Voltage Drop and Conductor Sizing for a Long Feeder
A 200A feeder runs 150 meters from a main distribution panel to a remote subpanel in a warehouse. The load is 160A at 600V, three-phase, with a power factor of 0.85. The contractor initially installs 3/0 AWG copper conductors. The voltage drop at the subpanel is measured at 5.5%, which exceeds the recommended maximum of 3% for feeders. The contractor argues that the Canadian Electrical Code does not mandate a specific voltage drop limit, so the installation is compliant. The owner complains that equipment at the subpanel is malfunctioning due to low voltage. The contractor suggests reducing the load by shutting off some equipment, but the owner needs full capacity. The contractor then proposes to increase the conductor size to 250 kcmil. The ambient temperature is 30°C, and the conductors are run in free air.
1. Is the contractor correct that the Canadian Electrical Code does not require a specific voltage drop limit? Discuss the code requirements and the practical implications.
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The contractor is partially correct: the Canadian Electrical Code does not mandate a specific maximum voltage drop percentage for feeders, but it does require that voltage drop be considered to ensure proper operation of equipment. Rule 8-102 states that voltage drop shall not exceed 5% from the service entrance to the load, and 3% for feeders and 2% for branch circuits is a common recommendation. However, the code does not enforce these as absolute limits. Nevertheless, a 5.5% drop exceeds the recommended 3% for feeders and can cause equipment malfunction, as observed. The contractor's argument that compliance alone is sufficient ignores the practical performance requirement. The code requires that the installation be done in a manner that ensures safe and proper operation. Therefore, the contractor should address the voltage drop to meet the owner's needs and ensure equipment operates within its rated voltage tolerance.
2. Calculate the voltage drop for the proposed 250 kcmil copper conductors and determine if it meets the 3% recommendation. Assume the same load and conditions.
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First, determine the impedance of 250 kcmil copper conductors. From Chapter 9, Table 9 of the CEC (assuming similar to NEC), the resistance of 250 kcmil copper at 75°C is 0.068 ohms per 1000 feet, or 0.223 ohms per km. The reactance for non-magnetic conduit is about 0.05 ohms per 1000 feet, or 0.164 ohms per km. For 150 meters (0.15 km), the resistance is 0.223 * 0.15 = 0.03345 ohms, and reactance is 0.164 * 0.15 = 0.0246 ohms. The load current is 160A at power factor 0.85, so sinφ = 0.5268. Voltage drop (line-to-line) = √3 * I * (R cosφ + X sinφ) * L. First, calculate the impedance terms: R cosφ = 0.03345 * 0.85 = 0.02843; X sinφ = 0.0246 * 0.5268 = 0.01296; sum = 0.04139 ohms. Then Vdrop = 1.732 * 160 * 0.04139 = 11.47V. Percentage drop = (11.47 / 600) * 100 = 1.91%. This is below 3%, so it meets the recommendation. However, the contractor should verify the actual resistance and reactance values from the CEC tables and consider the temperature correction. Also, the 250 kcmil conductors must be checked for ampacity: base ampacity at 30°C is 255A, which is greater than 160A, so it is adequate.