Your calculations are a bit overthinking it, but you are scientifically correct. Don't expect an electrician to want to be so interested in your thinking. Do for sure consider aluminum wire vs copper. Also take in consideration actual voltage at the service vs bulb ratings. For example, if you start out at 125 volts you can afford to loose 5 volts and still be at a typical bulb ratings of 120 volts.

 

If you wanted to get fancy you could use two circuits (with a shared neutral and ground, so just one additional wire).

 

@dcwired The house is under construction so I don't know yet what voltage I will be starting with, although I could get an idea by checking the temporary service. I have one electrician's estimate that shows 4 awg aluminum is a little less than half the price of 6 awg copper for the wire (no difference in the labor). Using the various smaller gauge copper looks like it is about 15% cheaper than running 4 awg aluminum for all wires.

@Luke M I will probably be doing that, but the additional circuit will be for other stuff, e.g., LAN switches (I'm also running fiber optic cable to the drop locations) and security cameras, but they won't require much power. Maybe 10 watts for each LAN switch and 15 watts for each camera (using PoE).

 

The temporary voltage is a good likelihood as the POCO probably uses the same design voltage for all.
Under construction? The inspector may be uncertain about mixed wire gauges on 1 circuit.
Let's start with the actual voltage and determine if there is a reason to be concerned about voltage drop.

 

Way over thinking this.

I've been involved with installing streetlights and parking lot lights.

Determine how many fixtures are required and the amps per fixture.

#of fixtures X amps per fixture = amps required with no correction factors

Because the fixtures will be on for more than 3 hours per night they are considered a continuous load.

Multiply load amps by 1.25 (correction factor), this will be the Amperage rating of the wire.

But you're not done yet.

Now do the voltage drop calculation to determine the corrected wire size.

You cannot change the wire size unless you want to install fuses at the points where the wire changes size.

Here is a for instance.

Say you determine a 30 amp beaker is required in the panel, the section of #12 down in the last section will not be properly protected by the breaker.

The maximum rating of the #12 is 20 amps, so you'd be protecting the #12 with a 30 amp breaker, which is not allowed.

Correct way would be to install the same size wire the entire length of the run.

Your electrician is correct.

 

You wrote:-

I want to put five light sources (floodlights, driveway lamp post, etc.) fairly evenly spaced (every 300') along this driveway. I believe that 120 watts of 120V LEDs at each location would be more than enough, but it keeps the numbers "round". The wire will be run underground in conduit. Assume all the lights will be on at the same time.

120 watts (of any Voltage) LEDs "at each location" would be complete "overkill".
I believe that you may mean the "equivalent of 120 W "incandescent", which would be about 1600 Lumens each.

You need to stop thinking of "Incandescent Watts" and start thinking in Lumens.

LEDs giving the "Equivalent of 120 W Incandescent" would be about 12-15 W, at each location.
At 120 V this would be not more than 125 mA - at each location
Using 12 V "Garden Lighting", this would be not more than 1.25 A - at each location.

(With "120 watts of LEDs at each location", "five light sources (floodlights, driveway lamp post, etc.) fairly evenly spaced (every 300') " it would be much brighter than an Airport Runway !)

 

You wrote:-

120 watts (of any Voltage) LEDs "at each location" would be complete "overkill".
I believe that you may mean the "equivalent of 120 W "incandescent", which would be about 1600 Lumens each.

You need to stop thinking of "Incandescent Watts" and start thinking in Lumens.

LEDs giving the "Equivalent of 120 W Incandescent" would be about 12-15 W, at each location.
At 120 V this would be not more than 125 mA - at each location
Using 12 V "Garden Lighting", this would be not more than 1.25 A - at each location.

(With "120 watts of LEDs at each location", "five light sources (floodlights, driveway lamp post, etc.) fairly evenly spaced (every 300') " it would be much brighter than an Airport Runway !)

I had similar thinking. I assumed 120W equivalent which would be laughable at 300' intervals. LED street lights are easily 100W actual, which makes far more sense. FPL offers LED lights in the range of 50-300W. You might ask your PoCo if they'd be interested in quoting.

 

Frodo1, his wattage is not overkill considering we are talking about 300' between fixtures. This is a massively long driveway.

 

Old electricians are sizing the wire for incandescent or metal halide lights and not the new LED that require a fraction of the current. A 8 Watt LED puts out more than enough lumens for outdoor walkways. What is important is the height of the lights which will determine the distance they can illuminate and avoiding having them kill your night vision.

I would go with AC as DC will require a much heavier gauge wire for a run as long as you are planning. There are options for pole mounting and for recessed lights in the ground or a combination of the two.

 

I would go with AC as DC will require a much heavier gauge wire for a run as long as you are planning. There are options for pole mounting and for recessed lights in the ground or a combination of the two.

For the same Voltage (and, more importantly, Current), the Voltage drop for AC and DC will be the same.

However, you may mean "I would go with 120 V AC as 12 V or 24 V DC will require a much heavier gauge wire"

 

OK, you've gotten some good answers, so I'm going to drift into a bit of a rant.

Where are you moving to which has a 1500' driveway? And more importantly, do your neighbors WANT their town to be lit up like Times Square?

Obviously I don't know the answers for your situation. I can only share what I see in my (formerly) rural town. People from "away" come in and try to make it just like the city they moved from. If you like that sort of thing, then stay there! If you want to live out in the country, then show some consideration for those who chose to live that way. Some of us like to see the stars at night. Hey, we even have these things called headlights, so we don't need to light up every inch of our streets like daylight.

Sorry. Rant over. I can find my own way out.

 

I want to put five light sources (floodlights, driveway lamp post, etc.) fairly evenly spaced (every 300') along this driveway.

Firstly, unless you wish to put one right at the far end, and the first 300 feet from the premises
the first may be 150 feet from the premises and the last at 150 feet from the "entrance" (?)
Hence, the spacing may become 150', 300', 300', 300', 300' or 1350' in total

I believe that 120 watts of 120V LEDs at each location would be more than enough, but it keeps the numbers "round". The wire will be run underground in conduit. Assume all the lights will be on at the same time.

I think that means I need 5 amps at the first location, 4 amps at the second location, and so on, and the fifth/last location will need 1 amp.

The Southwire voltage drop calculator says I need to use 4 awg if I need 5 amps at the end of the circuit, but I really only need 1 amp at the end.

Please bear with me...

Working backwards from the 5th/last location to the 4th location, 300' of 12 awg wire will carry 1 amp with only a 1V drop. The wire between the 4th and 3rd locations will need to carry 2 amps (4th and 5th lights). The calculator says 10 awg will carry 2 amps for 300' with a 1.25V drop. Between 3rd and 2nd, 10 awg will carry 3 amps with a 1.9V drop. Between 2nd and 1st, 10 awg will carry 4 amps with a 2.5V drop. Finally, between the 1st light and the breaker, the calculator says 8 awg will carry 5 amps with a 2V drop.

The above adds up to a total voltage drop of 8.65V at the farthest distance. I understand this is more than is allowed, but given that the total amps will be closer to 2.5 amps instead of 5 amps, that should bring the total voltage drop down to (I think) allowable levels.

125 W at 120 V is 1.042 A
but
let us say 1 A

Many years ago I developed a Spreadsheet for this calculation for Halogen garden lighting and have adapted this for your situation.
While I cannot attach the Excel file I have attached a Photo Copy.

Initially, I did not used the Southwire calculator but used the Resistance per Foot as stated in https://en.wikipedia.org/wiki/American_wire_gauge - which seems to be more than somewhat different !
For #10 AWG it is given as 0.9989 mΩ/ft for each Conductor
so 1.9978 mΩ/ft for each Pair of Conductors.
I then re-did the calculations using the Resistance per Foot according to Southwire.

You may note that (using the Southwire value) I calculate almost the same result at the far end as you did.

The "Forward Calculation" is from the Premises to the Far End and the "Reverse Calculation" is the reverse (Obviously),
When these two results are Combined, by making the circuit into a "Ring", the "Combined Drop" can be calculated and is, of course, less.

I do not know if "Mains Voltage" Ring Circuits are allowed in your jurisdiction.
Also, you would use twice the length of Line and Neutral conductors.

However, when using certain LED devices, the voltage drop may not much matter.
Most LED devices include a Switch Mode Power Supply and can operate over a "range" of voltages - some from 100 V to 250 V. (This is a reason why many cannot be "dimmed".)
Hence, it may be possible for you to (virtually) ignore any Voltage drop up to even 20V.

Whether that would be "allowed" is another matter.

 

5 lights for 1500' of driveway is not being well lit. If you insist on only 5 lights then I wouldn't worry about voltage drop too much. There is no limit on voltage drop except for fire pumps. So if you start at 120-124v you will lose 9v at the furthest. That means that those lights will not be as bright as the others.

 

5 lights for 1500' of driveway is not being well lit.

I keep coming back to the idea of "well" lit. What makes lighting a good thing?

I sort of like this planet, where we have alternating periods of day and night. Unfortunately, humans have turned sections of it into permanent daylight. I don't see that as a good thing. But, if you're scared of the dark, there are plenty of places you can go live. Please don't add more light pollution where it wasn't before.

 

5 lights for 1500' of driveway is not being well lit.

Think I'll agree here. I just did some measurements on google earth. The massive lights at a nearby interstate interchange have 800' spacing. That's just an extreme example of what exists out there... those are probably $80k each. A local bridge has lights spaced at 300' and they are pretty useless once you get in between two, granted the city went with form over function for those. Another bridge (and most power pole mounted lights) are spaced at 150'. That seems to work pretty good with the classic "Cobra" light.

Where are you moving to which has a 1500' driveway? And more importantly, do your neighbors WANT their town to be lit up like Times Square?

I also agree with this, granted we really don't know your specific application here. Just keep in mind that a dark night sky is pretty rare and is a selling point of a house in the country. One nearby street light can ruin star gazing.

 

While I agree with many others that the "Project" seems to be "overkill", in the most recent post by the OP (Now, three days old)

@dcwired The house is under construction so I don't know yet what voltage I will be starting with, although I could get an idea by checking the temporary service. I have one electrician's estimate that shows 4 awg aluminum is a little less than half the price of 6 awg copper for the wire (no difference in the labor). Using the various smaller gauge copper looks like it is about 15% cheaper than running 4 awg aluminum for all wires.

If you wanted to get fancy you could use two circuits (with a shared neutral and ground, so just one additional wire).

@Luke M I will probably be doing that, but the additional circuit will be for other stuff, e.g., LAN switches (I'm also running fiber optic cable to the drop locations) and security cameras, but they won't require much power. Maybe 10 watts for each LAN switch and 15 watts for each camera (using PoE).

The "Shared Neutral" would carry the total Current
JustinLacy could use 240 V, where the Voltage Drop would be less important.

However, as I have pointed out in Post #13,
"the voltage drop may not much matter.
Most LED devices include a Switch Mode Power Supply and can operate over a "range" of voltages - some from 100 V to 250 V. (This is a reason why many cannot be "dimmed".)
Hence, it may be possible for you to (virtually) ignore any Voltage drop up to even 20V. "

A small amount of "research" shows that many 120W LED Flood Lights available in North America can operate on Voltages from 100 V to 240 V - (or even 85 V to 277 V).

e.g. Waterproof Outdoor LED Flood Light 10W-300W
and

LED Flood Light 120W 5000K IP65

www.ledretrofit.ca

 

Sorry for the delayed response, and thanks for all the comments. I will try to answer the questions...

The property is a little over 11 acres, approximately 1500' deep, from 300' to 400' wide. Light closest to house is 300' from house, then four more at approximately 300' separations. I live outside the city limits. There are no electrical inspections done, unless I pay to have an independent inspection. The driveway is not straight. The locations are 1) at the gate, 2 and 3) at curves in the driveway, 4) at the middle of a 700' mostly straight run, and 5) at the "final" curve, 300' from the house. The idea is not to light the driveay up at night so that you can safely drive it without headlights. It's to provide a reasonable amount of light so someone not familiar with it will have a basic idea of the driveway. Also, initially I will probably only install lights at the ends, then "fill them in" later if we decide they are needed. However, the drops at each location will be done now to make it easier to add lights at the intermediate locations and facilitate the cable pulling.

There's very little ambient light spillover from adjacent properties due to the heavy foliage on the property lines, especially near the back of the properties where the homes are located.

I meant 120 watts of bulbs, not 120 watt equivalent bulbs. I also mentioned that it will probably only be 2.5 amps total (i.e., 60 watts per location), and it may be less than that. I've actually taken a floodlight to the property that can switch between 40, 50, and 60 watts (not equivalent watts), to make sure that 60 watts (7800 lumens) is enough, and it is. It may be too much, but I won't know that until later. I don't intend to light up the sky. I will almost certainly use dimmable bulbs. I'm also hoping that some long range z-wave dimmers will work at these distances to allow me to easily dim them. For example, keep them very dim (and/or some of them off) until the driveway gate is opened at night.

I used 120 watts (of power) lights because it made it easier to calculate (e.g., 1 amp at 120V), knowing that if my calculations were correct for 120 watts, they would be good for 60 watts.

I'm leaning towards using two 30 watt dimmable LED corn bulbs in a 12' or 14' driveway lamp post fixture (looks matter). As someone posted earlier, many of these bulbs work at 100V-277V.

There will be an enclosure at each location to hold the fiber optic terminations, LAN switch, etc. I can possibly put a DIN mounted breaker (supplementary protector?) at each location where the wire size changes to protect the smaller downstream wires. i.e., when the wire changes from 8 awg to 10 awg and from 10 awg to 12 awg. It may also be that using 10 awg for the final drop is cheaper than using 12 awg and a breaker.

At Wire & Cable Your Way: Electrical Wire By the Foot 4500' of 6 awg costs $4140. 3600' of 10 awg costs $1188 and 900' of 8 awg costs $540, for a total of $1728 or 41% of the cost of using all 6 awg.

I need to ask the electrician how he came up with using 6 awg. My guess is he calculated for 5 amps at the end of the run. if the 8/10 awg design will work, certainly an all 8 awg design will work and be about $1400 cheaper than all 6 awg.

 

There will be an enclosure at each location to hold the fiber optic terminations, LAN switch, etc. I can possibly put a DIN mounted breaker (supplementary protector?) at each location where the wire size changes to protect the smaller downstream wires. i.e., when the wire changes from 8 awg to 10 awg and from 10 awg to 12 awg. It may also be that using 10 awg for the final drop is cheaper than using 12 awg and a breaker.

That makes no sense, just use a 15A breaker from the beginning.

 

For lumen output, the towable light generator things have 4 lights at 135k lumens (for metal halide). I forget at this point how much we’re shooting for, but that can be a reference. Can even rent one to test it out. Some can turn on individual bulbs.

 

Not a question in my mind that I would drive the highest voltage possible and ignore any consideration for 120V over that distance!!! NOPE.

 

Yeah using 240V for everything isn't a bad idea. That would minimize wire cost. In fact it would be possible to use 14awg given lights that work over the "universal" voltage range (100V-240V). Voltage drop would be irrevelant, only downside of smaller wire would be wasted power.