Additional feed for outbuilding

[Heathcb]

Is it acceptable to run an extra length of 6mm t+e to allow me to run 10mm swa to my outbuilding?

I currently have an outbuilding with a 6mm t+e feed. I want to future proof the outbuildings supply as I intend to set up a workshop. I think a workshop off of a 32A feed is a bit underrated.

My idea is to run a second feed from by the consumer unit to the outside wall of the house. Then, in a suitable ip rated box, join both 6mm t+e (effectively a ring?) to the 10mm swa I will run to the outbuilding consumer unit.

I will get a spark to connect it all in the new year, but want to run the cable while ceilings are down.

Thanks in advance.

 

[ericmark]

There are strict rules for conductors in parallel.

433.4.2 Unequal current sharing between parallel conductors
Where the use of a single conductor is impractical and the currents in the parallel conductors are unequal, the design current and requirements for overload protection for each conductor shall be considered individually.
NOTE: Currents in parallel conductors are considered to be unequal if the difference between the currents is more than 10% design current for each conductor. Refer to paragraph 2 of Appendix 10.

523.8 Conductors in parallel
Where two or more live conductors or PEN conductors are connected in parallel in a system, either:
(i) measures shall be taken to achieve equal load current sharing between them
This requirement is considered to be fulfilled if the conductors are of the same material, have the same cross-sectional area, are approximately the same length and have no branch circuits along their length, and either:
(a) the conductors in parallel are multicore cables or twisted single-core cables or non-sheathed cables, or
(b) the conductors in parallel are non-twisted single-core cables or non-sheathed cables in trefoil or flat formation and where the cross-sectional area is greater than 50 mm² in copper or 70 mm² in aluminium, the special configuration necessary for such formations is adopted. These configurations consist of suitable groupings and spacing of the different lines or poles or (ii) special consideration shall be given to the load current sharing to meet the requirements of Regulation 523.1.

This regulation does not preclude the use of ring final circuits with or without spur connections.

Where adequate current sharing is not possible or where four or more conductors have to be connected in parallel consideration shall be given to the use of busbar trunking.

I would not want to run two cables of unequal sizes myself and if asked to commission one some one else had done I would refuse.

That's not saying you can't but my view would be it's your job it's either all or nothing so either you do all the inspecting and testing and put your name on the form or do nothing there is really no middle ground.

To my mind if it needs 16mm then use 16mm not 10 and 6.

 

[Heathcb]

Thanks you for the quick reply.

Sorry I may have not described it very well. I am proposing to have 2x 6mm t+e from cu to external wall. Then changing to a single run of 10mm swa from wall to outbuilding.

I would then assume that because the 6mm in parallel will both run the same length and be same size, this is ok? I am only doing it this way as there is already one length of 6mm run and I have a roll in the garage.

Thanks.

 

[EFLImpudence]

Well 6mm² can carry (method C - clipped or buried in masonry) 47A so do you need a second?

It is not clear - does the first 6mm² cable already go the whole way to the garage?

 

[Heathcb]

The 6mm I bought from Screwfix (Prysmian) says it is rated to 32A. The cable at the moment is run all the way in 6mm and on a 32A mcb. If I could get away with a 50A mcb then I would be ok.

 

[EFLImpudence]

The 6mm I bought from Screwfix (Prysmian) says it is rated to 32A.

It does. It is wrong.
As long as it is not in conduit nor running through thermal insulation it is 47A.
So a second is not necessary.

The cable at the moment is run all the way in 6mm and on a 32A mcb. If I could get away with a 50A mcb then I would be ok.

If the load in the garage is less than 47A - e.g. 32A for sockets and 6A for lights - then you could fit a 50A MCB at the house.

If you run the 10mm² swa alongside the present cable then it (the present cable) may be used if needed for any bonding required.

You really should ask the opinion of an electrician before you start the work as there are things with outside supplies that you probably don't know exist.