help needed with 17th edition please

[i=p/u]

so when would you do this adding of 1.2 in the real working world, if you have we example

you don't need to know thength to calculate the voltage drop of a circuit!
The simple formula ; r1 + rn x 1.2 x Ib = vd

 

[i=p/u]

i just know IxR=V

 

[telectrix]

well that's what he means . V ( volt drop) = I x R, where R is R1+R2, then correct for temp. with the 1.2 factor.

 

[i=p/u]

what reg is this telectrix? so when would you use this, would you just not measure it, give you awnser ... or is this when designing