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

 

[davesparks]

If the description of single phase or three phase phase wasnt obvious enough then consider that you will only be using two of those three cores that you have, therefore treat it as a two core cable. The ccc tables are only concerned with the cores which are carrying current under normal conditions for, what are hopefully, obvious reasons.

Do not use the values from table 4e4a for the voltage drop calculation, that table details the current carrying capacity of the cable. Use the values from table 4e4b on the following page which is clearly identified as being the table of values of voltage drop per ampere per metre.

 

[Sparky_marky2]

I defo owe you a pint mate!

 

[buzzlightyear]

for that loading sub cable would be 10mm.

 

[Sparky_marky2]

Even if the max demand was 23.2A?

 

[buzzlightyear]

What do you think?

 

[davesparks]

Even if the max demand was 23.2A?

It's a distribution circuit, not a final circuit.

 

[Sparky_marky2]

Is this calculated in a different way? apologies for the nativity of the question.

 

[davesparks]

Is this calculated in a different way? apologies for the nativity of the question.

The calculations are the same, its your metnod which is wrong.

Your apologies are irrelevant, you should have finished making this kind of mistake before you started dsigning circuits for the real world without supervision.

Trusting to a bunch of strangers on the internet will almost certainly invalidate your insurance and not be a valid defence in court should it all go wrong.

 

[Deleted member 9648]

Taking pity...a few hints.
This is a distribution circuit. Therefore if you calculate allowing for the maximum volt drop on the distribution circuit only that will not allow for volt drop on any final circuits connected to the distribution circuit.
You also need to take into account that where a lighting load is to be supplied the maximum permitted volt drop requirement is not 5%.
Refer to App 12.

 

[Pete999]

Why run SWA in galv conduit? the mind boggles

 

[Deleted member 9648]

Why run SWA in galv conduit? the mind boggles

Something I do regularly where an SWA drops down a wall into the ground. A meter or so of galv with a bushed top protects the cable from damage from strimmers and such like.

 

[westward10]

Is this the same house as your other thread.

 

[telectrix]

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

where did 53 come from.? the mV/A/m for 6mm is 7. so:

( 7 x 40 x 22 ) /1000 = 6.16.

 

[Deleted member 9648]

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

where did 53 come from.? the mV/A/m for 6mm is 7. so:

( 7 x 40 x 22 ) /1000 = 6.16.

He's used the CCC for 3 or 4 core 6.0mm from table 4E4A....53a!
That would rather skew the result.

 

[telectrix]

He's used the CCC for 3 or 4 core 6.0mm from table 4E4A....53a!
That would rather skew the result.

tables are useful only if you know how to use them.

 

[Pete999]

tables are useful only if you know how to use them.

My kitchen table has a wonky leg, but it's easy to use if you know how.

 

[telectrix]

i sympathise with your kitchen table. i got 2 wonky legs (even when sober).

 

[Pete999]

i sympathise with your kitchen table. i got 2 wonky legs (even when sober).

Yes, but that's self inflicted, mind you I was wobbly this morning, to much vino

 

[Sparky_marky2]

Hi guys, thank for you for all the helpful posts! it was late lastnight when i was posting so i was looking at the wrong page for the Vdrop and actually looking at the current carrying capacity as Davesparks pointed out.

But this has now raised up another related issue about voltage drop with the sub board distribution cable, I've calculated that i need to up it to 10mm SWA for such a run of 22m but also there's the combined Vdrop from the circuits on the sub-board that i need to account for EG. the Vdrop for the length of the circuit from the Main CU to the Sub-board + the Vdrop from the Sub-board MCB to the end of the lighting circuit on that board.. How do you combine two different voltage drops? (ie the lighting circuit must be within 3% and the distribution circuit must be within 5%, or do you treat thee two circuits independently?
likewise for the ring circuit on the sub-board they would both be 5% do they have to both add up to be less than 5% or is there another figure to go from?

any insight would be greatly appreciated.

Something I do regularly where an SWA drops down a wall into the ground. A meter or so of galv with a bushed top protects the cable from damage from strimmers and such like.

I'v chosen to run a part of the SWA in galv conduit as it will run past an outdoor step and be about 2 inches from the ground just for added mechanical protection from people kicking it or from some garden tools hitting it (just trying to account for worst possible situation)

Is this the same house as your other thread.

my other thread on here about alternate cables for external use is for the same house but is not related to the sub-board/sub-distribution cable.