Hi, Can I run this past some people to find out if I'm doing this correctly... I'm want to run power to an outbuilding that I'm going to use as a workshop. (ok it's a shed, but outbuilding sounds grander!) My plan is to run cable from a spare MCB in my house's consumer unit to the shed (under a patio, so it'll be well protected) On entering the shed it'll go into an auxilery consumber unit with build in RCD. This consumner unit will have 3 MCBs to supply the lighting, power points and a water feature in the garden I've read around on the net and I like the idea of a consumer unit in the outbuilding, but I was wondering if this was an ok way of doing it. If so, what rating should the MCB be at my house's consumner unit end? and also what do the ratings of RCD's actually mean? I've seen 63A 30ma and 80A 30ma etc and I'm a tad confused... :-S
Thanks in advance for any help you can give, Cheers, Rob
How far is the cable run? What are you likley to use in the workshop in the sockets? is the spare MCB in the house protected with an RCD of just an isolator?
The A rating is the current they can cope with, and the mA rating is the fault current they will trip at.
Recently discussed here Rob: do a Google search on uk.d-i-y to see others in a similar predicament.
63A 30mA RCD (or so) means it can supply 63Amps of current but if the difference between what flows out of the RCD and what is returned into the RCD (difference between Live and Neutral) exceeds 30 thousandths of one amp then the device will trip - the RCD presumes that this difference is flowing down your left leg to ground (!)
I've always thought that an analogy of electricity with a hot water radiator might help folks understand some of the simpler principles. Not that it goes
*too* far, but I'm expecting to draw comments from others with this. So the "live" is the hot water flowing in to the wall radiator (burny burny, don't touch). And the "neutral" is the tepid water flowing out of the radiator back to the boiler. If 63 gallons of water flow in to the radiator but only 57.96999 flow out then there is a discrepancy of 0.031 gallons (or 31 milligallons) hence the safety system shuts the boiler down. How am I doing with the analogy folks?
The cable will run about 10m or so from the house's consumer unit to the shed. I've already wired the shed in a radial circuit with 6mm cable (a little big maybe, but I had it to hand). That circuit is only about 4m.
The sockets in the shed will have to cope with a (small) table saw, bench drill, uv exposure box and a tv. I'm working on some video electronics stuff (12volts I'm ok with... but 240 _scares_ me!). Not all of the above will be on at the same time tho
As for the MCB at the house end, the whole story is that I'm going to change over the consumer unit in the house (before I connect up the shed) to one with RCD protection because at the moment all my sockets upstairs and down run off a single fuseway, as do the lights (I'll change it to a split load one with the lighing in the house running off
2 unprotected MCB's and the remaining of the ciruits (sockets down and up, shower and shed) off the protected ones.
Having an auxiliary CU in the shed is entirely reasonable. The main points to consider are:
- Power requirements at shed
- Cable type and sizing from house
- Earthing arrangements
You need to determine likely current requirements for all equipment that will be used in or from the shed first and to base the supply rating on that plus a margin.
For an installation with underground cable, the convention is to use
3way + shield SWA armoured cable. This needs to be sized sufficiently large to meet current carrying requirements as well as the maximum allowed volt drop for the length to be used - so you need to know length as well. The reason for this is in part to make sure of an adequate supply voltage under load for the equipment, but also so that in the event of a short circuit at the shed CU or in the cable, that the MCB at the house end will trip, cutting off the supply and protecting the cable. Note that there is no problem in oversizing.
For earthing, there is a choice of exporting the house earth as long as the distance is short or separating the earth (using an insulating fitting at the shed CU) and installing a ground rod at the shed.
If you export the house earth, you can fit either an MCB (either side of an RCD in the house CU if there is one) or an RCBO (combined RCD and MCB). Note that RCDs, on their own do not trip on excessive current. Their rating in Amps is only their carrying capacity. If you use an RCD or RCBO at the house, then any current imbalance fault in the shed will cause all power to the shed to be cut. You might not want that - e.g. lights going out or freezer going off. In that case an MCB only should be used in the house CU and a 30mA RCD in the shed CU with lights upstream of it etc.
If you go for the local earth rod option, then you are creating an equivalent to TT earthing of the main supply - i.e. what happens with overhead supplied power to a house. In this case, an MCB is fitted at the house CU and a 100mA time delay RCD is used on the supply input at the shed CU. You then install MCBs and 30mA RCD as before.
It is worth reading through the relevent sections at
First step is to ascertain the load required. If you are not using the saw and drill simultaneously, and don't have any heating, 20A is probably adequate for the shed overall. So, fit a 20A MCB to the non-RCD side of your new house CU and run an appropriate size of SWA (steel wired armour) cable, which is suitable for direct burial, to the shed.
You can check the appropriate size of cable required using the calculator on
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technical section.
At the shed, fit a CU with MCBs for lighting, power and pond. If you have a whole-shed RCD I would strongly recommend that you put an emergency light unit in, so that if the RCD trips you are not left in sudden darkness with power tools spinning.
Earthing: if your house is TT (earth rod) or TN-whatsit (sheath of supply cable) earthing you can probably export the earth to the shed. If the house is PME (combined neutral and earth) you must not export the earth. In this case, the SWA cable sheath is earthed at the house end but insulated at the shed end. The shed will require a local earth rod and will be a TT installation. Refer to the uk.d-i-y electrical FAQ on earthing etc.
If you are in England and Wales, Part P of the building regs covers replacing a CU and outdoor installations.
Given all of this, it's reasonable to export the house earth and use a
20A MCB in the house CU.
The only remaining thing to consider is whether you care if all power in the shed goes off as a result of a trip on the house RCD. The implication is that a fault in one could affect the other, so you need to think about where essential equipment is connected.
Just to point out, under Part P, you are not permitted to just do this on your own, unless you get building control involved (because it is a new circuit, and also because it is also outside) you are also not permitted to change the CU. "Am I Bovvered" I hear you all cry :-)
It would be best to run the shed off the non RCD side really, or a problem in the shed will probably trip both the shed RCD and the house one, this is not unsafe, but is bloody irritating!
As you are changing the CU, I would also recommend you run a radial to the kitchen fixed appliances (Freezer, fridge at least), the boiler and alarm if you have one, from the non RCD side too (So if you are away, and something trips the RCD you do not loose the freezer's content, or the alarm) again, Part P says you can't just do this one either, as it is in the Kitchen and also because it would be a new circuit.
So, back to the shed, sorry, workshop.
I assume the workshop is a wooden construction?
The 6mm cable is a little on the large side, 2.5mm would have been fine here, however, this is not a problem (except when you try to jam in a load of cables into one socket/spur etc., then you run out of room faster!)
As it is only 10M of cable, then you can use the house earthing (otherwise you would need to install a TT system in the workshop, providing local earth with an earth rod etc.)
It would probably be better to opt for a non RCD overall protected consumer unit in the workshop, and use RCBO's here (these are a combined RCD and MCB, but are quite expensive!)- the reason for this is mainly because you mention potentially dangerous power tools (the table saw). If the lighting and this saw were both needed (you were sawing something when it was dark outside) and the RCD tripped, you would be plunged into darkness with a table saw still spinning down. I have taken the emergency light route in my workshop (overall protection is an RCD, and the emergency light is wired up to the same circuit as the main lighting, so in the event of either the lighting MCB or the RCD tripping, the emergency light illuminates) If cost is a factor, then the emergency light route will be cheaper (It was when I did it, and the fact I don't use this type of tool that often, made be choose it!) It will also illuminate if the workshop MCB in the house trips.
It would probably be best to run SWA cable (Steel Wire Armour) from the house to the workshop, as it provides very good protection (and can be directly buried) If the run from the cable entry point in the house to the CU is a bit tricky, then just terminate the SWA in a box (In the house, or a box screwed to it) and run the rest in normal T&E (This is what I have done, as it would have been a major pain in the arse to run SWA through the house!) I found the three core SWA (really meant for three phase) was more easily available than the two core (The earth would have run through the armour on two core) so I used this, and sleeved the wires each end accordingly. I did use the third core as the earth, but also earthed the armour)
If you are going to dig up the patio, it may also be a good idea to lay some conduit under there so you can run additional wires (network, phone, doorbell etc.) at a later date (It is best to lay a draw string in there too, but if you forget, or it breaks, a vacuum cleaner will usually suck one through later!)
I would use 6mm SWA cable, but you could get away with 4mm (6mm will give you room to expand a bit later) I would use a 32A MCB in the house A 20A MCB or RCBO for the sockets (as it is a radial) A 6A MCB for the lighting A 6A MCB or RCBO for the water feature (assuming this is enough, and you don't have an imitation Niagara falls/Trafalgar square in the garden!
Sparks (Who is NOT a qualified electrician by the way!)
The A figures are max AC current capacities given in rms Amps. The mA figure is the rated sensitivity of the trip mechanism to respond to a naughty L-N imbalance. The lower, the faster the shut-off is in mSecs. The Test button on an RCD or an older ELCB actually simulates such a leakage by shunting a resistor in appropriate kOhms from L to E when it should immediately trip! (This fault-making test should be practised every 2 months)
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