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gozzer66

Power To Shed

Question

gozzer66

Hello all, any advice on this is appreciated.

I want to install power via a cable to a large shed.

The intake at the house is 3 phase.

The measured run is 115 metres.

The load in the shed will be approx 4KW, to inc water heater (2000w) panel heater (1500w)internal lights (72w) external lights (28w) extractor fan (36w) and a couple of sockets.

The consumer unit in the shed will be protected by 100m/a, 63amp rcd and the circuits by 2 30m/a rcds.

An earth rod will be installed at the shed.

I have the following questions.

1. What cable to use for the run? 35mm or 25mm 3 core or 16mm 4 core, (using 2 cores for live 2 for neutral)

2. Is a load test required at the intake, a single phase consumer unit powers the house circuits but various outbuildings are also supplied through 100amp isolators.

3. Is it okay to attach shed cable through henley blocks at 25mm cables that supply the consumer unit. Thank you.

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Blue Duck

Supply type?

Ze?

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steptoe

If you are going to TT shed why not use 2core,? 


Even better, use 4core and take your 3phase to shed, if it's a large one as you say. 

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apprentice87

If you have a three phase supply, do you mean you have all the loads off the one phase??? Why do you not have the house off one phase, the outbuildings off another, and the new shed off the third.

 

No, you cannot just attach the cable to henleys. You need overcurrent protection for the cable so it must be connected through a switch fuse or similar.

 

You only need 2 core cable. You need to earth the cable armour at the house end, [to protect the cable] then, yes, you could TT the shed.

 

There is a LOT more to this than you imagine....

 

The setup of the RCD's you describe is pointless. You only need the one, a 30mA one, and it MUST be a plastic CU in the shed...

 

Try to post a photo of the setup you already have at the house end.

 

john..


Yes, as Steptoe says, why NOT take the 3 phase to the shed, be a far better job...

 

john...

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steptoe

Correction John,

If it's done 'properly' and a 100ma TD with OCPD  at house it could be a MC CU at shed end.


;)

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apprentice87

Yes, you COULD do it like that, but what would be wrong with using the house earth if NOT TNCS to protect the cable, and then convert to TT at the shed. A plastic "domestic" type CU at the shed would be a lot cheaper too, so it makes no difference..

 

john..


Hi Gozzer,

 

You only need a 16mm cable for that load....

 

john..

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steptoe

Even if it is TNCS it should be used to protect the cable,

I'd still fit a TD at the house though.

 

What's VD like on a 16mm John,? Does it leave much room? 

I'd be more concerned with the load rising, tbh,  you'd want to leave some spare capacity if possible. 

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apprentice87

Yes, I would myself [use tncs for the cable] but at that distance away [115M] some folks might disapprove. Still, if you stuck it into plastic box with a nylon compression gland, there would not even be an exposed bit of gland at the far end, [people might moan about a potential difference between tncs earth and standing next to the shed earth, you know what i mean] so that sorts that!!

 

16mm gives a 6 volt drop at 18 amps, so 1 volt inside 3%

 

Know what you mean about the load growing though.... I would DEFINITELY use a 4 core 16mm and have the three phases in the shed too.. Twice the cable price, but three times better!!

 

The 16mm 2 core would be £322

 

25mm 2 core would be £486

 

and the 4 core 16mm would be £539

 

john...

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kerching

Is it definitely a three phase supply?

Around here they ran short on single phase heads, so fitted 3s!

A few people have found that out the expensive way

Just saying

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steptoe

Yer,

I had a friend once that wired up a 3phase nightclub only to discover that it was only a 2phase supply, :| 

Luckily one of the barmen worked for the dno and him and his mate sorted it, only a 18" trench to get across the pavement to the service cable. 

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apprentice87

OMG!!!!! I would have a heart attack!!! Just shows... Never take anything for granted......

 

john...

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Murdoch

If you have a three phase supply, do you mean you have all the loads off the one phase??? Why do you not have the house off one phase, the outbuildings off another, and the new shed off the third.

 

No, you cannot just attach the cable to henleys. You need overcurrent protection for the cable so it must be connected through a switch fuse or similar.

 

You only need 2 core cable. You need to earth the cable armour at the house end, [to protect the cable] then, yes, you could TT the shed.

 

There is a LOT more to this than you imagine....

 

The setup of the RCD's you describe is pointless. You only need the one, a 30mA one, and it MUST be a plastic CU in the shed...

 

Try to post a photo of the setup you already have at the house end.

 

john..

Yes, as Steptoe says, why NOT take the 3 phase to the shed, be a far better job...

 

john...

 Why do you always jump in with detailed information before you've established the competence of the poster. He's probably a builder.

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apprentice87

Or a farmer!!!!

 

john..

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gozzer66

Thanks so much, a lot of info there.

The shed isnt that big, not enough to warrant 3 phase supply. All loads are spread across the 3 phases.

I meant using henley blocks to split the tails at the board and then into switch fuse etc.

not worried too much about needing extra capacity, but 16mm will allow some extra capacity even over that distance?

To summarise, 2 core 16mm split at consumer unit tails, through opd, cable earthed via armour. Shed end, tt with rod and 16mm earth into plastic cu with one 30ma rcd protecting all circuits? I thought this was against 17th ed regs and a 100ma rcd was needed to protect the whole installation?

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steptoe

use a 100mA TD upfront at house end, that will also offer some protection to your cable


oh,

16mm will offer virtually NO room for further loading,

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gozzer66

might go for 25mm then. No split board in shed? Thanks Steptoe

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kerching

I would think about an RCBO board in the shed.....

Just saying

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gozzer66

Cheers Kerching, will do that. Couple more questions.

Is it ok to put swa in same trench and next to water pipe?

If its 16mm cable it could come from spare way in house board, would a 20 amp breaker suffice? predicted shed load is 18amp. Ta muchly.

If i henley the house board tails and take shed cable from there and through OCPD, what rating would i use for switch fuse and can i incorporate the 100m/a rcd into this device?

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