Commercial Tenant Buildout: 100A Subpanel
Design a 100A tenant subpanel for a 3,000 sq ft office space — load calculation, transformer tap, and panel schedule.
Scenario Overview
Design the electrical distribution for a 3,000 sq ft commercial office tenant space in an existing building. The landlord provides a 100A, 208Y/120V tap from the building transformer for the tenant panel.
Given Information
- •3,000 sq ft office space
- •100A, 208Y/120V, 3-phase subpanel
- •Lighting: LED throughout (0.8 W/ft² actual, NEC requires 1.5-3.5 VA/ft²)
- •20 computer workstations
- •Break room with microwave (1,500W) and refrigerator (800W)
- •2× mini-split HVAC units (20A each, 208V)
- •No electric heating
Calculation Steps
General Lighting Load (NEC 220.12)
Office occupancy: NEC Table 220.12 = 3.5 VA/ft² (required minimum for load calc regardless of actual LED wattage).
3,000 ft² × 3.5 VA/ft² = 10,500 VA.
Result: 10,500 VA lighting (code minimum)
Receptacle Load
NEC 220.14(I): First 10 kVA of receptacles at 100%, remainder at 50%.
20 workstations × 180 VA/outlet = 3,600 VA (general use).
Additional general receptacles: estimate 30 × 180 VA = 5,400 VA.
Total receptacle: 9,000 VA (all at 100% since under 10 kVA).
Result: 9,000 VA receptacle load
Apply Lighting + Receptacle Demand
Combined: 10,500 + 9,000 = 19,500 VA.
NEC Table 220.44: First 10,000 VA at 100% = 10,000. Remaining 9,500 at 50% = 4,750.
Demand: 14,750 VA.
Result: 14,750 VA after demand factors
HVAC Load
2× mini-split units: 20A × 208V = 4,160 VA each.
Total: 8,320 VA. Both at 100% (no demand factor for HVAC in commercial).
Result: 8,320 VA HVAC
Kitchen/Break Room Equipment
Microwave: 1,500 VA.
Refrigerator: 800 VA.
Total: 2,300 VA at 100%.
Result: 2,300 VA break room
Total Demand and Panel Verification
Total: 14,750 + 8,320 + 2,300 = 25,370 VA.
Panel current: 25,370 / (208 × √3) = 25,370 / 360 = 70.5A.
100A panel has 30% spare capacity — adequate for growth.
Result: 70.5A demand on 100A panel — 30% spare ✓
Final Answer
Total demand: 25,370 VA (70.5A on 208V 3Φ). 100A, 208Y/120V panel is adequate with 30% spare capacity. Use 1 AWG copper or 2/0 AWG aluminum feeders from building tap to tenant panel.
Key Takeaways
- •NEC lighting load uses code-minimum VA/ft² from Table 220.12, not actual LED wattage
- •Commercial calculations use NEC Table 220.44 demand factors for non-dwelling lighting
- •Always leave 20-30% spare capacity in commercial panels for future tenant changes
- •208V three-phase provides less power per amp than 240V single-phase — verify calculations use correct voltage
Calculators Used
NEC References
- NEC 220.12 — Lighting by Occupancy
- NEC 220.14 — Receptacle Loads
- NEC Table 220.44 — Lighting Demand Factors (non-dwelling)