"Wiring into an external ..." revisited

[BeeLZeebub]

Thoroughly enjoyed last week's epic BAS / Paul_C dong dong on the "outdoor lighting" regs. (Got through some serious amounts of popcorn). Anyway, I have an additional question on outdoor cabling.......

Just moved into a house that appears to have a radial circuit downstairs (well, it has 20mcb protection). Once I've eventually got a (spark to fit a) new CU in with RCDs, I was wondering ....

Assuming it is a radial, then putting aside notification, is it possible to spur off it (via FSU and SWA) to power a shed 150 feet away? Or does this contravene the square footage allowable for such a circuit?

My current plan is to get said spark to turn it into a ring and put the shed feed on a separate RCD protected circuit ..... but I just wanted to know if this is the only option. Advice appreciated .....

 

[Taylortwocities]

Sure it can be extended as a spur from either a radial or a ring final but (don't forget) that the FCU will limit the total current to 13amp so don't think about running a car lift or MIG Welder in the outbuilding.

50metres length may require a decent sized SWA cable depending on your expected demand; Your electrician will be able to design and advise as necessary.

 

[BeeLZeebub]

Thanks TTC

The load will be minimal - couple o lights and a lawn mower tops. But I raised the question because every time I've seen radials discussed, its pointed out that you have to bear in mind the square footage of the circuit's total coverage. However, this cable would be in a straight line down to the shed, so I wondered how limits would be calculated in this instance .....

 

[Taylortwocities]

The maximum square footage comes from a GUIDANCE to assist in assessing socket layouts in houses. There's a figure for radials and another for ring finals.

As with all electrical installations, the designer would take a view on the total loading and its impact on the circuit, and the rest of the installation.

Your designer will tell you not to worry about it, given the load your are talking about the effect is as much as plugging in a 13A extension lead.

The electrician who is installing the cabling will be the one who has to sign off the design, installation and testing declaration, so he/she is the one who will make the judgement call.

 

[BeeLZeebub]

Thanks - much appreciated

 

[JohnW2]

Just moved into a house that appears to have a radial circuit downstairs (well, it has 20mcb protection). Once I've eventually got a (spark to fit a) new CU in with RCDs, I was wondering .... Assuming it is a radial, then putting aside notification, is it possible to spur off it (via FSU and SWA) to power a shed 150 feet away? Or does this contravene the square footage allowable for such a circuit?

I agree with the advice you've already been given. However, I just wondered what this '(assumed) 20A radial circuit downstairs' actually serves. Is it serving all of your downstairs sockets (other than kitchen)? If so, 20A is not very much (one could theoretically plug 20A worth of load into one double socket), so might not leave much for your shed.

My current plan is to get said spark to turn it into a ring and put the shed feed on a separate RCD protected circuit ..... but I just wanted to know if this is the only option. Advice appreciated .....

IMO, to increase the capacity of the present circuit and to give the shed it's own dedicated supply from the CU would certainly be the preferable route, but you need to discuss the options with your electrician. As has been said, regardless of where the supply comes from, you might need fairly meaty (aka costly) SWA for a 150 feet run in order to achieve compliance with the regs (particularly as regards voltage drop for the shed lighting - although I do confess that, whatever the regs may say, I do wonder whether one really 'needs' to get too concerned about voltage drop for lighting in a shed!).

Kind Regards, John.

 

[BeeLZeebub]

Hi John - I couldnt say for certain what the circuit covers to be honest - not been in more than a week. (I just noticed it when knocking off the power on day 1 in order to remove a plugged flex from my daughter's room - it went through the wall, above the skirting board (!!), and connected outside to a 300w PIR lamp, armed with a 13-amp fuse ) However, you're right - it leaves bugger all to power the shed and that's why there's no question I wont get the CU sorted, but I just wondered 'what if ? ...'.

As for the SWA, irrespective of voltage drop etc I'd certainly prefer to go belt and braces ... I may eventually put in that car lift TTC was talking about !! Course, I think I'd need a bigger shed .... cheers

 

[JohnW2]

Hi John - I couldnt say for certain what the circuit covers to be honest - not been in more than a week. (I just noticed it when knocking off the power on day 1 in order to remove a plugged flex from my daughter's room - it went through the wall, above the skirting board (!!), and connected outside to a 300w PIR lamp, armed with a 13-amp fuse ) However, you're right - it leaves b*****r all to power the shed and that's why there's no question I wont get the CU sorted, but I just wondered 'what if ? ...'.

Fair enough.

As for the SWA, irrespective of voltage drop etc I'd certainly prefer to go belt and braces ... I may eventually put in that car lift TTC was talking about !! Course, I think I'd need a bigger shed .... cheers

Again, that's fair enough, and probably sensible, too. If you wanted to achieve an acceptable voltage drop (for lighting) at a total load of, say, 20A, and with a run of ~50m, you'd probably be looking at 10mm² SWA, which would set you back best part of £200. If you wanted capability of more current than that, and moved up to 16mm² SWA, it would probably be about £250 - not all that much more, so maybe well worth it for 'future-proofing!

Kind Regards, John.

 

[BeeLZeebub]

Cool - thanks John

 

[Paul_C]

The maximum square footage comes from a GUIDANCE to assist in assessing socket layouts in houses. There's a figure for radials and another for ring finals.

And while the suggested maximum for domestic rings has remained pretty constant ever since they were introduced (changing only from 1000 sq. ft. to 100 sq. m with the metric conversion in 1970), the suggested areas and numbers of 13A sockets for radials has changed quite regularly over the years.

At one time it was a two sockets on a 20A circuit, or three if not in a kitchen and serving an area no greater than 200 sq. ft. The latter figure was then increased to six sockets for no more than 300 sq. ft. and not a kitchen, changed to 30 sq. m after metrication. In more recent times, it became any number of sockets serving not more than 50 sq. m.

There were similar fairly frequent changes to the suggested limits for 30A radials as well.