Okay, let's break down your landscape lighting project and see if your planned setup with the 10/2 wire, 300W/25A transformer, and 13 spotlights over 750 feet is likely to work. Voltage drop is the key concern here.

Understanding Voltage Drop

Voltage drop is the decrease in voltage that occurs along a wire due to the wire's resistance. If the voltage drops too much, your lights will be dim, unevenly lit, or potentially not even turn on at the end of the run.

Your Setup



Transformer

300W / 25A, 12V output

Wire

750 feet of 10/2 copper strand (meaning 750 feet out, 750 feet back, for a total loop of 1500 feet).

This is crucial. Spotlights

13 x 12V spotlights. The total wattage of each spotlight will determine the total load

Steps to Analyze Your Setup1. Determine the Wattage of Each Spotlight

This is the most critical piece of missing information. Are they 5W each? 10W each? This significantly impacts the calculation. Let's use a few examples.2.

Calculate Total Wattage

Multiply the number of spotlights by the wattage of each spotlight. Example 1: 13 x 5W = 65W total. Example 2: 13 x 10W = 130W total. Example 3: 13 x 20W = 260W total.3.

Calculate Total Current Draw

Divide the total wattage by the voltage (12V). Example 1: 65W / 12V = 5.42A Example 2: 130W / 12V = 10.83A Example 3: 260W / 12V = 21.67A4.

Voltage Drop Calculation

Here's where using a calculator is helpful. But let's illustrate the concept. I'll provide some estimates and then recommend using a specific online calculator. We need to know the resistance of the wire. A 10 AWG copper wire has approximately 1 ohm of resistance per 1000 feet at 75 degrees Fahrenheit. Since you have 1500 feet, you'll need to account for this to calculate the resistance Resistance for 1500 feet (loop) = 1.5 ohmsVoltage Drop = Current (Amps) Resistance (Ohms)Using the example currents that we calculated above Example 1 (5.42A): Voltage Drop = 5.42A 1.5 ohms = 8.13V drop Example 2 (10.83A): Voltage Drop = 10.83A 1.5 ohms = 16.25V drop Example 3 (21.67A): Voltage Drop = 21.67A 1.5 ohms = 32.51V dropVoltage at the last light = 12V - Voltage Drop Example 1 (5.42A): 12V - 8.13V = 3.87V Example 2 (10.83A): 12V - 16.25V = -4.25V Example 3 (21.67A): 12V - 32.51V = -20.51VVoltage Drop Percentage = (Voltage Drop / Source Voltage) 100 Example 1 (5.42A): (8.13V / 12V) 100 = 67.75% Example 2 (10.83A): (16.25V / 12V) 100 = 135.42% Example 3 (21.67A): (32.51V / 12V) 100 = 270.92%As you can see from these examples, the lights at the end of the run would be very dim or not turn on at all, especially with the 10W or 20W spotlights.
5.

Acceptable Voltage Drop

For landscape lighting, a generally accepted rule of thumb is to keep voltage drop below 10%. Exceeding this will lead to dim and uneven lighting.

Recommendations and Next Steps


PROVIDE SPOTLIGHT WATTAGE

Tell me the wattage of EACH spotlight. This is essential for a definitive answer.2.

Use a Voltage Drop Calculator

Southwire's calculator is good, but make sure you are inputting all the values correctly. Specifically: Voltage: 12V Wire Gauge: 10 AWG Wire Type: Copper Circuit Length: 750 feet

(IMPORTANT: some calculators want the total loop length which would be 1500 feet). If in doubt, experiment by doubling the distance and halving the current to see if the result stays the same. Current: The total current you calculated in Step 3 (based on the spotlight wattage).3. Consider Alternatives



Reduce Spotlight Wattage

If your current spotlights are too high wattage, consider lower wattage options. LED spotlights are very efficient.

Shorter Runs

If possible, break the 750-foot run into two shorter runs, each with its own transformer or a midpoint feed. This will drastically reduce voltage drop.

Multiple Transformers

Consider using two smaller transformers and splitting the load.

Larger Gauge Wire

While 10/2 is a good start, depending on the total wattage, you might need 8/2 or even 6/2 for a 750-foot run, especially with higher wattage spotlights. (This will be more expensive).

Parallel Wiring

Consider wiring the lights in parallel. In a parallel circuit, each light receives the full 12V of power. This is the best way to ensure that all your lights are bright and consistent, regardless of their distance from the transformer.4.

Test Before Burying

Never* bury the wire before testing your setup. Lay everything out, connect all the lights, and turn on the transformer. Observe the brightness of each light. If the lights at the end of the run are noticeably dimmer, you have a voltage drop problem.5.

Over-Sizing the Transformer (Generally Good)

You've got a 300W transformer. That's good, as it gives you some headroom. Never run a transformer at its maximum capacity. Aim to use no more than 80% of its rated wattage.

In summary: Without knowing the wattage of your spotlights, I cannot definitively say if your setup will work. However, with a 750-foot run, you're very likely to have voltage drop issues unless you are using very low wattage spotlights and/or take measures to mitigate voltage drop.

Please provide the wattage of each spotlight so I can give you a more accurate assessment! Also, feel free to share the results you get from the voltage drop calculator. Flag for review