be 3m.

... As you can tell, I'm still at the planning stage :0)

Jim A

[Jim Alexander]

Not necessarily so for a 20A circuit. Some people may want/need more power but garage consumer units are typically 16A power, 6A lighting. 2.5sqmm easily satisfies volt drop rules at the OPs design distance.

Jim A

[LittleMissTerious]

Thanks, Christian!

I will be running SWA through the house wall, under the driveway and into the garage, glad I can go straight into the garage consumer unit with the SWA. The consumer unit I have already been looking at is the same one you suggested!

BUT - do I definitely I need a 32A MCB? When we re-wired the house and replaced the consumer unit a few years back, the people at Newey & Eyer sold us a 20A MCB to take power out to the garage (at a later date), so that is all I have! Money is a serious problem at the mo, so IF i can use the 20A I have, then I need to!

Also, why do I need to take 6mm T&E out of the house consumer unit....I only have 2.5mm T&E that the rest of the house is wired with? The only place I used heavier T&E was for the kitchen cooker, from a 45A MCB??

Thanks

L x

[David Hansen]

On 21 Jun 2006 00:58:11 -0700 someone who may be "LittleMissTerious" wrote this:-

Many buildings rely on the earth through the steel armour of the external supply cable back to the sub-station. I am sitting in one at the moment. This has been done for a very long time and does not mean huge numbers of deaths and injury.

I'm not convinced the fashion for separate copper cables with green and yellow plastic over them everywhere increases safety noticeably in many situations. There are some situations where it does, like items subject to robust environments, but many where it does not.

A search engine will pull up much discussion on this. The answer to the question is, "it depends."

[Christian McArdle]

The earth straps used by the supplier's TN-S earthing are usually in a different league to the crappy locknuts you get on a typical SWA gland. However, those Piranha locknuts suggested elsewhere look like a very worthwhile improvement.

Christian.

[Christian McArdle]

Whilst you can, with short cable runs, use 2.5mm. Why bother? The cost is in the digging and laying, not the cable. You might as well run a 6mm run that will happily draw 32A than save a couple of percent to get something like adaptable.

Christian.

[Christian McArdle]

No, you can use lower, but 32A and 6mm is more likely (but certainly not guaranteed) to discriminate with the 16A MCB in the garage. Certainly, you can use the 20A one until you have saved up the five pounds for a 32A one!

It reduces voltage drop and allows a higher current circuit. If the house wiring is easy to replace, feel free to use the 2.5mm inside with a 20A MCB and switch to 6mm for the much harder to replace buried section. The 6mm SWA is tougher, too.

Remember to do a voltage drop calculation for the entire circuit, though. If you do use 2.5mm cable throughout, you can very easily run out of volts.

Christian.

[Christian McArdle]

I meant "less adaptable".

Christian.

[Roger Mills]

I'm proposing to do something similar. I currently have a radial circuit which was put in for an electric shower but, in the event, we opted for a power shower using stored hot water instead. This circuit is wired in 6mm or more (I need to check) T&E, and goes via a junction box in the attic to the shower pump in the en-suite.

I'm planning to build a detached garage at the side of the house, about

1100mm from the house with a door between for access to the back garden. To get power to the garage, I'm planning to 'pinch' this shower circuit - rewiring the shower pump to a spur on a ring main. I can then join some 6mm SWA cable to the T&E in the attic, and take it out through the wall and across the top of the frame of the door which I mentioned earlier and into the garage.

Does anyone see a problem with this? In particular, assuming that the existing T&E *is* 6mm, will its earth conductor be adequate?

[Andy Wade]

In general the sounds OK. Adequacy of the CPC depends on how the circuit will be protected, and the type of earthing:

- supply system earthing to house: TN-S, TN-C-S or TT?

- type and rating of intended overcurrent device?

- sub-main circuit length, split into its T&E and SWA sections?

[Jim Alexander]

If its 6mm T&E the earth will be 2.5sqmm. The armour of 6mm SWA has a slightly lower impedance than 2.5sqmm copper. In answer to your question the existing earth conductor will be adequate but you will have to specify a protective device appropriate to your earth fault loop impedance. Personally I would advocate running the cable across the face of the frame rather than the top so that it is obvious.

Jim A

[David Hansen]

On Wed, 21 Jun 2006 23:03:42 +0100 someone who may be "Roger Mills" wrote this:-

Forgive me, but I do find use of such measurements a bit odd. Houses are not built to anything like the nearest mm. This trend is particularly odd in civil engineering. I have seen the dimensions of bridges quoted in mm, which is foolish because the bridge will expand and contract by many mm between day and night.

Personally I would consider pulling back the T&E cable and poking it through a conduit into the garage, saving the cost of the SWA cable. Provided it is fixed in an appropriate way that is an acceptable way of bridging such a small gap.

[Christian McArdle]

That's not what it is about. In the SI system, you are supposed to use only units using a factor of a power of 3. Under this system, 1100mm and 1.1m are appropriate. If it went above 10m, then you should only really use m.

Christian.