Installation reference methodsWhy does the same cable carry less current in insulation?

The reference method describes how and where a cable is installed. This affects how much current it can carry, because heat has to escape. More insulation around a cable means more heat trapped, which means more resistance and a lower current rating. You may then need a bigger cable.

• A cable surrounded by thermal insulation for a short length is derated by its length (cables up to 10 mm², BS 7671 Appendix 4, section 2.6, p.451): 50 mm → 0.88, 100 mm → 0.78, 200 mm → 0.63, 400 mm → 0.51. Totally enclosed for 0.5 m or more means the rating is half that of clipped direct (0.5).
• Examples from the table: 1.0 mm² T&E clipped direct = 16 A (enclosed in insulation, about half), and 2.5 mm² clipped direct = 27 A. The rating drops for method B and the 100-series methods.
• PVC-sheathed cable must not touch polystyrene, which degrades it over time.
• Twin and earth can go in conduit or mini-trunking for extra protection. Councils often run mini-trunking along skirting and ceilings instead of chasing. But more cables bunched together means more heat.

Electrician's Guide to the Building Regs · 2.3.1 Cable installation methods A, B, C Electrician's Guide to the Building Regs · Reference methods 100, 101, 102, 103 · pp.39–41 Electrician's Guide to the Building Regs · Table 2.3.1 T&E current ratings · p.41 BS 7671 · Appendix 4 · section 2.6 · cables surrounded by thermal insulation · p.451