Landscape Lights

[NYHigh]

This is actually a continuation of an old post I came across. I thought I had the voltage drop calculations down to a science until I read it. To simplify and make things easy I'll give exact numbers to eliminate confusion even though i'm the one who is confused here. Please with all due respect, serious inquiries only as to answer my questions correctly, you must take the time to look at what i am doing here if you will. Thank you.

EXAMPLE 1) Ok, so If you have one 60w lamp at the end of a 1000' run, I would do it like this:

2X12.9X.5aX1000'/3.6v = 3583cm(#14awg will be sufficient)

EXAMPLE 2) This is where i got confused. If four 60w lamps total were in that 1000' run, each 250' apart(the first lamp 250' from power source), i would do it like this:

To find wire size from power source to first light i would do this:
2X12.9X2aX1000'/3.6=14333cm(#8awg will be sufficient from power source to first light)

To find wire size from 1st light to 2nd light i would do this:
2X12.9X1.5aX750'/3.6v=8062cm(#10awg will be sufficient from 1st to 2nd light)

To find wire size from 2nd light to 3rd light i would do this:
2X12.9X1aX500'/3.6v=3583cm(#14awg will be sufficient from 2nd light to 3rd light)

To find wire size from 3rd light to 4th light i would do this:
2X12.9X.5aX250'/3.6v=895cm(#14awg will be sufficient from 3rd light to 4th light.

Questions:
1) Are my calculations correct?

2) Assuming my calculations are correct, if i do the reciprical of EXAMPLE 2 assuming an exact 120v power source, "WHY" do i exceed the 3% permissable voltage drop by using these size wires?

Power source to 1st light wire size check:
2X12.9X2aX250'/16510cm(#8awg)=.78134464vd.....Therefore 120v(at power source) - .7813446vd=119.2186554v(at 1st light)

1st light to 2nd light wire size check:
2X12.9X1.5aX250'/10380cm(#10awg)=.932080925vd...Therefore 119.2186554v(at first light) - .932080925vd = 118.2865745v(at 2nd light)

2nd light to 3rd light wire size check:
2X12.9X1aX250'/4110cm(#14awg)=1.569343066vd...Therefore 118.2865745v(at 2nd light) - 1.569343066vd = 116.7172314v(at 3rd light).

3rd light to 4th light wire size check:
2X12.9X.5aX250'/4110cm(#14awg again)=.784671533vd...Therfore 116.7172314v(at 3rd light) - .784671553vd = 115.9325598v(at 4th light) WHICH EXCEEDS the 3% voltage drop permitted which would be 116.4v?

What did I do wrong?

 

[LarryFine]

What did I do wrong?

It's obvious: you shouldn't have broken the pattern. You should have used #12 from the second light to the third.






Just kidding. Did you use the reduceed voltage at each node for the next run's calculation?

I'm honestly too tired to do any math, but I would have done the 8 - 10 - 12 - 14 gauge without doing any calculations. Check that out and see what happens.

 

[Dennis Alwon]

NY High-- I am not the best at this either but it appears you have the right idea. Using your formula it does not appear that you will have more than a 3.6v drop at the end of that circuit. With incadescent lighting it probably wouldn't hurt to use 5% (6V) drop and I believe you can use #10 all the way.

 

[steve66]

I'm not familiar with your formula, but it looks like its made to calculate the wire size necessary to keep the voltage drop to less than 3%.

But from a quick look, it looks like when you use the formula several times, it is made to calculate a maximum 3% drop at the first light, then ANOTHER 3% drop to the second light, and ANOTHER 3% at the third, etc, etc.

It doesn't look like this formula was made to be used this way.

Steve

 

[BAHTAH]

Voltage Drop

Voltage Drop

NY High-- I am not the best at this either but it appears you have the right idea. Using your formula it does not appear that you will have more than a 3.6v drop at the end of that circuit. With incadescent lighting it probably wouldn't hurt to use 5% (6V) drop and I believe you can use #10 all the way.

Using the "Load-Center Length" to determine the total distance related to ampere/ft of each load I get: 3.11 VD (2.59%) using #10 for the circuit with (4)60w lamps 250ft apart.
I get 4.94VD(4.12%) using #12. I calculated the LCL to be 625ft.

 

[cyriousn]

Voltage Drop

Voltage Drop

Assuming that you have 120v at the start of the circuit.....Aren't you supposed to find the drop at 250' with 60w. Take that new voltage then do the calc on 250' at 120w. Then take that new voltage then do a calc on 250' at 180w. Take that new number then do one more calc at 240w at 250'? If so with a #10 wire i got a final voltage of 117.83v which is only a 1.8 percent drop. Going with 8,10,14,14 yielded 113.69v which is a 5.25 percent drop....Critisicm is welcome as I use this forum as a learning tool! Thanks guys.

 

[Dennis Alwon]

Assuming that you have 120v at the start of the circuit.....Aren't you supposed to find the drop at 250' with 60w. Take that new voltage then do the calc on 250' at 120w. Then take that new voltage then do a calc on 250' at 180w. Take that new number then do one more calc at 240w at 250'? If so with a #10 wire i got a final voltage of 117.83v which is only a 1.8 percent drop. Going with 8,10,14,14 yielded 113.69v which is a 5.25 percent drop....Critisicm is welcome as I use this forum as a learning tool! Thanks guys.

First off, if you don't live in NJ the FPN is not code as stated by Bob. Secondly, if you do the vd formula VD= (2 x 12.9 x L x I) / CM
You would get this

VD= (25.8 x 250 X .5) / 16510 = .2 vd for #8 wire
VD= (25.8 X 250 X 1) / 10380 = .31 vd for #10 wire
VD= (25.8 X 500 X .5) / 4110 = 3.14 vd for #14 wire

Total = 3.65 VD which is right at the 3.6 VD suggested at 120 volts.

I think the op has it correct.

 

[hurk27]

Dennis you have to upload the file to the web site first before you can use BB code to display it:grin: the file has to have a web address, not a location on your computer

 

[Dennis Alwon]

Dennis you have to upload the file to the web site first before you can use BB code to display it:grin: the file has to have a web address, not a location on your computer

I didn't upload anything. I tried to copy and paste what is typed below all that goobly goop and nothing showed up till it posted .

I would get rid of it but time has past. Everything is there that "LINK" is not necessary.