Voltage Drop Question Via a PM

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iwire

iwire

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A question I received via a PM.


kpdci said:
My question about voltage drop deals with this.

My run will be 60' from a building where at that point I plan to splice a tee and run a line going East which will be 190' and the other run going West will be 220'.

In my calculation, would I add up all three numbers or just the 60' + 220' in determining my total length?

Thank you,
Kpdci
Click to expand...
 
I will go out on a limb here since this is not my area of expertise.

I would figure the VD for both the 60' and 220' and also for the 60' and 190' and see which is greater. The wire size for the 60' run could be based on that. You could then do another calculation for the each of the 190' and the 220' and possibly reduce the wire size for those runs.

This makes sense to me on a logical level however I am not an engineer so I may be missing something.
 
Do you want or are you required to have the same size conductor for the entire length?

Figure the VD from the origin to the T at full load. Calculate and size wire at no more than 2%. Now calculate the VD at 3% for the load going East and West. Size those wires accordingly. You now have no more than 5% VD for each load.

Lots of other things to consider but in a nutshell that will work.
 
ptonsparky said:
Do you want or are you required to have the same size conductor for the entire length?

Figure the VD from the origin to the T at full load. Calculate and size wire at no more than 2%. Now calculate the VD at 3% for the load going East and West. Size those wires accordingly. You now have no more than 5% VD for each load.

Lots of other things to consider but in a nutshell that will work.
Click to expand...

One question.
What are the loads for the two different legs?
 
As mentioned, calculate the total voltage drop from the branch breaker to the tee using total current, then calculate the remaining voltage drop from the tee to the load. Add the voltage drop from the source to the respective load. From the tee out to the two loads, the voltage drop will not effect each other (loads), but the overall load on the origin to tee portion does matter.
 
ptonsparky said:
Now calculate the VD at 3% for the load going East and West. Size those wires accordingly.
Click to expand...
That might not be possible. The OP did not mention the intent to install additional overcurrent protection at the "split point," and we don't have enough information to decide if any of the tap rules apply.
 
it depends entirely on the current flow in the east and west legs, which was not stated.

figure the resistance (both ways) of the east and west legs in Ohms. Multiply that by the current in amps. That will give you the VD on each leg. the VD on the short leg is the sum of the 2 currents times the resistance of that leg.
 
Voltage Drop

Voltage Drop

Thanks to everyone that helped me on this. I was saying 230 vac when the voltage is really 208 vac.

The required current for each of the two directions is ((2x400 = 800 M-H Watt bulbs / 208 vac = 4 amps) from the Tee for a total of 2x4 = 8 amps at 208 vac single phase.

I just wasn't sure if I was suppose to figure the length for each run, the longest run, or each run from the Tee.

The voltage drop should be within the 3% allowance at 70' from inside the building to the Tee and at 8 amps. If calculating the one direction at 200 feet at 4 amps, this should be within the 3% allowance right?

Does everyone agree with this?
 
worst case is the 220' leg.

if you are using #12 wire and have 4A of current, the VD in that run is approximately 4*1.93*440/1000 = 3.4V.

1.93 being ohms/1000 foot from annex a
440 being the total amount of wire in the run.

the 60' leg VD is approximately

8*1.93*120/1000 = 1.85 V.

The total VD is approximately 5.5 V.

If using #14;

vd = 4*3.07*440/1000 + 8*3.07*120/1000 = approx 8.7V.
 
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