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

[Sparky_marky2]

Hi guys I've attached a photo, at the roof of the bungalow (i've marked this in red) is where the SWA will come out of and down the wall, where it is marked in blue is the area I am concerned maybe 'at risk' from kicks and other problems from people walking past. The client has stated that the bit in blue that runs horizontally in front of the step will be concreted over once his outhouse has been completed.

 

[Pete999]

Hi guys I've attached a photo, at the roof of the bungalow (i've marked this in red) is where the SWA will come out of and down the wall, where it is marked in blue is the area I am concerned maybe 'at risk' from kicks and other problems from people walking past. The client has stated that the bit in blue that runs horizontally in front of the step will be concreted over once his outhouse has been completed.

View attachment 37579

What size galv conduit are you intending to use? 10mm2 2 core it will be a tight job to run that in conduit.

 

[Sparky_marky2]

Pete999 - I'm going to spec the job again later today as things have changed somewhat from initially using 6mm to now using 10mm SWA, after reading everyone's advice I may leave out the galv conduit and run the cable clipped direct.

 

[telectrix]

calculate VD for the sub first. from my previous post, for 6mm this would be 6V assuming a 40A load ( which is a bit pessimistic. then, calc. the shed VD.. add them together.

 

[telectrix]

Pete999 - I'm going to spec the job again later today as things have changed somewhat from initially using 6mm to now using 10mm SWA, after reading everyone's advice I may leave out the galv conduit and run the cable clipped direct.

or just use some plastic duct. and i think 10mm is overkill.

 

[Toneyz]

What size galv conduit are you intending to use? 10mm2 2 core it will be a tight job to run that in conduit.

Let alone the bend.

 

[Sparky_marky2]

calculate VD for the sub first. from my previous post, for 6mm this would be 6V assuming a 40A load ( which is a bit pessimistic. then, calc. the shed VD.. add them together.

I've calculated the Vdrop for the 10mm SWA on 40A MCB from the main board at 22m length run which comes to 3.8VD (1.68% of the allowable 5%)

The issue is the circuits coming off the sub-board.. for instance I've calculated the 2 double sockets that the client wants needs a run of 2.5 twin and earth on 32A MCB and 12m length run which comes to 5.76VD (2.5% of the allowable 5%)

IF i add these two figures together 1.68% and 2.5% i get 4.18% which is just below the allowable 5% (11.5V)

Am i right to add it like this and then do the same for the other circuits (lighting VD + 10mm SWA VD and air con VD + 10mm SWA) on the sub-board or should I treat them all individually and not add the figure for the VDrop figure for the 10mm SWA

 

[telectrix]

personally, i add the voltages first, then work oyt the %. i'd not be too worried if the figure was over the 3% anyway. you got 10V to play with due to the silly 230V nominal voltage rubbish figure.

 

[Sparky_marky2]

personally, i add the voltages first, then work oyt the %. i'd not be too worried if the figure was over the 3% anyway. you got 10V to play with due to the silly 230V nominal voltage rubbish figure.

Thanks for the help, how do you know this figure? is this in the BS7671.. i just want to do things by the book etc

 

[Deleted member 9648]

The total volt drop from the origin of the installation must not exceed 3% for lighting. For example if your distribution circuit is expected to carry a maximum load of 40a then the volt drop of both distribution circuit and the furthest point of the lighting circuit combined must not exceed 3%. Therefore you will calculate the maximum volt drop on your lighting circuit and add it to the calculated voltage drop on the sub. In total it should not exceed 6.9v
Put another way if your lighting circuit will have a VD of 2v from the CU then your distribution circuit fully loaded will have a permitted volt drop of 4.9v.

 

[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.