Voltage Drop Calculations

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demapanp

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I'm really confused about these voltage drop calculations and about finding a proper cable size to use for my situation.

I have 5 lights that need to be powered using a 120/240 volt single phase 3 wire source. The lamps are 120V, 300 watts each. The distance (one-way) from the source to the lamps is 3170 ft. Is the full load current just 1500W/120V = 12.5 Amps? Or is it different since I am coming out with a 120/240 V source?Can someone PLEASE send me the equations I need to calculate the line to line and line to neutral voltage drop and the %Voltage drop. Thank You.
 
Re: Voltage Drop Calculations

Holy Cow. 3170 feet :eek: :eek: is that one way?

I think you can give up on using 120V right now. I don't have the formula but the spreadsheet I made years ago says that to get under 3% drop you need to run 500 mcm. I think Mike had a voltage drop calculator in his free stuff page a while back. If not google it.

If you are using 300W fixtures, you can probaly go with 480V if you have it available. If not start looking at some transformers.
 
Re: Voltage Drop Calculations

Each of the two phases would carry half the load (for an even number of fixtures). You will have three fixtures on one circuit and two on the other. The neutral would only see the unbalance.
Calculate the volt drop for each 120V circuit individually.
The circuit with 3 fixtures would carry 900W or 7.5A @ 120V. It will be the worst case. You would need approx. 3/0 copper for that phase. Although the neutral would carry only unbalance, you need to upsize it to compensate for volt drop.
Don't forget to upsize the equipment ground conductor too.
 
Re: Voltage Drop Calculations

Thanks For Your Comments. Could you tell me if this is the correct formula to use.

Vdrop(line-to-line) = (2 x one-way circuit length x Load Current x Resistance of Cable per 1000 ft)/1000 ft. This was taken from the 2005 NEC Handbook.

Is this the equation I should be using for single phase three wire. Thanks Alot for Your Help
 
Re: Voltage Drop Calculations

3/0 cu would be correct if all the load was at the end of the circuit. If the lights are spaced out every 634' then the wire size would #1 cu.
I thought i heard someone say that airport light were run in series...is that correct?
 
Re: Voltage Drop Calculations

I believe the runway lights are run in series. However, these are for the Runway Alignment Indicating lights and they are not in series. These lamps are about 500 feet from each other. Do you know if the equation above is correct because I need to submit the calculations to my supervisor ASAP.
 
Re: Voltage Drop Calculations

Ok I broke down my spreadsheet and here is the formula I use:
Note 3 phase voltage drop is approx sqrt(3)/2 * single phase drop

Single phase voltage drop =
(L/100)*(I/n)*((R*pf)+(Xl*sin(Acos(p.f.))))/5
Where:
L= length (one way)
I= Load current
n= Number of parrallel feeders
pf= power factor
R and Xl are functions of the cable size used. see table below

AWG Xl R

12 0.068 2
10 0.063 1.2
8 0.065 0.78
6 0.064 0.49
4 0.060 0.31
3 0.059 0.25
2 0.057 0.2
1 0.057 0.16
1/0 0.055 0.12
2/0 0.054 0.1
3/0 0.052 0.079
4/0 0.050 0.063
250 0.052 0.054
300 0.051 0.045
350 0.050 0.039
400 0.049 0.035
500 0.048 0.029

edited to add note that I can't make the table look any better sorry

[ November 16, 2005, 08:02 PM: Message edited by: sceepe ]
 
Re: Voltage Drop Calculations

So, if the light are 500' apart then, the distance from the source to the first light should be 1170'+500'+500'+500'+500' = 3,170'

that would reguire 1/0 cu for VD% of 2.76
source voltage of 120/240v
 
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