Low Voltage Transformer Calculator
Add up the fixture load and estimate cable voltage drop for a 12 V or 24 V circuit. Compare the proposed supply and cable with the fixture specifications.
Size Your Transformer
Enter your fixture details below. The calculator updates results in real time as you type.
System Overview
Browse the range on Tonghua Lighting, our main manufacturer website.
This tool adds a 20% allowance to the lighting load and rounds up to a listed transformer size. Its voltage-drop estimate treats the full load as if it were at the end of a two-conductor copper run.
Calculation method
- Load W = fixture count × watts per fixture
- Minimum planning capacity = load W × 1.20
- Current A = load W ÷ supply V
- Voltage drop V = 2 × one-way length (ft) × current A × resistance (Ω/1,000 ft) ÷ 1,000
Worked example
Ten 7 W fixtures draw 70 W. The tool adds 20% to get 84 W, then selects 100 W. At 12 V over 50 ft of 14 AWG copper (2.53 Ω/1,000 ft), estimated drop is 1.48 V, or 12.3%.
Magnetic vs Electronic Transformers
Two technologies, different tradeoffs. Pick based on your installation.
| Feature | Magnetic (Core & Coil) | Electronic (Solid State) |
|---|---|---|
| Lifespan | 15 to 20 years | 5 to 8 years |
| Weight | Heavy (5 to 15 lbs for 300W) | Light (1 to 3 lbs for 300W) |
| Cost | Higher upfront ($80 to $200) | Lower upfront ($30 to $80) |
| Dimming | Compatible with most dimmers | Needs specific electronic low voltage dimmers |
| Heat & Noise | Runs warm, slight hum | Runs cool, silent |
| Minimum Load | No minimum, handles 0 to 100% | Needs 20 to 30W minimum load |
| Best For | Permanent installations, long wire runs, low voltage switchgear | Small installs and tight spaces |
Three Costly Mistakes
Check total load, cable size, and branch layout together. An adequate transformer can still leave distant fixtures under-supplied if the cable loss is too high.
No Headroom
A supply operated outside its loading or temperature limits may overheat or trip. Leave the margin required by the manufacturer.
Undersized Wire
Voltage drop rises with cable length and current. Enter the actual load before deciding whether a conductor is large enough.
Hub Wiring
Daisy-chaining low voltage lights puts the farthest fixture at the lowest voltage. A hub or T-junction layout keeps wire runs shorter and voltage more uniform across all fixtures.
Low Voltage Wire Gauge Selection Chart
Use the table as a starting point, then calculate voltage drop for each branch. Cable ampacity, burial rating, and connection method must also suit the installation.
| Wire Gauge | Max Load at 12V | Max Load at 24V | Recommended Max Run | Resistance |
|---|---|---|---|---|
| 18 AWG | 25W | 50W | 25 ft / 7.6 m | 6.51 Ω/1000ft |
| 16 AWG | 50W | 100W | 40 ft / 12 m | 4.02 Ω/1000ft |
| 14 AWG | 100W | 200W | 60 ft / 18 m | 2.53 Ω/1000ft |
| 12 AWG | 150W | 300W | 100 ft / 30 m | 1.59 Ω/1000ft |
| 10 AWG | 250W | 500W | 150 ft / 46 m | 0.998 Ω/1000ft |
| 8 AWG | 400W | 800W | 200 ft / 61 m | 0.628 Ω/1000ft |
Transformer Sizing FAQ
What size transformer do I need?
Add the fixture watts and the loading allowance required by the supply manufacturer, then choose a suitable rated unit. This tool uses a 20% planning allowance: ten 7W fixtures give 84W before rounding up.
12V or 24V for landscape lighting?
Use the voltage specified for the fixtures. At the same power, 24V draws half the current of 12V and has less cable loss, but a 12V fixture cannot be assumed compatible with a 24V supply.
How do I calculate voltage drop?
For a simple end-loaded DC circuit, voltage drop = 2 × one-way length × current × resistance per unit length. With 70W at 12V, 100 ft of 14 AWG copper at 2.53 Ω/1,000 ft gives about 2.95V loss. Distributed loads need branch-by-branch assessment.
What wire gauge for landscape lighting?
Choose wire from current, length, permitted voltage drop, and the cable’s installation rating. A fixed distance rule cannot cover every load.
Can I mix different wattage fixtures?
You can mix wattages if the fixtures use the same compatible supply and the combined load stays within its limits. Check voltage drop on each branch.
What if my transformer is too small?
An undersized supply may overheat, trip, or produce unstable output. Check the nameplate load and instructions; surface temperature alone cannot tell you the loading percentage.