Opinion On Cable Ampacity in Thermal Insulation

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I don't understand how you don't reduce the rating if the cable is run straight through thick insulation. Surely people realise the effect it has?

We just don't... to my knowledge it doesn't appear to cause any problems. Maybe our insulation just isn't that great. No idea to be honest, but the fact BS7671 requires derating is just as much of a surprise to me.

 

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I don't know for sure, but if the normal cable current capacity rating is for a 60C rise (30C ambient to 90C max insulation rating) then doubling the current is 4 times the heat dissipation so you would see 270C conductor temperature (on the above crude assumptions).

For us 60*C, 75*C and 90*C is the temperature rating of the wire insulation itself. Nearly all modern wire is rated 90*C, but typically restricted to the 75 or 60*C column based on the temperature rating of the terminals and certain restrictions in rules going beyond 320. The 90*C column is basically used for de-rating purposes.

Here is Table 310.15 B 16:

https://imgur.com/Rd6tcPl

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In reality the cable doesn't actually reach 60*C, let alone 90*C. I'd imagine the cable doesn't go above these values in deep insulation.

 

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Don't get me wrong- we do have some rules that kick in around thermal insulation- just not as restrictive. Here is an example for romex:

https://imgur.com/Zyj1G6g

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And as stated in the rule- Table 310.15 (B) (3) (a):

https://imgur.com/N23PvwL

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Rule 310.15 (A) (2):

https://imgur.com/Qrgthhr

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