7.3 Commercial & Multifamily Load Calculations

7.3 Commercial & Multifamily Load Calculations

Quick Reference:

• Continuous Load Multiplier (NEC 210.20 & 215.2): Commercial lighting operates $\ge 3\text{ hours}$ continuously $\rightarrow$ calculate feeder ampacity at 125% of continuous load.
• Commercial Receptacles (NEC 220.14(I)): Minimum 180 VA per strap (yoke) regardless of whether single, duplex, or triplex.
• Commercial Receptacle Demand Factors (NEC Table 220.44):
  • First 10 kVA (10,000 VA): 100%
  • Remainder over 10 kVA: 50%
• Multioutlet Assemblies (Plugmold, NEC 220.14(H)):
  • Intermittent Use: 180 VA for each 5 ft (or fraction thereof)
  • Heavy / Simultaneous Use: 180 VA for each 1 ft (or fraction thereof)
• Commercial Kitchen Equipment (NEC Table 220.56): 1–2 units = 100%; 3 units = 90%; 4 units = 80%; 5 units = 70%; 6+ units = 65%. Feeder demand cannot be less than sum of top 2 loads!
• Show Window Lighting (NEC 220.14(G) & 220.43(A)): 200 VA per linear foot of show window.
• Electric Signs (NEC 220.14(F) & 600.5): Minimum 1,200 VA branch circuit for commercial occupancies with public pedestrian/vehicle access.
• Multifamily Optional Method (NEC 220.84): Applicable to $\ge 3$ units with electric cooking and electric HVAC; demand factor scales from 45% (3 units) down to 23% (62+ units).

Commercial electrical systems differ profoundly from single-family dwellings in physical scale, operating hours, load diversity, and electrical distribution topology. Commercial occupancies routinely operate on 3-phase alternating current systems (such as 208Y/120V or 480Y/277V) and subject conductors to sustained continuous currents that accelerate thermal heating. For an E-2 Unlimited Journeyperson candidate in Connecticut, Article 220 calculations for commercial buildings and multi-unit apartment complexes demand precise mathematical competence across continuous load derating, non-dwelling receptacle demand schedules, commercial kitchen equipment tables, and multifamily optional methods.

---

1. Commercial General Lighting & The 125% Continuous Rule

Unlike residential general lighting, which uses a uniform 3 VA per square foot unit load, commercial occupancies vary dramatically in visual task difficulty and lighting power density.

NEC Table 220.12 Commercial Unit Loads

NEC Table 220.12 establishes unit loads (volt-amperes per square foot) categorized by occupancy type:

The Continuous Load Factor (NEC Articles 100, 210.20, 215.2 & 230.42)

Article 100 defines a continuous load as a load where the maximum current is expected to continue for 3 hours or more. In retail stores, office buildings, grocery stores, and schools, lighting systems remain illuminated continuously throughout the entire business day. Sustained current produces steady-state thermal rise in conductors and overcurrent device terminals.

To prevent nuisance tripping and premature thermal degradation, NEC 215.2(A)(1) and NEC 210.20(A) enforce the 125% Continuous Load Multiplier:

$\mathbf{\text{Minimum Feeder Ampacity} = \text{Non-Continuous Load} + \left(1.25 \times \text{Continuous Load}\right)}$

$\mathbf{\text{Continuous Lighting Calculation (VA)} = \text{Square Footage} \times \text{Unit Load (VA/sq ft)} \times 1.25}$

Exception: If the overcurrent protective device assembly (switchboard, panelboard, and circuit breaker) is specifically listed for operation at 100 percent of its rating, the 125% multiplier is waived. In standard field work and licensing exams, assume standard equipment requiring the 125% factor.

---

2. Commercial Receptacle Loads & Demand Factors

In non-dwelling occupancies, convenience receptacles are not bundled into the general lighting unit load. They are governed by dedicated rules under NEC 220.14(I) and NEC Table 220.44.

Sizing Receptacle Outlets (NEC 220.14(I))

Except where specific appliance loads are known, receptacle outlets in commercial occupancies must be calculated at not less than 180 volt-amperes for each single or multiple receptacle on one strap (yoke).

                    COMMERCIAL RECEPTACLE STRAP EVALUATION
   +------------------+  +------------------+  +----------------------------------+
   |     [ o o ]      |  |     [ o o ]      |  |     [ o o ]      [ o o ]         |
   |     [ o o ]      |  |                  |  |     [ o o ]      [ o o ]         |
   +------------------+  +------------------+  +----------------------------------+
     DUPLEX RECEPTACLE     SINGLE RECEPTACLE           QUADPLEX RECEPTACLE
       (Single Strap)        (Single Strap)           (TWO Separate Straps)
      Calculated Load:      Calculated Load:             Calculated Load:
     180 VOLT-AMPERES      180 VOLT-AMPERES      2 x 180 VA = 360 VOLT-AMPERES

• Duplex Receptacle: Mounted on one metal yoke $\rightarrow$ 180 VA (a frequent exam trap is multiplying $2 \times 180 = 360\text{ VA}$; this is incorrect).
• Triplex Receptacle: Mounted on one single yoke $\rightarrow$ 180 VA.
• Quadplex Receptacle: Consists of two duplex yokes installed in a 2-gang box $\rightarrow 2 \times 180\text{ VA} = \mathbf{360\text{ VA}}$.

Applying Receptacle Demand Factors (NEC Table 220.44)

In a large office building with hundreds of receptacles, simultaneous use of all outlets is impossible. NEC Table 220.44 permits the following demand factors applied to non-dwelling receptacle loads computed at 180 VA:

$\mathbf{\text{First 10 kVA (10,000 VA) or less} \longrightarrow 100\% \text{ Demand Factor}}$ $\mathbf{\text{Remainder over 10 kVA (10,000 VA)} \longrightarrow 50\% \text{ Demand Factor}}$

Worked Example: Commercial Office Receptacle Demand

Problem: A commercial office suite contains 120 duplex convenience receptacles installed on individual straps. What is the calculated feeder demand load for these receptacles under NEC Table 220.44?

• Step 1: Determine total connected receptacle load:
  $\text{Connected Load} = 120 \times 180\text{ VA} = 21,600\text{ VA (21.6 kVA)}$
• Step 2: Apply Tier 1 (First 10 kVA @ 100%):
  $10,000\text{ VA} \times 1.00 = 10,000\text{ VA}$
• Step 3: Apply Tier 2 (Remainder over 10 kVA @ 50%):
  $\text{Remaining Load} = 21,600\text{ VA} - 10,000\text{ VA} = 11,600\text{ VA}$ $11,600\text{ VA} \times 0.50 = 5,800\text{ VA}$
• Step 4: Sum the demand tiers:
  $\text{Net Receptacle Feeder Demand} = 10,000\text{ VA} + 5,800\text{ VA} = \mathbf{15,800\text{ VA}}$

---

3. Multioutlet Assemblies (Plugmold) — NEC 220.14(H)

In laboratories, school computer labs, commercial test benches, and retail display counters, surface metallic or nonmetallic multioutlet assemblies (commonly called "plugmold") are installed to provide closely spaced receptacle outlets.

Under NEC 220.14(H), multioutlet assemblies are calculated using one of two distinct rules based on expected operational usage:

                       MULTIOUTLET ASSEMBLY LOAD SIZING
   =========================================================================
   CONDITION 1: Normal Intermittent Use (Office credenzas, retail display)
   - Outlets unlikely to be used simultaneously.
   - Calculate: 180 VA for each 5 FEET (or fraction thereof).
   
   CONDITION 2: Heavy / Simultaneous Use (Workbenches, test stations, labs)
   - Multiple equipment pieces plugged in and running simultaneously.
   - Calculate: 180 VA for each 1 FOOT (or fraction thereof).
   =========================================================================

Calculation Comparison: 32-Foot Multioutlet Assembly

• If Intermittent Use:
  $\text{5-ft Segments} = \frac{32\text{ ft}}{5\text{ ft}} = 6.4 \longrightarrow \text{Round up to } 7 \text{ segments}$ $\text{Load} = 7 \times 180\text{ VA} = \mathbf{1,260\text{ VA}}$
• If Heavy Simultaneous Use:
  $\text{1-ft Segments} = 32 \times 180\text{ VA} = \mathbf{5,760\text{ VA}}$

---

4. Commercial Kitchen Equipment Demand Factors (NEC Table 220.56)

Commercial kitchens in restaurants, hotels, hospitals, and institutional cafeterias feature high-density electrical cooking and sanitizing equipment: commercial electric deep fryers, griddles, dishwashers, booster water heaters, steam tables, and conveyor ovens.

NEC Table 220.56 Demand Scale

Under NEC Table 220.56, demand factors are permitted to be applied to all thermostatically controlled or continuously operated commercial electric kitchen equipment:

The Mandatory Statutory Floor Rule

Table 220.56 contains an imperative statutory restriction that candidates must memorize: In no case shall the feeder demand calculated under Table 220.56 be less than the sum of the two largest kitchen equipment loads!

Step-by-Step Calculation: Restaurant Kitchen Feeder Demand

Problem: A restaurant kitchen contains 6 commercial electric appliances:

1. Booster water heater: 12 kW
2. Conveyor pizza oven: 10 kW
3. Commercial dishwasher: 6 kW
4. Electric deep fryer: 5 kW
5. Steam table: 4 kW
6. Commercial soup warmer: 3 kW Calculate the feeder demand load under NEC Table 220.56.

• Step 1: Calculate total connected nameplate:
  $\text{Total Connected} = 12 + 10 + 6 + 5 + 4 + 3 = 40\text{ kW (40,000 VA)}$
• Step 2: Apply Table 220.56 demand factor for 6 units (65%):
  $\text{Calculated Demand} = 40\text{ kW} \times 0.65 = \mathbf{26.0\text{ kW}}$
• Step 3: Check against the statutory two-largest rule:
  • Largest load: Booster heater = 12 kW
  • Second largest load: Conveyor oven = 10 kW
    $\text{Sum of Two Largest} = 12\text{ kW} + 10\text{ kW} = \mathbf{22.0\text{ kW}}$
• Step 4: Verify compliance:
  Because $26.0\text{ kW} \ge 22.0\text{ kW}$, the calculated demand of 26 kW (26,000 VA) is legally valid.

---

5. Specialized Commercial Loads: Show Windows, Signs & Motors

Commercial services must account for specific exterior and mechanical branch loads:

Show Window Lighting (NEC 220.14(G) & 220.43(A))

Show window illumination is calculated at not less than 200 volt-amperes per linear foot (660 VA/m) of show window, measured horizontally along its maximum width. Because show windows operate continuously, this load is subject to the 125% multiplier when sizing branch circuits and feeders.

Commercial Signs and Outline Lighting (NEC 220.14(F) & 600.5)

Every commercial occupancy accessible to pedestrians or vehicles at street level must be provided with at least one 1,200 volt-amperes (minimum) branch circuit for sign or outline lighting. When evaluated in feeder calculations, this continuous load is taken at $1,200 \times 1.25 = \mathbf{1,500\text{ VA}}$.

Commercial Motor Loads in Feeder Calculations (NEC 220.14(C) & 430.24)

When motors (such as air conditioning compressors, rooftop exhaust fans, and chilled water pumps) are fed from a commercial distribution panel, the feeder conductors must be sized to carry 125% of the highest rated motor full-load amperes (FLA) plus the sum of the full-load amperes of all other motors on the feeder:

$\mathbf{\text{Feeder Motor Load} = \left(1.25 \times \text{FLA}_{\text{largest}}\right) + \sum \text{FLA}_{\text{remaining}}}$

6. Multifamily Dwelling Unit Calculations: Standard vs. Optional Method

Multifamily buildings (apartment complexes, condominiums) present unique electrical engineering challenges because load diversity increases exponentially as the number of dwelling units increases.

Standard Method for Multifamily Dwellings

Under Part III, each apartment is calculated using individual unit rules, then aggregated onto the main building service:

1. General Lighting: 3 VA/sq ft across all units, combined with small appliance ($3,000\text{ VA/unit}$) and laundry ($1,500\text{ VA/unit}$), then subjected to Table 220.42 demand tiers.
2. Cooking Equipment: Sized collectively using Table 220.55 Column C (e.g., 20 ranges = 35 kW total demand!).
3. Clothes Dryers: Sized collectively using Table 220.54 demand factors.
4. House Loads: Sized separately at 100% (or 125% for continuous loads like corridor lighting, central elevators, fire pumps, exterior site lighting).

Optional Method for Multifamily Dwellings (NEC 220.84)

To simplify large apartment complexes, NEC 220.84 provides a unified Multifamily Optional Calculation Method.

                      NEC 220.84 MULTIFAMILY ELIGIBILITY
   +-------------------------------------------------------------------------+
   | 1. The building must contain THREE OR MORE dwelling units.             |
   | 2. Each unit must have ELECTRIC COOKING equipment (or optional range). |
   | 3. Each unit must have ELECTRIC SPACE HEATING and/or AIR CONDITIONING.  |
   | 4. No dwelling unit may be supplied by more than ONE feeder.           |
   +-------------------------------------------------------------------------+

Table 220.84 Demand Factors

When the four eligibility criteria are met, the electrician calculates the total connected load of all apartments (3 VA/sq ft lighting + 3,000 VA small appliance/laundry + all appliance nameplates + HVAC) and applies a single unified demand factor from Table 220.84:

• 3 Units: 45%
• 4 to 5 Units: 43%
• 6 to 7 Units: 40%
• 8 to 10 Units: 38%
• 11 to 13 Units: 35%
• 14 to 17 Units: 33%
• 18 to 20 Units: 31%
• 21 to 25 Units: 30%
• 26 to 30 Units: 28%
• 31 to 40 Units: 26%
• 41 to 61 Units: 24%
• 62 Units and over: 23%

(House loads such as common laundry rooms, central boilers, and corridor lighting are computed separately and added at 100% to the feeder demand).

---

7. Comprehensive Worked Commercial Feeder Sizing Example

Let us execute a complete real-world commercial feeder calculation on a 3-phase, 4-wire, 208Y/120V system supplying a commercial retail tenant space.

Specification Data

• Occupancy: Retail Sales Suite (8,000 sq ft)
• Electrical System: 3-Phase, 4-Wire, 208Y/120 Volts
• General Lighting: 8,000 sq ft @ 3.0 VA/sq ft (Table 220.12), operating continuously ($\ge 3\text{ hours}$)
• Show Window: 30 linear feet of show window lighting (continuous)
• General Receptacles: 50 duplex receptacle outlets on individual straps
• Multioutlet Assembly: 20 feet of plugmold along an electronics technician repair bench (used simultaneously)
• Commercial Kitchen / Breakroom Equipment: 4 pieces of commercial kitchen equipment:
  1. Booster water heater: 4,500 VA
  2. Commercial warming oven: 3,200 VA
  3. Commercial soup kettle: 2,500 VA
  4. Commercial dishwasher: 1,800 VA
• Commercial Sign: One 1,200 VA sign branch circuit (continuous)
• HVAC Unit: 3-Phase, 208V rooftop unit with rated full-load current of 28 amperes

---

Step-by-Step Feeder Calculation Execution

Step 1: Commercial Lighting with Continuous Multiplier

• Baseline lighting: $8,000\text{ sq ft} \times 3.0\text{ VA/sq ft} = 24,000\text{ VA}$
• Apply 125% continuous multiplier per NEC 215.2(A)(1): $\text{Lighting Load} = 24,000\text{ VA} \times 1.25 = \mathbf{30,000\text{ VA}}$

Step 2: Show Window Lighting (Continuous)

• Baseline: $30\text{ ft} \times 200\text{ VA/ft} = 6,000\text{ VA}$
• Apply 125% continuous multiplier: $\text{Show Window Load} = 6,000\text{ VA} \times 1.25 = \mathbf{7,500\text{ VA}}$

Step 3: Receptacle Loads & Demand Factors

• Duplex Receptacles: $50 \times 180\text{ VA} = 9,000\text{ VA}$
• Multioutlet Assembly (Simultaneous Use): $20\text{ ft} \times 180\text{ VA/ft} = 3,600\text{ VA}$
• Total Connected Receptacle Load: $9,000 + 3,600 = 12,600\text{ VA}$
• Apply Table 220.44 Demand Factors:
  • First 10,000 VA @ 100% = $10,000\text{ VA}$
  • Remainder ($12,600 - 10,000 = 2,600\text{ VA}$) @ 50% = $1,300\text{ VA}$
  • Net Receptacle Demand: $10,000 + 1,300 = \mathbf{11,300\text{ VA}}$

Step 4: Commercial Kitchen Equipment Demand (Table 220.56)

• Connected equipment: $4,500 + 3,200 + 2,500 + 1,800 = 12,000\text{ VA}$
• Table 220.56 demand factor for 4 units = 80%: $\text{Demand} = 12,000\text{ VA} \times 0.80 = \mathbf{9,600\text{ VA}}$
• Check against statutory two largest loads: $4,500\text{ VA} + 3,200\text{ VA} = 7,700\text{ VA}$.
• Since $9,600\text{ VA} \ge 7,700\text{ VA}$, the 9,600 VA demand governs.

Step 5: Commercial Sign Circuit (Continuous)

• Baseline = 1,200 VA
• Apply 125% continuous multiplier: $\text{Sign Load} = 1,200\text{ VA} \times 1.25 = \mathbf{1,500\text{ VA}}$

Step 6: 3-Phase HVAC Unit Load

• Rooftop unit operates at 208V, 3-phase, 28A FLA: $\text{HVAC Load (VA)} = \sqrt{3} \times V_{\text{line}} \times I = 1.732 \times 208\text{ V} \times 28\text{ A} = \mathbf{10,087\text{ VA}}$

Step 7: Master Feeder Load Synthesis

\text{General Lighting (125 percent)} &= 30,000\text{ VA} \\ \text{Show Window Lighting (125 percent)} &= 7,500\text{ VA} \\ \text{Receptacle Net Demand (Table 220.44)} &= 11,300\text{ VA} \\ \text{Commercial Kitchen Net Demand (Table 220.56)} &= 9,600\text{ VA} \\ \text{Commercial Sign (125 percent)} &= 1,500\text{ VA} \\ \text{3-Phase HVAC Load} &= 10,087\text{ VA} \\ \hline \mathbf{\text{TOTAL CALCULATED FEEDER LOAD}} &= \mathbf{69,987\text{ VA}} \end{aligned}$$ #### Step 8: Sizing 3-Phase Feeder Conductors & Overcurrent Protection Calculate line current for a 3-phase, 208Y/120-volt feeder: $$\mathbf{I_{\text{feeder}} = \frac{\text{Total VA}}{\sqrt{3} \times V_{\text{line}}} = \frac{69,987\text{ VA}}{1.73205 \times 208\text{ V}} = \frac{69,987}{360.27} = 194.26\text{ Amperes}}$$ - **Feeder Overcurrent Protection Selection (NEC 240.6(A)):** With a calculated design current of 194.26 amperes, the minimum standard overcurrent protective device rating is **200 amperes**. - **Conductor Selection (NEC Table 310.16, 75°C Copper THHN):** - 3/0 AWG Copper has an allowable ampacity of **200 amperes** at 75°C, perfectly matching the 200A breaker. - Feeder Makeup: **Three 3/0 AWG Copper THHN phase conductors**, **one 3/0 AWG Copper neutral conductor** (sized to maximum unbalanced load), and **one 6 AWG Copper equipment grounding conductor** per NEC Table 250.122, installed in **2-inch EMT**.