zs=ze+(R1+R2)

[Ash]

When you do R1R2, it's not live, so are you not accounting for the resistance caused by heat when it goes live?

it's around 0.8 you work it out at or so in the regs I believe

 

[IQ Electrical]

When you do R1R2, it's not live, so are you not accounting for the resistance caused by heat when it goes live?it's around 0.8 you work it out at or so in the regs I believe

No, you're getting confused with the 'rule of thumb' 80% value which is applied to the BS7671 maximum Zs values to allow for increased conductor operating temperatures.