What this tool does
This calculator helps you determine whether your low-voltage landscape lighting system will experience excessive voltage drop between the transformer and the last fixture on the run. Voltage drop is the gradual loss of electrical pressure as current travels through wire, and it is the most common cause of dim or uneven landscape lighting. By entering your transformer voltage, total fixture wattage, wire run length, and wire gauge, the calculator tells you exactly how much voltage reaches the end of each run and whether you need thicker wire or a shorter run to fix the problem.
How it calculates
The calculator uses standard electrical engineering formulas for copper wire resistance to compute voltage drop.
**Current draw:** \`\`\` Current (amps) = Total Wattage / Source Voltage \`\`\`
**Voltage drop across the wire:** \`\`\` Voltage Drop = 2 x Wire Length x Current x (Resistance per 1000 ft / 1000) \`\`\` The factor of 2 accounts for the round-trip path. Current travels out to the fixtures and back to the transformer through both conductors.
**Copper wire resistance values (ohms per 1000 ft):** - 10 AWG: 1.018 - 12 AWG: 1.619 - 14 AWG: 2.575 - 16 AWG: 4.094 - 18 AWG: 6.510
These are standard NEC values for solid copper conductors at 68 degrees F (20 degrees C). The calculator uses three threshold levels for voltage drop: under 5% is good, 5% to 10% is acceptable but not ideal, and over 10% is too high and requires a change.
Wire gauge selection guide
Choosing the right wire gauge is critical for landscape lighting performance. Thicker wire (lower AWG number) has less resistance and allows more current to flow with less voltage loss.
**Common guidelines:** - **18 AWG** — Only suitable for very short runs under 25 feet with low wattage. Often included with cheap lighting kits. - **16 AWG** — Good for runs up to 50 feet with moderate loads. Common for small residential installations. - **14 AWG** — A solid choice for runs between 50 and 100 feet. Works well for most residential setups. - **12 AWG** — The most popular choice for professional landscape lighting installations. Handles runs up to 150 feet with typical loads. - **10 AWG** — Best for long runs over 150 feet or high-wattage applications. The thickest wire commonly used in low-voltage landscape lighting.
When in doubt, go one size thicker than you think you need. The cost difference in wire is small compared to the labor of replacing undersized wire later.
How to use
1. Select your transformer voltage — most landscape transformers output either 12V or 15V 2. Enter the total wattage of all fixtures on the run by adding up the wattage of every light on that circuit 3. Enter the wire run length in feet, measured from the transformer to the last fixture 4. Select the wire gauge (AWG) you plan to use or already have installed 5. Review the results showing voltage drop percentage, voltage at the end of the run, and current draw 6. If voltage drop exceeds 10%, follow the recommendation to upgrade to a thicker wire gauge 7. Use the "Add Another Zone" button to calculate multiple lighting runs from the same transformer
FAQs
Q: What is an acceptable voltage drop for landscape lighting? A: Industry best practice is to keep voltage drop under 5% of the source voltage. Between 5% and 10% is technically acceptable but lights may appear noticeably dimmer, especially LED fixtures. Over 10% is considered too high and can cause fixtures to flicker, change color temperature, or fail to turn on at all.
Q: Why do my landscape lights get dimmer further from the transformer? A: This is caused by voltage drop. As electrical current flows through wire, some energy is lost to resistance in the wire itself. The longer the wire and the thinner the gauge, the more voltage is lost before reaching the fixture. Fixtures at the end of a long run receive less voltage than those near the transformer, causing them to appear dimmer.
Q: Should I use a 12V or 15V transformer? A: A 15V transformer provides extra voltage headroom to compensate for voltage drop across long wire runs. If your lighting runs are over 75 feet, a 15V transformer can help ensure all fixtures receive close to 12V even after accounting for wire losses. Most LED landscape fixtures are rated for 10 to 15 volts, so the extra voltage from a 15V transformer is safe and helps maintain consistent brightness.
Q: Does LED lighting reduce voltage drop compared to halogen? A: LED fixtures draw significantly less current than halogen for the same light output, which does reduce voltage drop because the formula depends directly on current. However, LED fixtures are actually more sensitive to voltage variations than halogen bulbs. Even small voltage drops can cause noticeable color shifts or flickering in some LED fixtures, making proper wire sizing even more important.
Q: Can I connect fixtures in a loop instead of a single run? A: Yes, a loop or T configuration feeds power from both directions, effectively cutting the wire run length in half and reducing voltage drop significantly. This is a professional technique that allows you to serve more fixtures on the same wire gauge. This calculator models a single home-run configuration, so for loop layouts, enter half the total loop length as your wire run.
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