SWA armour supply to outbuilding, what CSA cable? and Reference Method?

[Sparky_marky2]

Hi guys,

I am running a supply to an outbuilding that requires a 22m run from the Main CU (in the garage) to the CU in the outbuilding, at the Main CU it will be a 40A MCB supplying an 63ma RCD protected sub board which will then supply:

6A type B MCB for 12 downlights and an extractor fan
32A type B MCB for 3 double sockets
20A type B MCB for a 3.5kw split air condition unit

I've calculated the voltage drop if using 6mm 3 core SWA clipped direct (table 4E4A p372 BS 7671) 53 x 40A x 22m / 1000 = 46.64 Vdrop .. thats a crazy figure

And the supply will be feeding a sub board initially based on what I've been taught I calculated maximum demand by adding the breakers amp size and x them by 40% so 6+32+20 x 40% = 23.2A

The SWA will run up the outside wall from the garage into the loft and out the other end, and down galvanised conduit, into the ground and into and up a wall with insulation (2m) and into the outbuilding

The queries I have are:

The majority of the run will be clipped direct and only a small part of the length will be in insulated brick wall am I right/wrong to assume Reference method C (clipped direct)?

And as it is SINGLE PHASE 3 CORE cable I'm using I'm a little confused as the only two options under clipped direct in table 4E4A (p372 BS 7671)is '3 CORE 3 phase AC' and '1 two core cable SINGLE PHASE AC' my installation does not fall perfectly into either option, which should I select?

And how best would you recommend lowering the Voltage drop to the stipulated 5% 11.5V?


Thank you in advance for helping,

Mark

 

[telectrix]

Seeing your nice pic has reminded me there is a run inside the house. If there's insulation involved that may add weight to the 10mm. Just saying

Another thought :

55mm x 55mm Cable Guard - 2Mtr Length - Black - https://www.tlc-direct.co.uk/Products/CG552BK.html?ad_position=1o4&source=adwords&ad_id=45425533757&placement=&kw=&network=g&matchtype=&ad_type=pla&product_id=CG552BK&product_partition_id=174053380627&test=finalurl_v2&gclid=EAIaIQobChMIz5emr6-u1QIViLvtCh355guSEAQYBCABEgLg7PD_BwE

View attachment 37583

that will look smart nailed on his £50/roll wallpaper.

 

[davesparks]

And I still don't know why you are thinking about running 10mm armoured in conduit. The steel armour inside is there to protect the conductors. Good luck getting armoured around a bend

The point is protect the cable from damage, any damage to the sheath allows water ingress which will rot the armour and make the cable unsafe.
The steel armour is there to connect a metal object penetrating the cable to earth, if the sheath is damaged and the armour rusts then it will no longer perform this function properly.

Luck is not required, just the correct sized conduit.

 

[davesparks]

And I still don't know why you are thinking about running 10mm armoured in conduit. The steel armour inside is there to protect the conductors. Good luck getting armoured around a bend

The point is protect the cable from damage, any damage to the sheath allows water ingress which will rot the armour and make the cable unsafe.
The steel armour is there to connect a metal object penetrating the cable to earth, if the sheath is damaged and the armour rusts then it will no longer perform this function properly.

Luck is not required, just the correct sized conduit.

 

[davesparks]

I've just calculated this and it IS 10mm, it can't be longer than your previous measurement and be a smaller conductor!!. If your friend says it's overkill, I hope can be at the enquiry with you to explain the fire

What fire are you predicting here? 6mm SWA has a ccc of 49A single phase clipped direct at an ambient temperature of 30c which is well above the nominal current of the circuit.

 

[Richard Burns]

What you need to do is calculate your volt drops for the distribution circuit and the final circuits based on your design currents for those circuits.
The VD on the distribution circuit will add to the VD on each final circuit giving you three values, one will be for lighting and must not exceed 3% (6.9V) the others are non lighting and must not exceed 5% (11.5V).
Select your distribution cable based on one that complies in all cases.
Check that you are not likely to exceed CCC (very unlikely) or Zs limits.
E.g. If you had 230W (1A) of lighting/extraction on a 10m final circuit, two sockets each possibly taking 10A (very variable, normally you take the In for sockets) one at 7m and one at 12m and 15A for the AC on a 10m circuit then the calcualtion coudlba as the attached diagram.

Note you have no short circuit discrimination between the supplying MCB and the final circuits, if you are installing 2 double sockets why is it a ring, a 20A radial may be easier.

 

[dinger809]

What fire are you predicting here? 6mm SWA has a ccc of 49A single phase clipped direct at an ambient temperature of 30c which is well above the nominal current of the circuit.

Ok, bad turn of phrase, I wasn't saying this would start a fire. The point I was trying to make is that if you do a calculation according to the BYB stick to what it says, dintbthen second guess it. That is your safety net. If you stay with that, then nothing should go wrong and if it does, you can always say, I followed BS 7671 to the letter and you are fully covered.

 

[Dave OCD]

that will look smart nailed on his £50/roll wallpaper.

Do they use that round your way on the outside of houses Tel ??

 

[Sparky_marky2]

At 30m the volt drop would exceed the permitted maximum though..(going by the book)....though in practice we all know 6.0mm would be perfectly OK.

I know right this is what i've been taught from sparks i've worked with on site, "6mm does that job" but now im doing the jobs on my own i find myself questioning what I've learnt a lot more....

 

[Sparky_marky2]

yeah, but OP has used 40A for his Ib. in practice 25 - 30A would be more realistic.

Do recommend dropping the MCB size? I had thought about 32A

 

[Sparky_marky2]

40A through 6.0mm SWA is well within it's capacity. that's why i said 10.0mm was overkill.

Thanks for the reply Telectrix always appreciate your input

 

[Richard Burns]

Do recommend dropping the MCB size? I had thought about 32A

I do not think dropping the MCB size is the intent it is mainly using the expected design current for the calculation rather than the maximum current the MCB can take.