Why does design current affect vd

[Eric12]

Hi, apologies if im having a bald moment but anyone know why Ib effects vd ? is it related to ohms law ?

 

[pc1966]

Basically it is ohm's law, though for large cables it is slightly more complicated as both resistance and inductance (i.e. impedance) are significant.

Ib is the design current, so what the circuit is expected to carry. That might be a fixed load, or it might be the maximum for a set of sockets, etc, determined by the supply breaker rating. At that current the impedance of the cable results in a drop in voltage which may or may not be acceptable, so the factor are:

• Cable impedance per unit length (basically inversely related to the size of the cable)
• Cable length
• Design current Ib
• Nominal voltage Uo

The nominal voltage is a factor as most specifications for VD give it as a percentage of the nominal supply. Hence very low voltages like 12V are impacted far more by a given drop than typical AC supplies at 230V, etc.

 

[Eric12]

At that current the impedance of the cable results in a drop in voltage

Now you put it like that makes absolute sense, really should have known. Im becoming complacent doing the formulas / cable calcs that the principles are eluding me. I hope I dont go bald ! Thanks for the refresher !