Why does insulation effect cable sizes?

[mildmanneredjanitor]

Just been reading through various posts and the subject of insulation came up again.

Why does insulation effect cables and reduce the current handling capacity???

I can't get my head around that one?


Has it got anything to do with the insulation and chemical reaction?


So much too learn...

 

[plugwash]

because it significantly reduces heat disapation so the cable will run hotter at the same current

also there are some types of insulation that cable should NEVER be in direct contact with because they attack the plastic polystyren is the best known of theese

 

[masona]

So is this why SWA has a higher load rating than T&E ?

 

[plugwash]

xple swa has a higher current rateing because of its higher core temp rating (90 celcius as apposed to 70 celcius)

i dunno about pvc swa i don't have the figures but i can't imagine it's much diifernt to pvc twin and earth though the different cable structure may count for something

 

[mildmanneredjanitor]

because it significantly reduces heat disapation so the cable will run hotter at the same current

I'm not disagreeing with you, but surely cables don't heat up that much??? If they are then surely they are inadequately rated?

Or am I missing the point... They don't heat up normally because the heat is dissapated, but within thermal insulation, the heat cannot escape...

Think I inderstand now!

Are there limits to how many cables are bundled together???

Or does this not have a significant effect?

And finally, does this also count if the cable is resting on insulation and not within insulation?

 

[plugwash]

there are ratings for different basic installation methods
and then further derating factors beyond that

here