Multi-Family (12-Unit) Service Calculation
Calculate the service for a 12-unit apartment building — individual unit loads, demand factors, and building service sizing per NEC 220.
Scenario Overview
Calculate the total building service for a 12-unit apartment complex. Each unit is identical: 1,200 sq ft with electric range, dryer, A/C, and water heater. NEC provides generous demand factors for multi-dwelling buildings.
Given Information
- •12 identical dwelling units, each 1,200 sq ft
- •Each unit: electric range (12 kW), dryer (5 kW), water heater (4.5 kW)
- •Each unit: A/C (24A, 240V = 5,760 VA)
- •House loads: hallway/exterior lighting (8 kW), elevator (15 HP)
- •Service: 208Y/120V, 3-phase
Calculation Steps
Per-Unit General Lighting + Small Appliance + Laundry
General lighting: 1,200 ft² × 3 VA/ft² = 3,600 VA.
Small appliance: 2 × 1,500 = 3,000 VA.
Laundry: 1 × 1,500 = 1,500 VA.
Per unit total: 8,100 VA.
12 units: 8,100 × 12 = 97,200 VA.
Result: 97,200 VA lighting/receptacle (12 units)
Apply NEC Table 220.42 Demand
First 3,000 VA at 100% = 3,000.
3,001 to 120,000 VA at 35%: (97,200 − 3,000) × 0.35 = 32,970.
Total after demand: 35,970 VA.
Result: 35,970 VA after lighting demand
Range Demand (NEC Table 220.55)
12 ranges, each 12 kW: Column C for 12 appliances = 27 kW.
Note: this is dramatically less than 12 × 12 = 144 kW connected!
Result: 27,000 VA range demand (from 144 kW connected)
Dryer Demand (NEC 220.54)
12 dryers, each 5 kW: 12 × 5,000 = 60,000 VA.
NEC 220.54: use demand factors from Table 220.54.
12 dryers: 47% demand factor → 60,000 × 0.47 = 28,200 VA.
Result: 28,200 VA dryer demand
Water Heater + A/C (per unit)
Water heaters: 12 × 4,500 = 54,000 VA. With 4+ fixed appliances per unit: 75% → 40,500 VA.
A/C: 12 × 5,760 = 69,120 VA. No heating, so use A/C at 100%.
Result: 40,500 VA water heaters + 69,120 VA A/C
House Loads
Hallway/exterior lighting: 8,000 VA at 100% (no demand for house loads).
Elevator: 15 HP, 3Φ, 208V. FLC = 46.2A → VA = 46.2 × 208 × √3 = 16,647 VA.
125% of largest motor: 46.2 × 1.25 = 57.8A → add 25% = 4,162 VA.
Result: 28,809 VA house loads
Total Service Demand
Lighting: 35,970
Ranges: 27,000
Dryers: 28,200
Water heaters: 40,500
A/C: 69,120
House loads: 28,809
Total: 229,599 VA.
Service amps: 229,599 / (208 × √3) = 637.6A.
Select: 800A service (provides growth capacity).
Result: 638A demand → 800A service
Final Answer
Total building demand: 229,599 VA (638A at 208V 3Φ). Recommended: 800A, 208Y/120V, 3-phase service. NEC demand factors reduced connected load from over 500 kVA to 230 kVA — a 54% reduction.
Key Takeaways
- •Multi-family demand factors are extremely generous — 12-unit buildings see 45-55% load reduction from NEC tables
- •NEC Table 220.55 for ranges drops 144 kW connected to only 27 kW demand for 12 units
- •Always calculate both per-unit and building-level demand factors — they compound
- •House loads (common areas, elevators) are at 100% demand with no reduction
Calculators Used
NEC References
- NEC 220.42 — Lighting Demand
- NEC 220.54 — Dryer Demand
- NEC 220.55 — Range Demand
- NEC 220.84 — Optional Multi-Family Method