Voltage Drop vs Ampacity on a Long 480 V Pump Feeder
A 480 V, 3-phase feeder supplies a 120 A continuous load at a remote pump station 1,100 ft from the main switchboard. The existing feeder is three 1/0 Cu THWN conductors in 1-1/4 in. PVC buried conduit, with 75°C terminations, protected by a 150 A breaker. The conductor ampacity at 75°C is 150 A, and the calculated load is 120 A. During commissioning, the voltage at the pump station is 451 V with the pump running; the switchboard is 480 V. The operator wants to add a 30 A lighting load and replace the 150 A breaker with a 200 A breaker to get more capacity. A voltage-drop calculation using 1/0 Cu at 0.122 ohm per 1,000 ft gives about 5.8% feeder drop. The existing 1-1/4 in. PVC contains only the three phase conductors, with no neutral.
1. Is the existing feeder compliant for the 120 A continuous load, and is the voltage drop acceptable?
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The feeder is code-minimum acceptable for the 120 A continuous load. 1/0 Cu at 75°C is 150 A, and 125% of 120 A is 150 A, so the ampacity just matches. The 150 A breaker is also appropriate for the calculated load. Voltage drop, however, is poor. Using the given resistance, R = 0.122 ohm per 1,000 ft × 1,100 ft = 0.134 ohm; three-phase drop is √3 × 120 A × 0.134 ohm ≈ 27.9 V, or 5.8% of 480 V. The measured 451 V is a 6.0% drop. The NEC does not generally mandate a 3% feeder limit, but the fine-print notes recommend keeping feeder plus branch-circuit drop within about 5% for reasonable efficiency and equipment operation. At 5.8% on the feeder alone, motor starting, contactor hold-in, and pump controls may be marginal. The feeder is legal but not a good design for this length.
2. Determine a conductor size that limits feeder drop to about 3% and check whether the existing conduit can be reused.
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Target 3% feeder drop: allowable drop = 0.03 × 480 = 14.4 V. Required total resistance = 14.4 / (√3 × 120) = 0.0693 ohm. For 1,100 ft, the maximum conductor resistance is 0.0693 × 1,000 / 1,100 = 0.0630 ohm per 1,000 ft. 3/0 Cu at about 0.079 ohm per 1,000 ft gives 3.8%, too high. 4/0 Cu at about 0.0608 ohm per 1,000 ft gives about 2.9%, so 4/0 is the minimum practical size. The existing 1-1/4 in. PVC is too small: three 4/0 THWN conductors exceed the 40% fill, so the raceway must be increased, typically to 2 in. PVC. Verify lug and breaker terminal sizes; 4/0 is within the 150 A breaker, but the breaker does not need to be increased just because the conductor is larger.
3. What should be done about the proposed 30 A lighting load and the plan to install a 200 A breaker?
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Adding 30 A changes the calculation. If the new total is 150 A continuous, the minimum conductor ampacity is 125% × 150 = 187.5 A, and 1/0 at 150 A is already inadequate; the 150 A breaker is also too small. A 200 A breaker could only be used if the conductor ampacity and load calculation support it. 4/0 at 75°C is about 230 A, so it could carry 150 A continuous, but the voltage drop at 150 A with 4/0 is about 3.6%, still above 3%. A larger conductor or a local transformer may be needed. Never change the breaker to get more capacity without recalculating ampacity, continuous-load sizing, voltage drop, and conduit fill. A 200 A breaker on the existing 1/0 feeder would not be permitted and would not fix the voltage problem.