Suggestions for best way to wire underground supply please

[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

 

[Wilko]

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

I know what you mean! Looking into it a bit (feedback appreciated)
Table 4B3 gives rating factors if we know the soil thermal resistivity on the site. Otherwise we use the tabulated ccc of Table 4D4A column 6 (say) which are worked out for 2.5K.m/W. Looking on internet (no warranty there) for thermal conductivity and using 4B3 for rating factors :
- dry soil 3.3 - rating factor 0.9
- damp sandy soil 0.5 - rating factor 1.88
So local conditions could change the rating quite a lot, and it looks like BS7671 has conservatively punted for dryer soil conditions.

 

[Guitarist]

I've just done the exact same job, except from a detached garage on an old property. Gas central heating, so used 16mm 3-core swa with a 60amp switch fuse.