Sizing Conductors for a Long Run with Voltage Drop
A 240 V, 50 A single-phase subpanel is to be installed 180 ft from the main panel. The load is a 40 A continuous welder. The available conductors are 6 AWG copper with 75°C insulation. The terminals are rated 75°C. The calculated voltage drop using 6 AWG is 4.8%. The customer wants to use the existing 6 AWG wire to save money. The local inspector has flagged voltage drop as a concern but acknowledges it is not a code requirement in the NEC for this installation. The equipment nameplate specifies a minimum of 90% nominal voltage for proper operation.
1. Is 6 AWG copper acceptable for this installation based on ampacity alone? Show the calculation and state the code basis.
Read the worked answer
Continuous load: 40 A × 1.25 = 50 A. 6 AWG copper at 75°C has an ampacity of 65 A (per NEC Table 310.16). 65 A ≥ 50 A, so ampacity is acceptable. Terminals are rated 75°C, so the 75°C column applies. Therefore, based on ampacity, 6 AWG is acceptable.
2. Given the 4.8% voltage drop and the equipment nameplate requirement, what is the actual voltage at the equipment, and what recommendation should the technician make?
Read the worked answer
Nominal voltage = 240 V. Voltage drop = 4.8% of 240 V = 11.52 V. Voltage at equipment = 240 V − 11.52 V = 228.48 V. Nameplate requires at least 90% of 240 V = 216 V. 228.48 V > 216 V, so the equipment will operate within its nameplate requirement. However, 4.8% is higher than the commonly recommended 3% for branch circuits. The technician should inform the customer that while code-compliant and within nameplate tolerance, the voltage drop may cause inefficiency and heating. If the customer wants optimal performance, upsizing to 4 AWG would reduce voltage drop. The decision should be documented.