I am in the process of having a large 'shed' installed to act as a small workshop and study which naturally needs power. The total load including electric heaters is around 24A but certainly shouldn't exceed 30A.
Now I intend to lay an underground cable in due course, but this will require a small digger (solid clay around here) and I have no way of gaining access to my property with one. At least not until the council fill in the 4ft deep 8ft wide trench behind my property where they are culverting a small brook.
A temporary solution would appear to be to suspend the cables. The distance is only 20m so shouldn't present any major problems, but are there any regulations I should be aware of before dangling cables between my house and the outbuilding?
Before anyone suggests bunging the digger operator working on the council job some cash they're already long gone...
This is going to cause problems with a span of 20 mtrs. I'd personally take the power supply under ground where ever possible over that sort of distance. You'll find that straining forces will be absolutely enormous to be able to hold anything up and away to a safe working height for a mains electrical supply, and you'll need at least posts between the gap to take up the sag.
The full Regs have an extensive section on temporary supplies for construction sites, according to Steward & Stubbs "Modern Wiring Practice" which says it's section 604. It's possible there'd be a minimum recommended height, but the "obvious" answer is "out of the way of likely traffic" and "adequately protected against mechanical damage". Said Section also apparently has plenty to say about disconnection times, use of ELV (extra-low-voltage) supplies, and the like. Using that section of the Regs as a model would be a pretty gold-plated demonstration of adherence to Good Practice, but might be non-sensibly expensive.
In my utterly unqualified opinion, it boils down to "how temporary is temporary", and on the actual risks of mechanical damage. At one extreme, let's say you'll be putting in a permanent SWA cable in a trench in 2-3 months time; and no-one but you and immediate family members use the space between you and the outbuilding; and that you'll only want power in the outbuilding a couple of times a week. In that case (and with a vague memory of you saying the max load for now is twenty-something amps, yes?), it would not be unreasonable to string a 2.5mmsq blue arctic cable (or similar) along a more or less out of the way route - along a fence - and have it disconnected most of the time.
At the other extreme, if you have public access between house and outbuilding, and/or employees/contractors working for you in the outbuilding, you shouldn't be contemplating any kind of shortcut: if you can't trench for now, running SWA along the ground covered in (expensive) road-traffic-duty cable protectors (like a small speed bump) is the very minimum you could try getting away with
- and even then I wouldn't want to think about the liability; the Electricity At Work laws kick in at this end of the scale.
Somewhere inbetween (basically your own private land, but with limited access for visitors and people doing jobs for you, to whom you owe a duty of care) you might string some 2.5mmsq or larger HiTuf up along some poles at least, oh, 8 ft overhead, or secured to any boundary wall/fence you might have for most of the distance. (HiTuf is notably lighter for rigging up high than SWA would be, and though you're supposed to use separately earthed metal catenary wire for a permanent suspended installation, with supports every couple of yards it'd be adequately self-supporting, and too stiff to sway in the wind, though aesthetically naff. If you do have walls or fences to secure to for most of the run, and negligible access to the run, you could use the SWA you're going to bury, temporarily fixed up. For a permanent install, HiTuf or SWA attached to walls is OK, but not to fences which wobble and maybe blow down...)
From memory, it would limit you to 110V appliances too, for all appliances less than (IIRC) 4kW (although I have heard a rumour that EU law has now overriden this restriction;-)
Cable heights are in the On-Site Guide, and depend if the area crossed is just pedestrian access, vehicle access, or public highway (again from memory -- don't have either regs or OSG on me at the moment). However, these are just guidelines, and may not be suitable in all cases.
"Jason Arthurs" wrote | Now I intend to lay an underground cable in due course, but this will | require a small digger (solid clay around here) and I have no way of | gaining access to my property with one. At least not until the council | fill in the 4ft deep 8ft wide trench behind my property where they are | culverting a small brook.
You don't have any military pals who could lend you a temporary bridge?
There is no specific instruction in "the regs" as far as I can see, but table 4B (p104) in the On Site Guide is very relevant...
For a PVC cable "supported by a catenary wire" there is "no limit" on the maximum length of span. Non-supported PVC gets just 3m.
At road crossings there must be 5.8m miminum height (obviously taking any sag into account), at other positions "accessible to vehicular traffic" 5.2m, in positions inaccessible to vehicular traffic (except in agricultural premises) 3.5m - just to keep it out of reach one assumes.
For a temporary installation I wouldn't worry about the effect of UV on PVC cables, but if it's to be for 5 years or more it may be worthwhile investigating a cable that isn't susceptible to UV deterioration.
I lost my copy of the overhead line regulations when we moved some time ago but this post reminded me of the "good old days" when hard drawn copper strung between D-irons and porcelain insulators was the norm for overhead services. Somewhere in a shed I believe I have an old linesmans tensioning vice too
I watched the tensioning of the replacement 250,000V lines over our industrial estate. The 'tensioning device' was a winch truck same size/shape as one of the airplane tugs but with a winch built in, and anchored with about half a dozen large skips full of rubble to stop it being dragged along the ground.
I think his user name may indicate that he was joking and had escaped the clutches of the evil empire, or was at least working towards that, nineteen years ago ...
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