Parking Lot Lighting Circuit Design
Design outdoor parking lot lighting circuits — pole layout, voltage drop for long runs, and photocell/contactor control.
Scenario Overview
Design the lighting circuits for a 120-space parking lot with LED pole-mounted luminaires. This involves calculating the number of fixtures, sizing circuits for long conduit runs, and addressing voltage drop.
Given Information
- •Parking lot: 120 spaces, approximately 40,000 sq ft
- •Target illuminance: 1.0 fc (IES RP-8 for open parking)
- •Luminaires: 150W LED, 20,000 lm each, on 20 ft poles
- •Spacing: approximately 80 ft between poles
- •Power source: 277V, single-phase from building panel
- •Longest circuit run: 400 ft
- •Underground PVC conduit
Calculation Steps
Determine Number of Luminaires
Using lumen method: N = (E × A) / (Φ × CU × MF).
E = 1.0 fc, A = 40,000 ft², Φ = 20,000 lm, CU = 0.35 (outdoor parking), MF = 0.85.
N = (1.0 × 40,000) / (20,000 × 0.35 × 0.85) = 40,000 / 5,950 = 6.7 → 7 poles.
With site layout adjustments: use 8 poles for uniform coverage.
Result: 8 poles with 150W LED luminaires
Calculate Total Load
8 fixtures × 150W = 1,200W total.
At 277V: I = 1,200 / 277 = 4.33A.
Continuous load (lighting > 3 hours): 4.33 × 1.25 = 5.41A.
Split into 2 circuits: 4 poles per circuit = 600W (2.17A per circuit).
Result: 4.33A total (2 circuits × 2.17A each)
Check Voltage Drop (Worst Case)
Longest circuit: 400 ft, 4 fixtures, 2.17A (but fixtures are distributed along the run).
Conservative calculation (all load at end): VD = 2 × 400 × 2.17 × 2.08 / 1000 = 3.61V (using 12 AWG, R=2.08).
VD% = 3.61 / 277 = 1.30% — acceptable.
Even with 10 AWG (R=1.29): VD = 2.24V (0.81%). Use 12 AWG — adequate.
Result: 1.30% voltage drop with 12 AWG ✓
Conduit Sizing
Per circuit: 2× 12 AWG THWN-2 + 1× 12 AWG EGC in PVC Schedule 40.
3 conductors at 12 AWG THWN-2: area = 0.0133 in² each.
Total: 3 × 0.0133 = 0.0399 in². 40% of ½" PVC Sch40 (0.217 in²) = 0.087 in².
½" PVC Schedule 40 — adequate for each circuit.
Result: ½" PVC Schedule 40 per circuit
Control (Photocell and Contactor)
Photocell sensor at exterior location with clear sky view.
Lighting contactor (mechanically held): 2-pole, rated for LED ballast loads.
NEC 410.104(A): switching devices for fluorescent/LED must be rated for the load type.
Timer override optional for after-hours security lighting on a separate zone.
Result: Photocell + lighting contactor + optional timer
Final Answer
8 × 150W LED pole luminaires on 2 circuits, 12 AWG THWN-2 in ½" PVC Schedule 40 underground, 20A breakers (277V), controlled by photocell and lighting contactor. Voltage drop: 1.30% on longest 400 ft run.
Key Takeaways
- •LED parking lot lighting significantly reduces circuit sizing compared to HID — 150W LED replaces 400W HPS
- •Voltage drop is critical for long outdoor lighting runs — always check at design stage
- •Lighting is always a continuous load — apply 125% for OCPD and conductor sizing
- •Underground PVC for parking lot lighting minimizes cost and is standard practice
Calculators Used
NEC References
- NEC 220.12 — Lighting Loads
- NEC 210 — Branch Circuits
- IES RP-8 — Parking Lot Lighting
- NEC 300.5 — Underground Burial Depth