How to calculate the tabulated current (It) for circuits with multiple Reference Methods (A and B)?

[AReynolds]

I am trying to select a suitable conductor size from Fig 4D1A of the BS 7671 Book, and am encountering a problem because the circuit does not conform to one Reference Method. The circuit conductors leave the distribution board in wall mounted steel trunking, then run through some steel conduit that passes through about 20cm of concrete wall, then finally through some PVC wall mounted trunking and two more 20cm concrete walls.

Technically, the points at which the cable pass through the three walls are Reference Method A, while the rest of the circuit would be considered Reference Method B. Given that the tabulated current using Reference Method A will be more stringent, do I have to consider the entire circuit as being Reference Method A for the purpose of determining the tabulated current, or can I consider it as being Reference Method B, given that 60cm of the 20m cable run are Reference Method A, with the remaining 18.2m being Reference Method B?

 

[Wilko]

Hi AR - Unfortunately, I think the worst case should be used. But perhaps the route in conduit through the wall is method # 41/42 or 59/60 from Table 4A2 which are all B ?

 

[westward10]

As it is a concrete wall I would use Method B throughout.

 

[AReynolds]

I have had a look at Table 4A2, and while the wall does not contain thermal insulation, it does appear that the concrete blocks have a thermal conductivity of less than 10W/mk (most of the websites I have looked at quote values between 0.70-2 W/mk). I cant believe that I am going to have to return to the drawing board with the nasty Ci correction factor and the more stringent Reference Method A tabulated current, which in turn will increase the cable size from 4mm to 6mm- which is ridiculous for a 20m socket radial- all for 1.8m of wall!

Would it be acceptable to leave the circuit as Reference Method B if I tell the person drilling the conduit holes in the wall to double their diameter or something?

 

[westward10]

Treat it as Method B. If the wall forms part of a fire compartment it will require fire stopping so don't oversize the hole.

 

[Strima]

Unless you run the trunking through the walls...

 

[Wilko]

If the masonary has conductivity of 2W/mK this is a resistivity of 0.5 mK/W which means it qualifies as Number 57 in Table4A2 which is Ref method C, but please confirm