Industrial Warehouse Lighting Circuit Voltage Drop Assessment

Scenario

A new 12,000 m² automated warehouse in Phoenix, AZ is being commissioned. The lighting system uses LED high-bay fixtures supplied via a dedicated 400 V three-phase distribution board. Due to architectural constraints, the longest radial circuit run from the panel to the farthest fixture bank is 85 m — routed through a hot mezzanine space (ambient ~45°C). NEC 210.19(A)(1) mandates ≤3% voltage drop for branch circuits supplying lighting.

Given Data

• Nominal Voltage: 400 V
• Current: 78 A (calculated load for 24 fixtures × 1.2 kVA each, PF = 0.95)
• Conductor Length: 85 m
• Resistance per km: 0.31 Ω/km (25 mm² copper THHN, derated for 45°C ambient using IEEE 80–2013 correction factor of 1.15 → base 0.27 Ω/km × 1.15 ≈ 0.31 Ω/km)

Calculation

Voltage drop is calculated as:

Voltage Drop = 2 × conductor_length (km) × current (A) × resistance_per_km (Ω/km)
              = 2 × (85 / 1000) × 78 × 0.31
              = 2 × 0.085 × 78 × 0.31
              = 4.09 V

Percentage Voltage Drop = (4.09 / 400) × 100 = 1.02%

Result and Decision

The calculated 1.02% voltage drop is well within the 3% NEC limit. No conductor upsizing is required. The design was approved for installation using 25 mm² copper THHN in EMT conduit, with thermal derating verified.

Lesson

Ambient temperature significantly impacts conductor resistance — skipping temperature correction can underestimate voltage drop by >10%. Always apply NEC Table 310.16 ambient correction factors before inputting resistance into voltage drop tools.