Tables for T&E Reference Method B?

[imroberts]

I'm replacing a number of switches and sockets in my (domestic) garage and I'm intending to fit metal clad accessories with cables enclosed in 20mm plastic conduit rather than clipped as they are currently.

I'm a bit confused how I determine the current-carrying capacity for twin&earth enclosed in conduit.

Table 4D5 provides ratings for twin&earth specifically, for clipped direct and enclosed in insulation, but nothing for conduit.

My instinct is to use Table 4D2A which is for "Multicore cables having thermoplastic insulation, non-armoured, with a conductor temperature of 70C", which feels appropriate for standard T&E, and provides ratings for method B as I require. If I compare ratings for A and C in tables 4D5 and 4D2A, I find that the ratings in 4D2A are generally slightly lower, so if anything by using 4D2A I'll be slightly safer than strictly necessary - however if this table is an appropriate reference that begs the question why the ratings are different at all.

What ratings are deemed the most appropriate?

 

[Adam_151]

Table 4D5 Was added in much later (probably about 15 years ago?) to specifically cater for twin and earth and some of the medthods it might be installed in 100, 101, 102, 103, etc. Before that we used 4D2A, the reason the values are likely different, is that they are empirically derived, and having the conductors flat actually works slightly better thermally, you get a bigger surface area through which to shed heat. You are free to use 4D2A, because as you you note, it'll increase the margin of safety, rather than reduce it. There was an IET wiring matters article on this around the time, but my google skills are failing me (I'm not sure their site goes back far enough)