Supply to outbuilding

[Dan Carroll]

Hi team.

A couple of questions for you.
Just to explain as well. I've just completed the level 2 & 3 diploma, and I'm now trying to work out in the big wide world.

All of this is in regards to a studio outbuilding. The outbuilding is located about 25 meters away from the house.
It is fed via a fused spur taken from a ring main. The ring main is protected with a 32 amp mcb, no RCD.
The feed cable is only 2.5mm 3 core SWA.
The feed goes to a CU in the studio which is RCD protected with 2 circuits, a light on a 6amp and radial on a 16a.
The studio is also on a TT

So firstly, is the general set up ok? To my limited experience it would seem so as the feed is protected so the cable is never going beyond is safe level with regards to current carrying capacity.

Secondly though is volt drop.
When I have worked it out, it seems to be well over the 3 limit for the lighting, but when tested against the inside of the house, there is basically no drop. The confusing part for me as well is the wording in the 7671. When it refers to the origin of the installation, do you take that as the tails into the main CU or the studio CU. The studio is an installation and is origin is there.

Many thanks as always team

 

[telectrix]

the studio is part of the house installation so the origin is at the meter/cut-out in the house. feeding a sub-main CU from a final circuit is bad practice. it should be fed from the origin, either on a OCPD in the house CU, not RCD protected if SWA, or by splitting the tails with henly blocks and a stand-alone OCPD/isolator for the sub-main. the existing 2.5mm cable may be enough for what load is there at present, but does not allow for expansion.

 

[Pretty Mouth]

How did you calculate your volt drop?

 

[Dan Carroll]

How did you calculate your volt drop?

From the book. mV/A/m * Ib * L divide by 1000.

 

[Deleted member 105166]

When I have worked it out, it seems to be well over the 3 limit for the lighting, but when tested against the inside of the house, there is basically no drop.

Whilst you don't indicate the cable length from the origin to the FCU, I'm struggling as to how you are calculating "well over" the 3% for lighting, unless the cable length from the origin is significant. I calculated the "submain" section from the FCU as 18mV*6A*25m=2.7V, to which you need to add the VD for the origin to the FCU (if you insist on wiring it this way - see Tel's advice above).
The likely reason you're not seeing this level of VD when testing is your draw is nowhere near 6A.

 

[Dan Carroll]

unless the cable length from the origin is significant

Its probably about about 20 meters from CU to where the FCU is, so 45 meters to the studio CU.
I'm calculating the Ib of the feed as the lighting and power circuit which would add up to 22. (Feel free to correct, we don't go to in depth on the finer points) so 18 * 22 * 45 over 1000 is 17.82 VD. already at 7% without the studio wiring

 

[westward10]

I assume this is an existing installation so why the interest. Even at 13A loading and only considering the swa cable it does not exceed 3%.

 

[Dan Carroll]

I assume this is an existing installation so why the interest. Even at 13A loading and only considering the swa cable it does not exceed 3%.

This is my first project. I'm trying to figure it all out but unfortunately don't have all the answers. And even if it had nothing to do with me, I would still show an interest in something I didn't quite understand, so that I could better myself in case something comes up in the future.

My maths has 17v at around 7.5%, what are you doing different to get less than 3% please

 

[Pretty Mouth]

From the book. mV/A/m * Ib * L divide by 1000.

Its probably about about 20 meters from CU to where the FCU is, so 45 meters to the studio CU.
I'm calculating the Ib of the feed as the lighting and power circuit which would add up to 22. (Feel free to correct, we don't go to in depth on the finer points) so 18 * 22 * 45 over 1000 is 17.82 VD. already at 7% without the studio wiring

You can calculate the volt drop from CU to FCU using R1+RN of the ring, and the design current of the ring. It will give a slightly pessimistic result. Your design current of the ring will have to include the design current of the studio, it being spurred from the ring.

Volt drop from FCU to studio CU calculate in the usual way. The design current for the studio cannot be more than 13A.

 

[Dan Carroll]

The design current for the studio cannot be more than 13A

Is that because its coming from the FCU?

Out of interest, why is it not best practice to run the outside feed from a FCU to a CU? The FCU is protecting the cable. Could there not be a case of installing a larger swa?
Remember I'm still new to this
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not RCD protected if SWA

By the way, why? I thought pretty much everything has to be RCD protected. I know there are exceptions

 

[Pretty Mouth]

Is that because its coming from the FCU?

Yes, assuming it's been designed correctly. Otherwise breaking reg 433.1.1 (i)

 

[telectrix]

the main problem is the low ccc of the cable and the fact that it's protected by a 13A fuse. this means that the studio is limited to a basic socket circuit and a few lights. any hungry appliances would need a bigger supply.

 

[DPG]

By the way, why? I thought pretty much everything has to be RCD protected. I know there are exceptions

An SWA submain would not need RCD protection, but the circuits that it feeds most likely would.

 

[Dan Carroll]

the main problem is the low ccc of the cable and the fact that it's protected by a 13A fuse. this means that the studio is limited to a basic socket circuit and a few lights. any hungry appliances would need a bigger supply.

I appreciate that. It is only going to be for an art studio thats well insulated, so phone charging, a small heater and maybe a couple of desk lamps. I work out they should have 2500watts (rounded down). It is literally just for her to sit in and paint. Maybe a few drinks when ever we have nice weather.

So really would the best options be to install something like 6mm (not done the calc yet for VD) SWA, back to the main DB. The DB is in the middle of the house, under the stairs.
Or knowing that it is only going to get light use, upgrade the SWA to 6mm from the FCU.
I do like the henley block method, however the current layout already has a couple of units under the stairs and access to the middle of the house would be problemattic.

I really appreciate all the comments. Its just that sometimes the book isn't exactly the clearest in what you can and cannot do. I have come from 15 years as a mechanical helicopter engineer, where the regs where substantially larger than these, but also more defined. I like to have good and clear knowledge of what I'm trying to achieve

 

[telectrix]

pointless upgrading to 6.0mm cable from a 13A FCU. if it's too difficult to get direct to the CU then leave as is, it's safe. just advise on the load limitations.

 

[DPG]

No point running a 6mm SWA from the FCU.
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Bet me to it Tel.

 

[Dan Carroll]

pointless upgrading to 6.0mm cable from a 13A FCU. if it's too difficult to get direct to the CU then leave as is, it's safe. just advise on the load limitations.

But then, as far as I can work out, I'm still out on volt drop. I'm over 3% by the time I get to the studio CU.

 

[telectrix]

i wouldn't worry about that, esp if LED lighting is used. bear in mind that you probably have > 230V anyway.

 

[Dan Carroll]

i wouldn't worry about that, esp if LED lighting is used. bear in mind that you probably have > 230V anyway.

My worry is that it is against 7671. I'm hoping that either I will be in a position to certify it or will have to get someone to do it.

 

[DPG]

Is this a job for a customer, or at your own property?

 

[Dan Carroll]

For a customer

 

[DPG]

Are you doing the testing and filling in the installation certs yourself?

 

[Dan Carroll]

Hopefully, I've just completed the part p course, so going to give a body a call, (prob niceic, unless you can advise otherwise), so should be able to do it myself. Concern would be if I have an assessor.

 

[DPG]

Ah right. I'm with you. So you're using this job as your assessment job to join a scheme.

 

[Baddegg]

Hopefully, I've just completed the part p course, so going to give a body a call, (prob niceic, unless you can advise otherwise), so should be able to do it myself. Concern would be if I have an assessor.

There will always be an assessor on your first job @Dan Carroll, I’d advise an easier job as your first assessment mate.....once you are a member the technical department will give you all advice you need, have only skimmed through the thread but why are you using 22 as your lb?

 

[Dan Carroll]

That's an added bonus, but primarily its for the work experience.
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There will always be an assessor on your first job @Dan Carroll, I’d advise an easier job as your first assessment mate.....

I'm going to phone them tomorrow. Can I get away with wiring a plug

 

[Baddegg]

That's an added bonus, but primarily its for the work experience.
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I'm going to phone them tomorrow. Can I get away with wiring a plug

Not sure you could write a large enough cheque for that mate ha ha ha but seriously being realistic a small Cu change is gonna be easier from a design and calcs perspective than extending to an out building...

 

[Dan Carroll]

Not sure you could write a large enough cheque for that mate ha ha ha but seriously being realistic a small Cu change is gonna be easier from a design and calcs perspective than extending to an out building...

True.

I have, in my time ignored, cut corners, and done my own thing, mechanically on helicopters, BUT it was always safe, I knew the procedures and the risks, and it was only on the item's that where not essential or important. Where you could apply common sense to get the correct result.
I feel (and again, stop me if I'm out of order) that volt drop is like that.
I appreciate that there is a time and place where it is vitally important, but to run a few LEDs and a phone charger, doesn't seem to be one of them

 

[Baddegg]

True.

I have, in my time ignored, cut corners, and done my own thing, mechanically on helicopters, BUT it was always safe, I knew the procedures and the risks, and it was only on the item's that where not essential or important. Where you could apply common sense to get the correct result.
I feel (and again, stop me if I'm out of order) that volt drop is like that.
I appreciate that there is a time and place where it is vitally important, but to run a few LEDs and a phone charger, doesn't seem to be one of them

I’m sure your corner cutting came after many years in the trade with lots of experience, I dare say you’d never even consider it at the beginning of your time? I think your doing the right thing here and trying to understand and do it properly, trouble is here once you’ve touched it and signed it off you are now responsible for it, is this install already here or are you designing it now? If so follow @telectrix advice mate...

 

[Taylortwocities]

It’s also rather pointless having a cu in the outbuilding with a 16A MCB if the origin has a current limit of 13Amp.
You are supposed to provide discrimination of protective devices.

 

[davesparks]

Out of interest, why is it not best practice to run the outside feed from a FCU to a CU? The FCU is protecting the cable. Could there not be a case of installing a larger swa?
Remember I'm still new to this

By the way, why? I thought pretty much everything has to be RCD protected. I know there are exceptions

Creating a circuit which is both distribution and final circuit in one goes against common sense before you've even looked at the regulations,as does sharing a final circuit between a house and outbuilding.
Any fault or overload in the outbuilding could cause a loss of the final circuit in the house.
Feeding a CU from a 13A fuse is pointless, especially when the CU has an ocpd rated higher than 13A,

Whilst we all appreciate that you are new and none of us ever stop learning there is a basic level of knowledge and experience needed before you can safely go out on your own. It's not fair on a customer for them to think they are employing a qualified professional when they are actually paying for someone to practice and get their designs checked/reviewed by a bunch of strangers on a forum.

Some things require RCD protection but certainly not everything, RCD protection is generally an additional thing required for certain things. Socket outlets 32A or below and not for a specific documented purpose, TT systems, bathrooms etc require RCD protection. The only place that you would usually consider everything needing rcd protection as a blanket requirement is the domestic sector.

 

[Pretty Mouth]

It’s also rather pointless having a cu in the outbuilding with a 16A MCB if the origin has a current limit of 13Amp.
You are supposed to provide discrimination of protective devices.

Good point. Do you think in a situation such as this it would be reasonable to do away with the FCU, and have the whole outbuilding on a 16A breaker?

 

[davesparks]

Good point. Do you think in a situation such as this it would be reasonable to do away with the FCU, and have the whole outbuilding on a 16A breaker?

No, doing away with the CU in the outbuilding and using a SFCU with 3A fuse for the light would be reasonable in my opinion.

 

[Dan Carroll]

It's not fair on a customer for them to think they are employing a qualified professional when they are actually paying for someone to practice and get their designs checked/reviewed by a bunch of strangers on a forum.

The client is actually a friend, who knows exactly where I am with regards to my current position. At no point have they been misled into thinking I'm anything other than what I actually am.
I appreciate some people out there would take everything said on the internet as gospel, however I assure you I'm not. I'm seeking people's opinions along side trying to work safely within the regulations.

basic level of knowledge and experience needed

I hope you can appreciate they didn't cover this scenario on either the 2365 lever 2 or 3, or the 18th edition, or the part P courses.
Creating an atmosphere of hostility can cause people to keep their heads down and not ask questions. This in turn can lead to mistakes being made, when if they had just asked 'the stupid question' it could have been avoided

Many thanks for your concern though.

Thank you everyone for your help and advice. I have learnt a bit more and you have helped clear the muddy water.

Look forward to picking you're brains again sometime