Data Center UPS Feeder Upgrade in Singapore

Scenario

A Tier III data center in Jurong East, Singapore upgraded its 2N UPS output feeder to support new high-density server racks. The existing 35 mm² copper cables were overheating during extended runtime tests. The upgrade required a single-run, 15 m cable from the UPS output busbar to the PDU input, operating continuously at 400 V AC (line-to-line, three-phase assumed balanced). Ambient conditions: air-conditioned but localized hot spots near HVAC ducts (measured up to 42°C). Critical constraint: voltage drop must remain ≤0.5% (≤2.0 V) to prevent PDU input undervoltage alarms.

Given Data

• Voltage: 400 V
• Current: 125 A (per phase, based on 86 kW total 3φ load ÷ √3 ÷ 400 V)
• Length of Cable: 15 m
• Ambient Temperature: 42 °C
• Conductor Temperature Rating: 90 °C (XLPE-insulated Cu — standard for modern data center feeders)

Calculation

Using the Cable Size Calculator:

• Input values: voltage=400, current=125, length=15, ambient_temperature=42, conductor_temperature_rating=90
• The tool applies thermal derating for XLPE at elevated ambient (derating factor ≈ 0.87 at 42°C vs. 30°C reference per IEC 60287 Annex G), then evaluates voltage drop using ρ = 0.0172 Ω·mm²/m for Cu and AC resistance correction.
• 70 mm² Cu: ampacity = 132 A (≥125 A), voltage drop = 1.87 V (≤2.0 V) → passes.
• 50 mm² Cu: ampacity = 108 A (<125 A) → fails thermal check.
• 70 mm² also satisfies short-circuit withstand (validated separately per IEC 60364-5-54).

Result and Decision

The calculator recommended 70 mm² copper cable. The engineering team selected 70 mm² XLPE-armored cable (IEC 60502-2), replacing the undersized 35 mm² feeders. Post-installation thermal imaging confirmed conductor surface temps <65°C under full load, well below the 90°C rating.

Lesson

In high-current, precision-power applications like data centers, ambient temperature derating can reduce effective ampacity by >10%—never assume nameplate ratings apply directly; always recompute capacity for actual site conditions.