6.4 Commercial & Industrial Feeder/Service Calculations

6.4 Commercial & Industrial Feeder/Service Calculations

Unlike residential installations, commercial and industrial occupancies feature substantial continuous lighting, specialized display window loads, dedicated exterior signage, high-density receptacle layouts, three-phase commercial kitchen cooking appliances, and heavy motor equipment.

Designing commercial services requires applying the specific rules in NEC Article 220 Parts II and III alongside three-phase power mathematics.

For this section, use the 2023 NEC. Keep the general-lighting table allowance, other continuous loads, demand factors, and motor allowances distinct; applying a percentage twice overstates the load.

1. Commercial Lighting Loads (2023 NEC Table 220.42(A))

Multiply floor area by the applicable minimum unit load. These table values already include the continuous-load allowance.

Do not apply another 125% multiplier to these tabulated minimum lighting loads. This does not remove the applicable continuous-load sizing requirements for separately calculated loads such as signs or show-window lighting.

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2. Show Window & Commercial Sign Loads

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

Show windows used for retail merchandise display must be calculated at not less than 200 VA per linear foot (660 VA/m), measured horizontally along the base of the window.

Connected Show Window VA = Linear Feet x 200 VA/ft Continuous Design VA = Linear Feet x 200 VA/ft x 1.25

Sign and Outline Lighting (NEC 220.14(F) & 600.5)

Under NEC 600.5(A), each commercial building and each commercial occupancy accessible to pedestrians must be provided with at least one outlet for sign or outline lighting on a dedicated 20A branch circuit. The branch circuit load must be calculated at a minimum of 1,200 VA (or actual rating of connected sign equipment). For the continuous sign load assumed here:

Sign Circuit Load = 1,200 VA x 1.25 = 1,500 VA

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3. Commercial Receptacle Loads & Demand Factors (NEC 220.14(I) & Table 220.47)

In non-dwelling occupancies, convenience receptacle outlets are calculated separately from lighting:

• Individual Receptacles: Each single or multi-receptacle device mounted on a single yoke/strap is calculated at not less than 180 VA (NEC 220.14(I)).
• Duplex Receptacle: Mounted on one yoke = 180 VA.
• Triplex Receptacle: Mounted on one yoke = 180 VA.
• Quadruplex (Two Duplexes on 2 yokes): Mounted on two yokes = 2 x 180 = 360 VA.

+-----------------------------------------------------------------------------+
|               NEC TABLE 220.47 RECEPTACLE FEEDER DEMAND FACTORS             |
|                                                                             |
|   TOTAL CONNECTED RECEPTACLE LOAD = Total Yokes x 180 VA                    |
|                                                                             |
|   PORTION OF RECEPTACLE LOAD        | DEMAND FACTOR | CALCULATION METHOD    |
|   ----------------------------------+---------------+-------------------   |
|   First 10 kVA (10,000 VA) or less  |     100%      | First 10,000 VA x 1.00|
|   Remainder over 10 kVA (10,000 VA) |      50%      | (Load - 10,000) x 0.50|
+-----------------------------------------------------------------------------+

Example: An office building contains 150 duplex receptacles on single yokes. Calculate the receptacle-count component first:

• Total Connected Load = 150 x 180 VA = 27,000 VA = 27 kVA.
• First 10,000 VA x 1.00 = 10,000 VA.
• Remaining 17,000 VA x 0.50 = 8,500 VA.
• Net Feeder Receptacle Demand Load = 10,000 + 8,500 = 18,500 VA. For an office, NEC 220.43 also requires comparison with 1 VA/sq ft; use the larger result. Floor area is therefore needed before this component can be accepted as the final office receptacle load.

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4. Commercial Kitchen Cooking Equipment (NEC Table 220.56)

In commercial kitchens (restaurants, cafeterias, hotels, institutional dining), electric cooking equipment—including fryers, griddles, ovens, steamers, microwave ovens, booster heaters, and commercial dishwashers—is subject to the demand factors in NEC Table 220.56.

+-----------------------------------------------------------------------------+
|            NEC TABLE 220.56 COMMERCIAL KITCHEN EQUIPMENT DEMAND             |
|                                                                             |
|   NUMBER OF COMMERCIAL UNITS        | DEMAND FACTOR (PERCENT OF NAMEPLATE)  |
|   ----------------------------------+-------------------------------------  |
|   1 to 2 Units                      | 100%                                  |
|   3 Units                           |  90%                                  |
|   4 Units                           |  80%                                  |
|   5 Units                           |  70%                                  |
|   6 or More Units                   |  65%                                  |
+-----------------------------------------------------------------------------+

5. Balanced Three-Phase Current

For a balanced three-phase apparent load, line current is:

$$I = \frac{S}{\sqrt{3}\,V_{LL}}$$

Use volt-amperes for S and line-to-line volts for V. The divisor is approximately 360.27 at 208 V, or 831.38 at 480 V. Check individual phase loading when single-phase loads are unevenly distributed.

6. Restaurant Example: Known Loads and Missing Motor Data

Given loads:

• Restaurant floor area: 3,600 sq ft.
• Show window: 25 linear feet; one sign circuit; 60 duplex receptacles on individual yokes.
• Six cooking appliances: fryer 12.0 kW, oven 8.5 kW, griddle 6.0 kW, dishwasher/booster 9.5 kW, steam table 4.0 kW, microwave 2.0 kW. Total: 42.0 kW.
• At 208 V three-phase: rooftop AC unit 15.0 kVA, heating blower 3.0 kVA, cooler compressor 4.8 kVA, freezer compressor 3.6 kVA, exhaust/make-up-air fan 2.4 kVA.

Calculation assumptions: Lighting uses the 2023 table minimum; show-window and sign loads are continuous. Cooking nameplate kW equals kVA for this exercise. Cooling and heating do not operate together, the other listed motors can run with cooling, and the listed HVAC/motor loads do not yet include any extra largest-motor allowance. Treat the final service load as balanced when converting VA to current.

Why the Largest Motor Is Not Yet Known

The 15.0 kVA rooftop unit is an equipment total. It may contain a compressor, fans, and other loads. The problem does not give their individual ratings, so it does not establish that the 4.8 kVA cooler compressor is the largest motor. Nor can the whole 15.0 kVA automatically be treated as a single motor.

NEC 220.50 directs motor calculations to Article 430 and hermetic air-conditioning compressor loads to Article 440. Obtain the applicable motor/compressor currents and equipment nameplate data, and check whether a supplied equipment load already includes a required allowance. Do not add the same allowance twice.

For a simplified case where the stated base loads are appropriate and exclude the extra allowance, let M be the VA corresponding to the largest applicable individual motor/compressor load. Then:

$$S_{\text{calculated}} = 76{,}650 + 0.25M$$ $$I = \frac{76{,}650 + 0.25M}{\sqrt{3}\times208}$$

Conditional check: Only if the missing rooftop data confirms that M = 4,800 VA is the governing motor does the result become 77,850 VA and approximately 216.1 A. That condition is not established by the given unit total. Resolve it, phase loading, conductor conditions, and equipment requirements before selecting a final service, conductor, or OCPD rating.