Voltage Drop Different cables in circuit

Designer69

Senior Member
Need verification:
If I have a branch circuit consisting of say; a 50’ length #12AWG and 100’ of #6AWG.
To calculate total voltage drop, you calculate the voltage drop on the 50’ #12, calculate vd on the 100’ #6 and add them together to get total?
thanks
 

Fitzdrew516

Senior Member
Location
Cincinnati, OH
Need verification:
If I have a branch circuit consisting of say; a 50’ length #12AWG and 100’ of #6AWG.
To calculate total voltage drop, you calculate the voltage drop on the 50’ #12, calculate vd on the 100’ #6 and add them together to get total?
thanks
The answer to what I think you are asking is yes. Here's an example to make it more clear.

Circuit voltage = 120V

Total circuit length = 150'

First run is 100' of #6 CU and has (2) 150w lights that need powered at that location.

Second run is 50' of #12 CU and has (1) 150w light that needs powered.
--------------------------

The first run would have 3 lights/450w on it (or 3.75A). So you calculate that voltage drop over 100' with #6's which equals about .372 total volts dropped over that run.

Then do the second run the same way except you only need to calculate 1 light/150w load on this run (1.25A). Calculated over 50' with #12's is about .252 volts dropped.

Add both together (.372+.252) = .624 Volts dropped on the circuit.
 

Designer69

Senior Member
thanks guys but would arrangement matter?

what if the smaller #12 is first then the larger #6 terminates at the load?
 

Fitzdrew516

Senior Member
Location
Cincinnati, OH
thanks guys but would arrangement matter?

what if the smaller #12 is first then the larger #6 terminates at the load?
You would drop more voltage because that would mean the #12's would be carrying all of the load for the first 100' (in this example) and then the number 6's would only carry the load of one light for 50'. It wouldn't make any sense to do it this way really. I can't think of an application where this would be used unless it was some sort of a half existing application or something like that.
 

iwire

Moderator
Staff member
Location
Massachusetts
You would drop more voltage because that would mean the #12's would be carrying all of the load for the first 100' (in this example) and then the number 6's would only carry the load of one light for 50'. It wouldn't make any sense to do it this way really. I can't think of an application where this would be used unless it was some sort of a half existing application or something like that.
A typical situation for me would be to leave a 20 or 30 amp breaker with 10 AWG for some length, join onto a 6, 4 maybe 2 AWG underground out to site poles and then reduce wire size from the pole base up to the fixture.

The length of the 10 AWG is kept short enough to not be a problem.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
thanks guys but would arrangement matter?

what if the smaller #12 is first then the larger #6 terminates at the load?
If you have a single load at the end of the run, it does not matter, but if you have multiple loads, you need the larger conductor to be at the beginning of the run.
 

Carultch

Senior Member
Location
Massachusetts
Need verification:
If I have a branch circuit consisting of say; a 50’ length #12AWG and 100’ of #6AWG.
To calculate total voltage drop, you calculate the voltage drop on the 50’ #12, calculate vd on the 100’ #6 and add them together to get total?
thanks


If they are in series, and all the current is drawn at the end of the combined run, you add up voltage drop percentages of each portion of the circuit.

If current branches off throughout the run, each portion gets considered with its wire size and corresponding current. Applicable portions that are in series accumulate the voltage drop percentages.

If they are separate loads, you keep track of them separately, and pay attention to worst case scenario.
 
Last edited:

Carultch

Senior Member
Location
Massachusetts
If current branches off throughout the run, each portion gets considered with its wire size and corresponding current. Applicable portions that are in series accumulate the voltage drop percentages.
Example:
Load 1 and Load 2 are both 5A loads.
Circuit 1 is the main branch circuit to both Load 1 and Load 2.
Load 1 is connected immediately at the end of Circuit 1.
Circuit 2 continues from the position of Load 1, onto Load 2.

Voltage drop across circuit 1 is 1%, based on the gauge/length of Circuit 1 and a combined load of 10A.
Voltage drop across circuit 2 is 0.8%, based on the gauge/length of Circuit 2 and a load of 5A.

The voltage drop from the source to Load 1 is 1%.
The voltage drop from the source to Load 2 is 1.8%.
 
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