6.4 Commercial & Multi-Family Load Calculations (Cooktops, Dryers, HVAC & Stores)
Key Takeaways
- Commercial receptacle loads are calculated under NEC 220.14(I) at 180 VA per strap/yoke (whether single, duplex, or triplex on a single yoke), with demand factors under Table 220.47 allowing the first 10 kVA at 100% and the excess over 10 kVA at 50%.
- Commercial kitchen equipment (cooking, dishwashing, booster heaters, water heaters) is calculated under NEC 220.56 and Table 220.56 with demand factors scaling from 100% for 1-2 appliances down to 65% for 6 or more appliances, provided the calculated feeder load is never less than the sum of the two largest appliances.
- Show window lighting requires a minimum of 200 VA per linear foot under NEC 220.14(G) and 220.43(A); commercial sign lighting mandates at least 1,200 VA per 220.14(F) and 600.5; and track lighting is sized at 150 VA per 2 feet of track under 220.43(B).
- Multi-family dwelling feeder and service loads can be calculated using the Optional Method of NEC 220.84 if the building has 3 or more dwelling units equipped with electric cooking and heating/cooling, applying Table 220.84 demand factors based on total unit count.
- Continuous commercial loads (such as general store lighting, illuminated signs, and display windows) must be multiplied by 125% per NEC 210.20 and 215.2, and service feeders supplying motor loads must include 125% of the largest motor full-load current per NEC 430.24.
Commercial & Multi-Family Load Calculations (NEC Article 220)
Commercial and multi-family electrical installations present complex load dynamics that differ significantly from single-family residential dwellings. Commercial facilities incorporate continuous retail lighting, high-density receptacle layouts, dedicated show windows, commercial kitchen equipment, and three-phase motor loads. Multi-family buildings feature high unit diversity that allows substantial demand reductions on main services.
On the Oklahoma Journeyman Electrician examination, questions in this domain test your ability to apply commercial demand factors, size three-phase services, and execute multi-family calculations.
1. Commercial Receptacle Loads (NEC 220.14(I) & 220.47)
In non-dwelling occupancies, receptacle loads are calculated on a volt-amperes per yoke (strap) basis:
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| COMMERCIAL RECEPTACLE LOAD RULES (NEC 220.14(I)) |
| |
| [YOKE / STRAP RATING] |
| • 180 VA for each single receptacle or duplex receptacle on ONE yoke. |
| • 360 VA for a two-gang box with two duplex receptacles (TWO yokes). |
| • Multi-outlet Assemblies (220.14(H)): 180 VA for each 5 ft standard, |
| or 180 VA for each 1 ft where appliances are used simultaneously. |
| |
| [TABLE 220.47 RECEPTACLE DEMAND FACTORS] |
| • First 10 kVA (10,000 VA) or less: 100% (1.00) |
| • Remainder over 10 kVA (10,000 VA): 50% (0.50) |
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Commercial Receptacle Calculation Example:
A commercial office has 150 standard duplex receptacle outlets installed on individual yokes.
- Connected Receptacle Load: $150 \times 180\text{ VA} = 27,000\text{ VA}$.
- Apply NEC Table 220.47 Demand Factors:
- First $10,000\text{ VA} @ 100% = 10,000\text{ VA}$
- Remaining $17,000\text{ VA} (27,000 - 10,000) @ 50% = 8,500\text{ VA}$
- Net Feeder Receptacle Load: $10,000 + 8,500 = 18,500\text{ VA}$.
2. Commercial Kitchen Equipment (NEC 220.56 & Table 220.56)
Commercial food service establishments (restaurants, cafeterias, hotel kitchens) operate thermostatically controlled and duty-cycled electric cooking, dishwashing, and water heating equipment. NEC 220.56 permits demand factors from Table 220.56:
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| NEC TABLE 220.56 COMMERCIAL KITCHEN DEMAND FACTORS |
| |
| NUMBER OF EQUIPMENT UNITS DEMAND FACTOR (%) |
| ----------------------------------------------------------------------- |
| 1 Unit 100% |
| 2 Units 100% |
| 3 Units 90% |
| 4 Units 80% |
| 5 Units 70% |
| 6 or More Units 65% |
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The "Two Largest Loads" Rule (NEC 220.56): "In no case shall the feeder or service load be less than the sum of the largest two kitchen equipment loads."
Qualifying Equipment:
Commercial ranges, deep fryers, griddles, ovens, conveyor toasters, commercial dishwashers with booster heaters, electric water heaters, and steam tables.
Kitchen Demand Calculation Example:
A restaurant kitchen contains 6 commercial appliances:
- Range ($15\text{ kW}$), Deep Fryer ($10\text{ kW}$), Convection Oven ($8\text{ kW}$), Dishwasher ($6\text{ kW}$), Booster Heater ($9\text{ kW}$), Food Warmer ($4\text{ kW}$).
- Total Connected Nameplate: $15 + 10 + 8 + 6 + 9 + 4 = 52\text{ kW} = 52,000\text{ VA}$.
- Apply Table 220.56 Demand (6+ units = 65%): $52,000\text{ VA} \times 0.65 = 33,800\text{ VA}$.
- Check Minimum Limit (Sum of 2 Largest): Range ($15\text{ kW}$) + Deep Fryer ($10\text{ kW}$) = $25\text{ kW} = 25,000\text{ VA}$.
- Since $33,800\text{ VA} \ge 25,000\text{ VA}$, the net calculated feeder load is $33,800\text{ VA}$.
3. Specialized Commercial Loads
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| SPECIALIZED COMMERCIAL LOAD SIZING RULES |
| |
| [SHOW WINDOW LIGHTING (NEC 220.14(G) & 220.43(A))] |
| • Sized at 200 VA per linear foot of show window measured at the base. |
| • Continuous load: Sized at 125% for branch circuits & feeders. |
| |
| [OUTLINE & SIGN LIGHTING (NEC 220.14(F) & 600.5)] |
| • Minimum 1,200 VA rating per required commercial sign circuit. |
| • Continuous load: Sized at 125% for branch circuits & feeders. |
| |
| [TRACK LIGHTING (NEC 220.43(B))] |
| • In non-dwelling occupancies: 150 VA for every 2 ft of track or fraction.|
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4. Multi-Family Dwelling Calculations (NEC 220.84 Optional Method)
For multi-family residential apartment buildings, NEC 220.84 provides an Optional Multi-Family Calculation that applies high diversity factors to the aggregate building load.
Eligibility Criteria (NEC 220.84(A)):
- Each dwelling unit is equipped with complete electric cooking equipment.
- Each dwelling unit is equipped with electric space heating and/or air conditioning (or individual gas heating with individual metering).
- The building contains three (3) or more dwelling units supplied by a common service or feeder.
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| NEC TABLE 220.84 MULTI-FAMILY OPTIONAL DEMAND FACTORS |
| |
| NUMBER OF DWELLING UNITS DEMAND FACTOR (%) |
| ----------------------------------------------------------------------- |
| 3 to 5 Units 45% |
| 6 to 7 Units 44% |
| 8 to 10 Units 43% |
| 11 to 13 Units 42% |
| 14 to 17 Units 41% |
| 18 to 21 Units 40% |
| 22 to 25 Units 39% |
| 26 to 30 Units 38% |
| 31 to 35 Units 37% |
| 36 to 40 Units 36% |
| 41 to 45 Units 35% |
| 46 to 50 Units 34% |
| 51 to 55 Units 33% |
| 56 to 61 Units 32% |
| 62 Units and Over 30% |
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5. Master Worked Commercial Example: Retail Clothing Store
Commercial Store Specifications (120/208V 3-Phase 4-Wire Service):
- Retail Floor Area: $6,000\text{ sq ft}$ ($3.0\text{ VA/sq ft}$ standard general lighting)
- Show Window: $30\text{ linear feet}$
- Track Lighting: $60\text{ linear feet}$
- General Receptacles: 50 duplex yokes ($180\text{ VA}$ each)
- Commercial Sign Outlet: 1 sign circuit ($1,200\text{ VA}$ minimum)
- Breakroom / Kitchenette Appliances (3 units):
- Commercial Microwave: $1,800\text{ VA}$
- Built-in Dishwasher: $1,500\text{ VA}$
- Electric Water Heater: $3,000\text{ VA}$
- Rooftop HVAC Units (3-Phase 208V):
- Two 5-ton packaged units, full load current = $18\text{ A}$ each at $208\text{V}$ 3-phase
- Total HVAC Apparent Power: $2 \times (208\text{V} \times \sqrt{3} \times 18\text{A}) = 2 \times 6,485\text{ VA} = 12,970\text{ VA}$
- Largest motor 25% addition ($18\text{A} \times 0.25 = 4.5\text{A}$): $208\text{V} \times \sqrt{3} \times 4.5\text{A} = 1,621\text{ VA}$
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COMMERCIAL RETAIL STORE LOAD CALCULATION WORKSHEET
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1. GENERAL LIGHTING LOAD (NEC 220.42 & Continuous 125% Factor):
• 6,000 sq ft × 3.0 VA/sq ft = 18,000 VA
• Continuous Factor: 18,000 VA × 1.25 = 22,500 VA
-> Net General Lighting Load = 22,500 VA
2. SHOW WINDOW LIGHTING (NEC 220.14(G), 220.43(A) & Continuous 125%):
• 30 linear feet × 200 VA/ft = 6,000 VA
• Continuous Factor: 6,000 VA × 1.25 = 7,500 VA
-> Net Show Window Load = 7,500 VA
3. TRACK LIGHTING (NEC 220.43(B) & Continuous 125%):
• 60 ft / 2 ft = 30 segments × 150 VA = 4,500 VA
• Continuous Factor: 4,500 VA × 1.25 = 5,625 VA
-> Net Track Lighting Load = 5,625 VA
4. RECEPTACLE LOAD (NEC 220.14(I) & Table 220.47 Demand):
• 50 yokes × 180 VA = 9,000 VA
• Table 220.47 Demand (First 10,000 VA @ 100%) = 9,000 VA
-> Net Receptacle Load = 9,000 VA
5. COMMERCIAL SIGN OUTLET (NEC 220.14(F), 600.5 & Continuous 125%):
• 1 Required Sign Circuit = 1,200 VA
• Continuous Factor: 1,200 VA × 1.25 = 1,500 VA
-> Net Sign Load = 1,500 VA
6. KITCHEN APPLIANCES (NEC 220.56 & Table 220.56):
• Microwave (1.8k) + Dishwasher (1.5k) + Water Heater (3.0k) = 6,300 VA
• Apply Table 220.56 (3 appliances @ 90% demand): 6,300 × 0.90 = 5,670 VA
• Check 2 Largest (3,000 + 1,800 = 4,800 VA < 5,670 VA -> OK)
-> Net Kitchen Appliance Load = 5,670 VA
7. ROOFTOP HVAC & MOTOR LOADS (NEC 430.24):
• Base HVAC Load (Two 18A 208V 3-Phase Units) = 12,970 VA
• Largest Motor 25% Addition = 1,621 VA
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-> Net HVAC & Motor Load = 14,591 VA
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TOTAL CALCULATED COMMERCIAL SERVICE LOAD = 66,386 VA
THREE-PHASE SERVICE ENTRANCE CURRENT CALCULATION (120/208V 3-Phase):
Current (Amperes) = Total VA / (Line Voltage × √3)
Current (Amperes) = 66,386 VA / (208 Volts × 1.73205)
Current (Amperes) = 66,386 VA / 360.267 Volts = 184.27 Amperes
STANDARD COMMERCIAL SERVICE RATING: 200A or 225A 3-Phase 4-Wire Service
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A commercial office floor contains 120 standard duplex receptacle outlets installed on individual yokes. What is the calculated feeder receptacle load after applying the demand factors of NEC Table 220.47?
A restaurant installs six commercial kitchen cooking and water heating appliances with a total connected nameplate rating of 60 kW. Under NEC 220.56 and Table 220.56, what is the demand factor and net calculated feeder load for this equipment?
A retail department store has a 40-foot continuous glass show window along its exterior walkway. Under NEC 220.14(G) and 220.43(A), what is the minimum calculated continuous branch circuit and feeder load for this show window?
An electrical contractor is sizing the main electrical service for a 12-unit multi-family apartment building under the optional method of NEC 220.84. If the total connected load of all 12 units combined is 360,000 VA, what demand factor from Table 220.84 must be applied, and what is the calculated service load?