Another Shed Supply Question With A Twist

[Darrenm200]

I am considering running a permanent electrical feed to my shed which is aprox. 2M from house / CU.

This is the intented plan 6mm T&E connected to a 32A (b) mcb in the non RCD side of the CU, through the house wall in to a waterproof junction box where it will connect to 4mm twin core SWA cable this will then pass under the ground in 60mm conduit in to my shed. This will then connect in to a garage RCD CU to be spilt via a 6A (b) and 16A (b) mcb to feed a 1.5mm T&E lighting and a 2.5mm T&E radial socket circuit respectively.

The cable lengths will be approx 6mm - 1M, 4mm SWA - 4M, 2.5mm Radial circuit 5M

Part P aside can anyone see any problems with cable sizing, type, connections etc.

Now the twist, please dont jump down my throat until you've read it all!!!

I would like to make the join between the 6mm T&E and 4mm SWA in the Electricity board meter box, I am not talking about making any sort of connection in to the incomer or meter tails. I am purely talking about siting the waterproof junction box inside the EB enclosure. Is this in any way illegal or gonna upset the EB.


All comments / derogatory comments welcome and appreciated.

Cheers, Darren.

 

[breezer]

why not run it all in swa?

 

[Darrenm200]

Dont fancy making a conection in to the house CU in SWA as it's inside a cupboard it will just get messy, plus bending it to pass through the wall / in to the CU will cause problems.

 

[ban-all-sheds]

Why 6mm² then 4mm²?

Why not 3-core SWA?

And it might upset the supplier.

 

[Darrenm200]

6mm T&E because according to TLC 4mm is only rated to 32A which is a bit tight with a 32A mcb. 4mm SWA because it's rated to 39 A i.e in excess of the MCB. Twin core SWA because I dont need 3 core as i can use the wire armour as CPC. Unless you suggest otherwise?

(I get the feeling i'm teaching my grandmother to suck eggs as you probably know these figures of by heart)

 

[Darrenm200]

Ban,

Regarding upsetting the supplier, is that upsetting as in legal action. In my view the enclosure is part of my propertyas long as I dont interfere with EB equipment in any way. Previous enclosures I have seen have had plugs and secondary fuseboards in etc.

 

[ban-all-sheds]

6mm T&E because according to TLC 4mm is only rated to 32A which is a bit tight with a 32A mcb.

You should use the proper ratings for cables that take account of how they are installed, not the generic ones on websites that sell them.

4mm² could take 37A, or less than half that depending on how it's installed.

6mm² might take less than 32A, depending on how it's installed.

OK - in both cases the low value is unlikely, but the capacities for 4mm² and 6mm² for the worst-case Reference Methods are 26A & 32A respectively.

But what's your design current? Would a 20 or 25A breaker not be OK?

4mm SWA because it's rated to 39 A i.e in excess of the MCB.

PVC insulated SWA isn't, and nor is XLPE insulated if you're limited to 70° because of what it's connected to...

Twin core SWA because I dont need 3 core as i can use the wire armour as CPC. Unless you suggest otherwise?

I'd always suggest 3-core rather than relying on the armour.

 

[ban-all-sheds]

Regarding upsetting the supplier, is that upsetting as in legal action.

Doubt it.

In my view the enclosure is part of my property

Yeah, but what's their view? Who actually owns it?

 

[Darrenm200]

Okay point taken about the cable ratings I understand there are many factors influencing it's capacity, however they are there as a general guide.

Okay given the installation i.e very short length of 6mm (2M max)T&E passing through cavity wall with no cavity insulation, potentially having 2 or three bends (not severe) does this sound okay?

The 4mm SWA is XLPE again a relatively short lenth 5m with approx. 1m passing under ground in a 55mm conduit with 2 radius'd bends. Does this sound okay. I dont quite understand the temperature comment regarding the SWA.

To me given the installation the cable ratings are up to spec, if anyone feels different please say.

Again quoting catalogue figures 2 core SWA has slightly higher rating than 3 core.

 

[ban-all-sheds]

Okay point taken about the cable ratings I understand there are many factors influencing it's capacity, however they are there as a general guide.

Here's a better guide: http://www.kevinboone.com/cableselection_web.html

Okay given the installation i.e very short length of 6mm (2M max)T&E passing through cavity wall with no cavity insulation, potentially having 2 or three bends (not severe) does this sound okay?

Use heavy-duty round conduit where it goes through the wall, in case insulation is later installed.

The 4mm SWA is XLPE again a relatively short lenth 5m with approx. 1m passing under ground in a 55mm conduit with 2 radius'd bends. Does this sound okay.

Where's the other 4m?

I dont quite understand the temperature comment regarding the SWA.

When current passes through a conductor (unless it's a superconductor) it gets hot. The more current, the hotter it gets.

For PVC cables the limit on current is that which takes the conductor to 70°. XLPE can withstand 90° which is why it can carry more current, but if you are connecting it to PVC cable, or anything that can't withstand more than 70° then you can't use the higher current rating of XLPE cable.

FYI, mineral insulated bare copper sheathed cable not exposed to touch nor in contact with combustible materials can go to 105°, and in free air 4mm² is rated at 60A.

Again quoting catalogue figures 2 core SWA has slightly higher rating than 3 core.

TLC assume 3-phase for 3-core cables.

 

[plugwash]

FYI, mineral insulated bare copper sheathed cable not exposed to touch nor in contact with combustible materials can go to 105°.

and for short durations at least it can go much higher than that. almost to the melting point of copper.

 

[Darrenm200]

Thanks for the link had a quick glance will read it in more depth later.(After sleep)

The other four meters of SWA will be cleated to the outside wall of the house / inside the shed up to the 'garage' CU. Big U shape

I always use conduit through cavity walls when joining to the back of waterproof enclosure (creates a better seal especially when more than one cable enters through the same hole) but thanks for pointing out the insulation side.

Sooo your basically saying that the cable ratings are okay? or is the 4mm XLPE SWA still cutting it a bit fine in which case as i've already ordered it it might as you suggested back at the start be cheaper to de-rate the circuit by using a 25 A MCB at the house CU

The only reason i'd plummed for a 32 A was incase I wanted to up the 16A MCB in the shed to a 20A (use a ring circuit rather than a radial).

I understand that passing current through a cable heats it up (i'm a mechanical engineer by trade did quite a bit of electrical theory years ago, remember most the fundamentals) asked the question because I mis read the "what its connected to bit of what you said" - so I understand basic electrical theory I just cant read thats all.

Mineral insulated copper sounds cool you could crank up the wattage and cook your dinner on it at the same time. A multi tasking cable, what next.

FYI rightfully or wrongfully 07 copy of said catalogue quotes 39A for 4mm PVC SWA. - Doesn't flex (multi strand core) as per SWA have slightly higher capacity than solid copper conductors of the same CSA (something to do with surface area)?

 

[briwire]

You should have the RCD on the inside of the house, as the way you sugested would not protect the cable running to the shed.

For a few pence extra, use 3 core and the earth will be a dedicated core plus the armouring. This is a requirement in afcory supplies to machines so I would expect that somewhere it would apply yo domestic electrics.

 

[RF Lighting]

You should have the RCD on the inside of the house,

No you shouldn't

as the way you sugested would not protect the cable running to the shed.

Which is the correct way to do things.

For a few pence extra, use 3 core and the earth will be a dedicated core plus the armouring. This is a requirement in afcory supplies to machines so I would expect that somewhere it would apply yo domestic electrics.

Which regulation specifies that then?

 

[RF Lighting]

Hello Briwire?

I'm still anticipating your answer.