Applying Demand Factors for Schools per NEC 220.86

A proper school load calculation is a fundamental skill for any journeyman electrician or master electrician, ensuring the safety and efficiency of a facility’s electrical infrastructure. According to the nec code book, specifically NEC 220.86, electricians can use an optional method calculation for sizing the feeder and service load for schools. This method simplifies the process by applying specific demand factors to the building’s total connected load, provided the school has electric space heating, air conditioning, or both. The calculation involves determining the load in VA per square foot, applying tiered demand factors from Table 220.86, and then calculating the final service size. Understanding this optional method is crucial for accurate bidding, design, and passing certification exams, and is a core component of advanced electrician training.

What is the Optional Method for School Load Calculation in NEC 220.86?

The National Electrical Code (NEC) provides two primary ways to determine the service size for a commercial building: the standard method calculation (outlined in NEC Article 220, Part III) and various optional methods. NEC 220.86 provides one such optional method specifically for schools. This approach acknowledges the diversity of electrical usage in a school environment, recognizing that it’s highly unlikely all lights, equipment, and HVAC units will be operating at full capacity simultaneously. It allows for a more realistic, and often smaller, service size compared to the standard method, which can lead to significant cost savings on equipment like service conductors and switchgear.

The key condition for using this method is that the school must be equipped with either electric space heating, electric air conditioning, or both. If the school uses other fuel sources for heating and cooling, this optional calculation is not permitted, and the standard method must be used. This is a critical first check for any electrician approaching a school project.

Key Loads Included in the Calculation

Before applying demand factors, you must first determine the total connected load. According to NEC 220.86, this includes a comprehensive list of the building’s electrical systems at 100% of their rating. It’s important to note that when summing these initial loads, you do not apply the 125% multiplier for continuous loads as you would in other calculations. The total connected load includes:

• All interior and exterior lighting
• Receptacle and power loads
• Water heating equipment
• Cooking equipment
• Other miscellaneous loads
• The larger of the electric space heating or air conditioning load (not both). This concept of accounting for non-coincident loads is central to the calculation’s efficiency.

This optional method does not apply to the load of relocatable structures or portable buildings. These structures must have their loads calculated separately, typically using the standard method, as their load characteristics differ from the main building.

Step-by-Step School Load Calculation Using NEC 220.86

Performing a school load calculation using the optional method is a systematic process. For those preparing for an exam or designing a system, following these steps ensures accuracy and compliance. This process is a cornerstone of advanced online electrical courses focused on commercial applications.

1. Calculate the Total Connected Load: Sum all loads as listed in the section above. Remember to only include the larger of the heating or A/C load. This value should be in volt-amperes (VA). For example, let’s assume a 150,000 sq. ft. school has a total connected load of 3,750,000 VA (3,750 kVA).
2. Determine Load Density (VA per Square Foot): Divide the total connected load by the total area of the school in square feet. This gives you the initial load density, which is the key figure for applying demand factors.
   Example: 3,750,000 VA / 150,000 sq. ft. = 25 VA per square foot.
3. Apply Demand Factors from Table 220.86: The NEC applies demand factors in tiers to the VA/sq. ft. value you just calculated.
   • The first 3⅓ VA/sq. ft. are calculated at 100%.
   • The next portion, from over 3⅓ VA/sq. ft. up to 20 VA/sq. ft. (a range of 16⅔), is calculated at 75%.
   • The remainder over 20 VA/sq. ft. is calculated at 25%.

   Example using 25 VA/sq. ft.:
   – First 3⅓ VA/sq. ft: 3.333 x 1.00 = 3.333 VA/sq. ft.
   – Next 16⅔ VA/sq. ft: 16.667 x 0.75 = 12.5 VA/sq. ft.
   – Remainder over 20 VA/sq. ft: (25 – 20) = 5 x 0.25 = 1.25 VA/sq. ft.

4. Calculate the Final Demand Load: Add the results from each tier to get the final calculated demand load in VA per square foot. Then, multiply this new, lower value by the school’s total square footage to find the total calculated feeder and service load. A precise final number is easier to find than using a generic size electrical wire calculator without these inputs.
   Example:
   – Final demand in VA/sq. ft: 3.333 + 12.5 + 1.25 = 17.083 VA/sq. ft.
   – Total Calculated Service Load: 17.083 VA/sq. ft. x 150,000 sq. ft. = 2,562,450 VA or 2,562.5 kVA.

In this example, the optional method reduced the calculated service load from 3,750 kVA to 2,562.5 kVA—a reduction of nearly 32%. Mastering calculations like this is essential for anyone following a master electrician exam prep study plan. For a deeper dive into code specifics, our guide to NEC 2023 load calculations provides additional context.

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Important Considerations and Related NEC Sections

While NEC 220.86 provides the core framework, other code sections are relevant. Understanding these nuances is what separates a proficient electrician from an expert. To become an expert, it is also recommended to study related topics such as how the 2023 NEC changes dwelling unit load calculations.

• Neutral Load Determination: NEC 220.86 explicitly permits the neutral load determination to be made according to NEC 220.61. This section allows for reductions in the neutral conductor size for loads in excess of 200 amps, which can lead to further cost savings.
• Feeders Within the School: The optional method in 220.86 applies to the building’s main service or the main feeder load for the entire school; it is not permissible for calculating individual feeders downstream from that point. Those feeders must be calculated using the standard method from Part III of Article 220, which ensures they are sized appropriately for the specific loads they serve.
• Voltage Drop: Although not part of the load calculation itself, the final circuit length and load will impact voltage drop. Always use a reliable voltage drop calculator to ensure that conductor sizes are sufficient to maintain proper voltage at the furthest point of use.
• Code Reference: For any professional, having a quick way to reference code is essential. A guide to key NEC tables is an invaluable resource on the job.

Primary Sources

• NFPA 70, National Electrical Code (NEC), Article 220, Part IV, Section 220.86

Frequently Asked Questions (FAQ)

1. When can I use the optional school load calculation from NEC 220.86?

You can only use the optional method for a school’s feeder and service load if the facility is equipped with electric space-heating, air-conditioning, or both. If it uses non-electric systems for heating and cooling, you must use the standard method calculation as detailed in NEC Article 220, Part III.

2. How are continuous loads handled in the NEC 220.86 calculation?

When calculating the initial total connected load for the optional method, you do not multiply continuous loads (like lighting) by 125%. You include all loads at 100% of their nameplate rating. The demand factors applied later in the calculation account for the load diversity, replacing the 125% rule used in the standard method.

3. Can I use the NEC 220.86 optional method for portable classroom loads?

No, the loads of relocatable structures or portable buildings are specifically excluded from this optional calculation. These separate structures must have their electrical loads calculated independently, typically using the standard method, which would account for their continuous lighting and HVAC loads at 125%.

4. What’s the difference between the standard method and the optional method school load calculation?

The standard method calculation involves a more granular process of applying different demand factors to various types of loads (e.g., lighting, receptacles, kitchen equipment) individually before summing them. The optional method calculation under NEC 220.86 simplifies this by first summing nearly all loads into a single total connected load, converting it to a VA per square foot figure, and then applying a single, tiered set of demand factors from Table 220.86.