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Hi Guys I have been following this forum over a number of years reading endless discussions and picking up things along the way.
I am a JIB gold card spark and have worked mainly in commercial/ industrial and high end apartments.

within this next year i am looking to go full time solo and work for myself and aim to join the NIC. ( have to try, have had enough of working on site )

A friend of mine is building an outbuilding\shed and i will be doing the install, now as you probably know working for big contractors on site , all drawing, calculations design is all done for us. we just install and get on with it. so bear with me if i have gotten anything wrong.

to start off this job will be used as one of my NIC assessments

Out building is roughly 30m from the meter cupboard at the front of the house
Out building will have 7 LED Downlights, 2 LED Wall Lights 3 Double sockets and a 2 FCU unit supply a freezer( 100 odd watts ) and potato peeler. ( 250 watts )


Will run a 10mm 3 core armoured ( future proofing ) & incase there is any extraneous conductive parts in the future the or extra demand. 10mm earth would be sufficient ( not opting to TT the outbuilding as in my opinion incoming earth is more reliable) . It is a PME system .

Armoured will be part buried, and part clipped onto the side of the house.
I am thinking of splitting the meter tails, The tails go from the meter to a 100amp Switch fuse - from switch fuse to a junction box splitting it to CU and PV System Via henley blocks.

question is do i add another junction before the 100amp switch fuse or take it from the main junction which does and CU and PV system & whichever i choose i will be putting a 60A domestic switched fused unit
Wylex 60A Domestic Switched Fused Unit - https://www.screwfix.com/p/wylex-60a-domestic-switched-fused-unit/4848J?tc=DT7&ds_kid=92700055281954493&ds_rl=1249401&gclid=Cj0KCQjwk4yGBhDQARIsACGfAetmei2XrUVAxFl8QV2Dja3I1xjp4RzQEVps-SsJtqhxAx5RxKFZlAwaAlKIEALw_wcB&gclsrc=aw.ds#BVQAWidgetID

I would also like to add yes i know you're not technically supposed to touch the Main incoming Fuse.

small garage board with a 63A RCD incoming switch.
2.5mm Ring of a 20amp breaker and 6amp for lighting.

or do i opt for a 32 breaker instead?

I have attached an image, any help or advice would be appreciated.

One last final question, as this has probably been asked thousand times, What do people charge per point roughly? Im assuming when they charge per point its for bog standard plastic?

Thank you
 

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Julie.

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I would split before the 100A SF because if you have a 60A fuse downstream of the 100A fuse there is no guarantee they will discriminate, under certain fault levels both fuses could blow.

This issue still exists for the incoming fuse of course.

So if you are constrained for space splitting after the 100A SF wouldn't be bad, just not as flexible.

BTW You really need to check the MD for the site - the incoming fuse may only be 60A
 

pc1966

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As @Julie. says you might have a 60A cut-out fuse and so the fused-switch for this out building would be quite capable of taking both out!

If you are looking at a 60A BS1361 fuse then OSG Table B5 has you max end-of-sub-main Zs at 0.53 ohms, so check what the supply Ze is and add your R1+R2 for the feed to check it is OK. Also remember to earth both ends of the armour properly using a banjo, or Earthing nut, and flying lead to your earth terminals where the 3rd core CPC goes, thus any live-armour internal cable fault Zs is kept down.

It sound like the outbuilding has very little load for now, so potentially you could look at fitting a 45A fuse instead, that is going to be fine for total selectivity with a 80A or more supply, but not for a 60A supply fuse (as you typically need a 1.6:1 ratio).

However, as you drop the sub-main fuse then you have greater risk of a final fault taking it out. Not for 13A sockets/FCU, they will be totally selective with any similar fuse of 30A and above, but fixed wiring protected by MCB/RCBO.

Typically (looking at Hager table I have to hand) you would be seeing something like a limit of the order of 0.5kA fault selectivity for 32A MCB and 45A BS88 fuse (so a hard fault meeting 5% VD will potentially take it out), rising to around 1.1kA fault for 6A MCB (which I doubt you would see at the final circuit, so selectivity probably would be fine).
 

pc1966

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If you go for 20A MCB/RCBO then you can do a radial in 2.5mm if that is easier.

However, going for the RFC would allow 32A for any future larger loads as well as the (to me at least) advantage of two CPCs. If you drop the supply fuse to 45A then 20A makes more sense to keep max selectivity, even if you still do a RFC.

If there are no windows and/or a real risk of plunging in to darkness when doing anything dangerous (power tools, hot cooking, etc) you might want to include an emergency light, can often be part of one of the planned LED lights.
 
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As @Julie. says you might have a 60A cut-out fuse and so the fused-switch for this out building would be quite capable of taking both out!

If you are looking at a 60A BS1361 fuse then OSG Table B5 has you max end-of-sub-main Zs at 0.53 ohms, so check what the supply Ze is and add your R1+R2 for the feed to check it is OK. Also remember to earth both ends of the armour properly using a banjo, or Earthing nut, and flying lead to your earth terminals where the 3rd core CPC goes, thus any live-armour internal cable fault Zs is kept down.

It sound like the outbuilding has very little load for now, so potentially you could look at fitting a 45A fuse instead, that is going to be fine for total selectivity with a 80A or more supply, but not for a 60A supply fuse (as you typically need a 1.6:1 ratio).

However, as you drop the sub-main fuse then you have greater risk of a final fault taking it out. Not for 13A sockets/FCU, they will be totally selective with any similar fuse of 30A and above, but fixed wiring protected by MCB/RCBO.

Typically (looking at Hager table I have to hand) you would be seeing something like a limit of the order of 0.5kA fault selectivity for 32A MCB and 45A BS88 fuse (so a hard fault meeting 5% VD will potentially take it out), rising to around 1.1kA fault for 6A MCB (which I doubt you would see at the final circuit, so selectivity probably would be fine).

Hi, thank you for your detailed reply, apologies in advance as i am totally brain dead this Sunday morning

you might be right, incoming fuse might only be 60a. i forgot that you should not go by what is labelled on the side

And if it is 60a?


My original thinking was to Split before the 100a SF- but speaking to a spark at work, which is a old timer compared to me. he advised to split at the current Junction box which is after the 100a SF.

Yes, I do a lot of armoured glanding and terminating at work so i will assure its earthed correctly.

So your suggesting fit a 45A fuse SF instead splitting it before the 100a SF as the total load is pretty low for now,



for the board in the shed, which would you opt for? I think I have decided i will put it on a RFC either way.

4 way board with 100a main switch- 32 RCBO and 6 RCBO ?
4 way board 40a RCD main and 32mcb and 6 ?
 

pc1966

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Hi, thank you for your detailed reply, apologies in advance as i am totally brain dead this Sunday morning

you might be right, incoming fuse might only be 60a. i forgot that you should not go by what is labelled on the side

And if it is 60a?
The label says 100A but that is not necessarily what is in there!

My original thinking was to Split before the 100a SF- but speaking to a spark at work, which is a old timer compared to me. he advised to split at the current Junction box which is after the 100a SF.

Yes, I do a lot of armoured glanding and terminating at work so i will assure its earthed correctly.

So your suggesting fit a 45A fuse SF instead splitting it before the 100a SF as the total load is pretty low for now,
Looking at the photo you would be hard pressed to fit anything in to that path from cut-out to 100 fused-switch, and it means the seal faeries need to be called upon (or DNO out to isolate, etc).

Providing you have adequate load limits on all paths then taking after the main switch-fuse is not a bad idea, as it also allows a single point to isolate everything.

for the board in the shed, which would you opt for? I think I have decided i will put it on a RFC either way.

4 way board with 100a main switch- 32 RCBO and 6 RCBO ?
4 way board 40a RCD main and 32mcb and 6 ?
Personally I would go with a switch and RCBOs, ideally double pole. You ought to be looking at SPD there as well, so it might be an idea to look at the likes of the Wylex or Fusebox compact RCBO ranges and matching small home CU that come with SPD, unless it needs an IP rated CU! But it sounds more like a liveable area, not an "exposed to the weather when doors are open" garage.
 

pc1966

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This sort of idea:
 
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The label says 100A but that is not necessarily what is in there!


Looking at the photo you would be hard pressed to fit anything in to that path from cut-out to 100 fused-switch, and it means the seal faeries need to be called upon (or DNO out to isolate, etc).

Providing you have adequate load limits on all paths then taking after the main switch-fuse is not a bad idea, as it also allows a single point to isolate everything.


Personally I would go with a switch and RCBOs, ideally double pole. You ought to be looking at SPD there as well, so it might be an idea to look at the likes of the Wylex or Fusebox compact RCBO ranges and matching small home CU that come with SPD, unless it needs an IP rated CU! But it sounds more like a liveable area, not an "exposed to the weather when doors are open" garage.

Fusebox price looks really good, i have seen more and more people opting to use Fusebox

Quick question regarding the Current carrying capacity for the 10mm SWA 3 core.

As seems to be mixed opinions regarding this

Current Capacity D (Ducting/Direct Burial) - 71 Amps (2 Core) 58 Amps (3+4 Core)

Considering I will be using one of the cores as the CPC. ( and its single phase )
does it count as 2 cores? ( which this is my understanding ) as there is only two cores that are 'live' or does it count as three core ( three phase)

And if it is counted as three core, does that mean 60A fuse would be wrong and need to downsize accordingly perhaps to a 50A ? i mean 2 amps out is it that much of an issue considering?
 

pc1966

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Current Capacity D (Ducting/Direct Burial) - 71 Amps (2 Core) 58 Amps (3+4 Core)

Considering I will be using one of the cores as the CPC. ( and its single phase )
does it count as 2 cores? ( which this is my understanding ) as there is only two cores that are 'live' or does it count as three core ( three phase)
As above, it is the number of live cores (i.e. current-carrying) as they are the contributors to the cable heating.

So in single-phase you have 2 cores carrying the current and heating the cable, in 3-phase you have 3 cores so you have 50% more heat. In order to keep the cable max temperature at the same limit (say 70C PVC or 90C XLPE) you need to reduce the heating by 2/3. As heat generated is I2R you have to reduce the current by SQRT(2/3) = 0.816

E.g. 71A single-phase * 0.816 = 57.97A = 58A as you find.

With 4-wire 3-phase you have other possibilities relating to the neutral heating but it won't really get above the 3-phase balanced case for any sane load (huge 3rd harmonic can heat the neutral significantly).

Also in some cases the reduction in current is not as much as 0.816 because the 3 or 4 core cable is significantly larger overall than 2 core and so able to dissipate heat better.

And if it is counted as three core, does that mean 60A fuse would be wrong and need to downsize accordingly perhaps to a 50A ? i mean 2 amps out is it that much of an issue considering?
Here it is OK as the 2-core limit applies.

However, in general you also need to be weary of using the highest 90C cable rating as then you need the termination accessories to be rated for 90C operations, and not all are. If you rely on the cable's 90C rating for sections that are more thermally insulated (e.g. some cable runs in an insulated wall, etc), but the end bits are below the 70C rating for the same CSA of conductor, you are fine as the ends will be cool enough for all normal accessories.
 

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