Was just leafing through the Dorling Kindersley DIY bible, "DIY Know-How With Show-How", and came across this item [1] on taking power to an outbuilding. Am I losing the plot or is this terrible advice?
2.5mm radial on a 30A MCB?
And what is with "Do not connect the circuit cable's earth core to the consumer unit earth terminal. Instead run a 6mm single-core earth cable from this earth terminal to an earth electrode driven into the ground." Do they want us to convert the outbuild to a TT supply, with all the complications that entails, just for the hell of it?
Time to share your horror stories. What is the worst DIY advice you've ever seen published?
Double socket 20A max plus one lighting point 100w is hardly a problem for 27A rated 2.5mm cable on a 30A mcb. The circuit would also be safe whether earthed via the house or a local rod, if their advice to use an RCD is followed. 6mm is overkill.
Its fair to say that diy programs and books mostly give rubbish advice, but this one's quite harmless.
If you read the page posted though, its not limiting the load to one socket. It shows the 2.5mm submain feeding a outbuilidng CU thence multiple sockets as an option.
Might be acceptable for a limited install with say one socket and a light... then all the head end MCB would need to do is provide fault protection. However since they go on to suggest multiple sockets and/or a downstream CU its getting more dodgy.
(chances are in a shed that only ever feeds a lamp and a mower, its unlikely to end badly even if not really to a proper standard)
I don't know if the book goes into the length of the circuit either - but voltage drop will bite quite hard in 2.5mm^2 cable before long.
It would seem to be the case, although from the page you posted it is very unclear about which "earth terminal" they are talking about at any given time - I could see someone adding an earth rod to their main earth terminal in the house, and leaving the outbuilding unearthed!
The advice to use 6mm^2 single seems odd, since that is not a typical size used in any TT installation.
Perhaps they are trying to avoid the complexities of exporting a TN-C-S earth and its associated equipotential zone into locations where this may not actually be possible.
Based on some of the other posts here: maybe a lamp, an electric still running for 2 or 3 hours, & a mower & various other appliances to kill time while waiting for the still to finish.
Its not a time limited load of 30A - the downstream CU may have many MCBs. Even one 30A MCB could be a continuous load of In x 1.45.
So the upstream MCB will be responsible for overload protection as well as fault protection. Hence in the configuration they describe it would be potentially allowing >40A continuous, on the 2.5mm^2 submain.
Most sheds that have electricity have no mcbs. The ones with their own CU usually have 2 mcbs, as I believe they were proposing. What would one want multiple mcbs for in a shed?
I've never heard of anyone running a continuous 44A in their shed! I can't imagine what you're proposing that would use that much for a long period.
The situation's similar to a double socket run on a single 2.5mm cable off a 30A mcb. In principle its possible to abuse it if determined enough, in reality its not a significant issue. I'm curious about this long running 10.5kW load.
Firstly before you go off on one, read the comment I posted in my
*original* reply to the OP:
"(chances are in a shed that only ever feeds a lamp and a mower, its unlikely to end badly even if not really to a proper standard) "
i.e. I am agreeing with you; if you limit the scope of the exercise to a shed etc then although the advice given is theoretically flawed, it it very unlikely to cause a problem in real life.
Could I ask that you actually go back and read the scanned page in question? You will note that it describes taking power to an "outbuilding". It does not limit the scope to "sheds".
If in the definition of outbuildings we are to include workshops, or pool houses (electric shower anyone?), granny annex (electric cooking and heating) or any number of other applications one could easily conceive of, then the proposed solution is (as it is presented) not adequate. (it may be there is more information on the preceding pages etc that mitigate this - but I can't see them!)
Lekky shower in a granny flat...
500W lighting, 2kW dust extraction, 3kW heating 5kW lathe in a workshop...
12kW pottery kiln in a pottery shed...
Fast charger for your electric car in the garage...
I am sure you can think of more.
(none of the above are likely to be "plugged in" to a socket)
That is only the start of the problems; the design as proposed has an RCD at the head end and hence no protection of downstream circuits from the effects of nuisance trips - so a CH boiler or freezer in the garage could be screwed, or a man in workshop could be plunged into darkness etc.
Then there is voltage drop - not addressed. If the building is a shed some 40m away and he is running a powerful electric mower, then it might just shag the mower since modern ones tend to use induction motors, and they don't take kindly to sustained undervolts.
Then there is the very confusing advice on earthing practice. Following the guidance presented could easily end up with an unsafe installation.
So in summary, as basic advice - yes you will get away with it for a good many installations. As general "power to outbuildings" explanation, it falls well short IMHO.
indeed, and that's the point I was making. Best advice of course it isnt, b ut the OP was touting it as terrible advice, and its clearly far from that.
True, though I would seriously expect someone wiring up a substantial outbu ilding would realise there's a bit more to it than 'run a lead.' Even if th ey didnt, 27A rated cable can handle even the theoretical 44A you proposed, even if definitely not recommended. Poor advice of course, but hardly a di saster.
Quite. Anyone running such things will need to find out how to fit them, an d will discover elsewhere what they need.
yes, but not a significant safety issue
not a significant safety issue
How would 6mm to a local earth rod and an RCDed feed create a danger?
Its not just about personal safety though is it? If you come back to a wrecked house after your winter holls, because your shed roof leaked, a circuit tripped, and the heating went off, and pipes froze & burst etc, that would still be classified by many as a "disaster".
No, but a shagged mower is not desirable either. These are deeper issues than "will it kill you" at play here.
Read the description:
"Do not connect the circuit cable's earth core to the consumer unit earth terminal. Instead, run a 6-mm2 single-core earth cable from this earth terminal to an earth electrode driven into the ground beneath the building."
It does not make clear which CU its talking about - the one at the origin or in the outbuilding. Or which circuit cable's earth should be connected to the spike. Only that the downstream CU should not be connected to the circuit's earth conductor (again which one?). Now you or I could work out from context what it means, but if following the advice blindly without a priori understanding of the concepts, one could end up doing something daft.
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