Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning a greenhouse and outdoor workshop for a home, and the electrical supply to them, is answered by BASEC:
The greenhouse needs a supply suitable for electric heating, light/heat light, and irrigation system controllers etc. The shed will be used as a workshop, so may have lathe/mill, arc welder 60A (I think) and other power tools.
There will be water supplied to both the shed and the greenhouse via plastic MDPE pipe with metal tap fittings. There is no gas or other services to either shed or greenhouse.
I expect to have an electrician to connect up the services etc..., but want to lay the cable in advance. I plan to use SWA cable and don't mind buying three-core to have the option for a separate earth conductor. I plan to have a T-junction in the cable at the greenhouse such that one arm of the T goes to the greenhouse and the other to the shed. The cable will be buried directly in the ground at 450mm depth.
Two questions:
1) What size / type / insulator type do I need for the cable (house is rated at 100A incoming supplier fuse), noting that I want 100A available in the shed)?
2) Can I use an underground junction box for the T location, and what type of box / connection do I need?
I'm prepared to over specify to be sure it is suitable, as the cable trench will be covered over some months before the electrician comes to connect up.
Answer: The system design needs the services of a qualified electrician now, as this is outdoors in a domestic premises, so the requirements of Building Regulations Part P apply. You will need to engage a Part P registered electrician or consult local Building Control.
Underground junction boxes for cables are not really recommended. Given the loadings you describe at the shed, the cable should probably go in one run straight to the shed (which is the major use area), and then a separate cable could run from there to the greenhouse (which has lower loadings).
It is also likely, given the distances, that a proper subsidiary consumer unit will be required in the shed, with its own circuit protection, RCD and earthing arrangements. This would enable the greenhouse to have its own circuit breaker.
Cable sizing should be undertaken by the electrician. Suitable types of cable would be BS 5467 (SWA), or BS 6724 (SWA) if a low smoke cable is needed. As you have high peak current motor start-up to consider, the circuit breaker calculations and the earthing arrangements also need to be carefully assessed.
I am sure that the electrician will not mind if you bury the cable for him, and the actual installation be delayed for some months, but it would be wise to engage the electrician as early as possible to get the design right. It is important that the ends of SWA cable are sealed if left un-terminated for some period, to avoid water penetration.
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