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TP EIMSA study materials
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TP EIMSA: Wiring Methods and Materials

TP EIMSA · Work through your exam, one topic at a time.

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Wiring Methods and Materials

This domain is about choosing cable, containment and accessories that carry the load, survive the environment and stay testable for the life of the installation. Questions combine current-carrying capacity with installation method, grouping, ambient temperature and thermal insulation.

The capacity chain

Every circuit must satisfy Ib is less than or equal to In, which is less than or equal to Iz. The tabulated capacity It is corrected to give Iz: Iz = It x Ca x Cg x Ci x Cd, where Ca covers ambient temperature, Cg grouping, Ci thermal insulation and Cd depth of burial. Correction factors are applied to the tabulated value, never to the device rating or the design current.

Cables and where they belong

  • Harmonised H07V-K single core in conduit, H05VV-F flexible cords, and flat twin-and-earth for dry internal runs, but not outdoors or in contact with expanded polystyrene.
  • Steel wire armoured cable to BS 5467 or BS 6724 for underground and exposed routes, with the armour earthed through a brass gland and banjo.
  • Fire-resisting cable such as FP200 for life-safety circuits; low-smoke zero-halogen sheathing where smoke and acid gas are the hazard.
  • Minimum copper sizes of 1.5 mm2 for lighting and 2.5 mm2 for power, with the usual small-signal exceptions.
  • Bending radii follow manufacturer guidance, commonly 4 times overall diameter for unarmoured and 6 times for armoured cable.

Worked example

A 32 A Type B MCB feeds a 4.0 mm2 thermoplastic multicore clipped direct, tabulated It = 37 A. The route passes through a 35 C plant room (Ca = 0.94) and is bunched with one other loaded circuit (Cg = 0.80). Corrected capacity is 37 x 0.94 x 0.80 = 27.8 A, below the 32 A device, so 4.0 mm2 fails. Moving to 6.0 mm2 (It = 47 A) gives 47 x 0.94 x 0.80 = 35.3 A, which satisfies Ib <= In <= Iz. Re-routing out of the hot area would be cheaper if the grouping can also be reduced.

Traps that cost marks

  • Applying correction factors to In or Ib instead of to It.
  • Using 90 C XLPE ratings when the terminations or accessories are only rated at 70 C.
  • Forgetting Ci where a cable is completely surrounded by thermal insulation, where a factor of 0.5 is typical.
  • Assuming a cable is protected because the device rating matches the tabulated It without any derating.