AC Voltage drop

[Mike pagano21]

Hi everyone, Question on voltage drop calculations. I understand vd and the calculations and use them alot in the field and school. My question is, when you figure vd out on, lets say a parking lot lighting project. 4 lights in a row. 4 amps per pole@ 208 volts. First pole @ 200 ft then each pole 100ft between each other for a total of 500ft. Now on a purley resistive ck the current drops off at each pole so at the last pole there is no current. The first pole would be #10 wire then after the 2nd pole the cm goes down even thought the distance increases. I have always taken the calculation to the last pole first which makes the conductors larger. On a purley resistive load the conductors will be smaller. Now on an non-linar load such as parking lot lights how do you all calculate your voltage drop? Because of the inductance should the Neutral not be dropped? Confused a bit. Please help. Thanks Mike

 

[steelersman]

Hi everyone, Question on voltage drop calculations. I understand vd and the calculations and use them alot in the field and school. My question is, when you figure vd out on, lets say a parking lot lighting project. 4 lights in a row. 4 amps per pole@ 208 volts. First pole @ 200 ft then each pole 100ft between each other for a total of 500ft. Now on a purley resistive ck the current drops off at each pole so at the last pole there is no current. The first pole would be #10 wire then after the 2nd pole the cm goes down even thought the distance increases. I have always taken the calculation to the last pole first which makes the conductors larger. On a purley resistive load the conductors will be smaller. Now on an non-linar load such as parking lot lights how do you all calculate your voltage drop? Because of the inductance should the Neutral not be dropped? Confused a bit. Please help. Thanks Mike

well forst of all you wouldn't have a neutral would you if the circuit is 208 volts. As for the resistive and non-linear thing I'm not sure i can help with that. I've always just done the voltage drop calculation and that's that. The wire should be larger usually by just one size to accomodate most voltage drop calculations. As for the current being zero at the last pole that would be news to me since they should be wired in parallel they should all have about the same voltage barring voltage drops between them which should mean each light should have the same current draw.

 

[Mike pagano21]

ac voltage drop

ac voltage drop

Sorry I meant return leg on 208 or neutral@120v. In a parrallel ac circut the current drops off at each load not voltage, try it. In a series dc circut voltage drops off at each load. When I figure vd i use the formula 2 x 12.9 x length x current divide by vd permitted. Going one size up on voltage drop is not always going to do it. Thanks for the comment

 

[steelersman]

Sorry I meant return leg on 208 or neutral@120v. In a parrallel ac circut the current drops off at each load not voltage, try it. In a series dc circut voltage drops off at each load. When I figure vd i use the formula 2 x 12.9 x length x current divide by vd permitted. Going one size up on voltage drop is not always going to do it. Thanks for the comment

so these lights are 120 volt?

 

[steelersman]

In a parrallel ac circut the current drops off at each load not voltage, try it.

I understand that. on the neutral coming off the last light you should have the sum of the currents of all 4 lights or roughly 16 amps. Each light you said uses approximately 4 amps.

 

[charlie b]

For a long string of parking light loads, I usually do the VD calculation in stages. Do the VD from the source to the first light (or the first group of lights), using the current associated with all the lights. Do the next VD from that point to the next group of lights, using the smaller current that flows downstream from that point. It's a hassle, I admit. But you don't wind up counting VD to the last light, using the current from all the upstream lights, since that much current doesn't flow all the way to the last light.

Welcome to the forum.

 

[Mike pagano21]

yea charles thats what I am asking. I was taking the length for the second light and third..... alway back to the panel so the last light would be 500ft. My orginal question was with the inductance would that make a difference in the calculation. Otherwise if it were 120v lighting would the neutral be overloaded as we dropped conductor sizes between the lights due to harmonics. Lets say from #8s to #12s. But yea I see your point and just wondered if everone was doing it the same way. Its easy on a single load form sure! Thanks again to both of you for your input. Mike

 

[steelersman]

yea charles thats what I am asking. I was taking the length for the second light and third..... alway back to the panel so the last light would be 500ft. My orginal question was with the inductance would that make a difference in the calculation. Otherwise if it were 120v lighting would the neutral be overloaded as we dropped conductor sizes between the lights due to harmonics. Lets say from #8s to #12s. But yea I see your point and just wondered if everone was doing it the same way. Its easy on a single load form sure! Thanks again to both of you for your input. Mike

I don't think you have anything to worry about as far as harmonics or upsizing the neutral is concerned. You can go here: http://www.mikeholt.com/documents/freestuff/other/VoltageDropCalculator.xls and scroll towards the bottom and download the free calculator. Hope this helps. Welcome to the forum.

 

[mull982]

I understand that. on the neutral coming off the last light you should have the sum of the currents of all 4 lights or roughly 16 amps. Each light you said uses approximately 4 amps.

would this 16A necessarily be measured at the last light? I would think that looking at a typical parallel circuit this 16A would be measured between the nuetral on the first light and the source.

When putting lights in series the voltage would drop across each light as was stated so the last light would see much less than the source voltage because of the drops through all the other lights. Does less voltage across the lights at the end of the series loop effect the lights?

 

[steelersman]

would this 16A necessarily be measured at the last light? I would think that looking at a typical parallel circuit this 16A would be measured between the nuetral on the first light and the source.

When putting lights in series the voltage would drop across each light as was stated so the last light would see much less than the source voltage because of the drops through all the other lights. Does less voltage across the lights at the end of the series loop effect the lights?

Yes you are correct. The current would only read the full 16 amps after the first light and source. I realized that a little while after posting that. Thanks for pointing that out.