Steel Conduit as the Circuit Protective Conductor (CPC)
This episode examines whether steel conduit can be used as the CPC on it's own, or whether an additional copper wire needs to be installed inside the conduit.
The Video
Regulations in BS7671
Steel conduit is permitted for use as a circuit protective conductor (CPC). In the 18th edition of BS7671, regulation 543.2.1 states:
'A protective conductor may consist of one or more of the following: ... (vi) A metal conduit, metallic cable management system or other enclosure or electrically continuos support system for conductors.'
For connecting to accessories such as switches and sockets, regulation 543.2.7 requires that '... the earthing terminal of each accessory shall be connected by a separate protective conductor incorporated into the associated box or other enclosure.'.
Metal backboxes for sockets and switches already include such a terminal, so this is just installing a short piece of green/yellow insulated copper wire from that terminal to the earth terminal on the switch or socket.
Round conduit boxes have a 4mm threaded hole in the base. A ring crimp and 4mm screw can be used to attach a short piece of wire to connect to the accessory. This would typically be used in the case of ceiling lighting.
Minimum size of protective conductors
For all protective conductors, the minimum size can either be determined from Table 54.7, or by calculation as described in 543.1.3
This process is described in more detail in the video, but with either method, the worst case is that for copper, the CPC is the same size as the line conductor.
As steel has significantly less conductivity than copper, the cross sectional area required is approximately three times larger than for copper.
Using a size of 16mm² which in reality is too large to use with 20mm diameter conduit, this results in a CPC size of about 48mm².
20mm diameter steel conduit has a cross sectional area of 92mm², so even in the worst case situation, is still around twice as large as the minimum required.
Other considerations
Use of steel conduit as the CPC does require that it is installed and jointed correctly. It must also be suitable for the environment where it is installed, so that factors such as corrosion are considered. However these apply to any electrical installation, whether using conduit or not.
