Ninth-Floor Chiller Feeder: Ampacity, Voltage Drop and a Re-Bonded Neutral
A twelve-storey office tower is being fitted out. The tenant's chiller plant on the ninth floor has three 350 kW chillers with variable-frequency drives, giving a calculated continuous demand of 420 A per phase at 400/230 V, three-phase four-wire. The feeder route from the basement main distribution board to the ninth-floor panel is 220 m. The contractor installed one 250 mm² copper XLPE (90 °C) run per phase plus neutral in a single conduit, protected by a 500 A molded-case breaker. The Code ampacity table gives 455 A for 250 mm² copper at 90 °C for three current-carrying conductors at 30 °C ambient; the electrical room runs at 40 °C. Neutral is counted as a current-carrying conductor because of the drive harmonics. At commissioning, the neutral bar in the ninth-floor panel was found bonded to the enclosure with a green screw. The tenant reports that the chillers drop out intermittently when the lifts start.
1. Determine whether the installed conductor and breaker arrangement complies, showing the correction factors.
Read the worked answer
Required ampacity. The load is continuous, so the conductor must carry 125 % of it:
1.25 × 420 = 525 A
Derated ampacity of the installed conductor.
- Table ampacity at 30 °C, three current-carrying conductors: 455 A
- Ambient correction for 40 °C: 0.91
- Conduit fill correction for four current-carrying conductors: 0.80
- Derated: 455 × 0.91 × 0.80 = 331 A
Result. 331 A is far below the 420 A load and below the 525 A continuous requirement. The installation does not comply, and the 500 A breaker does not protect the conductor: the conductor is overloaded to roughly 127 % of its derated ampacity, which drives insulation degradation and is a fire risk.
The mistake was sizing from the table value at 30 °C with three conductors, ignoring both the 40 °C room and the harmonic-loaded neutral. A second, subtler error is that the neutral was counted, correctly, but the correction was never applied.
Remedy. Two parallel runs of 185 mm² per phase, each carrying four current-carrying conductors in its own conduit:
370 A each × 2 = 740 A; 740 × 0.91 × 0.80 = 539 A, which exceeds the 525 A requirement.
The 500 A breaker is then permissible, with the conductor ampacity exceeding the continuous requirement.
2. Calculate the voltage drop of the proposed two-run arrangement and check it against a 3 % feeder limit. Use R = 0.10 ohm/km and X = 0.08 ohm/km per run, and a load power factor of 0.90.
Read the worked answer
Parallel impedance. Two identical runs halve the impedance:
R = 0.10 / 2 = 0.05 Ω/km; X = 0.08 / 2 = 0.04 Ω/km
For 220 m: R = 0.011 Ω, X = 0.0088 Ω
Voltage drop (three-phase).
ΔV = √3 × I × L × (R cos φ + X sin φ)
With I = 420 A, L = 0.220 km, cos φ = 0.90, sin φ = 0.436:
ΔV = 1.732 × 420 × 0.220 × (0.05 × 0.90 + 0.04 × 0.436) ΔV = 160.0 × (0.0450 + 0.0174) = 160.0 × 0.0624 = 10.0 V
As a percentage of 400 V: 10.0 / 400 = 2.5 %
This is within the 3 % feeder limit, leaving margin for the branch circuit and the final connection to the chillers. For comparison, the single 250 mm² run (R = 0.09, X = 0.08 Ω/km) gives:
ΔV = 1.732 × 420 × 0.220 × (0.09 × 0.90 + 0.08 × 0.436) = 160.0 × 0.1159 = 18.5 V = 4.6 %,
exceeding the feeder allowance on its own.
The intermittent chiller dropouts when the lifts start are consistent with this: the lift starting current depresses the voltage further at the end of an already excessive feeder, and the drive undervoltage protection trips. Upsizing the feeder addresses both the ampacity and the voltage problem; the breaker cannot be uprated without upsizing the conductor first.
3. Explain the significance of the green bonding screw at the ninth-floor panel and state what must be done.
Read the worked answer
What it does. The bonding screw connects the neutral bar to the panel enclosure. At a panel that is fed from an upstream service or separately derived system, this creates a second neutral-to-ground connection in parallel with the one at the source. Neutral return current, which includes triplen harmonics from the drives, then divides and flows back through the equipment grounding conductor and the conduit rather than entirely through the neutral.
Why it matters.
- The grounding conductor and conduit become live under normal load. Anyone touching the panel enclosure or a bonded metallic part while in contact with ground can receive a shock.
- Circulating current causes electrolytic corrosion at joints and terminations.
- Upstream residual-current devices may nuisance-trip or, worse, may be blinded to a genuine ground fault because part of the fault current returns by an unintended path.
- The panel enclosure and its bonding path are not designed to carry load current.
Code position. The neutral is bonded to the enclosure only at the service equipment or at the source of a separately derived system, and never again downstream. The panel is a load-side panel, so the green screw must not be installed.
Action.
- Remove the bonding screw and separate the neutral and ground bars.
- Verify that the main bonding jumper exists at the correct point upstream, otherwise the system is ungrounded at its source, which is a separate and more serious defect.
- Inspect the neutral and grounding terminations for heat damage and retorque.
- Test the neutral-to-ground isolation downstream with the feeder isolated.
- Record the finding and photograph the corrected arrangement before energisation.