Discuss Supply to outbuilding in the Electrical Engineering Chat area at ElectriciansForums.net

Dan Carroll

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Trainee
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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
 
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telectrix

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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.
 
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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

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Trainee
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  • #6
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
 
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

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Trainee
  • Thread Starter Thread Starter
  • #8
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
 
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

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  • #10
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
Post automatically merged:

not RCD protected if SWA
By the way, why? I thought pretty much everything has to be RCD protected. I know there are exceptions
 
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telectrix

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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

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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

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Trainee
  • Thread Starter Thread Starter
  • #14
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

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telectrix

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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.
 
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