Maximum number of 150W LED streetlights on 100A breaker with 12 AWG THHN wire?

I have a streetlight project that I need help with choosing the wire gauge for economical reasons.  I don't want to be using a higher wire gauge than what is necessary.  My led light is 150watts.  My street post is distance approx 25 meters apart.  My wire is in an underground conduit in a 1/2 inch pvc pipe with concrete encasement. Im using THHN gauge 12 wire.  (gauge 14 is cheaper and gauge 10 is more expensive)  

Question 1.  How many 150 watts led light can I load on a 100AMP breaker using a gauge 12 wire?  

Question 2.  From the underground wiring going up the steel post, what gauge of wire can I use to wire the 150 watts led light? Can I use gauge 14?  Can I use a lesser gauge wire here?

 

Thank any help will be much appreciated

 

joey

Hello there, Joey! I think this "site":https://www.sprinklerwarehouse.com/Determining-Cable-Size-s/7929.htm will somehow help you determine the cable size of your design.

Joey...you don't say what the total circuit length is, but if it's long one trick you can use is a loop or ring circuit. have two pairs of wires going from your power source to the end, and connect every second lamp to each pair of wires (ie lamp 1 to pair 1, lamp 2 to pair 2, lamp 3 to pair 1, lamp 4 to pair 2...etc. Then at the end join + to + and - to - (I'm assuming from your currents you're using 12V)? This way you can get away with much thinnner wire, and it also makes if more fault tolerant (if a wire breaks it should still work).

Hi Joey,

The first critical thing you forgot to mention is the Voltage you are using for this. This will help to determine the answer your questions since it is the Amps that will determine the size breaker and wire gauge needed. For instance, if you are using 12 Volt, each 150 Watt light will use 12.5 Amps

To determine the gauge wire you need, you can use a table such as the ones on the web site below which tells you what distance you can run wires depending on the gauge and Amps. This table assumes that you can accept 5% voltage loss so you 12 Volts is only 11.4 Volts at the light. Looking at the table, for 15 Amps at 12 Volts, you can only run 8.5 feet (about 2.5 meter) so not enough for your use. In this case you would need #2 wire to get 25 meters.

https://www.solar-electric.com/learning-center/wiring-cabling/wire-loss-tables.html

I hope that you are doing this with much higher voltage since that will reduce the Amps required and you can use much smaller wire (and smaller circuit breaker)

Ok. My voltage is 240volts. My led light is 150 watts. The distance between light is 25 meters. I usually put 10 lights in one 100amps breaker and another 10 lights on the same 100 amp breaker going the other side. Which means my distance from my last light from the breaker is about 250 meters to 300 meters. I use 12 gauge wire from the breaker to the end and use series wiring for all the light. Am i using the correct wire size?

This gets complicated because the current is not constant through the entire 250 or 300 meters of wire

Watts = Volts x Amps so 150 Watts light at 240 Volts is 0.625 Amps

So the current in the first 25 meters is 10 x 0.625 = 6.25 Amps but the current in the next section is less....

Using the table here

http://www.ohmslawcalculator.com/awg-wire-chart

the resistance of 12 gauge wire is 5.21 Ohm/ kilometer so the resistance in 300 meters is about 3.2 Ohms (there and back). If we assume that the entire current is in the whole wire (to make it simple) then the maximum voltage drop in the wire is 3.2 x 6.25 = 20 Volts. This is almost 10% of your total voltage which is a bit much...

Of course the above is overstating but you probably want a heavier gauge wire at least for the first lights in the string

Ok each 150 watts light is always 25 meters apart. How many lights can i string with that gauge 12 wire.

WIRE SIZE WIRING DIAGRAM

x = 150 watts led lght
#14, #12, #10, #8 = AWG wire size

x---------x---------x--------x---------x--------x--------x---------x--------x--------x----POWER SOURCE
#14 #14 #12 #12 #10 #8 #8 #8 #8 240V with
100 AMP Breaker

how can i reduce cost by using a thinner wire? How much light can I string along using lower wire size? Can you show in diagram too?

The number of lights you can put in a string depends on what voltage drop you can accept. For each 25 meters, you will have 0.26 Ohms resistance in 12 gauge wire. So the Voltage drop in the first 25 meters section is N(number of lamps) x 6.25(Amps) x 0.26(Ohms). the drop in the second section is (N-1) x 6.25 x 0.26 and so on. then you add the drops together.

This would be much easier in a spreadsheet.

Depending on how the lights are physically laid out, it might make sense to have multiple branches. Unfortunately, that's hard to draw here.

Maybe im asking the wrong question. How much voltage drip is acceptable with this project if mine?

As a general rule, I would want to have less than 5% Voltage drop and I have seen some places where less than 3% is specified.

In your case, the amount of voltage drop you can tolerate depends on the specifications on the lights that you are using.

I guess the next logical question is in order to add nore to the string i should upgrade the gauge of the wire as i go further from the source. Say gauge 10 wire. What is the maximum number of lights i can string on gauge 12 to not go beyond the 3% voltage drop?

Hi Jo,

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So another way to look at this:

For 12 gauge wire with 5.21 Ohm/kilometer, the resistance in 25 meters (round trip) is 0.26 Ohms

For each150 watts light at 240 V you will have 0.625 Amps

For 240 V, 3% is 7.2 Volt

Using Ohms law you can calculate the voltage drop of each 25 meters segment by Voltage drop = 0.26 Ohms x 0.625 Amps x number of lights using that segment

So starting from the end furthest from the power:
first segment drop = 0.26 x 0.625 = 0.1625 volt
2nd segment drop = 0.26 x 0.625 x 2 = 0.325 volt Total so far is 0.4875
3rd segment drop = 0.26 x 0.625 x 3 = 0.4875 volt Total so far is 0.975
4th segment drop = 0.26 x 0.625 x 4 = 0.65 volt Total so far is 1.625
5th segment drop = 0.26 x 0.625 x 5 = 0.8125 volt Total so far is 2.4375
6th segment drop = 0.26 x 0.625 x 6 = 0.975 volt Total so far is 3.4125
7th segment drop = 0.26 x 0.625 x 7 = 1.1375 volt Total so far is 4.55
8th segment drop = 0.26 x 0.625 x 8 = 1.3 volt Total so far is 5.85
9th segment drop = 0.26 x 0.625 x 9 = 1.4625 volt Total so far is 7.3125

So by the time we get to 9 lights in the string we exceed the 3% Voltage drop so we can only have 8 lights in the string.
Note that what I have here as the 9th segment is the one closest to the power and that it has the highest voltage drop. So you could get further by increasing the wire size on the sections closest to the power

Ok im finally getting am answer to my original question. Thank you very much. As a follow question, on the 9th light i have to upgrade to gauge 10 wire. I get that. How much light then can i continously string together from the 9th light with the gauge 10 wire before i go over the 3% voltage drop again. Can i wire 16 lights or more?