Advice on Electrical supply to Garden Please.

[mikescollan]

Having just laid a concrete pad to install a summer house, I would like to carry out the following work. Currently there is a power cable (conduited T&E) to the area I want to go to, but it is a spur from the attached garage, and I would prefer the external power to be on a seperate circuit. This circuit will be disconnected once the replacement is in place

1. Run a power feed from the house consumer unit to a secondary consumer unit in the summer house using armoured cable. Cable to be buried (Depth?)

1. Run (alongside the armoured cable but in a conduit) 2 data cables, a front door bell, and a telephone cable. I will include a length of thin Polyprop rope to allow for futher cables to be pulled later.

2. From the secondary consumer unit:- i. Individual Power for the pond pump and UV. To be security switched and needs some kind of visual indicator (like an led switch without the switch) ii. Individual Power run to each of 4 Garden lights (each to be individually switched from within the summer house. These will be spurs rather than a ring. iii. Individual Power run to each of 2 waterproof power outlets in the Garden. iv. A small 'ring-main' within the summer house (to be capable of running total comsumption of c10A). v. Lighting for the summer house itself.

I am intending protecting the circuits from the summer house with individual Residual Current breakers on the secondary Consumer unit. One breaker for the external lights, one for the power outlets, one for the pond stuff, one for the internal ring, and one fro the internal lighting.

With all the above in mind.....

Should I protect the Power run from the main consumer unit to the secondary consumer unit with an Residual Current breaker at the Main Consumer end?

Should the whole of the secondary consumer unit be protected at the inlet point with a resudual current breaker (I would prefer to avoid the pond stuff going off due to a fault on one of the other circuits)

Is conduited T&E sufficient for the external power runs to the pond, garden lights and power outlets.

Is the earth on the armoured cable sufficient or do I require an earth rod in the garden?

I intend to do alll the trenching and laying in of cabling and get a local sparks in to do the final hook-up. Can anyone think of anything I have ommitted? (I'm sure this is a common enough project that someone will have done something similar.

Sorry for all the detail, 1st posting here and would prefer to get as much info down as possible.

[Andy Wade]

Present advice in the IEE EGBR is 500 mm (or 600 mm if under a vegetable plot). Marker tape should be placed while back-filling, so as to be at approx 150 mm below the finished ground level.

Use some sort of seamless duct (intended for underground use), rather than electrical conduit, so it stays dry. I used green cable TV duct, scrounged from a helpful man working in the road, for mine. Blue MDPE water pipe (suitably marked) is another option.

A grid switch system (e.g. MK Grid Plus) is your friend here. Make yourself a control panel with switches (I'd suggest DP for all the outdoor stuff), secret key switches, neon indicators and fuses as required. Make sure everything is clearly labelled.

If you only want a few sockets, a 16 A or 20 A radial circuit in 2.5 mm^2 will suffice. No need for a ring.

RCBOs, you mean (combined MCB and RCD). An expensive option: are you sure that's not a little OTT?

Another option would be a split-load unit with a 100 mA RCD for all the fixed equipment and a 30 mA one for the two socket circuits. You'd then have (say) 5 MCBs: summerhouse sockets, outdoor sockets, pond (sub-fused if necessary at the control panel with BS 1362 fuses), summerhouse lights, outdoor lights.

Not unless that's necessary to protect the submain cable (unlikely in most cases). If you did use an RCD here is would have to be a time-delayed type.

Not if you use individual RCBOs, or multiple RCDs.

Yes, if it will stay dry and the T&E won't be exposed to direct sunlight. Generally though, it's a lot less bother to use direct buried

1.5 or 2.5 SWA for this sort of thing - and it's quite cheap.

More information required: cable size; length of run; circuit rating (32 A might be about right?); type of house earthing; any accessible Class 1 (metal cased, earthed) equipment to be installed outdoors; any electrical supply to a metal-framed greenhouse? Also "earth on the armoured cable" subdivides into two options - 2-core with armour as earth, and 3-core with one core plus armour as earth.

You'll find plenty of previous threads on this in the Google archive.

[mikescollan]

Thanks, should all be at 0.5m then.

I am getting some undergound condiut (courtesy of my father who works for a major electrical company - they dig a lot of roads up). It is seamless and I will be able to do the entire run in one piece.

Had looked at these. Nice to know I was on the right tracks.

Thats nicer. Saves a lot of cable

Thats what I had in mind. Would rather be OTT than underprotected. May now revise if it is way OTT though.

Not sure I understand the difference here.

Thanks, thought it was a bit overkill. The armoured cable is *tough*

Thats good. Would rather the pump didn't cut out if anyhing else did.

Doh! Didn't think of using smaller gauge. Direct burial would be a lot easier.

You are correct in that the Circuit is to be fused at 32A. Type of house earthing. No idea (how do I tell?) Multiple equipment types, but will include a mini server farm so will need to be earthed. No supply to green house. Summer House is wooden construction. Mains supply to summer House will be 3 core one core + armour as earth.

Google is my best friend at the moment.

Thanks for your time, Mike

[Andy Wade]

That sounds fine.

Some might say it's way OTT, I only said it might be a little OTT. If _you_ don't think it's OTT then /ipso facto/ it isn't, as you obviously think the cost's acceptable.

Your proposal is individual RCBOs, i.e. you'd have a main switch feeding the busbars, from which each final circuit would be taken off via an individual single device (one or two modules wide, depending on manufacturer) providing both residual current and overcurrent protection for that circuit only.

My suggested alternative option would involve a single main switch feeding two two-module-wide RCDs (providing only residual current protection); each RCD than feeds its own busbar and neutral bar, from which final circuits are taken off via individual MCBs, which provide the overcurrent protection. This way the (relatively expensive) RCDs are shared between a number of circuits.

I suggest you get hold of copies of the IEE On-Site Guide (ISBN

0863413749) and Electrician's Guide to the Building Regs (ISBN 086341463X) - both available from Amazon - as you'll find them useful references for this sort of thing.

Yes, but you still need to be sure that the fuse/MCB at the house end would blow if someone dug through it.

Just guessing. We still need to know the run length, and details at the house end (fuse or MCB, and what type) to select an appropriate cable size. For SWA submain cables this is seldom determined by current rating considerations, voltage drop and earth fault loop impedance are usually the determining factors.

See the FAQ. Basically there are three types:

TN-S - where the main earthing conductor from the CU is connected to the metal (usually lead) sheath of the supply cable with an earthing clamp. Normally found on older supplies (pre-mid-1970s) in urban areas.

TN-C-S (aka PME) - where the main earthing conductor is connected to a special sealed terminal in the supplier's fused cut-out unit. (Here the supply cable is 2-core and your earth is taken from the supply neutral, which is earthed at many points in the street.) Normally found on newer premises, or where the service cable has been replaced. Frequently there will be a PME warning label.

TT (obsolescent) - where the supplier provides no earth connection and you rely on an local earth rod (etc.) in conjunction with RCD(s) or an older ELCB ('earth leakage trip'). Normally found in rural areas with overhead supplies, although the overhead networks are being upgraded for PME, so recent overhead connections are likely to be TN-C-S.

Don't confuse the main bonding conductors, which go to the water and gas pipes, with the main earthing conductor. If in doubt post a picture of the supply position on some web space somewhere and the panel will offer opinions.

... and the length is?

[mikescollan]

15-20Metres (depending on precise route the cable takes) measured from the main consumer unit to the secondary consumer unit. Each of the power outlets will be about 10m from the secondary, the lights will be between 15mteres (furthest), and 5 metres (nearest). Pond is about 5 metres from the secondary. Will get back on the earth type.