I felt that the conductors supplying this board should be able to carry the instantaneous tripping current of the circuit breaker (5*6=30A) which, in this installation would require 4mm2 cable. My colleague felt that 2.5mm2 was satisfactory and it could easily carry 6A continuously.
Ignoring the possibility of omitting fault protection for up to 3m for the moment, in general to be OK for fault protection the conductor (including its insulation) needs to be able to survive the actual fault current for a long as it takes the fuse to open. That ability isn't really a feature of it's normal current carrying capacity but is calculated using the adiabatic (S=sqrt(I² t) / k) using the worst-case conditions. To do that properly you'll need to know the characteristics of the cable (to give k), likely fault currents (I) and the time taken for the fuse to open for those currents (t). But as a short cut, the OSG gives max Zs values for various fuse & conductor sizes. Looking at the table for 100A BS 1361 (BS 88-3) fuses - 6.0mm² has a max loop impedance of 0.15 Ohms (measured) and all smaller sizes are "NP" for not permitted (i.e. they fail the adiabatic or exceed 5s disconnection time even in the most favourable conditions). (Although the table is set out for earth faults, the same physics & maths applies for L-N faults, although obviously using L-N loop impedance rather than earth loop impedance).
So my initial conclusion is that at least a 6mm² is probably needed to withstand faults when protected by a 100A fuse.
In this situation though, BS 7671 does permit fault protection to be omitted where certain conditions apply as specified in 434.2.1 - i.e. conductors no longer than 3m, installed in such a manner to reduce the risks of fault to a minimum and to minimise the risk of fire or danger to persons - if all those conditions are met then your 2.5mm² could be acceptable.
- Andy.