How to Calculate Service Feeder Loads


Performing feeder and service load calculations is an important task performed by professional electricians. Before the installation of cable or conduit, branch circuits and feeders must be planned and calculated. The objective of the National Electrical Code is to safeguard people and property from the dangers of electrical usage. It is simple to perform branch-circuit, feeder and service load calculations; however, the National Electrical Code must be followed.

Things You'll Need

  • Paper
  • Pencil
  • Calculator
  • National Electrical Code
  • Calculate the base general lighting load. Figure the total number of square feet for the home. Multiply total square feet by the 3 volt-amperes per square foot requirement in NEC Table 220.12. The base general lighting load for a 3,000-square-foot home equals 3,000 X 3 volt-amperes = 9,000 VA.

  • Calculate the circuit load requirements for two small appliances. A minimum of one single laundry circuit and two small appliance circuits are required for a single-family dwelling by NEC 210.11. Proceeding to the circuit load requirements for two small appliances in NEC 220.52, note that 1,500 volt-amperes for each two-wire, small-appliance branch circuit are required. So multiply 1,500 volt-amperes X 2 = 3,000 VA. The circuit load requirements for two small appliances equals 3,000 VA.

  • Add the circuit load requirements for one laundry branch circuit to comply with NEC 220.52(B) which requires 1,500 volt-amperes for each laundry branch circuit. If there are two laundry branch circuits, add 3,000 volt-amperes. In this example there is one, so 1,500 volt-amperes will be added to the calculation of base electrical load in Step 4.

  • Calculate the base electrical load for the home. The base electrical load is the amount of electricity the home will use if all lighting, laundry and small appliances are in use at the same time. To calculate the base electrical load, combine all the steps: 9,000 (Step 1) + 3,000 (Step 2) + 1,500 (Step 3) = 13,500 VA. The base electrical load for a 3,000 square foot home equals 13,500 VA.

  • Calculate the demand factor using the base electrical load. Since all lights, laundry and small appliances generally are not in use at the same time, a demand factor is calculated as an adjustment to the base electrical load. Calculate the demand factor by subtracting 3,000 volt-amperes at a 1:1 ratio from the base electrical load and multiply the remainder by 35 percent. The base electrical load (from Step 4) is 13,500 VA. 13,500 - 3,000 = 10,500 VA. Multiplying the remaining 10,500 VA by 35 percent: 10,500 X 0.35 = 3,675 VA. Thus, the demand factor for the home equals 3,675 VA.

  • Calculate the general lighting load for the home. Now that the demand factor is known, the general lighting load for this single-family dwelling can finally be calculated. Use 3,000 square feet (from Step 1) + 3,675 (Step 6) = 6,675 VA. The general lighting load for the home is 6,675 VA.

Tips & Warnings

  • Other sections of the NEC code related to branch circuits:
  • Refrigeration and Air Conditioners 440.6, 440.31 & 440.32
  • Appliances 422.10
  • Space Heaters 424.3
  • Data Processing and Information Technology Equipment 645.5
  • Motors 430.22
  • Signs 600.5

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