How to Calculate Voltage Drop  

Voltage drop might not be the most exciting part of outdoor lighting design, but ignore it and you’re setting yourself up for service calls, flickering issues, or worse, luminaires or fixtures that don’t turn on at all. It's not rocket science, and knowing how to do this will elevate your designs. You just need a general plan, a spec sheet, and a little math. I'll show you how.

What is Voltage Drop? 

Back in the US Fire Service as a wildland firefighter, I was an engineer on an engine, our fires were huge, and we had to pump water long distances to battle the flames. Voltage drop is very similar to pumping water. The smaller the hose, the more friction loss and gallons per minute would drop dramatically over distance. We could correct this problem a few ways: by increasing the pressure to overcome friction loss but risk popping our hoses, deploy a bigger hose, or run hose lines in tandem to deliver the water we needed to battle the blaze. In the same way, the longer and thinner your wire the more resistance it creates, leading to lost voltage before it reaches the fixture. Increasing the AWG of your wire, running multiple lines in a trench, or upping the voltage tap can solve for voltage drop.

For low-voltage landscape lighting, ideally you'll choose a luminaire that gives you the most forgiveness when designing and installing your system. Choosing one that can operate from 9.5V-22V (like most Sterling Luminaries) you have more discretion on what wire gauge and voltage to send down the line. For purposes of calculating, we will say you want to keep things between 12V and 15V, but read the label on what your light needs to operate effectively. Remember if you choose a luminaire that has to have an exact voltage to operate correctly, it will limit the ways you can solve for voltage drop.

What You Need to Calculate Voltage Drop 

Before we do any math, let’s grab the right information and definitions:

• Fixture (Wattage for DC) (VA for AC) – This is how much power the fixture consumes. You can find this on the Sterling Lighting Specification Sheet. For example, the SL01 Up Light can run from 4.05VA (lowest setting) to 14.8VA (highest setting).
• Wire Gauge (AWG) – The thickness of your wire. Common sizes in landscape lighting are 18 AWG, 12AWG, or 10AWG.
• Wire Length (Feet) – The total round-trip distance (to the fixture and back to the transformer).
• Current (Amps)– Calculated by dividing VA by the voltage (VA ÷ 12V).
• Resistance per Foot – Different wire sizes have different resistance values, which we’ll use in our calculation:
  • 18AWG = 6.39 ohms per 1,000'
  • 12AWG = 1.62 ohms per 1,000'
  • 10AWG = 1.02 ohms per 1,000'
  • 8AWG = 0.64 ohms per 1,000'

Let's work through an example of how to calculate voltage drop.

Voltage Drop for an SL01 Up Light using the formula  for Voltage Drop (V) = (Wire Resistance per Foot × Total Wire Length × Current) and the numbers below.

Fixture: SL01 Up Light set at 9.72VA

Wire Gauge: 12AWG

Distance: 100 feet (200 ft round trip)

Transformer Voltage: 12V

Step 1: Calculate Current (Amps)

Current = VA ÷ 12V which is 9.72VA ÷ 12V = 0.81A

Step 2: Look Up Wire Resistance

For 12AWG, the resistance is 1.62 ohms per 1,000 ft. We divide by 1,000 to get the per-foot value.  1.62 ÷ 1,000 = 0.00162 ohms per foot

Step 3: Apply the Voltage Drop Formula

Voltage Drop = (0.00162 × 200 ft × 0.81A) Voltage Drop = 0.26V

Step 4: Check the Final Voltage at the Fixture

Transformer Voltage - Voltage Drop = Fixture Voltage  12V - 0.26V = 11.74V

Since 11.74V is within the 9.5V-22V range of the SL01, this setup works perfectly. If the drop was too high, we could use thicker wire (lower resistance), increase the Voltage from 12V to 13V-15V, add another run of wire, or locate a transformer closer to the where the fixture will be placed to shorten the run.

How to Use the Sterling Lighting Spec Sheet 

Sterling Lighting spec sheets give you all the necessary data:

• Operating Voltage Range – Always check this to know what voltage your fixture can handle. The SL01 operates between 9.5V and 22V.
• Wattage (VA) – Found in the table. Make sure to choose the right setting based on the driver pins.
• Lead Wire Length – Helps determine where your connections are happening (SL01 comes with a 10’ 18AWG lead wire).

 Quick Tips for Managing Voltage Drop 

1. Keep wire runs as short as possible, adding another transformer to a design can help, don't be afraid to have an electrician add an outlet if necessary.
2. Use a thicker wire for longer runs, or double up your 12AWG to keep inventory simple on your jobs.
3. Split up high-wattage fixtures across multiple runs.
4. Use a multi-tap transformer so you can adjust voltage as needed.
5. Test voltage at the Luminaire before finalizing the install.
6. Document what you did, and leave room to add, this will help later on when you have a service call in the future or a customer asks you to add on.  Trenching once is really nice, adding more wire doesn't cost that much.

Voltage drop isn’t something to guess at, it’s a predictable problem with a clear solution. Taking a few minutes to run the numbers will save you headaches down the road and ensure your installations look as good, operate safely and allow you to service and add-on to your systems easily.