new shed, new electrics!

[Steve]

right, here goes! im going to be running power to a shed in the next few months. In the existing shed, there is a scratty old piece of 2.5mm t+e, run under the paving slabs to the house 20A MCB. This supplies tumble dryer, occasional lawnmower, radio, small 11w light. No RCD.

Getting new shed, therefore new electrics.

shed is approx. 15m from house. we are getting a new split load CU under the stairs, so the house end will be up to date. I reckon we will need about 25m of cable to supply the shed. We need to supply fridge, freezer, and tumble dryer, as well as garden appliances. I know outdoor electrics need RCD protection, but with a fridge and freezer etc, how would we do this without nuisance tripping?

Here is my plan:
32A MCB at house CU (on non-RCD side)
4mm T+E to outside wall of house
then IP65 box to 4mm 3C SWA (all done in appropriate glands)
approx. 20m of SWA to shed
then terminate SWA in new consumer unit.

in new consumer unit: 63A incomer, 20A MCB to fridge and freezer (2.5mmt+e), 20A MCB to tumble dryer, 20A RCBO to socket for outdoor power, 5A MCB for shed lights (2 fluorescent tubes).

You guys think ive got this right? i mean, fridge and freezer only consume, say, 200w each, if that, and they hardly consume anything in winter! (allowing for starting current with 20A MCB) The tumble dryer (2500w) probably gets 1-2 hours use on average each day, not in summer though.

any comments appreciated. Cheers.

ps. i know all about part pee, lets not have any debates here. i know i am competent enough to do this work. building control can go narners, "i did it before 2005"

 

[ban-all-sheds]

a scratty old piece of 2.5mm t+e, run under the paving slabs to the house

I know outdoor electrics need RCD protection,

No - only socket circuits.

but with a fridge and freezer etc, how would we do this without nuisance tripping?

Don't have it on an RCD..

then IP65 box to 4mm 3C SWA (all done in appropriate glands)

TN-S?

Also, you haven't said how/where you will be running the cable - I assume properly buried? In which case, given that you only want to dig that trench once, I'd always advise 10mm², ready for the day you decide you want a hot tub/pottery kiln/particle accelerator out there...

in new consumer unit: 63A incomer, 20A MCB to fridge and freezer (2.5mmt+e), 20A MCB to tumble dryer, 20A RCBO to socket for outdoor power, 5A MCB for shed lights (2 fluorescent tubes).

Use a Type C MCB for the F/F - those compressors have one hell of a startup surge....

You guys think ive got this right? i mean, fridge and freezer only consume, say, 200w each, if that, and they hardly consume anything in winter!

Err... talking of winter, you have checked what the minimum ambient temperature is that your F/F will work in, haven't you...

 

[Steve]

BAS, we have a TN-C-S system.

The 2.5 T+E needs replacing, i know. I was thinking about running the armoured cable low along a wooden fence. Is this safe / acceptable? i doubt that the neighbours will be cutting holes through the fence.

The fridge and freezer are seperate units. The fridge wont be used in winter - its only for booze and meat at BBQs etc and the freezer cools to -22 anyway, so i would think it would work at ambient 0 degrees?

Type C is probably a good idea, since we wont notice if the 20A MCB has tripped from the house, and in winter, it might be left days before we notice!

Not sure if i like the idea of a fat 10mm SWA running along the fence. Though this would mean i could use a 45A MCB and have, say, a George Foreman grill or 2 instead of a charcoal BBQ lol. Dont think ill be needing a particle accelerator just yet. Might plump for 6mm SWA and fit a 40A MCB. According to http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html this is acceptable and good for 10.5kw at 25m. Dont know whether to trust this. 6mm seems rather thin for 40A?

 

[ban-all-sheds]

BAS, we have a TN-C-S system.

The 2.5 T+E needs replacing, i know. I was thinking about running the armoured cable low along a wooden fence. Is this safe / acceptable?

No - a wooden fence is not permanent enough.

The fridge and freezer are seperate units. The fridge wont be used in winter - its only for booze and meat at BBQs etc and the freezer cools to -22 anyway, so i would think it would work at ambient 0 degrees?

Probably not.

Not sure if i like the idea of a fat 10mm SWA running along the fence.

Nor do I.
Nor do the people who write the wiring regs.
Nor will LABC if you should ever get caught.
Same would apply to 6mm², 4mm², 16mm²...

Though this would mean i could use a 45A MCB and have, say, a George Foreman grill or 2 instead of a charcoal BBQ lol.

Go for gas....

Dont think ill be needing a particle accelerator just yet. Might plump for 6mm SWA and fit a 40A MCB. According to http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html this is acceptable and good for 10.5kw at 25m. Dont know whether to trust this. 6mm seems rather thin for 40A?

OK for SWA, but given the marginal cost difference, use 10mm²...

 

[keyplayer]

Crafty, you been getting advice off your dad again?

 

[Big_Spark]

Ban, all circuits outside the Equipotential Zone require RCD protection, whether sockets or not.

The Cable should have an RCD (Time Delayed 300 or 500mA) at the source, and at the opposite end you should install an apporpriately rated 30mA/100mA RCD (trip rating dependng on supply use) for the protection of circuits connected the the submain.

I agree with the comments about the fence, bowever if you intend to bury the cable crafty, it should be at least 18" (450mm) below the surface and covered with warning tape that a cable is below.

 

[Steve]

yes, i will be running it along the house wall to the corner of the conservatory, then down into a trench straight to the shed. Should i put anything over the wire before i bury it (eg. paving slabs or bricks) or can it just be left as it is? It will be under the lawn. I just dont like the idea of leaving it unprotected in the ground.

I have just costed on TLC for using 10mm 2c cable, its more than i wanted to spend, but probably worth it for future upgrades etc.

RCD arrangements will probably be as follows: Whole house protected by 100ma rcd on main incomer of new CU (probably going to scrap the split load idea in this case). 40A MCB, then 10mm cable to shed, then 20A 30ma RCBO just for outdoor power. Fridge and freezer, and tumble dryer on seperate 16A MCBs (fridge and freezer on a type C of course)

cheers for the comments!

 

[ban-all-sheds]

yes, i will be running it along the house wall to the corner of the conservatory, then down into a trench straight to the shed. Should i put anything over the wire before i bury it (eg. paving slabs or bricks) or can it just be left as it is? It will be under the lawn. I just dont like the idea of leaving it unprotected in the ground.

See here: http://www.tlc-direct.co.uk/Book/7.13.3.htm for info & diagram.

I have just costed on TLC for using 10mm 2c cable, its more than i wanted to spend, but probably worth it for future upgrades etc.

1) Don't use 2-core. Firstly it would mean that you would be relying on the quality of your terminations for the quality of your earth, and secondly the armour on 10mm² SWA is too small to be used as an earth.
2) There are cheaper places than TLC...

RCD arrangements will probably be as follows: Whole house protected by 100ma rcd on main incomer of new CU (probably going to scrap the split load idea in this case). 40A MCB, then 10mm cable to shed, then 20A 30ma RCBO just for outdoor power. Fridge and freezer, and tumble dryer on seperate 16A MCBs (fridge and freezer on a type C of course)

You don't need, or want your entire house to be RCD protected, and for the circuits where you should have RCD protection (shower, ground floor sockets etc) that should be 30mA, not 100mA. Stick with a split-load, or use RCBOs in the house.