Old workshop supply inadequate - suggestions welcome

[petey2]

Hello all.

I have an old workshop in my garden - it is older than the house. The previous owner of the house put an armoured cable to the workshop (6mm) about 50m away, but this is too small and the striplights go off and restart when I start my circular saw (takes about 12A on startup).

To compound matters there is another shed much nearer the house, and this is supplied with another length of 6mm armoured from the first workshop.

I haven't asked an electrician yet (which I do plan to do), but rather than digging up my wife's prize garden to add a new length of armoured all the way to the workshop, is it feasible (and allowed) to simply run a new supply to the small shed but keep the existing cables (putting new rcd's etc where necessary) so as to make the garden into one large ring main with two outbuildings? This would mean that the workshop would effectively be at the end of two lengths of 6mm armoured instead of one.

Alternatively, could I have the new supply added to the nearer shed, and then divide the workshop supply into two so that the saw is on one armoured run and the lights on a different one (with a new consumer unit in the workshop for the lighting etc)

The supply comes from a separate box straight from the meter in the house - both of these supplies would come from the same box and RCD, so would not be on different phases etc, just taking a different route.
We had the house rewired recently, and the workshop supply consumer unit in the house was replaced, but the electrician sucked in his breath when he saw the size of the cable to the workshop… He would only put a 20A trip in because of the size, and even then he wasn't happy.

Any thoughts anybody?

Thanks

Pete.

 

[Adam_151]

A ring feeder would be highly unconventional. I do know of sites of multiple buildings where a ring feeder is in use (designed by an ex supply authority chap I imagine) but its certainly not the right solution for your garden sheds. If you are going to the effort of digging up the garden you might as well sort it properly.

How far along is the middle shed in the run as the crow flies, I would suggest at a start an uprated supply to that first one, then possibly consider using the existing cable to feed the second one from that

 

[ericmark]

Using a fluorescent lamp with an inductive start and current control with rotating machinery is dangerous because of the possible stroboscopic effect. Swapping the lights to a fluorescent lamp with a high frequency ballast would much reduce the risk of stroboscopic effect and also will work with a higher range of supply voltages.

So step one is swap the inductive ballast for a HF ballast in the fluorescent lamps that will likely cure two problems.

As for cable size 6mm² at 50 meters will have a current limit of about 20A to keep within the 6.9 volt drop permitted for lights. With an incoming supply Ze of 0.35Ω I would expect Zs reading of around 0.69Ω. First step is to actually measure as I have in the past found a 2.5mm cable between the consumer unit and the 6mm² SWA cable.

An electrician should have the test equipment required so it would be a simple test to see if there is something causing the volt drop which you have missed.

 

[petey2]

Ericmark, I was planning to change the strips to led ones soon anyway, but that will wait a while. I have an old style light bulb in a batten holder over the saw because of the problem.
I had a look today, and the workshop is 50m from the house (approx) but there are at least 15m more of the armoured cable running up the house into the loft (the house is single storey at that point) and through to the supply point. There are no joins in the cable that I could see - the armoured goes all the way to the supply through the loft area and under a floor. I did have a good look for junction boxes.

Adam, the middle shed is relatively close to the house - probably only 15m at the nearest point, and no garden digging will be required - there are slabs laid on sand all the way, so it would be relatively simple to lift the slabs and dig a narrow trench down the middle.
When I realised that this shed was fed from the workshop I was very surprised, as it would be so much closer from the house. We have only lived here for just over a year, and have found so many surprises.

I will get an electrician to have another look to see what goes on and test things. The chap who rewired the house didn't want to even look at the workshop wiring as he was worried what he might find. He avoided touching that supply at all. We had to get another chap to put in the new supply cu for the workshop in the house because the first one didn't want to know, and he did the changeover live too.

Thank you both for your comments.

Pete.

 

[333rocky333]

How about a Bigger cable to the nearer workshop and reverse the 6mm so it supplies the main shed from the nearer shed

 

[plugwash]

I have to say I don't see any real problem with making it a ring, it will get the volt drop down and since the limiting factor here is volt drop not cable rating we don't have to worry about unbalanced loads causing overloading.

 

[Taylortwocities]

I agree. As already stated, it would not be a 'standard' circuit arrangement but would solve your problem.

 

[mfarrow]

Rings have been standard in the UK for many years, in fact yours is better than most "typical" ring mains in that each leg will be sized to the fuse protecting it! It will half your volt drop in an instant.

The ring needs to be fixed so make your no leg is independantly switchable. Other than that if you're sure the cables are both 6mm^2 (check markings) then get to it.

Electronic ballasts will be cheaper though! Make sure you plan out what goes where inside the fitting ans have some equipment wire and connector blocks to hand for any extensions.

 

[petey2]

Thank you again.

The florescent fittings are so old that I don't want to waste money on them. This is why I added a batten over the saw. They are hanging 6ft doubles, and are rusty… There are 6 doubles there, so would be a fair amount of work. I have just to find the LED equivalent at a good enough price. A friend has them in his workshop, and a single 5ft is the same light output as a 6ft double!

pete