Long 480 V Feeder Voltage Drop Before Adding a Second Compressor
A 480 V, 3-phase feeder supplies a remote compressor building. The feeder is 260 m of three 250 kcmil copper conductors and one 4 AWG copper bonding conductor in PVC conduit. The 200 A breaker is sized to the 250 kcmil ampacity. At no load, the source line-to-line voltage is 492 V. With the 185 A compressor running, the disconnect voltage is 468 V and the current is balanced. The conductor resistance at operating temperature is 0.17 Ω/km per phase. The compressor nameplate allows ±10% voltage and a minimum starting voltage of 414 V. The plant wants to add a second identical 185 A compressor on the same feeder. The calculated voltage drop by the original designer was 14 V, but the measured drop is 24 V.
1. Calculate the expected voltage drop and compare it with the measured value. Is the feeder meeting the common 3% feeder voltage-drop guideline?
Read the worked answer
For a balanced 3-phase feeder, VD = √3 × I × R × L. The per-phase resistance is R = 0.17 Ω/km × 0.26 km = 0.0442 Ω. Then VD = 1.732 × 185 A × 0.0442 Ω = 14.2 V. As a percentage of 480 V, that is 14.2/480 = 2.9%, essentially at the common 3% feeder guideline.
The measured drop is 492 V − 468 V = 24 V, or 5.0% of 480 V. That exceeds the guideline. The calculation and measurement differ by about 10 V, which is too large to dismiss as rounding. Possible causes include loose or damaged terminations, a splice or damaged conductor, the actual conductor temperature being higher than assumed, current or voltage readings taken at different moments, or a meter accuracy issue. The feeder may be functionally operating, but it is not performing as designed. The terminations should be inspected and thermally scanned, and the voltage and current should be re-measured simultaneously before accepting the feeder.
2. Can the second 185 A compressor be added without changing the feeder? Support your answer with calculations and list realistic remediation options.
Read the worked answer
At 370 A total, the calculated drop becomes VD = 1.732 × 370 A × 0.0442 Ω = 28.3 V, or 5.9% of 480 V. With a 492 V source, the disconnect would be about 464 V, which is within the −10% steady-state limit of 432 V, but the starting dip and the measured 24 V discrepancy leave little margin. More importantly, 250 kcmil copper at 75°C is about 255 A, so 370 A exceeds the conductor ampacity. The existing 200 A breaker would also be inadequate. Therefore the second compressor cannot be added on the existing feeder.
Remediation options include installing a new parallel feeder, replacing with larger conductors such as 500 kcmil, providing a separate service or transformer closer to the compressor building, or using reduced-current starting such as a soft starter. The first step should be to correct any termination or conductor defect found during the voltage-drop investigation. Any option must be checked for ampacity, voltage drop under starting, overload and short-circuit protection, and code compliance.