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voltage drop calculations and parking lot lights

9K views 36 replies 13 participants last post by  Squidbilly 
#1 ·
Basic Information:
volunteer work for a local church
parking lot lighting w/ 4 GE M400 400W fixtures (3.9A each w/ HPS bulbs) on 2 poles (2 fixtures per pole)
fixture operating voltages: 108V - 131V per GE tech. support
120V AC (1 PH) 3-wire, copper conductor with PVC conduit
One 20A circuit feeds the fixtures
All 12 AWG wire (already is place when I arrived)

Relevant Distances:
segment 1: panel to center of parking lot (this point is geometrically between the poles): 175 feet
segment 2: center point to base of pole 1 or 2: 60 feet
segment 3: pole base to top of pole: 31 feet
segment 4: pole out to fixture at arms end: 14 feet

Wiring:
3 wires run from panel to center point of parking lot (175 feet) and feed:
3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 2
These wires run to the base and up to the top of the pole (31 feet) where they feed:
3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2


My question: In calculating the voltage drop for each fixture via a spreadsheet, I did it in segments as follows:
using Vd=2kIL/Cm (with 12.9 for k and 6530 for Cm) for each segment

* segment 1 *
load amperage: 4 fixtures*3.9A = 15.6A
length: 175 feet

* segments 2 and 3 *
load amperage: 2 fixtures*3.9A = 7.8A
length: 60 + 31 = 91 feet

* segment 4 *
load amperage: 1 fixtures*3.9A = 3.9A
length:14 feet


I estimated the total voltage drop (sum of voltage drop from each segment) at each fixture to be 13.8V or 11.5% of the supplied 120V.
However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.
Based on the calculations (and I do realize they are mere estimates if they are indeed correct), the lights will burn dim if the even come on at all.

Since I am probably rusty, I just wanted some advice on my calculations before proceeding.

Thanks in advance
 
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#21 ·
We are all wondering why the lights are not wired 240 volts single phase when you have all the wires necessary to change the wiring to that configuration. You will need to change the breaker to a double pole that's all. this of course will depend on if your using 120/240 single phase or a 120Y208 3 phase panel.
Second thing on the m400 ballast your going to have a series of tap wires each wire is for a certain supply voltage. You need to make sure they have the right tap wire chosen. The others are capped and not used.
 
#22 · (Edited)
That was my recommendation: wire the luminaires for 240V and run #10 to the center point.

For some reason, they completely ignored both and went ahead with #12 wire and had the fixtures (@ 120V) mounted and the poles erected via the local power company. I got a call tonight from the pastor that the fixtures would not light. Reluctantly, I went over and confirmed voltage at the base of the both poles. There wasn't much more that I could do at the time.

The voltage drop is indeed quite excessive (~11.5%), but their should at least be some response if the fixtures had been wired properly ... right?
 
#13 ·
It has half the voltage drop, but 1/4 of the voltage drop *problem*. The important parameter with voltage drop is not the absolute drop in voltage (5V or 10V) but the percentage reduction in supply voltage (2% vs. 8%). Doubling the voltage reduces the percentage voltage drop by a factor of four.
 
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#4 ·
No way #12 is acceptable under those conditions. The voltage drop is too great even at the first pole. HPS lights won't really burn dim like incandescents will, since the ballast will try to operate at rated current no matter the input voltage. However, running low input voltage will make the fixture harder to start (reducing effective lamp life) and make the ballast run hotter. It's not a good thing. #12 wire would probably be acceptable if you use 240V and ballasts that are rated 208-240V.
 
#5 ·
However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.
You should tell them they need to find some more knowledge.

For the life of me WHY would anyone run FOUR 400W MH parking lot fixtures at 120V???
Being a parking lot automatically screams voltage drop. 240V has 1/4 as severe VD as 120V.
 
#12 ·
Sorry Petey, not buying the inverse squared relationship on voltage drop. It's a simple inverse. 240V has 1/2 the VD of the same load at 120V. An Example:

1200W load at 120V is 10A (as seen at the source)
1200W load at 240V is 5A (as seen at the source)

Say the wire resistance is 1 ohm.

Since Vd=I*R, the Vd for the 120V circuit is 10V and the Vd for the 240V circuit is 5V. Of course this is only valid for relatively small Vd, since the actual load voltages would then be 110V and 235V respectively and the power would be 1100W and 1175W.

I think you are confusing voltage and power. The power lost by voltage drop is I^2*R, so dropping the raising the voltage to 2 times, drops the current to 1/2 and the power lost in the wire to 1/4 (as is seen by the 1100W versus the 1175W). However, the voltage drop is only 1/2 (10V versus 5V).

Mark
 
#14 · (Edited)
If I had been forced to use 120 volts I would have compensated in the first run to the center of the parking lot between poles with larger wire, then used 12 awg to each pole. I think a most 20 amp breakers would takeup to a #8 awg. Anyway I would play with wire sizes to see if that would get me reasonable Vd using 120 volts.

All this 12 awg and poles are in place but I take it that it is not in operation yet.
 
#17 ·
Thanks everybody for your help thus far!

Yes, #12 is in place, but the lights are not in operation yet. They want to use a timer on the inside to keep the lights off on nights when they are not there, and that has yet to be put in place.

In terms of voltage drop and playing with wires sizes, what voltage drop should be deemed reasonable for 120V? 3%? I seriously do not understand why they did not wire them for 240V. Otherwise, it is going to be be difficult to get the Vd under or around even 5% unless you use #8 from the panel all the way to the base of the poles (that would give Vd~5.1% by my calculations).
 
#20 ·
Thanks again for all the responses.

Just an update .....

Against my strongest recommendations, they went ahead with the wiring as is (#12 all the way) and had the local power company stand up the poles. Upon testing them tonight, none of the four lights will burn. Now, they are back to me asking why.
 
#24 ·
It could be anything. From a bad photocell to miswired fixtures to voltage drop.

As a side question, why would the POCO set the poles?
 
#26 ·
#28 ·
I contacted the power company today, and they wired the luminaires for 208V. So naturally, they were not going to light at 120V. I don't know why this wasn't communicated before now, but I am glad to see somebody knew #12 was too small for those fixtures @ 120V. So, some changes at the panel, and they should have lights. I will post back with results. Thanks again.
 
#33 ·
I doubt if you have 208 in a small venue like you described earlier. I suspect you are going to need to grow wings [or beg your POCO buddies back out with their bucket truck] to change the taps on the ballasts to 240V.
Yes, it is indeed 208V 3 phase. I went over today and confirmed that myself after hearing the fixtures had been wired for 208V.
 
#31 ·
I would expect the ballasts to work fine on the 208V tap. Might want to measure the operating current to make sure it's not higher than expected. If the current is correct, then the voltage is close enough to the design point that the ballast is regulating properly.
 
#32 ·
I would expect the ballasts to work fine on the 208V tap. Might want to measure the operating current to make sure it's not higher than expected. If the current is correct, then the voltage is close enough to the design point that the ballast is regulating properly.
The ballasts for the M400 HPS that have the multi voltage option are designed for 120/208/240/277 so if no 208 he will have 240.
 
#37 ·
Just a few side questions as we still try to figure out these lights. I made the changes in the panel and confirmed that 208V was being delivered to the base of the poles. However, still no lights. Hopefully the power company will be out today or tomorrow to check their work.

Is the NEC 215.2(A)(4) informational note No. 2 the only part of the NEC that makes voltage drop recommendations (< 5%).

Also, does the NEC address/restrict the use of colored ungrounded conductors in a 3 phase (208Y120) panel? The panel has black, red, blue with white and green as neutrals and grounds respectively.

Thanks.
 
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