# Voltage Drop Calculation Question?

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#### floridasun

##### Member
When figuring VD for outsiding lighting poles not to exceed 3% with long distance. Example 175 watt 120v poles spaced first pole 100ft from power source then 500,1000 and 1750ft to the last pole 20amp circuit. Do you still figure the total amps of all the lights with the farthest lt for the calc. or do you do the calc. for each lt. then average the voltage drop between them for sizing conductor sizes?

#### bob

##### Senior Member
floridasun said:
When figuring VD for outsiding lighting poles not to exceed 3% with long distance. Example 175 watt 120v poles spaced first pole 100ft from power source then 500,1000 and 1750ft to the last pole 20amp circuit. Do you still figure the total amps of all the lights with the farthest lt for the calc. or do you do the calc. for each lt. then average the voltage drop between them for sizing conductor sizes?
You caculate all of the lamps(4) thru the 1st 100 ft. Then 3 thru the 500 ft,
2 thru the 1000 ft an 1 thru the 1750 ft. Add the VD in each section the get the total at the last lamp.

#### Dennis Alwon

##### Moderator
Staff member
bob said:
You caculate all of the lamps(4) thru the 1st 100 ft. Then 3 thru the 500 ft,
2 thru the 1000 ft an 1 thru the 1750 ft. Add the VD in each section the get the total at the last lamp.
Bob are you saying that the 3 lights thru the 500 foot have to be calculated at 500 foot distance or 400 feet. It seems to me that since they are calculated already for the first 100 feet you wouldn't need to do it again. I don't know I am asking!!! Of course the same question is asked for the remaining lights---- 2 lights at 500 feet and 1 light at 750

#### Smart \$

##### Esteemed Member
Dennis Alwon said:
Bob are you saying that the 3 lights thru the 500 foot have to be calculated at 500 foot distance or 400 feet. It seems to me that since they are calculated already for the first 100 feet you wouldn't need to do it again. I don't know I am asking!!! Of course the same question is asked for the remaining lights---- 2 lights at 500 feet and 1 light at 750
Ummm... There are only 2 light poles through 500 feet. But in calculating voltage drop, it's the number of light poles powered through each segment of the circuit:
• The first segment of 100 feet powers 4 light poles,
• The second segment of 400 feet powers 3 light poles,
• The third segment of 500 feet powers 2 light poles, and
• The fourth segment of 750 feet powers 1 light pole.

#### Dennis Alwon

##### Moderator
Staff member
Smart \$ said:
Ummm... There are only 2 light poles through 500 feet. But in calculating voltage drop, it's the number of light poles powered through each segment of the circuit:
• The first segment of 100 feet powers 4 light poles,
• The second segment of 400 feet powers 3 light poles,
• The third segment of 500 feet powers 2 light poles, and
• The fourth segment of 750 feet powers 1 light pole.
Thanks smart
That's what I thought it was. I had a typo on the 3 lights at 100 feet. I meant 4. (Edited--- actually I think I said it correctly the first time)

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#### bob

##### Senior Member
floridasun said:
When figuring VD for outsiding lighting poles not to exceed 3% with long distance. Example 175 watt 120v poles spaced first pole 100ft from power source then 500,1000 and 1750ft to the last pole 20amp circuit. Do you still figure the total amps of all the lights with the farthest lt for the calc. or do you do the calc. for each lt. then average the voltage drop between them for sizing conductor sizes?

Smart \$ said:
Ummm... There are only 2 light poles through 500 feet. But in calculating voltage drop, it's the number of light poles powered through each segment of the circuit:
• The first segment of 100 feet powers 4 light poles,
• The second segment of 400 feet powers 3 light poles,
• The third segment of 500 feet powers 2 light poles, and
• The fourth segment of 750 feet powers 1 light pole.
I may be reading the origional post wrong. It sounds like the 1st pole is 100 ft from the source, 2nd is 500 ft from the 1st pole, 3rd is 1000 ft from the 2nd pole and the last is 1750 from the 3rd pole. If the distances show are from the source, then Smart you are correct.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
As long as the loads can be treated like a straight, linear resistance, this is a simple resistor ladder network. Start at the end: you have the last load and its two conductors in series, with that group in parallel with the next-to-last load.

Figure out that paralleled resistance, then place that value in series with the pair of resistances of the next-to-last conductor pair. Then you parallel that resistance with the third-to-last load, add that section's resistances, etc.

Once you reach the source point, you can calculate the current of the entire ladder, which gives you the voltage drop across the first (nearest source) pair of conductors. The voltage at the load end of this section is the first load's voltage.

You then use that same voltage to calculate the current through, and voltage lost along, the second section of conductors, giving you the second load's voltage and starting voltage for calculating the third run's current and its voltage drop, etc.

You will eventually arrive at the voltage across the last load, where you need to determine if that voltage is acceptable. If not, you need to upsize the conductors, starting with the run nearest the source, as it carries the entire circuit's current.

You'll probably end up cascading conductor sizes, such as #10 on the last section, #8 for the next-to-last, and so forth. If someone asked me to design this without doing the math, that's what I'd probably do, but it's better to do it by the numbers.

#### Snorks

##### Member
What Larry is describing is termed as a segment method series voltage drop solution. In all honesty, it is a lot of work to correctly compute these conductor sizes and corresponding voltage drops.

There are two software programs that have series voltage drop modules. One is EDR. EDR computes the VD correctly but is limited by using only the terminal temperature rating as its temperature variable. This will produce a very conservative answer that will result in higher copper costs. The other is Volts by Dolphins Software. Volts utilizies both ambient and terminal temperatures as separate variables as well as all ampacity tables. This will produce the correct conductor sizes with in the voltage drop limit with all environmental conditions taken into account (ie: above ground, below ground, in ducts, etc.).

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