Load calculations for heating and air conditioning systems are crucial in electrical design and installation. These calculations ensure that electrical systems can safely and efficiently handle the power demands of HVAC equipment. Understanding how to perform these calculations is essential for passing the NEC electrical exam and for real-world applications in electrical work.

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Example Heating & AC Load Calculation Questions on NEC Electrical Exams

A residence has a 3-ton central air conditioner with a nameplate rating of 3,500 VA, and an electric furnace rated at 15 kW. The air handler for the AC system draws 800 VA. Calculate the total VA that should be included in the service calculations.
Calculate the load for a building with a 5-ton heat pump (nameplate: 6,000 VA) and supplementary strip heating rated at 10 kW. The heat pump has a 1,500 VA air handler. What is the total wattage to be considered for service calculations?
A small office has three 2-ton ductless mini-split systems, each with a nameplate rating of 2,400 VA. There is also a 7.5 kW electric baseboard heating system. What is the total VA to be considered for service calculations?

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How to Identify a Heating & AC Load Calculation Question on NEC Electrical Exams

Key phrases to look out for in Heating & AC load calculation questions:

  • "Non-coincident loads"
  • "Central air conditioner" or "heat pump" ratings
  • "Electric furnace" or "baseboard heating" wattages
  • "Service calculations" or "total load"
  • "Nameplate ratings" of HVAC equipment

When you spot these elements:

  • Confirm it's a heating and AC load calculation question, not a general load calculation
  • Refer to NEC sections 220.60, 220.50, and 220.51

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Load Calculation Articles: NEC 220.60, 220.50, 220.51

To correctly apply NEC Articles 220.60, 220.50, and 220.51, focus on these main elements:

  1. Non-coincident loads (220.60): Recognize that heating and cooling loads are non-coincident (they don't operate simultaneously).
  2. Electric heating (220.51): Calculate the load for electric space heating equipment.
  3. HVAC equipment (220.50): Determine the load for motor-operated HVAC equipment.

Summary of how to use the codebook:

  • Use 220.60 to understand that only the larger of heating or cooling loads should be included in the final calculation.
  • Apply 220.51 for electric heating loads, using the nameplate ratings.
  • Refer to 220.50 for air conditioning and heat pump loads, considering both compressor and air handler ratings.

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Walkthrough for a NEC Electrical Exam Heating & AC Load Calculation Question

Question: A residence has a 3-ton central air conditioner with a nameplate rating of 3,500 VA, and an electric furnace rated at 3.6 kW. The air handler for the AC system draws 800 VA. Calculate the total VA that should be included in the service calculations.

Step 1: Identify and convert all loads to VA

  • Air conditioner compressor: 3,500 VA
  • Air handler: 800 VA
  • Electric furnace: 3.6 kW = 3,600 VA (1 kW = 1000 VA)

Step 2: Calculate total cooling load

Total cooling load = Compressor VA + Air handler VA
Total cooling load = 3,500 VA + 800 VA = 4,300 VA

Step 3: Compare the loads

We should use the larger of the two loads since heating and cooling won't operate simultaneously.

  • Cooling load: 4,300 VA
  • Heating load: 3,600 VA

Step 4: Determine the final load

The electric furnace load (4,300 VA) is larger, so this is the value that should be included in the service calculations.

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