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]

and if the supply voltage drops within permissible limits, e.g. @peak times, that's a drop of 6% which makes a mockery of your 3%.

 

[Deleted member 9648]

and if the supply voltage drops within permissible limits, e.g. @peak times, that's a drop of 6% which makes a mockery of your 3%.

Cant disagree with that mate, but the OP said he wants to go by the book.

 

[dinger809]

Nobody has mentioned the reference method. Clipped direct has a higher ambient temperature than buried, which is usually lower as it's cooler below 500cm. So, go for the one which has the higher impact on the temperature of the cable, so at least choosing clipped direct was right. 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

 

[Pete999]

Nobody has mentioned the reference method. Clipped direct has a higher ambient temperature than buried, which is usually lower as it's cooler below 500cm. So, go for the one which has the higher impact on the temperature of the cable, so at least choosing clipped direct was right. 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

Think the OP has already binned that idea

 

[dinger809]

Think the OP has already binned that idea

I'll try to keep up!!!

 

[Pete999]

I'll try to keep up!!!

Next time you get lines or detention.

 

[Sparky_marky2]

Hi all, Thank you for your feedback you were right I have scrapped the SWA in galv conduit idea.
Just a quick one would I be able to use 6mm SWA as the subboard distribution supply as I have spoken to another spark I know who thinks this would be adequate hence why originally thought that would do, and also I remeasured the run and its 30m of SWA not 22m as stated before.

Regards,

Mark

 

[Sparky_marky2]

Also I say this as I got a reply from someone else saying 10mm SWA is "over kill"

 

[dinger809]

Also I say this as I got a reply from someone else saying 10mm SWA is "over kill"

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

 

[telectrix]

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

 

[Sparky_marky2]

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

Dinger - thank you for your help

 

[Deleted member 9648]

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

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.

 

[telectrix]

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.

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

 

[dinger809]

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

.
I would go by the book, because I know it would be majored in my annual NICEIC MOT, if they chose that one to look at

 

[Wilko]

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, or a bit more care to avoid it... 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