Circuit Design for Driveway Lights

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  #1  
Old 10-05-05, 01:47 PM
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Circuit Design for Driveway Lights

I learned much from reading these forums. Thank-you all. Now, it's my turn to ask the experts. I hope you all can help.

I'd like feedback regarding designing a Driveway Lighting Circuit. I have 11 single element lights at 100 foot intervals, starting at 500 feet from the house panel, for a total distance of about 1,500 feet. There is an outlet for Xmass lights on the last pole. I put the buried wire in the ground when we built the house several years ago for this. The wire is 10/3 and ends in the basement next to the panel. The voltage drop and potential 220 volt circuitry are new to me, so here are some questions.

First, how can I wire the lights to minimize voltage drop, and maximize bulb wattage? (I've heard first 7 on one phase, last 4 on another phase, alternate every other light??) Can you run two hot, on one common?

Second, what breaker should be used? (double pole 220v/20amp?)

Third, what size switch/timer should be used? (are there 20amp + 20amp = 40amp residential switches?)

Forth, can I use 12/3 gauge wire for the runs in the basement, ie. breaker, switch/timer, or must I stick with 10 gauge? Can I use 14 gauge outside on the lights?

There's a start. It might not seem complicated to you, but it's over my head. Thanks for any help.
 
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  #2  
Old 10-05-05, 01:57 PM
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I'll let someone else think about he voltage drop issue. Voltage drop is a concern. I've heard several theories, but nothing jumps out at me.

Is this wire just run in the ground now? How did you run it? In conduit? How do you intend to tap into it? Did you leave slack? You will need at least 12 gage wire to tap into it if you expect to install a 20 amp breaker.

They make 240 volt timers. Intermatic is one brand that comes to mind.

I would stick with 10-3 cable between the breaker and the timer.

If you expect to run a multi wire circuit (two hots, one return, one ground), then use a 240 volt breaker.
 
  #3  
Old 10-05-05, 02:34 PM
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I would make this a 20A multi-wire circuit, alternating every other light on each hot leg. Wire the whole thing using #10 if that's what you already have. You could use #12, but stay with #10 for consistancy.

You will have major voltage drop, as much as 20-30%, but with just incandescent lighting the only side-effect will be dimmer lamps. For example, a 100W bulb will shine with the same brightness at the end of the run as a 60W bulb would near the house. You can counter-act the voltage drop a little by using 60W bulbs in the lamps near the house, 75W midway and 100W bulbs near the street so long as your fixtures are rated for 100W bulbs. If you don't mind the dimming in the distance, use all 100W bulbs.

The breaker will be a 20A double-pole GFCI breaker; expect this to be a special order for about $100. The lamps will share the neutral.

For the timer, use a standard 240V hardwired timer such as an Intermatic. They are commonly sold for pool pump motor controls. Add up the maximum wattage of all your light fixtures and divide by 240 to get the minmum amperage of the timer.
 

Last edited by ibpooks; 10-05-05 at 02:46 PM.
  #4  
Old 10-06-05, 07:45 AM
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Thanks for the input. I think I'm getting it. As far as the voltage drop, a 20-30% reduction in voltage seems like a lot!

If I use a multi-wire circuit, does that reduce the Vdrop on each leg to 10-15%?

I was thinking of using the new electric ballast fluorescent lights. These only draw about 25 watts each. Dose anyone know if there will be enough voltage to spark them on when its cold out? Say 90 volts at 20 degreeF??

Will there be any lack of circuit effiecency if I use 12g wire in the basement, and 14g wire just at the poles? I think the wire gauge on typical lighting fixtures are just multi strand 18g?

Does the breaker HAVE to be a GFCI? That seems redundant? I was going to use a GFI at the outlet.

Thanks again all!
 
  #5  
Old 10-06-05, 07:59 AM
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You cannot use 14 gage wire anywhere if you want to use a 20 amp breaker.

Use all the same gage wire, especially in the basement. It avoids confusion and limits voltage drop. Yes, it's a little more expensive, and yes the voltage drop in a few feet is not a big deal, but your in the basement where you have plenty of room to work on the wiring and it's only a few connections.

as for the lights, you must use at least 12 gage wire on a 20 amp circuit. Period. No exceptions for this type of circuit. Use 12 gage if you want here, but you cannot go to smaller wire.
 
  #6  
Old 10-06-05, 08:43 AM
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To summarize a few points:

Stop mentioning 14 ga. You cannot uset this anywhere on a circuit protected by a 20 amp breaker. The actual wiring WITHIN a fixture will be smaller, of course, but any wire YOU INSTALL on this system must be appropriate for the breaker.

Voltage drop is is direct linear function of distance. You are only talking about single digit amps on each leg, I believe, but at 1500 feet there will be voltage drop. There are calculators available on the net to put numbers on this.

Using compact flourescent will reduce the amps to approx. 1/4 of that required of incandescent bulbs. Many compacts are rated for -20F.

New math: 20 amps + 20 amps in the electrical world is still 20 amps. That is a double pole 20 amp breaker is rated for 20 amps (EACH LEG). Now, in your case, we are talking about a multiwire 120 volt circuit, so yes, you would have 40 amps @ 120. Your timer would need to be rated as double pole, 20 amps.
 
  #7  
Old 10-06-05, 08:45 AM
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Originally Posted by Lanzi
If I use a multi-wire circuit, does that reduce the Vdrop on each leg to 10-15%?
Nope, I accounted for that already.

I was thinking of using the new electric ballast fluorescent lights. These only draw about 25 watts each. Dose anyone know if there will be enough voltage to spark them on when its cold out?
There are two problems with that. First, fluorescent lights are much more sensitive to low voltage than incandescent bulbs. Second, fluorescents do not perform well in the cold at all. I don't know where you're located, but I wouldn't use them outdoors here in Michigan; they barely work in my insulated garage. There are insulated bulb sleeves that help sometimes, but your experience may vary. I'm not sure about CF bulbs; you could chack with some manufacturers for specs.

Will there be any lack of circuit effiecency if I use 12g wire in the basement, and 14g wire just at the poles? I think the wire gauge on typical lighting fixtures are just multi strand 18g?
Stick with #10 in the basement and use nothing smaller than #12 at the lamp posts. You can't use #14 on a 20A circuit.

Does the breaker HAVE to be a GFCI? That seems redundant? I was going to use a GFI at the outlet.
There are two things to consider. First, if the wire is buried less than 24" deep, you must have a GFCI breaker. Also, some localities mandate GFCI for outdoor lighting as well as receptacles; if so, you will need the GFCI breaker. Otherwise, I think you could just use a GFCI receptacle at the end of the circuit.
 
  #8  
Old 10-07-05, 07:36 AM
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I am just going to deal with the voltage drop issue here.

With the extremely long run that you have, your voltage drop could be 25% or more. But the critical issue is that voltage drop is _not_ a constant.

Voltage drop depends upon the length of the wire, the size of the wire, the resistivity of the wire, and the current flowing. Percentage voltage drop additionally depends upon the supply voltage.

The longer the wire is, the greater the voltage drop.

The thicker the wire is, the less the voltage drop.

Aluminium wires will have more voltage drop than copper wires of the same size.

The kicker is that the greater the current flowing, the greater the voltage drop.

The higher the supply voltage, the less the percentage voltage drop for a given load.

The resistance of #10 copper wire at 20C is about 1 ohm per 1000 feet. In a 1500 foot circuit you have a total of 3000 feet of wire. So something plugged in to the end of that circuit will see 3 ohms of resistance from the supply

Voltage drop is calculated by simply multiplying current times resistance.

So if you were to connect a 10A load at the end of this circuit, you would see 30V of drop; this is probably not acceptable. But if you only connect a 1A load, then you will only see a 3V drop, which is perfectly fine. With nothing at all connected, you will meter the full 240V at the end of the line.

Multiple voltage drops simply add up; so you could calculate the voltage drop for each _segment_ of your complete circuit, and add them up to get the total voltage drop. A simple approximation is to assume that all of the load is concentrated at the end of your circuit, and calculate this (worst case) voltage drop. In other words, you can do the complex (but more accurate) calculation for _each_ of the distances to your lights, calculating the total current at each tap point; or you could do the simple calculation for load at the end of the line, which will over-estimate the total voltage drop. I am going to work with total voltage drop.

A good rule of thumb is to limit to a 5% voltage drop. This is an approximation, explicitly not mandated by code. If you can find lighting hardware which will tolerate a wider range of supply voltages, then greater voltage drop can be tolerated.

The maximum load that you can place at the end of a 1500 foot #10 Cu circuit with 240V supply and 5% voltage drop is 4A, with 912W delivered to the load. If you connect this circuit as a multiwire branch circuit, and keep the total connected load to less than 900W, roughly balanced between the two sides, then you will be fine. Because some of the paths are much shorter than the 1500 foot maximum, you are probably okay with 150W lamps in each of your 11 locations, though I have not done this calculation. But if you try to connect 400W lamps in each location, then voltage drop will cause significant problems. If you try to run more than a couple hundred watts of decorative lighting, you will need to unscrew some of the driveway lights.

-Jon
 
  #9  
Old 10-07-05, 09:17 AM
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This is all EXCELLENT information. Thank-you! A couple of clarifications:

So, it's not 1,500 feet, but 3,000 feet total because of the common run back to the panel? But if this is a multiwire circuit, the supplies share the same common. Does this matter, ie 2,250 feet each leg?

To get the shortest overall distance then, it would work out best to put the first 7 lights on one leg, and then the last 4 on the other leg. This would shorten up the overall distance by 200 feet total, while keeping the Vdrop/load more or less equivalent. This would also allow a little more Voltage for the outlet at the end. Am I correct, or would the unequal length and load mess up the multi wire circuit??

I didn't mention this before, but while I have all of your attentions, I'll verify:

The line from the house to the end of the driveway isn't linear. At 500 feet from the house it branches to two lights at the right, and the other 9 to the left down the driveway. So it's kind of an unequal "Y." Does this affect anything in a major way???

Finally, the 1,500 foot run includes a 200 foot run from the Electric Company's Transformer Box to the house panel (The Electric company installed this Box because the house is too far from their street line). I thought that the distance would be calculated from the source of 120V, not the house panel. Is this correct, or should I subtract the 200 feet from the total distance????

By the way, I was thinking of using those new electric ballast CF lights. But I need to find out if they can light with just 120-30=90 volts of electricity. Also, to make it worse, in the winter, too (0 degree F). If not, then I'll probably just use regular 60 watt bulbs.
 
  #10  
Old 10-07-05, 10:52 AM
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Originally Posted by Lanzi
So, it's not 1,500 feet, but 3,000 feet total because of the common run back to the panel? But if this is a multiwire circuit, the supplies share the same common. Does this matter, ie 2,250 feet each leg?
The MWC doesn't effect the distance; that's constant. A well balanced MWC is a pure 240V circuit and therefore experiences half the voltage drop of a 120V circuit all else being equal.

To get the shortest overall distance then, it would work out best to put the first 7 lights on one leg, and then the last 4 on the other leg....Am I correct, or would the unequal length and load mess up the multi wire circuit??
The MWC will work best with balanced load. The biggest problem with 7 on one leg, 4 on the other will be a jump in brightness between lamp #7 and #8. If you alternate lamps on each leg, they will get progessively dimmer down the driveway. I think it will look better, but the choice is yours.

The line from the house to the end of the driveway isn't linear. At 500 feet from the house it branches to two lights at the right, and the other 9 to the left down the driveway. So it's kind of an unequal "Y." Does this affect anything in a major way???
Not really.

Finally, the 1,500 foot run includes a 200 foot run from the Electric Company's Transformer Box to the house panel (The Electric company installed this Box because the house is too far from their street line). I thought that the distance would be calculated from the source of 120V, not the house panel. Is this correct, or should I subtract the 200 feet from the total distance????
I'm not entirely sure what you mean here, but you always consider voltage drop from your panel. What the power company does to feed your panel is their business. They should have already compensated for the long feed to your house by using properly sized feeders and transformer tappings so that you end up with 240V at your panel.
 
  #11  
Old 10-07-05, 10:55 AM
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Out to the end of the run, it is _3000_ feet, 1500 feet out and 1500 feet back.

The shared neutral does not in any way change this length.

What the shared neutral _does_ do is change the current flowing on the neutral, and thus the voltage drop.

If, for example, you placed a 4A, 120V load at the end of one half of this circuit, you would have 4A * 3 ohms = 12V drop; unacceptable in a 120V circuit.

But if you placed _two_ 4A, 120V loads at the end of both halves of this circuit, then you would have _zero_ current flowing in the neutral. You would have 4A flowing through 1500 feet of wire from one hot, with 1.5 ohms of resistance and a 6V drop. You would have 4A flowing through the other hot, again with a 6V drop. Net result: each of these 120V circuits would only see a 6V drop, which is acceptable.

Another way of thinking about this: if the two loads are perfectly balanced, then you could say that you have a single 4A 240V load. You still have 4A flowing through 3000 feet of wire, and a 12V drop, but now that 12V is coming out of the 240V supply, which is acceptable.

For your application, IMHO, you will want to keep both branches as well balanced as possible; 5 lights on one side, 6 lights on the other, simply alternating one branch then the other.

A _detailed_ calculation would consider each and every separate length of wire separately, and then you would add up all of the various voltage drops. You calculate the resistance of each length of wire, and the current flowing in that particular length, and then add up the individual voltage drops in each path that the current can follow. Each segment of wire is part of the entire voltage drop equation.

The branch that you described will make a difference; the current in one branch is not flowing through the other branch, so each has separate voltage drop. The voltage drop in each branch affects that branch only. The length from the transformer to your main panel does count; but these are much thicker wires (meaning less resistance) carrying your entire house current (and more current means more voltage drop). Depending upon what is running in your house, you may find that you only have 220/110V at your main panel. So your voltage drop considerations need to start with this reduced number. You can't simply add that 200 feet onto the length, nor can you ignore it; you need to count its voltage drop in a separate calculation.

As I said, my strong guess is that you are fine with 100W loads at each location, balanced as best you can.

My guess is that there are lots of different ballasts that you could use, some of which are designed to deal with low temperatures and bad supply voltage regulation. But you might have a hard time finding one that could deal with a 10% drop at 0C. If you only place 25W loads at each location (1/4A per load) then your voltage drop will be quite small, perhaps only 1 or 2%; remember that the less load you place on this circuit, the less the voltage drop will be.

-Jon
 
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