Andrew asks about the sizing of cables for a new build ring main installation. His question has been answered by the ECA (Theme – Regulations & Legislation):
Question: I'm installing some ring circuits in a new build. Two of the rings are at the opposite end of the house to the consumer unit. The cables will be installed between the floor joists with thermal insulation of 4 inches between the joists.
I was going to wire the long rings in 4.0mm twin & earth (T/E) cable, but have been told by another electrician that 2.5 mm T/E will be OK.
Does the length of the cable from the first and last leg count in the 100m2 floor area stated in the regulations?
Also, am I correct in protecting the circuit with a 25A RCD because of the thermal insulation?
Answer: 2.5mm or 4.0mm T/E cable could be OK in a domestic environment, subject to installation conditions with a 30A 30mA RCBO protective device.
Why are you using ring circuits? Have you considered using radial circuits?
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