voltage drop on lighting

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Old 11-15-07, 04:22 PM
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voltage drop on lighting

We just started wiring a furniture store and we have both 277v flourescent lighting in the showroom and 120v track lighting. My question is regarding the track lights. There are 12 sets of track lights in various spots... each section has 6 heads on it with a 150w par 38 lamp in them. The prints originally called for 12 seperate circuits... one for each section of track, but our company is refusing to put 12 switches in and has since talked it over with the owner and agreed on 6 switched circuits instead. Problem is we already had it roughed in for 12 and we only used 4 sets of 12/4 mc cable... 2 sets have since been relocated elsewhere since they eliminated half the circuits. I am a little worried about voltage drop playing into this and causing the lights to dim quite a bit. Due to the design of this building with no ceiling in it... we don't have the luxary of taking the shortest route to the lights so it has to be very neat... and because of it, I might have some runs in total from the panel of 160'-180'.... at the most 200' on one maybe. I have mentioned this too several people as well as our boss and he doesn't seem to concerned with it. So I am asking all of you.... will there be a noticeable issue with running the lights that far out on #12's? Will the distance eventually draw more amperage and trip the breaker possibly? When figured out, it is going to be 1800 watts/15 amps/120v. If so, since the wire is already installed to the switch and would be a pain to redo not to mention I doubt our boss will even consider re-running the existing stuff... would running #10's to the lights from the load side of the switch help out at all or is is already screwed from being #12's from the panel? I have never had and issue on long runs in 277v. and ballasted fixtures since I think they have more tolerance in voltage... however I don't ever do incandescent stuff so this is new to me.

Thanks in advance...
 
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Old 11-15-07, 06:00 PM
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Resistivity of #12 Copper wire is 5.3 ohms/km

So for 200' of #12 wire the resistance should be 0.061 km times 5.3 ohms, which equals 0.3233 ohms.

So, does that mean a drop of 4.5 V @ 15 Amps?
 
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Old 11-15-07, 06:20 PM
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1800 watts of lighting, at 200 feet... you're going to have some kinda dim lights. A rough rule of thumb is to upgrade to #10 at 100 feet distance, and to #8 at 250. So I'd definitely kick up the wire from the switches to the lights to #10. Don't worry if part of the run is in existing #12 though, I can understand that replacing switch legs is probably more than the management will stand for. If most of the run is in #10, the voltage drop won't be so bad.

As an aside, the reason this problem doesn't turn up as much with 277V circuits is that most 277V lighting circuits aren't heavily loaded. One 277V flourescent fixture typically uses around 0.4A. So twenty of 'em are only drawing 8 amps, whereas your incandescents are pulling 15. If you put forty such fluorescents onto a 200 foot home run, you'd get voltage drop issues.
 
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Old 11-15-07, 06:20 PM
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So you have 2 sets of track lights on one circuit? If that is the case, and you have 1800 watts of load with 15 amps like you said. Assuming both sets of lights are 200 feet from the panel, I am getting almost 10% (5.79 volts line to neutral) voltage drop at the end of the circuit, which is way too much. You generally want to limit your voltage drop on branch circuits to around 3%. If you have one set of lights closer than 200 feet then the voltage drop on that light will be less. The lights will not draw more current and will not flicker, but they will have less lumens output due to the decreased voltage. Any chance of getting each set of lights on their own circuit?
 
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Old 11-15-07, 06:35 PM
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i dont know where to start being this is a commercal building.

anyway let start with the distance 180-200 feet is pretty serious long run. with #12 wire with 1800 watt total load per circuit ? is that what i am readign right and someone mention that you have 12-4 MC cables if so is that set up to run in MWBC format.

but IMO 150 W par spotlight some case can be overkill most commercal places i intercounted that most used 90 w PAR halogen spots [ it about the same as plain jane 150W PAR can dish out ] that reduce the load some degress.

what it should be done in first place is run with #10 instead of #12 that will slove most of the issue with long run.

the 277 volt flourscent system dont have much issue with voltage drop with that long run i done more longer than that but the main point is that keep it to 3% max voltage drop [ most 277 V ballast can work slghtly lower voltage as long it withen the spec]

the other way you can verify it is check the voltage at end of run with the load on you can see how much votlage it can crank out at the end of run and check at the breaker panel to compared the voltage.

some case but it do show up from time to time the transformer can have wrong taps on it too. [ only if the breaker box votlage is reading too low then can tap up a notch.][ if have that feature ]

Merci, marc
 
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Old 11-15-07, 07:13 PM
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In order to give you something detailed to take to your boss, and because I'm feeling geeky, I'm gonna run down some math.
150W bulb = 96 ohms
12 bulbs (in parallel) = 8 ohms
100ft. home run, 12AWG wire = 0.396 ohms
Percentage of voltage drop on the wire = 0.396/8.396 = 4.7%
(Maximum recommended voltage drop on branch circuits is 5%, and even that assumes that your panel is at 120V to begin with, so this just barely skims by)
So at 100 feet, your bulbs are running on 95.3% of line voltage, or 114V.
At 114V, the "150W" bulbs will put out 135W.
200ft. home run, 12AWG wire = 0.792 ohms
Percentage of voltage drop on the wire = 0.792/8.792 = 9%
At 200 feet, your bulbs are running on 91% - 109V.
At 109V, the "150W" bulbs will put out 124W (and be noticeably dingy yellow)

Now let's replace the 200ft of 12AWG with 10AWG
Total resistance = 0.496 ohms
Voltage drop on the wire = 0.496/8.496 = 5.8%
Bulbs are running on 94.2% - 113V.
At 113V, the "150W" bulbs will put out 133W.

A 200 foot run is actually bumping into the point where you should switch to 8 AWG. Running it in #10 will still mean a noticeable voltage drop, but leaving it in #12 will mean your bulbs will be sickly yellow.
 
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Old 11-15-07, 11:05 PM
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Thank you everyone for your fast replies... Yea I know about wanting to keep it in the 3% max voltage drop area and with 12's there is just no possible way... not even with 10's for that matter. What I am thinking about doing is getting an accurate measurement on the wire runs we have now and what I have left to run in the morning and seeing what I realisticly have. What I might just end up doing is tearing out the 12/4's and buying some 10/2 and re-routing it all. Only issue is I am really going out on a limb doing that cause I am not in the mood for butt chewing just because I tried to do something right. Yes dzdave... it needs to be on it's own circuit... thats what it calls for but I am in a rock and a hard place on what the heck I should do and who I should listen to. I can see trying to go cheap on some stuff but this is just stupid in my opinion. All I know is to try to change this after the fact when the issue arrises when the walls are rocked and the ceiling grid is up is going to be a nightmare... I'll think about it and maybe bring it up to the boss again. But if he says one more time to do it his way... well hey his choice and his problem I guess.

I guess one last question I do have however is that with that voltage drop the circuit it naturally going to try to draw more amps to try to make up for it... not sure if it will be significant enough to make it trip the breaker over time but is there any issues on significantly overheating the wire if it is under continuous use like that? I am not worried about fire as I am sure if it was a big issue it would trip the breaker b/c of the excessive amp draw which may or may not happen... but is it just a loss of voltage because of the length that is something you have to worry about or is there a chance of the wire heating up and causing a saftey issue in a case like that... kinda asking for this and future referance?

Anyways I appreciate everyones input... helped me out... now I have more insight on what to go off of... thanks again and take care.
 

Last edited by morpheusoptic; 11-15-07 at 11:12 PM. Reason: added another question
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Old 11-15-07, 11:30 PM
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Do you work at the place normally or is this a contract?
Can they come back to you if you do it the way they want and say it sucks, the lights are yellow, we're gonna kill you?

I'd almost be tempted to get it in writing that you warned them , and that they are okay with the choice they made.
 
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Old 11-16-07, 03:30 PM
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Originally Posted by morpheusoptic View Post
I guess one last question I do have however is that with that voltage drop the circuit it naturally going to try to draw more amps to try to make up for it... not sure if it will be significant enough to make it trip the breaker over time but is there any issues on significantly overheating the wire if it is under continuous use like that? I am not worried about fire as I am sure if it was a big issue it would trip the breaker b/c of the excessive amp draw which may or may not happen... but is it just a loss of voltage because of the length that is something you have to worry about or is there a chance of the wire heating up and causing a saftey issue in a case like that... kinda asking for this and future referance?
This is actually a misconception. Motors behave the way you're saying, but not a resistive load like an incandescent bulb or a wire. Inside a motor you've got a pair of opposing magnetic fields seeking a balance point. Some power supplies, most notably switched DC computer supplies, alter their operation on the fly to ensure stable output and therefore vary their amperage input.
Your bulbs are just resistive loads in series with the resistance of the wire. The higher the wire resistance, the lower percentage of the voltage seen by the bulb. So your light output simply does down... in fact, since the total series resistance goes up with longer wire, the total amperage goes down as well. So heat isn't a problem, it's just the functioning of your device that'll be compromised.
 
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Old 11-16-07, 04:11 PM
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I work for the place full time fungku so yea they probably could still come back on me even if the boss said to do it a different way. Today I just said forget it and don't care what they want me to do. I just split it in to 12 circuits like it was originally supposed to be in the first place... and on runs longer than 150' I am going to put in #10's. I may get chewed out for it but than again I sure as heck am not fixing it after the fact when the problem arises because that would be a pain. I think I still am going a little further on the 150' run's in #12's than I should for the 3% voltage drop but ah I can live with that... and I feel alot better about it now even though I might be walking on thin ice when the job is done if they find out what I did. Either way I am a 4th year apprentice about 2 months away from taking my test and this is the first job I can say is totally mine... I have a state card letting me work alone so thats what they decided to do is get me out and see what I can handle. I have worked under alot of guys that skimped on quality before and I am just not going to allow that on something I do myself so thats why I have been questioning about it. Either way, problem solved for now. Thanks everyone for your help... I appreciate it.
 
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