Calculating Amperage for Voltage Drop

[Jaybird77]

I was wondering if anyone can tell me how amperage is calculated in regards to the voltage drop formula. For a designed circuit do you calculate off actual volts to be used without regard to what size breaker is used? Or would you have to use the available amperage according to breaker size? If voltage drop was calculated off of a expected 10A usage but extra lights were added to a circuit since a 30A breaker was installed it would change the end of line voltage drop (assuming the circuit length was not extended).

 

[480sparky]

Voltage drop calcs require the actual ampere load. Breaker size is not relevant to the equation.

 

[Jaybird77]

Perfect. That's what I was looking for. Thank you.

 

[kwired]

To add to what has already been mentioned - if you had a 30 amp breaker, the load is likely less then 30 amps. To calculate actual VD, you need to use actual current.

Now if that breaker is 30 amps to allow starting of a motor - you may have a running VD based on say 17 amps, but VD during starting very well is from current well above 30 amps for a short time - if critical to reduce effects of VD during such starting - then you need to know what that starting current is. For such things like that you can upsize a conductor and still have VD because of source impedance though.