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Twin and Earth cabling

courtney blythe

Hi,

I'm using twin and earth cables for a office building refurb. In the BS 7671 regs i can only see cable current carrying capacities for these cables in thermal ceilings and walls, this will not be the structure for the office building internally. How can i size the twin and earth cables?  I.e are there alternative tables?  I hope you can answer soon.

Regards

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    In relation to the OP, 4D2A was there long before 4D5.

    Indeed. Historically we used what's now 4D2 for T&E (as well as circular section multicore cables) but then as more thermal insulation began to be used in homes, problem arose. Partly it was realised that the allowance for thermal insulation in the regs wasn't entirely adequate, but also that the shape of T&E makes it not quite so bad as circular cables (larger surface area, less thermal insulation effect from the thinner sheath). This article from the IET (IEE as it was then) describes some of the background: electrical.theiet.org/.../establishing-current-ratings-for-cables-in-thermal-insulation.pdf In that context it's perhaps not surprising that table 4D5 concentrates on thermal insulation related installation methods. 4D2 remains the 'fallback' for anything 4D5 doesn't cover.

       - Andy.

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  • 0

    In relation to the OP, 4D2A was there long before 4D5.

    Indeed. Historically we used what's now 4D2 for T&E (as well as circular section multicore cables) but then as more thermal insulation began to be used in homes, problem arose. Partly it was realised that the allowance for thermal insulation in the regs wasn't entirely adequate, but also that the shape of T&E makes it not quite so bad as circular cables (larger surface area, less thermal insulation effect from the thinner sheath). This article from the IET (IEE as it was then) describes some of the background: electrical.theiet.org/.../establishing-current-ratings-for-cables-in-thermal-insulation.pdf In that context it's perhaps not surprising that table 4D5 concentrates on thermal insulation related installation methods. 4D2 remains the 'fallback' for anything 4D5 doesn't cover.

       - Andy.

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