Voltage Drop Design Current?

[junksamiad]

When calculating voltage drop and using the formula

mV/A/m x Design Current x length / 1000 = v

Do I use the design current value after rating factors have been applied?

Thanks again for any help.

 

[Andy78]

When calculating voltage drop and using the formula

mV/A/m x Design Current x length / 1000 = v

Do I use the design current value after rating factors have been applied?

Thanks again for any help.

No you would use your design current, Ib.
Design current after derating factors is no longer design current.

Think about the calc. Design current is what you want the circuit to be able to continuously handle.

Derated values only apply to the cable used for that job.

 

[pc1966]

Check your units! Basic computation is:

voltage = (design current) x (resistance per unit length) x (length)

For whatever reason the resistance[*] is often given as volt/amp/km or similar. The volts/amp part is ohms and if it is given per km then it is equivalent to milli-ohm per meter (1ohm/1000m => 0.001 ohm/m).

As already explained the current after the various derating factors is used to select a cable from a current carrying point of view due to heat and is not a 'real' value from a circuit point of view. Rather than try to adjust cable specifications according to the environment, it is simpler to compute an "equivalent heating current" that you can then use with tables of cable capacity under basic conditions.

* Strictly speaking it is impedance but for most cables it is mostly resistance so we will call it that!