Mad? bad? or OK?

[studentspark]

Large outhouse in need of a supply upgrade.

This is the circuit..

A 32a MCB in the main house CU protects a 6mm t&e cable
This cable goes to a 100a switch fuse.
16mm SWA from switch fuse then goes to the outhouse.

The outhouse has quite a lot of electrical equipment so the 32a MCB and 6mm cable could do with an upgrade.
I plan to take this circuit off the house cu and run a sub main. i.e. off the supply fuse

The run of 6mm2 is about 25 m

I could run a new cable, but I have been wondering about running an additional 6mm2 cable along side the 6mm2 already in place. Double up as it were.

Now, is this a bad idea, or a way of saving money and materials.
I feel a bit odd about it, but would like some opinions
Thanks

 

[deadshort]

Hi, an additional 6mm in parallel with existing is a no no.

What is the load in the out building? Has the 32a tripped frequently ?

Regards,

DS

 

[JohnW2]

A 32a MCB in the main house CU protects a 6mm t&e cable ... This cable goes to a 100a switch fuse.
16mm SWA from switch fuse then goes to the outhouse. ...
The outhouse has quite a lot of electrical equipment so the 32a MCB and 6mm cable could do with an upgrade. ... The run of 6mm2 is about 25m

A more specific indication of the size of the load would be helpful ... and how long is the SWA?

I plan to take this circuit off the house cu and run a sub main. i.e. off the supply fuse

I hope you mean 'off the meter'

I could run a new cable, but I have been wondering about running an additional 6mm2 cable along side the 6mm2 already in place. Double up as it were. ... Now, is this a bad idea, or a way of saving money and materials. I feel a bit odd about it, but would like some opinions

IMO, not at all 'nice' - parallel cables raise some potential issues. I would personally certainly want to 'do it properly'.

Kind Regards, John

 

[studentspark]

SWA about 20 m

Load has been relativly low for for years few sockets, lighting, electric radiator.

But they want to put a shower down there ! Plus laundry

Agree with doing it right and putting the correct size cable in. I was just wondering about doubling up and its possible merits and problems.

Thanks

 

[scott6471]

John raises some good points, but I think there are also other things that are being overlooked:

What's the point of the 100a switch fuse? Not currently offering any protection as far as I can see; the main protection is the 32a MCB or at least there wouldn't appear to be discrimination between the two devices.

As an outbuilding where's your additional protection I.e. RCD?

Considering the proposal of a new cable or "paralleling" cables would suggest a significant load is being considered for the outbuilding. What is the calculated design load combined I.e. House and outbuilding?

 

[JohnW2]

SWA about 20 m ... Load has been relativly low for for years few sockets, lighting, electric radiator. ... But they want to put a shower down there ! Plus laundry

That's a very substantial load. With 45m of 16mm² (25m T+E & 20m SWA), you'd have a VD of about 8.8V (3.8%) at 70A (a little over the guidine 3% for lighting) A 50A shower alone would result in a VD of about 6.3V (2.7%). Is there already an electric shower in the house? If so, you would probably theoretically need DNO 'permission' to install a second one. If there is to be a shower, there are presumably extraneous-c-ps in the garage, which means that you will have to consider the earthing arrangements.

Agree with doing it right and putting the correct size cable in. I was just wondering about doubling up and its possible merits and problems.

The only 'merits' are in terms of laziness and a little cost-saving. As for problems, for starters ... ensuring (continuing) equal sharing of current between the two cables can be a concern. If one of the two parallel cables is 'lost' (e.g. loose connection) it won't necessarily be immediately apparent but, if they both go through the same OPD (fuse/MCB) the remaining (quite possibly overloaded) one would end up seriously under-protected.

Kind Regards, John
Edit: misplaced decimal points corrected!!.

 

[JohnW2]

What's the point of the 100a switch fuse? Not currently offering any protection as far as I can see; the main protection is the 32a MCB or at least there wouldn't appear to be discrimination between the two devices.

We are now told that there is a potential maximum load of at least 70A. Continuing to deriving the supply from a 32A MCB will therefore clearly not be possible and, if this goes ahead (and if the DNO 'allows' it!), there would probably have to be an 80A,or perhaps even 100A. fuse in a switch-fuse. As you've said, there will be a need for RCD protection, ideally at the outhouse end, but possibly at the house end if any of the T&E is going to be buried in walls.

Kind Regards, John
Edit: incorrect statement about cable size removed

 

[EFLImpudence]

Do you have the decimal point in the wrong place, John?

 

[ban-all-sheds]

Hi, an additional 6mm in parallel with existing is a no no.

Why?

 

[JohnW2]

Do you have the decimal point in the wrong place, John?

Oh dear - so I have - I must say that I was 'surprised' by the answers I got! I blame the Merlot! .. I'll edit the posts accordingly, to avoid confusion

I therefore hereby more-or-less withdraw my comments about cable size - 16mm² would just about be OK (although, at 3.8%, the VD at 70A would be a little beyond guidelines for lighting).

However, everything else I said still stands!

Kind Regards, John

 

[JohnW2]

Hi, an additional 6mm in parallel with existing is a no no.

Why?

As I'm implied, I would not go as far as saying that it was a "no no" (it can be BS7671-compliant). However, as I said before, I don't consider it to be very 'nice', and would be uninclined to do it myself, not the least because of the concerns/hazards that I recently mentioned.

Kind Regards, John

 

[bernardgreen]

You could leave the existing loads as are on the 6mm and put the new loads onto a new cable from a new breaker in the house.

 

[ericmark]

It would depend on installation method using 100 your limited to 57 amp and the largest MCB for most boards is 45 but some will take 50 amp so working on 50 amp 16mm all the way volt drop is 5 volt. Assuming there will be wiring in the out building you can't really go to 6.9 volt limit.

I have run parallel cables using 2 x 95mm instead of a 180mm simply as near impossible to terminate 180 mm into the generator but both cables laid together exactly same route and length. As SWA the chance of damage was very low. But with 6mm you will need a risk assessment could one cable be damaged on it's own? Clearly method 100 would not be appropriate method C clipped direct to tray work would be OK but I would assume your not using tray so renewing with 16mm saves any problems.

Once one starts to talk about using fuses or moulded breakers the problem is they are not type tested for use in a domestic situation. Again risk assessment and you have to sign the document. Drawing over 50 amp to feed an out building one has to consider what is left for the house. Although the fuse carrier may say 100A it can have anywhere from 60 to 100 amp fuse fitted.

You have not said what the load is so it's all guess work. Using water storage a shower only needs a 13A supply we are assuming instance with a 32A - 50A supply. It is a balancing act deciding what to use stored hot water gives a better shower but the thermostatic mixing tapes make it more expensive to fit. However likely the existing 32A supply would then be good enough.

 

[JohnW2]

You could leave the existing loads as are on the 6mm and put the new loads onto a new cable from a new breaker in the house.

Such a separation would require an additional run of SWA (as well as an additional 6mm² T+E) which I imagine is the last thing that the OP would want to have to do if the existing 16mm² SWA is adequate.

Kind Regards, John

 

[bernardgreen]

I thought the existing was 6 mm and in-adequate for the loads being added to the building's electrics.