Transformer Calculator & Wire Gauge Guide
Start by entering the wattage and quantity to know your total watts and minimum transformer size, scroll down to the wire gauge guide to pick a safe, reliable cable for each run.
Transformer Calculator
Simply enter the wattage and quantity of each fixture, and the calculator will total everything for you.
How to Find the Wattage of Light Fixtures
- Before using the calculator, please check the wattage of each light fixture you plan to install.
- Go to the product page of the light you want to calculate.
- Scroll down to the Specifications section.
- Look for the field labeled “Wattage” or “Bulb Wattage.”
- Enter that wattage into the calculator, along with the number of fixtures you plan to use.
- Repeat the step for your second fixture type (optional), and the calculator will total everything for you.
Example
- Well Light 2207A (2.5W) × 6
- Path Light 2401 (3W) × 4
→ The calculator will add the total wattage and suggest the correct transformer size.
Cable Selection Guide
- After you calculate the total wattage and your minimum transformer size, use this guide to choose the right low-voltage cable for your installation.
1. Measure One Run at a Time
“Feet” refers to the one-way distance from your transformer to the last fixture on a single cable run.
Each run should be calculated separately, since different runs often have different lengths and loads.
2. Total the Watts on That Run
For each run, add up the wattage of all fixtures you plan to place on that line.
To leave a proper safety margin, divide the total watts by 0.8 (same as the transformer calculator):
Design Load ≈ Total Watts ÷ 0.8
Why the 80% Rule?
Low-voltage systems naturally experience voltage drop as wire length increases.
The “80% Rule” is a common guideline used by landscape lighting installers to ensure:
- More stable voltage to each fixture
- Reduced heat buildup in long wire runs
- Better long-term reliability for LEDs
This simple adjustment helps keep your system within the optimal 11V–12.5V operating range.
3. Match Your Results to the Table
Take your run length (ft) and design load (W), then compare them to the table to see whether you need 16/2, 14/2, or 12/2 cable.
If you’re using a multi-tap transformer, repeat this step for each individual run.
Every tap acts as its own “line,” and each line should be sized independently.
⭐Example
Example Scenario
You have one cable run with:
- Total fixture watts on this run: 48W
- One-way wire length: 120 ft
Step 1: Apply the 80% Rule
- Design Load = Total Watts ÷ 0.8
- Design Load = 48W ÷ 0.8 = 60W
Step 2: Locate Your Numbers in the Table and Check the Recommended Wire Gauge
Find the point where:
- Feet (left side) = 120 ft
- Watts (top) = ≈ 60W
- ➡️ At 120 ft and 60W, the table shows 12 gauge.
Result:
You should use 12/2 low-voltage landscape cable for this run.
4. Shortcut Option
If you’d rather not calculate each run:
- Use the Min. Transformer Size from the calculator above as a “worst-case scenario” (as if all fixtures were on a single line).
- Combine that wattage with your longest run length.
- Use the table to select a safe cable size.
This gives you a conservative, reliable choice—ideal when planning layouts quickly.
5. Quick Recommendation
For most residential LED landscape lighting systems, 12/2 low-voltage cable is a safe and future-proof option.
It offers:
- Lower resistance per foot
- Reduced voltage drop over long distances
- More capacity for adding fixtures later
- Better performance with multi-run systems
If you’re unsure which wire gauge to choose, you’re always welcome to contact our customer support—we’re happy to help review your layout.
6. Want a More Detailed Professional Explanation?
For an in-depth look at 12/2 vs 14/2 cable, voltage drop basics, and installer tips, see our full guide here:
Recommended Q&A
Q1. How do I know what size transformer I need?
Use the calculator at the top of this page.
Enter the wattage and quantity of each fixture, and the tool will give you the minimum transformer size.
We recommend choosing a transformer that is 20–30% higher than your total load for best performance.
Q2. What does the “80% Rule” mean?
Low-voltage lighting systems naturally lose voltage over longer wire runs.
The 80% Rule is a common guideline used by installers to prevent excessive voltage drop and overheating.
In simple terms:
- Only plan to use about 80% of your transformer’s capacity so your fixtures always receive stable voltage.
Q3. How do I measure the wire length for the cable guide?
Measure the one-way distance from the transformer to the last light on a single cable run.
Each run should be calculated separately.
Q4. What if I have a multi-tap transformer?
Treat each tap as its own individual run.
Calculate the watts and wire length for each run separately, then choose a wire gauge for each one using the table.
Q5. Can I use the “Min. Transformer Size” to pick a cable gauge?
Yes.
If you prefer a simple shortcut, treat the Min. Transformer Size as a “worst-case” value, combine it with your longest run length, and use the cable chart.
This gives you a conservative, safe choice—especially helpful for DIY users.
Q6. Is 12/2 cable always required?
Not always, but 12/2 is the safest all-purpose choice for most homes.
It has lower resistance per foot, reduces voltage drop, and gives you room to add more lights later.
Q7. What happens if my wire gauge is too thin?
Using a cable that’s too thin for the wattage or distance can cause:
- Dim or uneven brightness
- Voltage drop
- Flickering or early LED failure
- Overheating on long runs
Always match your wire gauge to the table to avoid these issues.
Q8. Do I need to match AC/DC wire types with the transformer?
No.
Low-voltage landscape cables (12/2, 14/2, 16/2 SPT-2 or SPT-1W) work with any 12V AC or DC transformer.
What matters is choosing the correct gauge based on load and length.
Q9. I’m still unsure which wire gauge to pick. Can you help?
Of course.
You can contact our customer support anytime—we’re happy to review your layout, run lengths, and wattage to help you choose the safest wire gauge.