5.4 Commercial & Multifamily Load Calculations

5.4 Commercial & Multifamily Load Calculations

Quick Reference: Commercial and multifamily electrical installations exhibit distinct demand profiles compared to single-family dwellings. In commercial buildings, lighting operates continuously, show windows demand dedicated linear-foot capacity, and commercial kitchens incorporate heavy cooking arrays subject to NEC Table 220.56. In multifamily apartment complexes, plans examiners must distinguish between tenant dwelling unit diversity under NEC 220.84 and un-discounted common House Loads.

1. Commercial General Lighting Calculations (NEC 220.42 & Table 220.42(A))

In commercial occupancies, general lighting loads are determined on a unit-per-area basis using NEC Table 220.42(A) based on gross floor area calculated from outside building dimensions.

+-----------------------------------------------------------------------------+
|            NEC TABLE 220.42(A) NON-DWELLING LIGHTING UNIT LOADS             |
+------------------------------------+----------------------------------------+
| Occupancy Classification           | Unit Load (VA/sq ft)                   |
+------------------------------------+----------------------------------------+
| Office (banks are office-type)     | 1.3 VA / sq ft                         |
| Retail (stores, barber/beauty)     | 1.9 VA / sq ft                         |
| Restaurant (clubs are restaurants) | 1.5 VA / sq ft                         |
| Warehouse                          | 1.2 VA / sq ft                         |
| School / university                | 1.5 VA / sq ft (was 3 in the 2020 NEC) |
| Hospital / health care clinic      | 1.6 VA / sq ft                         |
| Hotels and motels                  | 1.7 VA / sq ft                         |
| Manufacturing facility             | 2.2 VA / sq ft                         |
| Parking garage                     | 0.3 VA / sq ft                         |
+------------------------------------+----------------------------------------+

The occupancy names in Table 220.42(A) are energy-code names, so the table carries lettered notes that redirect familiar building types: armories and auditoriums are gymnasium-type, lodge rooms follow hotels and motels, industrial commercial loft buildings are manufacturing-type, banks are office-type, commercial storage garages are parking-garage-type, clubs are restaurant-type, and barber shops, beauty parlors, and stores are retail-type. A plans examiner who cannot find "bank" or "store" as its own row is expected to work the notes rather than guess.

The 125% continuous multiplier of NEC 215.2(A)(1) and NEC 210.20(A) still applies in full to continuous lighting loads that are not taken from Table 220.42(A) — most importantly show-window lighting under 220.46(A), sign loads under 220.14(F), and any design that uses the actual connected lighting load because it exceeds the table minimum.

2. Show Windows & Commercial Receptacle Demand (NEC 220.46, Table 220.47)

1. Show Window Lighting (NEC 220.14(G) & 220.46(A))

In retail occupancies, show window lighting branch circuits and feeders must be calculated at not less than $200\text{ VA}$ per linear foot ($660\text{ VA/m}$) of show window, measured horizontally along its base. Because show window lighting operates continuously, feeders must be sized for $200\text{ VA/ft} \times 1.25 = \mathbf{250\text{ VA/ft}}$.

2. Commercial Receptacle Loads & Demand Factors (NEC Table 220.47)

In commercial buildings, general-use convenience receptacle outlets are calculated at $180\text{ VA}$ per strap/yoke (single, duplex, or triplex receptacle on a single mounting strap per NEC 220.14(I)). For multioutlet assemblies (e.g., plug-mold / raceway), NEC 220.14(H) requires $180\text{ VA}$ per $5\text{ ft}$ (general use) or $180\text{ VA}$ per $1\text{ ft}$ (simultaneous heavy commercial use).

Feeder and service demand factors for commercial receptacles are established in NEC Table 220.47:

3. Commercial Kitchen & Cooking Equipment (NEC Table 220.56)

In commercial kitchens (restaurants, school cafeterias, hospital dietary centers, hotel kitchens), cooking equipment, dishwashers with booster heaters, water heaters dedicated to kitchen sanitization, fryers, griddles, steam tables, and food warmers exhibit operational diversity governed by NEC Table 220.56.

+-----------------------------------------------------------------------------+
|             NEC TABLE 220.56 COMMERCIAL KITCHEN EQUIPMENT DEMAND            |
+-------------------------------------+---------------------------------------+
| Number of Units of Equipment        | Demand Factor (%)                     |
+-------------------------------------+---------------------------------------+
| 1 to 2 units                        | 100%                                  |
| 3 units                             | 90%                                   |
| 4 units                             | 80%                                   |
| 5 units                             | 70%                                   |
| 6 or more units                     | 65%                                   |
+-------------------------------------+---------------------------------------+

The Mandatory "Never Less Than Two Largest" Rule:

Under the footnote to NEC Table 220.56, in no case shall the calculated feeder or service demand load be less than the sum of the ratings of the two largest units of commercial cooking equipment.

4. Multifamily Building Calculations: Standard vs. Optional Method (NEC 220.84)

When calculating the main electrical service for a multifamily apartment building, engineers may choose between the Standard Method (Part III) and the Multifamily Optional Method (NEC 220.84).

Qualifying Conditions for NEC 220.84:

1. The building must contain three or more dwelling units.
2. Each dwelling unit must be equipped with electric cooking equipment. The Exception to this condition is not a gas-cooking allowance: where a multifamily building without electric cooking produces a larger Part III calculated load than the same building would produce under Part IV with electric cooking added at 8 kW per unit, the designer is permitted to use the lesser of the two results.
3. Each dwelling unit must be equipped with electric space heating and/or air conditioning.
4. No individual dwelling unit is supplied by more than one feeder.

+-----------------------------------------------------------------------------+
|                 NEC TABLE 220.84 MULTIFAMILY OPTIONAL DEMAND FACTORS        |
+-----------------------+---------------------+-------------------------------+|
| Number of Units       | Demand Factor (%)   | Number of Units | Demand (%)  ||
+-----------------------+---------------------+-------------------------------+|
| 3 to 5 units          | 45%                 | 26 to 27 units  | 34%         ||
| 6 to 7 units          | 44%                 | 28 to 30 units  | 33%         ||
| 8 to 10 units         | 43%                 | 31 units        | 32%         ||
| 11 units              | 42%                 | 32 to 33 units  | 31%         ||
| 12 to 13 units        | 41%                 | 34 to 36 units  | 30%         ||
| 14 to 15 units        | 40%                 | 37 to 38 units  | 29%         ||
| 16 to 17 units        | 39%                 | 39 to 42 units  | 28%         ||
| 18 to 20 units        | 38%                 | 43 to 45 units  | 27%         ||
| 21 units              | 37%                 | 46 to 50 units  | 26%         ||
| 22 to 23 units        | 36%                 | 51 to 55 units  | 25%         ||
| 24 to 25 units        | 35%                 | 56 to 61 units  | 24%         ||
|                       |                     | 62 and over     | 23%         ||
+-----------------------+---------------------+-------------------------------+|
+-----------------------------------------------------------------------------+

Separation of Multifamily House Loads (NEC 220.84(B))

House loads (loads not located inside individual apartments) are strictly excluded from Table 220.84 demand factors. House loads include:

• Corridor, stairwell, lobby, and architectural egress lighting
• Parking lot and landscape lighting
• Common clubhouse, leasing office, and fitness center
• Common tenant laundry rooms (commercial coin-op dryers/washers)
• Elevators, fire pumps, booster pumps, and common HVAC central plants

5. Step-by-Step Worked Commercial & Multifamily Calculations

Example 1: Commercial Restaurant / Retail Tenant Fit-Out

• Parameters: A $4,000\text{ sq ft}$ commercial restaurant with a $30\text{ ft}$ show window. System is $208\text{Y}/120\text{V}$, 3-phase, 4-wire.
  • Restaurant floor area: $4,000\text{ sq ft}$ (Table 220.42(A) unit load $= 1.5\text{ VA/sq ft}$)
  • Show window: $30\text{ linear feet}$
  • General Receptacles: 40 duplex receptacles ($40 \times 180\text{ VA} = 7,200\text{ VA}$)
  • Commercial Kitchen: (5) units totaling $42\text{ kW}$ (two largest units are $12\text{ kW}$ and $10\text{ kW}$)
  • HVAC: $10\text{-ton}$ Rooftop AC unit ($208\text{V}$, 3-phase, $38\text{ A FLA} = 13,690\text{ VA}$, largest motor)
• Step 1: General Lighting (Table 220.42(A) — 125% already included) $$\text{Lighting Load} = 4,000\text{ sq ft} \times 1.5\text{ VA/sq ft} = \mathbf{6,000\text{ VA}}$$ Do not multiply this by $1.25$ again. The Note to Table 220.42(A) states that the 210.20(A) continuous-load multiplier is already built into the unit load.
• Step 2: Show Window Lighting (Continuous $125\%$) $$\text{Show Window Load} = 30\text{ ft} \times 200\text{ VA/ft} \times 1.25 = \mathbf{7,500\text{ VA}}$$
• Step 3: General Receptacles (Table 220.47)
  • Gross receptacle load $= 7,200\text{ VA}$ (less than $10,000\text{ VA} \rightarrow 100\%$) $= \mathbf{7,200\text{ VA}}$
• Step 4: Commercial Kitchen Equipment (Table 220.56)
  • 5 units $= 70\%$ demand factor: $42,000\text{ VA} \times 0.70 = 29,400\text{ VA}$
  • Check 2 largest units floor: $12,000\text{ VA} + 10,000\text{ VA} = 22,000\text{ VA}$
  • Since $29,400\text{ VA} > 22,000\text{ VA}$, use $29,400\text{ VA}$.
• Step 5: HVAC & Largest Motor Adder (NEC 220.11(A))
  • HVAC load $= 13,690\text{ VA}$
  • Largest motor adder: $13,690\text{ VA} \times 0.25 = 3,423\text{ VA}$
  • Total HVAC $= 13,690 + 3,423 = \mathbf{17,113\text{ VA}}$
• Step 6: Total 3-Phase Calculated Load & Service Sizing $$\text{Total VA} = 6,000 + 7,500 + 7,200 + 29,400 + 17,113 = \mathbf{67,213\text{ VA}}$$ $$\text{Service Current} = \frac{67,213\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{67,213}{360.25} = \mathbf{186.6\text{ Amperes}}$$
  • Service Selection: Standard $200\text{ A}$, 208Y/120V 3-phase commercial service ($3/0\text{ AWG Cu}$ per Table 310.16 at 75°C).

Example 2: 20-Unit Multifamily Building (NEC 220.84 Optional Method)

• Parameters: 20 identical apartments ($900\text{ sq ft}$ each) supplied by a common $208\text{Y}/120\text{V}$ 3-phase service.
  • Each unit connected load: Lighting ($2,700\text{ VA}$), SABC ($3,000\text{ VA}$), Range ($9,000\text{ VA}$), Water Heater ($4,500\text{ VA}$), Dishwasher/Disposer ($2,000\text{ VA}$), A/C ($3,500\text{ VA}$). Total per unit $= \mathbf{24,700\text{ VA}}$.
  • Common House Loads: Common lighting ($4,000\text{ VA}$ continuous), Common laundry ($8,000\text{ VA}$ non-continuous), Elevator ($15,000\text{ VA}$ motor load).
• Step 1: Calculate Total Dwelling Unit Connected Load $$\text{Total Connected Dwelling Load} = 20\text{ units} \times 24,700\text{ VA} = \mathbf{494,000\text{ VA}}$$
• Step 2: Apply NEC Table 220.84 Demand Factor
  • For 20 dwelling units, Table 220.84 falls in the 18-to-20-unit tier at a $38\%$ demand factor.
  $$\text{Net Dwelling Unit Demand} = 494,000\text{ VA} \times 0.38 = \mathbf{187,720\text{ VA}}$$
• Step 3: Calculate Common House Loads (Part III)
  • Continuous Lighting $= 4,000\text{ VA} \times 1.25 = 5,000\text{ VA}$
  • Common Laundry $= 8,000\text{ VA}$
  • Elevator + 25% motor adder $= 15,000 \times 1.25 = 18,750\text{ VA}$
  • Total House Load: $5,000 + 8,000 + 18,750 = \mathbf{31,750\text{ VA}}$
• Step 4: Total Building Service Sizing $$\text{Total Service Load} = 187,720\text{ VA} + 31,750\text{ VA} = \mathbf{219,470\text{ VA}}$$ $$\text{Service Current} = \frac{219,470\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{219,470}{360.25} = \mathbf{609.2\text{ Amperes}}$$
  • Service Selection: Standard $700\text{ A}$ or $800\text{ A}$ 208Y/120V 3-phase switchboard.

6. Plans Examiner Verification Checklist & Common Traps

• Continuous Lighting Multiplier: Verify the designer did NOT apply 125% on top of a Table 220.42(A) unit load - the multiplier is already in the table. Confirm 125% IS applied to show-window, sign, and actual-connected-load lighting.
• Show Window Linear Footage: Confirm 200 VA/linear ft (and 250 VA/ft continuous) is provided for retail display windows.
• Table 220.47 Receptacle Demand: Verify commercial receptacle demand (1st 10 kVA @ 100%, rem @ 50%) is applied only to general convenience receptacles and not dedicated equipment.
• Commercial Kitchen Two-Unit Floor: Check Table 220.56 calculations to ensure the total is never less than the two largest kitchen appliances.
• Multifamily House Load Isolation: Confirm house loads are never factored through Table 220.84 and are calculated under Part III at 100%/125%.