What size cable ?
- Thread starter sparkymj
- Start date
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If you are competent to run this, surely you are able to do the cable calcs?
There's a cable calculator here: http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
Use the twin and earth, not the SWA option.
185m is a very long way. You must be clear on what the total max load WILL be.
My opinion would be to have the supply provided where you need it.
There's a cable calculator here: http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
Use the twin and earth, not the SWA option.
185m is a very long way. You must be clear on what the total max load WILL be.
My opinion would be to have the supply provided where you need it.
Whoops, my error. Suffering from snow-blindness. 
I should have said
select 2-core PVC SWA from the drop-down menu.
I should have said
select 2-core PVC SWA from the drop-down menu.
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This is my first post here so sorry that its quite long, but after reading through a few posts thought that a few on here are very helpful and so decided to post this.
On the Voltage drop calculator where it says circuit type then says non lighting circuit or lighting circuit, if its for a sub main then of the CU there will be lights what shall i put?
Also where it says ambient temperature, what would this be for a swa buried underground?
Where it says number of circuits if the swa is only acting as a sub main and won't be run with anything else i take it the number of circuits will be one?
And finally the option for power, i take it that is the max load that will be used?
Sorry to ask so many questions.
Voltage drop allowed for lighting circuits is less, but in this case if it's a sub-main feeding a separate CU then use non-lighting.
Good question - I usually work on 15 or 20 degrees unless it's going somewhere definitely colder than the norm (on average).
Yes - one.
Yes. On the second CU, each 20A radial crcuit for sockets will be 4600W and a 6A lighting circuit will be 1380W (both peak load, not applying any diversity).
We all had to learn once upon a time.
YorksSparks and martinxxxxxx thanks you, pages like this are the best thing about the internet.
Once you have decided the size of cable, and say for example i had a load of 32amps i would then be ok to fit a 32 amp breaker as a 'main switch' even if the cable is say 25mm wouldn't i?
Also what is the formula for working out what size cable to use?
Once you have decided the size of cable, and say for example i had a load of 32amps i would then be ok to fit a 32 amp breaker as a 'main switch' even if the cable is say 25mm wouldn't i?
Also what is the formula for working out what size cable to use?
I would say select a lighting circuit - just because there aren't lights on the submain itself does not mean that you don't have to keep the voltage drop down to 3%, as there will be lights eventually. If it was a circuit just running to an FCU and then a light you'd do that - same difference.
There is no formula as such which will pop out the size of cable - you start with the load you're designing for, adjust for factors such as ambient temperature, grouping, type of protective device etc, then look up in the tables for the cable type you'll be using and the installation method you'll be using to see what size you need, and then you check voltage drop.
This on-line guide is to the 16th Edition of the Wiring Regs, not the 17th, but the methodology is the same - you can read more here: http://www.tlc-direct.co.uk/Book/4.3.1.htm
There is no formula as such which will pop out the size of cable - you start with the load you're designing for, adjust for factors such as ambient temperature, grouping, type of protective device etc, then look up in the tables for the cable type you'll be using and the installation method you'll be using to see what size you need, and then you check voltage drop.
This on-line guide is to the 16th Edition of the Wiring Regs, not the 17th, but the methodology is the same - you can read more here: http://www.tlc-direct.co.uk/Book/4.3.1.htm
volt drop is given in the cable tables in mV/A/m
now you have to work backwards..
max voltage drop is 3% for lighting.. plus 0.005% for every meter over 100m.. ( to a maximum of 0.5% )
so your max volt drop is.. 3.425%..
230 x 3.425% = 7.8775V ( lets call it 7.9 for arguments sake.. )
so you have the volt drop.. you have the distance and you have the current.. ( 32A right? )
( 7.9 / 32 / 185 ) x 1000 = your required mV/A/m
in this case 1.33..
so you look at the table for your required cable type and find one with a mV/A/m less than that.. which from 4D4A in the regs is 35mm² as 25mm² is 1.75 and 35mm² is 1.25..
that is of course unless my brain has gone off on one at this late hour..
I'm sure someone will correct me if I've messed up somewhere..
now you have to work backwards..
max voltage drop is 3% for lighting.. plus 0.005% for every meter over 100m.. ( to a maximum of 0.5% )
so your max volt drop is.. 3.425%..
230 x 3.425% = 7.8775V ( lets call it 7.9 for arguments sake.. )
so you have the volt drop.. you have the distance and you have the current.. ( 32A right? )
( 7.9 / 32 / 185 ) x 1000 = your required mV/A/m
in this case 1.33..
so you look at the table for your required cable type and find one with a mV/A/m less than that.. which from 4D4A in the regs is 35mm² as 25mm² is 1.75 and 35mm² is 1.25..
that is of course unless my brain has gone off on one at this late hour..
I'm sure someone will correct me if I've messed up somewhere..
Yo - in this case (I'd forgotten we were looking at a very long distance) you wouldn't really need to bother with all the current carrying capacity factors.
Mine was more of a generic answer...
Mine was more of a generic answer...
as an asside, how would we stand if we installed a step up transformer at the house end and a step down at the barn end???
obviously we'd have a TT installation at the barn end then.. and we'd have to get suitably hight voltage rated cable
obviously we'd have a TT installation at the barn end then.. and we'd have to get suitably hight voltage rated cable
That would work.
Not sure it would be worth the cost & the hassle though, just to get down to something under 35mm²...
Not sure it would be worth the cost & the hassle though, just to get down to something under 35mm²...
185m of 35mm² don't forget...
what's the cost of a couple of 10:1 32A capable transformers if it means you can get down to 6mm² ? ( please note these are figures plucked out of the air and are not intended to be advice to the OP.. )
what's the cost of a couple of 10:1 32A capable transformers if it means you can get down to 6mm² ?
( please note these are figures plucked out of the air and are not intended to be advice to the OP.. )
6mm² medium or high voltage cable, remember - what does that cost?
And the cost of the trench is the same. (Or more - would it need to be deeper? Would you need a wire rope above the cable?)
And you'd need someone capable of doing MV/HV work. (Are there legal requirements for qualifications or licences?)
And (I believe) you'd need safety systems.
My gut feel is still that it would not be worth the cost & the hassle.
And the cost of the trench is the same. (Or more - would it need to be deeper? Would you need a wire rope above the cable?)
And you'd need someone capable of doing MV/HV work. (Are there legal requirements for qualifications or licences?)
And (I believe) you'd need safety systems.
My gut feel is still that it would not be worth the cost & the hassle.
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