Article 220 — Load Calculations for Small Commercial Occupancies

Article 220 — Load Calculations for Small Commercial Occupancies

Why Load Calculations Matter

Before any service or feeder can be sized, you must calculate the computed load the electrical system must carry. The NEC requires this calculation so that conductors, overcurrent devices, and service equipment are large enough to handle the demand without overheating—yet not so oversized that they waste materials or mask faults. Getting this math right protects workers, building occupants, and property.

Safety note: Undersized services and feeders overheat conductors, damage insulation, and cause fires. Always apply demand factors exactly as the NEC states—never estimate. When working in an energized panel to verify an existing installation, follow your employer's lockout/tagout (LOTO) procedures and wear appropriate PPE: safety glasses and voltage-rated gloves rated for the system voltage present.

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Structure of NEC Article 220

Article 220 is organized into parts:

• Part I — General (scope and rounding rules)
• Part II — Branch-circuit load calculations
• Part III — Feeder and service load calculations
• Part IV — Optional calculation methods (not covered in this lesson)

For small commercial occupancies—a retail store, office suite, or similar space—Part III is the working section. It tells you how to combine individual loads, apply demand factors where permitted, and arrive at the computed load that governs conductor and equipment sizing.

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General Lighting Loads

For commercial occupancies, the NEC provides unit loads expressed in volt-amperes per square foot (VA/ft²). You look up the occupancy type in the applicable table in Article 220 to find the correct unit load value. For example, retail stores and office spaces each have specified values listed in that table.

Step 1 — Calculate gross floor area: Measure outside wall to outside wall. Do not deduct for columns, stairwells, or partitions.

Step 2 — Apply unit load: Multiply gross area (ft²) × unit load (VA/ft²) = general lighting load in VA.

For commercial occupancies, Article 220 generally does not allow a demand factor on the general lighting load the way it does for dwellings, so the full calculated value is typically used.

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Receptacle Loads

For commercial (non-dwelling) occupancies, Article 220 provides rules for how to count receptacle loads. General-purpose receptacle outlets are assigned a load value per outlet as specified in Article 220. After totaling the receptacle load, the article permits a demand factor to be applied once the total exceeds a threshold: the first portion is taken at 100%, and the remainder is taken at a reduced percentage as specified in the relevant Article 220 table. This recognizes that not every receptacle will be fully loaded simultaneously.

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Fixed and Special Loads

After lighting and receptacles, add fixed loads individually:

• HVAC: Use the larger of the heating or cooling load (they are not additive in most cases; consult Article 220 and Article 440 for the specific rule that applies).
• Sign circuits: Article 600 requires at least one 20-ampere branch circuit for a sign outlet at each entrance to a commercial tenant space. Include that load.
• Other fixed equipment: Water heaters, kitchen equipment, compressors—add nameplate VA or wattage at 100% unless a specific NEC section permits a demand factor.

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Sizing the Service or Feeder

Once you have the total computed load in VA:

1. Convert to amperes: I = VA ÷ Voltage. For a 120/240 V single-phase service, divide by 240. For a 208Y/120 V three-phase service, divide by (208 × 1.732).

2. Select the next standard ampere rating at or above the calculated amperes. Standard ratings are listed in Article 240.

3. Verify conductor ampacity from Article 310 tables, applying temperature correction and conduit fill adjustment as needed.

The computed load drives minimum size; you may always install larger equipment.

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Applying Results to a Panel Schedule

A completed panel schedule lists every circuit, its breaker size, and its connected load in amps or VA. To verify adequacy:

• Sum all connected loads.
• Apply any permitted demand factors.
• Confirm the result does not exceed the panel's rated ampacity.

If the computed load exceeds the panel rating, the panel is undersized and must be replaced or supplemented—regardless of whether the breakers themselves fit.

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Safety Reminders for Commercial Electrical Work

• Always de-energize and LOTO a panel before adding or moving conductors.
• Verify absence of voltage with a properly rated meter before touching conductors.
• Commercial services frequently operate at 208Y/120 V three-phase; touching two ungrounded conductors simultaneously exposes you to 208 V line-to-line.
• Wear arc-flash PPE appropriate to the incident energy level when working on or near energized equipment, per NFPA 70E requirements.
• Keep your NEC and any state amendments current; adopted code editions vary by jurisdiction.