I have to run some cable (direct bury) from the main panel in the house to two poles for lighting a barn area. Clients can’t afford anything bigger than #10 uf wiring ($.62/ft from my supplier)…First pole is 175 feet from panel..cable will come up to an ext. outlet from which they will plug in at least one 1000w halogen lamp..cable then continues to next pole, 100′ further away.. Then same deal here–outlet to power another 1000w halogen.
This set up is entirely their idea..the lights they intend to use are regular old shop/work lights from HD (pulled right off the tripod)..Let me just say that they rejected my plan for exterior HPS lights and higher gauge wire to mitigate the voltage drop.
Question 1: My voltage drop calculator says for the total load on the circuit (about 17 amps) and the total distance on the 10awg wire being 275 feet, the drop will be 11.3 volts, or 9%. But the total load is only ‘felt’ at the first pole–which is 175′ from the panel. So I reason that the first light pole will experience a lessor drop (17 amp total load on 175′ = -7.2v drop)..and then the next light being 100′ further down stream has to subtract another 2 volts (8.3a load on 100′ 10awg = -2v), for a total of -9.2 at the second light? Why doesn’t the math add up?
In other words, assuming both loads are on, does the second light experience more of a voltage drop (and power loss) than the first light? Or do they combine as a ‘system’ to both experience the same (total) voltage drop on the circuit?
Next question: what is the effect of an 11v drop on a 1000w halogen lamp? Will the light intensity be significantly–noticeably–less? Can I calculate how many lumens will be lost? Will the bulbs burn out faster? ..Am I being overly anal in my figuring for this job?
Replies
If they can't afford it, they can't afford it. Period.
The plan has a few flaws and code violations in it.
First of all, each of those lights is large enough to require it's own circuit. That is, two 15 amp circuits. No, you can't use a single 30 amp circuit ... unless you're also putting breakers in at the barn. Either way, this is no longer a single circuit, and the barn needs a ground rod.
But, with that distance, you won't get 30 amps through a #10. You need bigger wire.
The light choice is a poor one. Not only do halogens have a very short life, you don't really get much light for your money. Even a 250 watt Metal halide light will give better light than a 1000 watt halogen ... and a 400 watt MH is quite a bit brighter. Replacing the lights with metal halide, or even mercury vapor, lights will solve the electrical issues. The only 'drawback' is that the lights take a few minutes to come fully 'on.'
I'm sure HD has these lights at less than $60 each!
As far as wire costs are concerned, you can likely run PVC pipe and 'ordinary' wire for a similar price ... and then the customer will be able to upgrade later, without digging.
Indeed, digging is the major part of this job ... code wants the UF a good, honest 24" down ...
If the customer can't / won't have the job done right, it's best to give it a pass. Period. I guarantee that, once they have power out there for the lights, it will be used for other purposes as well- starting as soon as your truck leaves the drive! It is also possible - if they keep livestock - that the code has some special grounding provisions for the barn.
As for the effect of voltage drop on lighting ... that 11 volt drop will result in the lights being a LOT less bright than they ought to be. Maybe only half as bright.
You said they want to use lights from a work light stand. This makes me wonder as to how the lights will be mounted, and connected to the power. If you're putting in receptacles, then they need to be GFI type- with 'bubble' covers.
My 'hillbilly' alarm is ringing loudly on this one, and my first impulse is to decline the work.
Thanks for the info, reno...Yea, hillbilly is right. They initially had 12g extension cords running the whole 275' (and then some). They complained that the fan they were running was "going real slow". ;-) And the 'breaker kept popping. So I explained all this to them and had planned for HPS lights (total load of 9 amps) and a #6 wire to a sub-panel..Cost was "way over budget".. Of course. Final possibility is running a 10/3 connected to two 20a circuits in the main panel giving a separate circuit for each pole (w/ a shared neutral). V drop goes to -3.5 on the first light and -5.5 on the second light (if they still use the crappy halogens). Does this sound feasible?If not, I give up.
Yes, what you plan would work, and can be done legally.
I was going to suggest a 10/3 multi-wire circuit. Protect it with a 240/120 GFCI (and you need GFCI at the receptacles anyway) and you can bury the wire at 12".But they are still going to be limited to what they can draw at that distance, but should be OK for JUST the lights. But I have not calculated the VD.But what they will pay for in a years worth of replacement bulbs and the extra power will doing it correctly..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thanks everyone for the replies..Yea, they are cheap people. I hardly feel like helping them for all the trouble I've gone through trying to do the 'right' thing and the 'best' thing---they could care less..They want it done for the cost of a subway sammich.Bill, you said "Protect it with a 240/120 GFCI (and you need GFCI at the receptacles anyway)"..you mean if I use a GFCI dual breaker I don't need the GFCIs on the outlets, right?
It is not hard to do this job right but you seem to have basic problems.I think you got the voltage drop wrong. And you got the amps wrong also.Perhaps you should let a professional do the work.
>>I think you got the voltage drop wrong. And you got the amps wrong also.I didn't calculate the voltage drop...this site did: http://www.electrician2.com/vd_calculator.htmBut it seemed pretty accurate in my limited experience. And the amperage was strictly the manufacturer's rating on the 2 1000w lights (8.3a each). >>Perhaps you should let a professional do the work.Obviously, they couldn't afford a professional...that's why they hired me! ;p
Yes, I see that I wored it poorly.The receptacles need GFCI protection in any case. So instead of getting one or two GFCI receptacles (if a multiwire circuit) you can instead use the 120/240 GFCI breaker..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thanks..that's what I understood you to be saying anyway.
Also, for my future reference, Does anyone have an answer to my previous question...i.e., for both lights on the same circuit, "assuming both loads are on, does the second light experience more of a voltage drop (and power loss) than the first light? Or do they combine as a 'system' to both experience the same (total) voltage drop on the circuit?"
Real curious about that..
Yes, the 2nd light would see extra drop.Start at the first light and figure the voltage drop starting from nominal line (120) and the total current for both lights.Then for the 2nd legs figure the drop starting with the voltage at the first light and using the load for just the 2nd light.That is the right calculations, but will be off a little as light bulbs are very non-linear and at the lower voltage they won't be as hot and the resistance a little higher. But I don't have an curves on them off hand.Also at the lower voltages the halogen cycle won't be as effective and the bulbs will have a shorter life..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Also, for my future reference, Does anyone have an answer to my previous question...i.e., for both lights on the same circuit, "assuming both loads are on, does the second light experience more of a voltage drop (and power loss) than the first light? Or do they combine as a 'system' to both experience the same (total) voltage drop on the circuit?"
You are correct, the two lights are in parallel and they both draw current through the wiring connecting the first light, and the second light carries current only through the wiring from the first light to the second light, so that leg would have a lower voltage drop given the same wire size as the first run.
I read this post and cannot believe how cheap some people are. Why would they hire somebody to do the job if they can't afford even the materials? Unless you need to pay your rent yesterday, I wouldn't do it.