C7 Low Voltage Landscape & Outdoor Lighting Systems — Questions and Answers
Question 1: A low voltage landscape lighting transformer is rated at 150 watts. The designer wants to run two cable runs from it. Best practice limits each run to what maximum percentage of the transformer's total wattage?
- 100% on one run, 0% on the other
- No more than 80% of transformer rating per run
- Each run should not exceed 50% of the transformer rating (Correct answer)
- Wattage per run is unlimited as long as total does not exceed the rating
Correct answer: Each run should not exceed 50% of the transformer rating
For balanced loading and to minimize voltage drop on any single run, each cable run from a multi-tap transformer should not exceed 50% of the total transformer wattage. This ensures no single run is overloaded and keeps voltage consistent across all fixtures. For a 150W transformer, each run should be limited to 75W maximum.
Question 2: When calculating voltage drop for a low voltage (12V) landscape lighting cable run, why is voltage drop especially critical compared to 120V circuits?
- 12V systems are not subject to voltage drop since they are low voltage
- A small absolute voltage drop represents a much larger percentage of the total 12V supply, causing noticeable dimming (Correct answer)
- Voltage drop causes overheating on 12V systems but not on 120V systems
- 12V landscape lights are designed to operate at any voltage between 5V and 15V without effect
Correct answer: A small absolute voltage drop represents a much larger percentage of the total 12V supply, causing noticeable dimming
A 1-volt drop on a 12V system is an 8.3% loss — enough to cause visible dimming in incandescent and halogen fixtures. The same 1V drop on a 120V circuit is less than 1% and imperceptible. This is why 12V landscape lighting runs are length-limited and wire gauge is carefully selected — the low operating voltage amplifies the impact of resistive losses.
Question 3: What is the maximum burial depth required by the NEC for a low voltage (12V) landscape lighting cable that is NOT in conduit?
- No burial required — surface-laid is permitted
- 6 inches (Correct answer)
- 12 inches
- 24 inches
Correct answer: 6 inches
Per NEC Table 300.5, low voltage (under 30V) landscape lighting cable rated for direct burial is required to be buried at least 6 inches deep. This is significantly less than the 12 or 24 inches required for 120V or 240V circuits, but burial is still required to protect cables from damage by tools, foot traffic, and lawn equipment.
Question 4: A landscape lighting design calls for ten 5-watt LED path lights on a single run from a 12V transformer, using 12 AWG cable over a 100-foot run. What is the approximate total load on this run?
- 5 watts
- 25 watts
- 50 watts (Correct answer)
- 100 watts
Correct answer: 50 watts
10 fixtures × 5 watts each = 50 watts total load on this run. This is a straightforward wattage summation. The result is then used to calculate current (I = W/V = 50/12 = 4.17A) and subsequently to calculate voltage drop using the resistance of the 12 AWG wire over the 100-foot run.
Question 5: A low voltage landscape lighting transformer connected to a 120V GFCI-protected outlet must be installed outdoors. What NEC requirement applies to the outlet supplying it?
- The outlet must be a twist-lock type rated for outdoor use
- The outlet must be GFCI-protected and have a weatherproof cover rated for 'in-use' (while-in-use) protection (Correct answer)
- A dedicated 20-amp circuit is required for any landscape lighting transformer
- Outdoor outlets do not require weatherproof covers if the transformer plug is rated for outdoor use
Correct answer: The outlet must be GFCI-protected and have a weatherproof cover rated for 'in-use' (while-in-use) protection
NEC 210.8(A) requires GFCI protection for outdoor 120V receptacles, and NEC 406.9(B)(1) requires a weatherproof 'in-use' cover (extra-duty) for outdoor outlets where the plug remains inserted during use — which describes a permanently-installed transformer. Standard flip-lid covers do not satisfy the 'in-use' requirement.
Question 6: When connecting multiple cable runs to a multi-tap landscape lighting transformer, which tap should be used for the longest cable run?
- The lowest voltage tap (e.g., 11V) to reduce energy consumption
- The highest voltage tap (e.g., 14V or 15V) to compensate for voltage drop over the longer distance (Correct answer)
- All runs should use the 12V tap regardless of distance
- The tap selection does not matter because LED fixtures self-regulate voltage
Correct answer: The highest voltage tap (e.g., 14V or 15V) to compensate for voltage drop over the longer distance
Longer cable runs experience greater resistive voltage drop, causing fixtures at the end of the run to receive less than 12V. By using a higher voltage tap (13V or 14V) for long runs, the transformer outputs a higher voltage that compensates for the drop, so fixtures receive closer to their rated 12V operating voltage. Short runs use lower taps to avoid over-voltage.
C7 Low Voltage Landscape & Outdoor Lighting Systems
A low voltage landscape lighting transformer is rated at 150 watts.
The designer wants to run two cable runs from it.
Best practice limits each run to what maximum percentage of the transformer's total wattage?