Site fabricated wooden trunking/capping

The real reason I raised the question about 4D5 was not to question whether a B32 is OK for this circuit or not - simply that I come across a lot of applications in which T&E is used, that aren't covered in 4D5 - causing a lot of debate about what to do next, or which method or other table is easier to use.


And of course I see training materials from different organisations with different approaches to that problem too ... so for example if one organisation chooses to well over-engineer and select Method 103 (13.5 A for 2.5/1.5 cable), they will get a vastly different answer to someone using Method 102 (21 A for 2.5/1.5 cable).


In reality, I think this is probably nearer to Method B as Andy alluded to.


However, how valid is it to use another table like Andy did? Let's have a look (apologies if this is teaching people to suck eggs, just thought it would be useful to readers in the future) ...

 

csa

Current carrying capacity

Ref Method C

Ref Method A

2.5 mm2 Table 4D2A

27.0 A

18.5 A

2.5 mm2 Table 4D5

27.0 A

20.0 A

In the table above, we see that clipped direct (Method C), both the 4D5 and 4D2A show the same current carrying capacity. However, when enclosed in a conduit in a thermally insulated wall (Method A), 4D2A round cable has a 7.5 % lower current carrying capacity than 4D5 T&E. This is because it's radiating surface area is less for the round cable.


So, Andy's approach of using 4D2A is no less safe than using 4D5 - it certainly errs on the side of caution, as the table above demonstrates ... but it's taken an extra couple of steps to see that.

The real reason I raised the question about 4D5 was not to question whether a B32 is OK for this circuit or not - simply that I come across a lot of applications in which T&E is used, that aren't covered in 4D5 - causing a lot of debate about what to do next, or which method or other table is easier to use.


And of course I see training materials from different organisations with different approaches to that problem too ... so for example if one organisation chooses to well over-engineer and select Method 103 (13.5 A for 2.5/1.5 cable), they will get a vastly different answer to someone using Method 102 (21 A for 2.5/1.5 cable).


In reality, I think this is probably nearer to Method B as Andy alluded to.


However, how valid is it to use another table like Andy did? Let's have a look (apologies if this is teaching people to suck eggs, just thought it would be useful to readers in the future) ...

 

csa

Current carrying capacity

Ref Method C

Ref Method A

2.5 mm2 Table 4D2A

27.0 A

18.5 A

2.5 mm2 Table 4D5

27.0 A

20.0 A

In the table above, we see that clipped direct (Method C), both the 4D5 and 4D2A show the same current carrying capacity. However, when enclosed in a conduit in a thermally insulated wall (Method A), 4D2A round cable has a 7.5 % lower current carrying capacity than 4D5 T&E. This is because it's radiating surface area is less for the round cable.


So, Andy's approach of using 4D2A is no less safe than using 4D5 - it certainly errs on the side of caution, as the table above demonstrates ... but it's taken an extra couple of steps to see that.