Power to outbuildings (again)

[jasonh]

Hi all,

I want to run power to some outbuildings. Unlike previous posts I can find, I want to provide power to several different locations via the same spur which complicates the calculations a little. I'll post details when I'm done with the maths in the hope that some kind person will check what I'm planning, but before I do I hoped to answer a couple of quick questions.

Reading various previous posts, there seems to be some uncertainty as to whether to use three core SWA (with earth on the third core and the armour), or whether to use two core and have just the armour as a earth. For the lengths of run involved the price difference is significant, but if there is any real benefit in the extra core I'll pay the money. Looks like I'll be getting the cable is from TLC in case the technical spec of the cable is relevant.

On a related note, just a curiosity really, but can anyone explain why the impendence and maximum current of three core cable is slightly different to two core of the same size?

And why the difference between PVC and XLPE - something to do with how the cable material affects the cable temperature when in use?

Before anyone mentions it - "Part P" does not apply. I live on the Isle of Man, and the nice man from Building Control has just rung to confirm that it has not yet been introduced (so I'd better get my skates on before it is!).

Many thanks,

Jason[/url]

 

[Adam_151]

Reading various previous posts, there seems to be some uncertainty as to whether to use three core SWA (with earth on the third core and the armour), or whether to use two core and have just the armour as a earth. For the lengths of run involved the price difference is significant, but if there is any real benefit in the extra core I'll pay the money. Looks like I'll be getting the cable is from TLC in case the technical spec of the cable is relevant.

Generally 2 core is sufficent, a third core is usful though if you want to run a submain and want to keep the Z's down on the sub board, obviously whether this is relevant depends on your Ze and the design of your sub circuits, etc. Or you are worried about the integrity of the armour being compromised, I believe its a reg to not rely on the armour in boat yards due to the spray and salt etc being corrosive, you may also want to avoid relying on the armour if there is a chance of the cable being pulled out the glands by catching on vehicles, etc, or if you just want some added reassurance that even if the gland works loose you still have an earth.

On a related note, just a curiosity really, but can anyone explain why the impendence and maximum current of three core cable is slightly different to two core of the same size?

Its based on the fact that all cores will be carrying the load, so and there is some correction for having 3 grouped current carrying conductors instead of 2, with regards to the volt drop, I'd guess its something to do with it being expected to carry 3 phase and the reactive component is a bit different

And why the difference between PVC and XLPE - something to do with how the cable material affects the cable temperature when in use?

PVC is rated upto 70 Degreess, XLPE upto 90 Degrees, however you must also consider the temperature rating of the switch gear terminals its terminated in

 

[jasonh]

Generally 2 core is sufficent, a third core is usful though if you want to run a submain and want to keep the Z's down on the sub board, obviously whether this is relevant depends on your Ze and the design of your sub circuits, etc. Or you are worried about the integrity of the armour being compromised, I believe its a reg to not rely on the armour in boat yards due to the spray and salt etc being corrosive, you may also want to avoid relying on the armour if there is a chance of the cable being pulled out the glands by catching on vehicles, etc, or if you just want some added reassurance that even if the gland works loose you still have an earth.

At which point I'm afraid I get a little lost in terminology. I suspect the best bet is to post my full plans when I'm done and take it from there. I don't think there is much chance of any damage to the cable once in place, but safety first and all that.

Its based on the fact that all cores will be carrying the load, so and there is some correction for having 3 grouped current carrying conductors instead of 2, with regards to the volt drop, I'd guess its something to do with it being expected to carry 3 phase and the reactive component is a bit different

So for the purposes of my voltage drop calculations, given that the third wire would not normally be carrying a load, I should use the figures for 2 core?

thanks,

 

[ban-all-sheds]

Generally 2 core is sufficent, a third core is usful though if you want to run a submain and want to keep the Z's down on the sub board, obviously whether this is relevant depends on your Ze and the design of your sub circuits, etc.

Also there are some sizes of SWA for which the armour csa is too low to be used as a cpc.

At which point I'm afraid I get a little lost in terminology. I suspect the best bet is to post my full plans when I'm done and take it from there. I don't think there is much chance of any damage to the cable once in place, but safety first and all that.

At which point your best bet is to put your plans on hold and start learning, as you need to know about all that stuff...

So for the purposes of my voltage drop calculations, given that the third wire would not normally be carrying a load, I should use the figures for 2 core?

Yes.

 

[jasonh]

At which point your best bet is to put your plans on hold and start learning, as you need to know about all that stuff...

That's what I'm trying to do. Perhaps you could elaborate, or point me at some useful definitions for the terminology you chaps like to use. You don't get far on google looking for "Z's"

I'm guessing Z's are impendence? Ze the impedance to earth? A spur becomes a submain if you have a CU (I get that one ) at the other end? Is a subboard another name for a CU?

Here's another question for you. If I protect the cable from overload by connecting it to an MCB in the main house CU, and the RCD in my shed trips because of a fault, would the main house RCD not trip as well? I've been pondering this one as wouldn't those 'plug in' RCD's also do the same?

cheers,[/url]

 

[Adam_151]

Also there are some sizes of SWA for which the armour csa is too low to be used as a cpc.

Not sure that there are that many times when its truely inadequate if the adiabatic is applied, and even if it is I'd prefer to go up a size rather than suppliment it with a core, what happens if someone sticks a spike into it and shorts phase to the armour, the extra earth core is not going to do much then [I suppose if the armour is connected to core at far end it'll do something, but it'd be tricky to calculate this things depend on how far along the fault occurs etc]

If you think about the amount of energy required to heat up a 17mm² steel conductor and a 2.5mm² copper one, its not usually the armour thats going to overheat in a fault...

 

[ban-all-sheds]

That's what I'm trying to do. Perhaps you could elaborate, or point me at some useful definitions for the terminology you chaps like to use. You don't get far on google looking for "Z's"

IMPORTANT NOTE - when buying books from Amazon, seek confirmation that they are the latest versions.

http://www.amazon.co.uk/exec/obidos/ASIN/0863413749

or http://www.iee.org/Publish/Books/WireAssoc/index.cfm?book=WR 261

http://www.iee.org/Publish/Books/WireAssoc/index.cfm?book=WR 500

http://www.amazon.co.uk/exec/obidos/tg/stores/detail/-/books/0953788547

or http://www.tlc-direct.co.uk/Main_Index/Electricians_Guide_Book/index.html <- you can start reading this online straight away

http://www.technicaltrainingsolutions.co.uk/publications.shtml

http://www.technicaltrainingsolutions.co.uk/publications.shtml


The Electricians Guide shown above is a good explanation of the regs, but if you fancy one with a bit more of the electrical engineering theory behind it all, I can recommend this:

http://www.iee.org/Publish/Books/WireAssoc/index.cfm?book=NS 031

Here's another question for you. If I protect the cable from overload by connecting it to an MCB in the main house CU, and the RCD in my shed trips because of a fault, would the main house RCD not trip as well?

Yes, it might trip as well, or instead. Which is why it's a good idea to supply the outbuilding from the non-RCD side.