Garden office/Shed Electrics

[ashtraywasp]

I am building an outbuilding with a garden office and a shed, and I need to know how to wire it.

The consumer unit in the house seems to have RCDs feeding MCBs for each subcircuit. I'm planning to have four subcircuits in the building, one for the outlets in the garden office, one washing machine and dryer, one for the lights on both sides, and one for the radiator in the garden office.

From research it seems that the best approach is an MCB in the main consumer unit and an RCBO per subcircuit with an SPD for the sub consumer unit in the outbuilding, using SWA cable run from the house to the subunit.

I don't know if that is overkill and I can use an MCB at the house unit and an RCBO feeding MCBs per subcircuit, although an SPD sounds good to protect my PC and other expensive equipment.

Any advice?

 

[ericmark]

Clearly new circuits, so step one is to consider which route, a compliance certificate (using scheme member) or completion certificate (using LABC) if the latter, the more detailed to application the better, as I found if you don't they may ask for something latter, which is harder to do when you have not planned to do it, in my case they wanted an extractor fan.

So the first question, is if it is possible to take a non RCD protected feed from the existing consumer unit. With washing machine, tumble drier, and radiator looking at least 32 amps, so at least 6 mm² SWA as a feed, but a 100 amp would need too large of a cable, so it will need some overload device in the main consumer unit.

Rather naughty, but even if you intend to DIY, I would get one quote for the work, A) as it may be cheaper than you think, and not worth DIY and flouting the law. B) as he will likely tell you how he would do it.

With a high integrity CU in the house, then a 32 amp MCB feeding the shed feed, without that, then looking at second mini CU in the house or renew the existing.

 

[ashtraywasp]

Ah I see I didn't know there were multiple ways to get it certified.

The current CU in the house seems to be made up of RCDs with downstream MCBs exclusively, so would it be fine to have a 63A RCD or RCBO added to a separate stream in the main CU, then down a 6 mm² SWA cable down to a subunit in the garden office, with a main switch and MCBs per subcircuit. When looking up information it seems to always recommend the opposite, an MCB in the main CU, and RCBOs per subcircuit in the subunit in the garden office.

 

[mikehammer69]

A garden office , you definitely don't want the supply cable connected to an RCD

 

[ericmark]

This from the IET is a bit outdated, Autumn 2005, but is a good start at understanding supplies to outbuildings.

In 2008 the rules on RCDs changed, and it has continued to change, this old consumer unit called a high integrity never worked out why that name, has three neutral bars, so can have two RCDs and also some RCBOs or MCBs which are not protected by the main RCDs.

Earlier and latter consumer units often have only one or two neutral bars, with an all RCBO box, only one neutral bar is required, and the early RCD models only had two neutral bars.

You can get kits of parts for current consumer units, but with older models, once bus bars have been cut to length, there is not much you can do.

You can't mix and match with a consumer unit, as they are type tested, to work with that makes devices, so an MK consumer unit can only use MK units inside it, and you want the work to be registored, so you can't ignor the rules.

I was lucky, I had a spare way in a all RCBO consumer unit when I wanted a supply to my shed, and I only wanted basics, so having the RCBO in the house, was not a problem for me.

If I was powering a larger shed, then I would want the RCD/RCBO to be in the shed. You're looking at design, not just simply banging in another socket, and with a washing machine looking at 2 to 3 kW, and a drier 650 watt to 3 kW, and a heater 500 watts to 3 kW, so the total load could be rather high.

A washing machine needs water, and it will depend on if metal or plastic for the water supply, on how the shed is earthed, also what the shed is made from, wood is easy, but a metal shed has to be treated the same as a caravan now, so a TN-C-S supply is no longer allowed.

The problem is, it is all too easy to miss some vital thing when giving advice.

 

[ashtraywasp]

Ah right I see, the structure is built out of a wooden frame so no metal involved. The washing machine is not a must for now as we would need to sort the water supply but the dryer will be going in.

My partner's dad is coming to help do the electrical installation but he's not an electrician so I wanted to make sure everything was planned before he came. We are fairly constrained for time and money so we wanted to do most of the work ourselves and just get ut certified after, but I don't know all the factors involved as you said.

I'll have a look at the way the current CU in the house is wired as I have only looked at it with the cover on (pic attached). But then a subunit in the garden office with an RCBO and downstream MCBs is a possibility, but then I still don't know how to wire that up to the main CU in the house.

I might try to get an electrician to come have a look but we really can't afford a full installation job.

 

[endecotp]

When looking up information it seems to always recommend the opposite, an MCB in the main CU, and RCBOs per subcircuit in the subunit in the garden office.

Because that’s the right thing to do. If you need that stuff in the shed at all - why not keep it simple and have everything on one circuit, with the lights on a 3A FCU?

Do you actually understand what you’re doing? Seriously, are you out of your depth? Do you understand what MCBs, RCDs and RCBOs actually do?

 

[ashtraywasp]

I understand that is the best thing to do, but most people seem to be fine using an RCD with downstream MCBs per subunit, that's why I ask. I also wanted to keep the circuits separated for in case of failure on one it doesn't take the whole thing down.

I understand what they do to an extent, I did electronics in university so I understand current and voltages just haven't dealt with AC to this extent before. It's why I have come here to ask for advice.

 

[Murdochcat]

Surely you built in the costs of having the electrics done before you started?

Your best approach would be to try and find a C local spark willing to work with you and let you do some of the work

FWIW your cu is about 18 years old and is now obsolete so adding 3rd party parts to it isn’t recommended

 

[ashtraywasp]

Well yes but the costs of the materials ballooned a fair bit before we even got to do the electrics, so room has slowly dwindled.

Yes that seems to be what I will aim for, ideally we can do the whole installation in the garden and they just connect it all according to law. We can wire up all the outlets and lights and just leave the CUs to the electrician.

Ah thanks for letting me know about the CU.

 

[endecotp]

most people seem to be fine using an RCD with downstream MCBs

To clarify, you do mean an RCD in the main CU, feeding SWA to the shed, with MCBs in the shed - right?

I don't know where you found these "most people", but I think "most people" shouldn't be working inside their consumer units.

 

[ashtraywasp]

Yeah you're probably right.

And yes I mean that setup, with an RCBO ideally upstream of the MCBs but downstream from the SWA, and within the subunit in the garden office.

I have seen that you actually aren't allowed by law to make any changes to the house CU, does that apply to the subunit in the office? Would I have to leave it all open wired for the electrician to do the CU themselves?

 

[ashtraywasp]

We do have an existing extension from the bedroom outlets coming out to the garden through an SWA cable already but that would be coupled to the 32A MCB in the CU. There is already two heaters in the bedroom so I don't think that would be enough to supply the entire garden office with outlets, heater and washing machine, even if they wouldn't be used all at the same time 90% of the time.

 

[pcaouolte]

you actually aren't allowed by law to make any changes to the house CU

Which law would that be?

 

[morqthana]

Ah I see I didn't know there were multiple ways to get it certified.

Some less feasible and/or more expensive than others.

The current CU in the house seems to be made up of RCDs with downstream MCBs exclusively, so would it be fine to have a 63A RCD or RCBO added to a separate stream in the main CU, then down a 6 mm² SWA cable down to a subunit in the garden office, with a main switch and MCBs per subcircuit.

Yes, if there's an unprotected section with capacity in the house CU, you don't mind losing all power to the outbuilding if there's a fault, and you don't mind schlepping back to the house to reset the RCD if it trips.

When looking up information it seems to always recommend the opposite, an MCB in the main CU, and RCBOs per subcircuit in the subunit in the garden office.

Yes, because most people do mind losing all power to the outbuilding if there's a fault, and do mind schlepping back to the house to reset the RCD if it trips.