View Full Version : Extra long wiring

10-23-2011, 02:55 AM
What do you guys do if you need to run an extra long run of wire?

I have a project I might do, the only issue is that there will probably be about 3000 feet or so between the transformer and the first fixture. There is no way to locate the transformer any closer to the lighting, and line voltage lighting is not an option.

Elegant Outdoor Lighting
10-23-2011, 02:58 AM
Over half a mile? What kind of project is this? How much light do you need?

10-23-2011, 05:23 AM
Ah crap... I really should stop trying to do math in my head this late at night... its going to be closer to 700 to 1000 feet, not sure exactly what I was thinkin. Its to put some lights in a flowerbed approximately 20'x20'. Cant really give any more details yet. If there's a solar power source I can get that I can remote the solar panels rather than have them at the lights themselves, that might be a better way to go.

10-23-2011, 12:45 PM
Use 8gauge wire and LED. Something that has a wide (low) range of voltage. If you stick with halogen on a 15volt tap 40-50 watts is about all you can run in that bed

Alan B
10-23-2011, 01:53 PM
VOLT Transformers have taps up to 22v which can be helpful for extra long runs. Using the voltage drop calculator you can go 1000' using 12-2 wire on and use up to 79 watts. You will have 10.5 volts of drop . The 22v tap will bring you down to 11.5v. If you go with halogen you will only have 2-4 fixtures. If you go LED you could essentially put upto 20+ fixtures out there.

-watts x cable length divided by cable constant = voltage drop.
-cable constant for 12-2 is 7490.
-79w x 1000 / 7490 = 10.5 Voltage drop.
22 tap on a VOLT trans - 10.5 = 11.5 volts.

If you want to use a transformer that only goes to 15v then you can only have 4 volts of drop so you could only use upto 30 watts so you would need to go LED.
30 w x 1000 /7490 = 4 volts of voltage drop.
15 v tap - 4 drop = 11 volts at fixture.

I would definitely go LED because I do not like using higher taps due to the "Cascading Effect". With the higher wattage draw of halogen it is common to have all your lamps burnout quickly and at once when using the higher taps. Eg if 1 lamp burns out the load drops and that means the voltage to the remaining lamps spike up a few volts. That makes the remaining ones burn out quicker from being over volted. The result is the cascading effect where one lamps blows then another, then quickly thereafter they all blow.

You situation is a perfect scenario to go LED. 12-2 cable, a VOLT transformer and our LED lamps can do the job easily. We have over 500 transformers in stock (stainless steel, toroidal core, best prices, bulletproof warranty), 5000+ LED lamps (250+ lumens, 3000K), miles of 12-2 wire (premium Superflex wire and only VOLT packages Superflex in free EZ pull boxes for easy handling and so you can free pull the wire right off the reel with out needing a spool holder). Everything ships same day.


10-23-2011, 05:07 PM
Alan, good of you to take the time to make these calculations, but you need to multiply by a factor of 2 to account for AC voltage loss (common mistake since your formula is correct for DC).

Maximum load for wire running 1,000 ft. to accomodate 22V tap:
- #12/2: 38W
- #10/2: 58W
- #8/2: 98W

I would, however, downgrade these values by at least 20%. Wire resistance increases in time (especially if wicking occurs, or if splices and connections are less than perfect). Degradation of the socket is another cause of increased resistance. Use of tin-coated wire helps to preserve wire conductivity. All these effects would lead to loss of voltage at the lamp.

Also keep in mind that while ambient temperature of the wire doesn't significantly change wire resistance, on longer runs it becomes more important. The change in resistance is about 8% per 40 degree F change. In the 1,000 ft. run scenario (22V tap with 10V loss), the wire in frozen ground would lose about 9V instead of 10V causing an increase of 1V at the lamp. (See article: Does Temperature Affect Landscape Lighting System Performance (http://www.cast-lighting.com/search/1/display-document/55)?)

Another reason to downgrade is that you can't be sure of the actual lamp wattage - mfg. numbers can be off by as much as 15% (though usually on the low end).

Keep in mind Alan's comments about the problem of successive burnouts. The formula to calculate this risk on a single wire run is:

'Voltage loss'/'number of fixtures' = voltage jump when one lamp burns out.

If you're using the 22V tap (10-volt loss), then any less than 10 fixtures on a run would present an extremely high risk that all incandescent lamps would burn out with a single lamp failure. LED's would present less of a risk, but you need to check voltage ranges to be sure. (See article: "Prevent and Treat Premature Lamp Burnout (http://www.cast-lighting.com/search/1/display-document/47)")

If you plan on using LED's, make sure to take power factor into account. Divide wattage by power factor (pf) to get volt-amps - use those numbers instead of wattage. (see explanation in "CAST Voltage Calculator - Daisy Chain (http://www.cast-lighting.com/search/1/display-document/146)")

Unfortunately, most LED mfgs. don't publish pf values; even if they do, you have to take into account that power factor changes with the following variables:
- Mfg. miss-reporting (confirmed by DOE Caliper Tests)
- Ambient temperature
- Operating temperature of LED driver
- Voltage
- Degradation of LED driver (esp. electrolytic capacitor)

In conclusion, I'd say the best approach for this long wire run project is to use #8/2 wire or two #10/2 wires and LED lamps with a wide voltage range.

Alan B
10-23-2011, 08:33 PM

Excellent post. Yes I did forget to multiple x 2. I also agree with many of your other points.

For a 20 x 20 bed you probably don't need more than 3-6 fixtures (don't know what you are lighting). If your using 3w LED lamps that still only uses up to 18w plus power factor. Even with 12-2 I think you have plenty of capacity if you use LED and a high tap.


RLI Electric
10-23-2011, 09:09 PM
Bear in mind though, you are not allowed to parallel wires under #1/0AWG per NEC 310.4.

Alan B
10-23-2011, 11:34 PM
One last note I also forgot... LEDs have a very wide range of voltages they can operate under. VOLTs Infiniti line of LED lamps for example operate using voltages anywhere between 10v and 18 v so even if you had a big voltage spike from 11v to 18v the lamp will still be fine. The operating range for LEDs varies from manu to manu but all of them have much wider ranges than the 10.5-12v of halogen.

10-24-2011, 09:44 PM
Steve, you have PM

10-26-2011, 07:52 PM
Ok... so now that everyone has put their wire size/voltage drop formulas to the test, may I recommend something else?

Years ago, when LED's first came out, I had a similar situation- a long run from the house to the end of the driveway and neither the client ( nor I) wanted to run 700 feet of cable to supply a few fixtures at the entrance. My solution which was about 8 years ahead of time, was to install a solar photovoltaic panel in the brush along the driveway.

The panel was simple to install and then there was a charge controller and two golf cart type batteries. The charge controller senses the daylight and regulates not only the charging of the battery, but the operational duration of the attached load- in this case- lighting

In theory, my idea was great and it worked fine. In reality, the LED's were too new and didn't produce nearly enough light that I would need. My solution was to upsize the photocell panel and use some 5 watt xenon lamps.

Moral of the story. If you have some area close by to the designated lighting area, it might be a good idea to consider using a solar powered, off the grid system. Of course, the LED's of today will provide enough light to do what you want to and then some.

Check online altestore.com is where I got my stuff long ago. They are very helpful if you call and tell them what you are trying to do, they will guide you to the right equipment.

I'm waiting to have some time to install my back yard fence/shed and downlighting all LED and all powered from the smaller 50 watt panel that I didn't use on that job.

Save yourself all that digging, and all the cost of wire, it's really not worth it considering the limited amount of lighting you will have available after all is said and done.

Good Luck