Long 120 V Lighting Run with Far-End LED Cycling
A 120 V single-phase roadway lighting circuit serves 14 LED luminaires rated 70 W at 120 V and spaced 35 m apart along a 490 m run. The conductors are 12 AWG copper, fed from a 20 A breaker at a service pedestal. At commissioning, the first six luminaires operate normally, but the last four cycle on and off at dusk and two drivers have failed. Open-circuit voltage at the pedestal is 122 V; with the circuit loaded, voltage at pole 10 is 104 V and at pole 14 is 96 V. The contractor suggests replacing the 20 A breaker with a 30 A breaker and increasing the photocell delay. The luminaires are on 8 m poles and the service is at one end of the run. No voltage-drop calculation is in the submittal.
1. Estimate the voltage drop and explain whether the symptoms are consistent with the circuit design.
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Approximate voltage drop. Each luminaire current is about 70/120 = 0.58 A at power factor 1; with driver losses and power factor below 1 it may be higher. 12 AWG copper is about 5.2 ohm/km per conductor, so each 35 m span has a loop resistance of about 2 x 5.2 x 0.035 = 0.36 ohm. The first span carries all 14 luminaires; the last span carries one. Summing current moments: 0.58 x (14 + 13 + ... + 1) = 0.58 x 105 = about 61 A. The drop is about 0.36 x 61 = 22 V. At 122 V open circuit, the last pole would be near 100 V, close to the measured 96 V; the extra drop comes from connections, power factor, and driver loading. This is roughly 18 to 22 percent drop, far beyond normal branch-circuit design targets of about 3 percent branch and 5 percent total. The symptoms fit: far-end drivers see undervoltage, drop out, restart, and fail.
2. Evaluate the contractor's proposal to install a 30 A breaker and increase the photocell delay.
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Reject the breaker change. A 30 A breaker does not reduce voltage drop and is not permitted to protect 12 AWG conductors unless the conductor ampacity and termination ratings are verified; 12 AWG is generally a 20 A conductor. Increasing photocell delay only masks cycling. The root cause is excessive impedance and load on one long run, plus possible loose connections. Check terminations, breaker and neutral torque, and measure voltage at each pole under load. Verify conductor ampacity, overcurrent protection, and grounding and bonding. If voltage drop is the issue, the fix is electrical redesign, not a larger breaker.
3. Propose a remediation plan and the measurements needed before energizing the revised circuit.
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Options include splitting the 14-luminaire run into two or more circuits fed from the service or from both ends, increasing conductor size after calculation, moving the service closer, or distributing at 240 V with listed drivers or transformers. For example, splitting into two 7-luminaire circuits reduces the current moment by about four times and cuts drop substantially; a recalculation with 8 AWG or 6 AWG may still be needed to meet the target. Recheck driver operating range, inrush, and photocell and contactor ratings. Before energizing, measure voltage at the last pole under full load and verify no driver cycling. Document as-builts and voltage-drop calculations.