Cable Resistance ( 20 Degree Vs 90 Degree Celsius) for DC Loads

[NickParker]

While sizing the cables for DC applications, Should I use cable resistance at 90 degree Celsius or 20 degree Celsius from the cable manufacturer catalogue?

Some other manufacturer catalogue shows both AC resistance (@ 90 degree Celsius) and DC resistance (@ 20 degree Celsius). Why not the DC resistance @ 90 degree Celsius?

 

[7anoter4]

For a.c. resistance you need to add skin effect, proximity and other [see for instance Neher and McGrath] factors which depend on d.c. conductor resistance at required temperature.
The d.c. resistance at any temperature it easy to calculate.
Rdc=Ro(K+Tc)/(K+Tref) where K it is a constant 234.5 for copper 228 for aluminium, Tc it is conductor required temperature and Tref it is the temperature of first resistance measuring[Ro], usually 20 or 25 degrees Celsius.

 

[7anoter4]

In IEC 60287-1-1 chpt. 2 Calculation of losses it is indicated the way to calculate these resistances, also.
2.1.1 DC resistance of conductor
The d.c. resistance per unit length of the conductor at its maximum operating temperature θ is given by:
R′ = Ro [1 + α20 (θ - 20)] where Ro is the d.c. resistance of the conductor at 20 °C (Ω/m) 1,7241 /10^8 for copper and 2,8264 /10^8 for aluminum [ohm.m]
It is simpler for ohm.mm^2/m for copper 1/58 and 1/35 for aluminum.Ro=leng[m]/area[mm^2]*ohm.mm^2/m
θ is the maximum operating temperature in degrees Celsius and α20 is the constant mass temperature coefficient at 20 °C [Table 1 copper 393/10^3 aluminum 403/10^3]