Suggestions for best way to wire underground supply please

[The General]

Morning all.

I'm after some suggestions please - I'm at the very early stages of a new build for a customer and just planning ahead for getting cables in the ground before the foundations go down.

There's a new PME supply cut-out which has been brought into a (very small) shed, and the plan is to run a cable underground for about 10 metres across to the C/U in the new house.

I'm thinking that I will put a Type S 100mA RCD in a small (metal) enclosure in the shed, and then run cables buried direct in the ground. Although, if it makes life easier, the client is happy to run a length of soil pipe or something similar to run cables inside.

So,
Do I run a length of SWA 25mm 2-core and also a length of 16mm earth, or is it going to be easier to run some SWA 3-core 25mm?

I'm concerned about working space for terminations inside the small enclosure at the supply end, so I was thinking that using the 2-core would allow me to run the earth straight to the MET rather than attempting to terminate a 25mm earth inside the enclosure which will probably only have 16mm holes in the earth bar.

Or, could I even just run some 25mm tails inside the soil pipe (or does it have to be armoured because it's buried?)

Does that make sense? Have I missed anything obvious? Would I also need to put in a 80A switch fuse at the supply end or am I covered by having the RCD?

If I go with the 2-core and separate earth, is the earth okay direct in the ground or does it need to go inside some conduit?

Sorry, just realised that's quite a few questions, but hopefully that all makes sense!
Thanks for your input and advice.

 

[Wilko]

Hi - from 522.8.10 with your proposed installation methods it needs to be SWA.

 

[telectrix]

opefully that all makes sense!
Thanks for your input and advice.
Do I run a length of SWA 25mm 2-core and also a length of 16mm earth, or is it going to be easier to run some SWA 3-core 25mm? use 3 core. if you fit a 80A sw/fuse, 16mm 3 core will be adequate.

I'm concerned about working space for terminations inside the small enclosure at the supply end, so I was thinking that using the 2-core would allow me to run the earth straight to the MET rather than attempting to terminate a 25mm earth inside the enclosure which will probably only have 16mm holes in the earth bar.install a metal (galv.) adaptable box under your Switch/Fuse. joint the earthing conductor to 16mm., and carry the L & N tails into the Sw/Fuse.

Or, could I even just run some 25mm tails inside the soil pipe (or does it have to be armoured because it's buried?) you could do, but SWA is preferable.

Does that make sense? Have I missed anything obvious? Would I also need to put in a 80A switch fuse at the supply end or am I covered by having the RCD? a switch/fuse is essential to protect the feed cable

If I go with the 2-core and separate earth, is the earth okay direct in the ground or does it need to go inside some conduit? non-starter.

Sorry, just realised that's quite a few questions, but h

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[westward10]

Why are you fitting an rcd.

 

[telectrix]

Why are you fitting an rcd.

in case he used T/E for the underground feeder.

 

[The General]

in case he used T/E for the underground feeder.

Also, before it reaches the new C/U I suspect that the route the cables are taking will probably mean that they're going to be buried in the fabric of the building at points, hence RCD.

 

[westward10]

It will be swa though which "should" have an earthed sheath and anyway a 100ma rcd does not provide additional protection.

 

[Ian1981]

Cables buried in walls <50mm requires additional protection.
I've never read that where cables are buried in the floor (concrete?)that additional protection is required.

 

[SparkyChick]

Just out of interest, is the supply head going to stay in the shed indefinitely? Sounds like it. What's the shed made of? What happens if the shed falls down or future owners want to move it?

Just playing devils advocate a bit and throwing a few thoughts out there.

 

[telectrix]

trust a woman to complicate the issue.

 

[Octopus]

I'm with sparkychick on this ........ Not that I'm taking sides!

 

[The General]

Just out of interest, is the supply head going to stay in the shed indefinitely? Sounds like it. What's the shed made of? What happens if the shed falls down or future owners want to move it?

Just playing devils advocate a bit and throwing a few thoughts out there.

Good question though!
Purpose built wooden frame/corrugated roof on a concrete base, specifically to house the incoming supply. It's a permanent structure.

 

[The General]

It will be swa though which "should" have an earthed sheath and anyway a 100ma rcd does not provide additional protection.

Ah yes. Good point, well made.

 

[The General]

So, the general consensus is that I can fit a 80A switch fuse, no RCD, terminate some 3-core 16mm SWA into a galv box and then run the cable buried direct into the ground. At the other end, terminate into a 30mA RCD if necessary to protect cables if buried in the walls <50mm deep.
Did I miss anything?!
Thanks for all your thoughts and comments.

 

[westward10]

Providing the protective device gives fault protection then yes. I would personally use an installation method that avoids the supply cables being in the wall as an overall rcd will lose your division of circuits. So either install surface or used earthed containment for buried cables.

 

[telectrix]

as it's a new build, the SWA can go direct to the CU, thus eliminating the need for RCD if cable is buried in wall/s for part of the run.

 

[westward10]

What size main protective bonding is required because this will influence the core cpc of the swa cable.

 

[Ian1981]

I'd imagine it's 10mm bonding conductor sizes being domestic and a maximum 100 amp supply

 

[The General]

I'd imagine it's 10mm bonding conductor sizes being domestic and a maximum 100 amp supply

Yes, domestic, 10mm bond to gas/water - thanks for checking though!

 

[westward10]

Needs to be 25.0 if your are burying it the ground on an 80A protective device.

 

[Ian1981]

Needs to be 25.0 if your are burying it the ground on an 80A protective device.

What does?

Edit
You mean the swa

 

[Ian1981]

Yes, domestic, 10mm bond to gas/water - thanks for checking though!

As it's a new build I doubt you will need to provide a protective bonding conductor to the water as the incoming supply externally will be in plastic but you'll have to see

 

[telectrix]

Needs to be 25.0 if your are burying it the ground on an 80A protective device.

why?

 

[Ian1981]

16mm swa buried is only 78 amps ambient temperature 20 degrees and that's without applying the 0.9 correction factor as cable is buried for ambient temperature for 30 degrees plus any other factors like depth etc

 

[westward10]

I always opt for the three phase rating with a three core but that is another debate but yes either way it is undersized if you use 16.0

 

[Wilko]

16mm swa buried is only 78 amps ambient temperature 20 degrees and that's without applying the 0.9 correction factor as cable is buried for ambient temperature for 30 degrees plus any other factors like depth etc

Just trying understand - Table 4D4A for direct buried at 0.7m, ambient 30, ground ambient 20 and conductor 70 is 78A. So question is, why pick 30 ground temp to further derate to?

 

[Ian1981]

Just trying understand - Table 4D4A for direct buried at 0.7m, ambient 30, ground ambient 20 and conductor 70 is 78A. So question is, why pick 30 to derate to?

0.9 is applied where overload protection is required and prevents the cable going over temperature when overloaded.
The factor is used because cable ratings for cables in the ground are based on a ground temperature of 20 degrees compared with 30 degrees for other reference methods.
See regulation 433.1.203

 

[telectrix]

which is all wrong. rarely will the ambient temperature rise above 20dg. C. in this country. once you are down 2 ft. underground the temp. is down to around 10dg.C.

 

[Ian1981]

What about the part of the cable that comes out of the ground?

 

[telectrix]

What about the part of the cable that comes out of the ground?

fit a fan.

 

[telectrix]

IET Forums - Underground SWA Current Rating? - http://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=27920

 

[telectrix]

as an experiment, try shoving 100A down some 16mm for an hour. then measure the temp. of the cable. if you read >40deg.C. then your temp gauge is wrong.

 

[Wilko]

What about the part of the cable that comes out of the ground?

That part would be ok - 89A clipped direct or 97A in free air ?

 

[Ian1981]

That part would be ok - 89A clipped direct or 97A in free air ?

Based on 30 degrees tho yes?

 

[Ian1981]

as an experiment, try shoving 100A down some 16mm for an hour. then measure the temp. of the cable. if you read >40deg.C. then your temp gauge is wrong.

Only if you pay my electric bill

 

[Wilko]

Deleted while thinking a bit more ... might be a while ...

 

[telectrix]

we not in the sahara . not even spain. 30deg. my arse. we'd all be like essex boys at that temp. but with real tans.

 

[Ian1981]

we not in the sahara . not even spain. 30deg. my arse. we'd all be like essex boys at that temp.

Well it errs on the side of safety but I didn't write the regs , send any complaints to jpel/64 ha

 

[telectrix]

haha, trouble is that the regs. are so pessimistic. written to cover arses. as a famous person once said." rules are for the strict adherence of fools, and for the guidance of wise men"

 

[Wilko]

Based on 30 degrees tho yes?

Yes, 4D4A is for ambient air temp of 30. Still thinking ...

 

[Ian1981]

Yes, 4D4A is for ambient air temp of 30. So for cable in free air Ref E - well the cable isn't buried any more - so 433.1.203 doesn't apply and that part of the cable doesn't need derating from what's listed in the table ?

The overall ccc of the cable needs to be the lowest rating if installed using different reference methods using the most onerous method

 

[Wilko]

Got it - thanks Ian

 

[Ian1981]

haha, trouble is that the regs. are so pessimistic. written to cover arses. as a famous person once said." rules are for the strict adherence of fools, and for the guidance of wise men"

I would assume tests would have been performed and the appropriate ccc recorded as we see them today.
Perhaps cables buried in walls and installed in trunking etc are more like at an ambient temperature of 30 as there's no free air?

 

[Ian1981]

Got it - thanks Ian

Don't thank me yet wilko as someone might come along and shoot me down in flames.
It's how I understand it and it's written in be 7671 so got to go with that don't we?

 

[Wilko]

Don't thank me yet wilko as someone might come along and shoot me down in flames.
It's how I understand it and it's written in be 7671 so got to go with that don't we?

Ha ha, always on the cards !
I needed to re read 433.1.203 a few times. Using an 80A fuse for overcurrent protection of a buried cable then the cable needs to be rated >=89A and clearly 16mm isn't when it's buried

 

[Ian1981]

Ha ha, always on the cards !
I needed to re read 433.1.203 a few times. Using an 80A fuse for overcurrent protection of a buried cable then the cable needs to be rated >=89A and clearly 16mm isn't when it's buried

Have a read of guidance note 6.
A lot more clear on the subject.
IET Guidance notes are our friends

 

[Wilko]

Have a read of guidance note 6.
A lot more clear on the subject.
IET Guidance notes are our friends

Good thought - I've 1, 3 and 8 now I've a reason for 6

 

[Ian1981]

Good thought - I've 1, 3 and 8 now I've a reason for 6

I'm the same except my 3 is to the green version of bs 7671 and only bought it for my testing exam.
Don't see the point in renewing it as all that's changed is the zs values in the book

 

[Dave OCD]

Why would buried in the ground be considered more onerous for current rating than clipped direct/'free air' . Doesn't make sense to me.

 

[Ian1981]

Why would buried in the ground be considered more onerous for current rating than clipped direct/'free air' . Doesn't make sense to me.

It's ability to dissipate heat is worse when it's buried would be my guess, especially if installed in a pvc duct.
Despite giving an ambient temperature of 20 degrees buried