5.3 Commercial Load Calculations & Non-Dwelling Demands

Commercial Load Calculations & Non-Dwelling Demands

Commercial electrical load calculations govern office buildings, retail spaces, restaurants, and schools. Unlike residential settings, commercial systems are characterized by prolonged continuous duty, intensive multioutlet assemblies, heavy kitchen equipment, and three-phase motor distribution. Mastery of NEC Article 220, the 125% continuous load multiplier, and demand factors is essential for the Alabama Journeyman examination.

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1. Commercial Lighting Loads & The 125% Continuous Rule (NEC 220.42)

2023 renumbering alert. In the 2023 NEC the non-dwelling lighting rule is 220.42, and the per-occupancy unit loads live in Table 220.42(A). Table 220.12 is the 2020-edition location. The unit-load values themselves were also revised between recent editions, so read the number out of your own 2023 codebook on exam day rather than trusting a memorized figure.

Under NEC 220.42 and Table 220.42(A), commercial lighting loads are determined from unit volt-amperes per square foot based on building occupancy. The table lists dozens of occupancy types (office, retail, warehouse, school, restaurant, hospital, parking garage, and so on); find the row that matches the occupancy in the question stem, then multiply:

$\text{Lighting VA} = \text{Floor Area (sq ft)} \times \text{Table 220.42(A) Unit Load (VA/sq ft)}$

Exam questions almost always hand you the unit load or expect you to look it up. The scoreable skill is the arithmetic that follows, not the memorized value.

Continuous Load Multiplier (NEC 210.20(A) & 215.2(A)(1))

Under Article 100, a continuous load operates for 3 hours or more. Commercial lighting operates continuously during business hours. Under NEC 210.19(A)(1), 210.20(A), and 215.2(A)(1), conductors and overcurrent devices must be sized for:

$\mathbf{100\% \text{ Non-Continuous}} + \mathbf{125\% \text{ Continuous Loads}}$

Consequently, general lighting volt-amperes must be multiplied by 1.25 for branch-circuit and feeder calculations.

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2. Show Windows, Signs & Multioutlet Assemblies (NEC 220.14)

Specialized commercial loads are governed by NEC 220.14:

• Show Windows (NEC 220.43(A)): Minimum 200 VA per linear foot (660 VA/m) along the base, sized at 125% continuous duty ($200\text{ VA} \times 1.25 = \mathbf{250\text{ VA/ft}}$). In the 2023 NEC, show-window and track lighting for feeder and service calculations sit in 220.43, not 220.14(G).
• Sign Circuits (NEC 220.14(F) & 600.5(A)): Commercial occupancies with pedestrian access require at least one 20A sign circuit, calculated at a minimum of 1,200 VA, treated as continuous ($1,200\text{ VA} \times 1.25 = \mathbf{1,500\text{ VA}}$).
• Multioutlet Assemblies (NEC 220.14(H)):
  • Ordinary Use (220.14(H)(2)): Where simultaneous use is unlikely, each 5 feet (1.5 m) or fraction is calculated at 180 VA.
  • Simultaneous Use (220.14(H)(1)): Where appliances operate concurrently (repair benches, testing labs), each 1 foot (300 mm) or fraction is calculated at 180 VA.

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3. Commercial General Receptacles & Demand Factors (NEC 220.14(I) & 220.44)

Under NEC 220.14(I), convenience receptacle outlets are calculated at 180 VA for each single or multiple receptacle on one yoke or strap:

• Single or duplex receptacle on one strap = 180 VA
• Two duplex receptacles on separate yokes in a 2-gang box = $2 \times 180 = \mathbf{360\text{ VA}}$

Non-Dwelling Receptacle Demand Factors (NEC Table 220.44):

Demand factors may be applied to commercial receptacle loads calculated under 220.14(I):

For example, 100 duplex receptacles ($18,000\text{ VA}$) yield: $10,000\text{ VA} @ 100% + 8,000\text{ VA} @ 50% = \mathbf{14,000\text{ VA}}$.

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

Commercial electric kitchen equipment—such as commercial dishwashers, water booster heaters, fryers, griddles, ovens, and warmers—is calculated under NEC 220.56 and Table 220.56:

The Two-Largest Unit Floor Rule:

NEC 220.56 mandates that the calculated feeder demand shall never be less than the sum of the two largest kitchen equipment loads. If applying the table percentage yields a smaller value, the sum of the two largest units must be used.

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5. Commercial Motor Loads (NEC 220.50 & 430.24)

Feeders supplying multiple electric motors are sized pursuant to NEC 220.50 and NEC 430.24:

$\mathbf{\text{Motor Feeder Load}} = (125\% \times \text{FLC of Largest Motor}) + \sum (100\% \times \text{FLC of Other Motors})$

Full-load current (FLC) ratings must be taken from NEC Table 430.248 (single-phase) or Table 430.250 (three-phase), never from the physical motor nameplate.

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6. Worked Commercial Service Calculation (208Y/120V, 3-Phase)

Calculate the service load and minimum ampacity for a 6,000 sq ft retail facility with a cafe served at 208Y/120V, 3-phase, 4-wire:

• Lighting: 6,000 sq ft retail; assume the Table 220.42(A) unit load for this occupancy is 1.5 VA/sq ft, continuous
• Show Windows: 40 linear feet continuous
• Sign: One 1,200 VA circuit continuous
• Multioutlet Assembly: 30 feet of repair bench (simultaneous use)
• Convenience Receptacles: 80 duplex receptacles on separate yokes
• Cafe Kitchen Equipment: 6 commercial appliances totaling 34,000 VA (largest two: 8,000 VA and 7,000 VA)
• Motors (208V, 3-Phase): One 5-HP air handler (FLC = 16.7A) and one 2-HP exhaust fan (FLC = 7.5A)

Step-by-Step Solution:

1. Continuous Lighting & Display:
   • Lighting: $6,000 \times 1.5\text{ VA} \times 1.25 = \mathbf{11,250\text{ VA}}$
   • Show Windows: $40\text{ ft} \times 200\text{ VA/ft} \times 1.25 = \mathbf{10,000\text{ VA}}$
   • Sign: $1,200\text{ VA} \times 1.25 = \mathbf{1,500\text{ VA}}$
2. Multioutlet & Receptacles:
   • Multioutlet (simultaneous): $30\text{ ft} \times 180\text{ VA/ft} = \mathbf{5,400\text{ VA}}$
   • Receptacles: $80 \times 180 = 14,400\text{ VA}$
     • First 10,000 VA @ 100% = $10,000\text{ VA}$; Remainder $4,400\text{ VA} \times 0.50 = 2,200\text{ VA}$
     • Net Receptacle Demand = $\mathbf{12,200\text{ VA}}$
3. Cafe Kitchen Equipment (Table 220.56):
   • 6 pieces @ 65% = $34,000\text{ VA} \times 0.65 = \mathbf{22,100\text{ VA}}$
   • Check: $8,000 + 7,000 = 15,000\text{ VA} \le 22,100\text{ VA}$ (governs).
4. Three-Phase Motors (208V):
   • Largest (5 HP, 16.7A): $16.7\text{ A} \times 208\text{ V} \times \sqrt{3} \times 1.25 = \mathbf{7,520\text{ VA}}$
   • Second (2 HP, 7.5A): $7.5\text{ A} \times 208\text{ V} \times \sqrt{3} = \mathbf{2,702\text{ VA}}$
   • Total Motor Demand = $7,520 + 2,702 = \mathbf{10,222\text{ VA}}$
5. Total Service Load & Line Current: $\text{Total VA} = 11,250 + 10,000 + 1,500 + 5,400 + 12,200 + 22,100 + 10,222 = \mathbf{72,672\text{ VA}}$ $I_{\text{line}} = \frac{72,672\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{72,672}{360.27} = \mathbf{201.72\text{ Amperes}}$ (Requires a standard commercial 225A, 3-phase, 4-wire service panelboard).