C7 Low Voltage Landscape & Outdoor Lighting Systems

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?

Accepted Answer

Each run should not exceed 50% of the transformer rating

Answer

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?

Accepted Answer

A small absolute voltage drop represents a much larger percentage of the total 12V supply, causing noticeable dimming

Answer

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?

Accepted Answer

6 inches

Answer

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?

Accepted Answer

50 watts

Answer

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?

Accepted Answer

The outlet must be GFCI-protected and have a weatherproof cover rated for 'in-use' (while-in-use) protection

Answer

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.

(C7) California C7 Low Voltage Systems Contractor Certification Practice Test

(C7) California C7 Low Voltage Systems Contractor Certification Practice Test

C7 Low Voltage Landscape & Outdoor Lighting Systems