Sunday, 4 September 2016

Overhead Power Cable to the Greenhouse.....


I recently decided to remove the solar panels from the greenhouse roof.  After several years of daily charging & discharging, the deep-cycle batteries were almost completely shot and to replace all three batteries would have cost around £150.

However, I still like the greenhouse to be illuminated by the LED growlights in the evenings, so I priced up a few bits and pieces and reckoned I could fix up an armoured mains power cable from the house to the greenhouse for only around £60 in total.

Running a mains cable will also let me use my two heated propagators in the late winter, to get a headstart on the vegetable seed sowing.

If I ran the cable underground from the house it would need to cross the concrete driveway. This whole hardstanding area to the front and side of the house could probably do with being completely replaced, but since there's more than 150 square metres in total it's a very big ticket project and therefore it's not a high priority.  Still, there seemed little point in routing an underground cable across there if it's likely to need digging up in the future.

So I decided instead to route the cable overhead, a distance of around 11 metres from the front corner of the house to the entry point at the greenhouse.

The first job was to establish a few basic design parameters and buy the necessary equipment; a 25 m length of 3-core 2.5 square millimetre cable with galvanised steel wire armouring (SWA), a pair of guide tubes, U-bolts and wire rope clamps.

cable with outer sheath stripped and armoured wires cut back


The guide tubes were cut from a 180 degree steel pipe bend, and mounted to lengths of angle iron, a short length at the house side which was bolted directly to the wall, and a longer length which sticks up above the greenhouse structure to create a high mast.   These guide tubes ensure the cable is bent smoothly and to a relatively generous radius, and they prevent kinking and damage to the cable armouring and conductors.

I could have simply welded the guide tubes to the angle iron brackets, but using the U-bolts allowed the tubes to be rotated during installation to ensure the exit & entry points at both ends of the suspended cable were accurately aligned.


guide tubes cut from 180 degree pipe section

The lengths of angle needed drilling to accept the guide pipe U-bolts and cable clamps, and also for the bolts to fix them to the walls.  After a trial fit, the steel brackets and guide tubes were painted with a few coats of black Hammerite.

Both brackets were then assembled, the walls drilled for fixings, and the brackets bolted in position.   On the house wall, I used 10 mm plastic plugs and long 8 mm diameter hexagon head coach screws, the sort of thing you can buy at DIY shops for mounting satellite dishes etc.  At the greenhouse, I used the same type of screws but fixed directly into the timber uprights at one corner.

The house wall was then drilled through both brick courses to pass the cable into the small bedroom at the front of the house.  This needed the 14 mm diameter 400 mm long masonry bit which I'd bought years ago.

The cable was routed through the house bracket, and the prepared end fed into the house.   After sufficient cable was passed through the guide tube, the cable was then fully clamped at the bracket.   

guide bracket & cable at corner of house

The other end of the cable was fed through the greenhouse mast guide tube, and pulled quite tight to establish a suitable catenary tension before initial clamping - too slack, and the cable would sag excessively, too tight and the mast might be overloaded.   There will also be expansion and contraction of the cable under seasonal temperature changes.

guide tube and first cable clamp at greenhouse mast


mast at greenhouse

I've previously carried out full catenary analyses on marine and offshore towing and riser installations, in connection with work projects, but it didn't seem worth bothering about for this simple job.  The cable weighs around 0.42 kg per metre length, and even factoring this for snow & ice loading gives an overall total suspended design weight of only around 10 kg or so, but this is not to underestimate the axial tension induced in the cable which is many times higher than the self-weight.

In the initial set-up condition, the cable was cut and prepared at the greenhouse, electrical connections were made at both ends and the system was tested.  However, the cable was left suspended in position to check the catenary and to allow the cable armouring to settle.  After a couple of days, it all seemed OK and so the mast cable clamps were fully tightened.

In the greenhouse, the cable was connected through the multi-pole isolator switch that I'd previously used for the solar panel installation.  In the house, the cable was simply plugged into an outlet socket in the bedroom using a plug fitted with a 3A fuse.

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