Data Center UPS Output Feeder Sizing in Singapore

Case Study 2: Data Center UPS Output Feeder Sizing in Singapore

Scenario

A hyperscale data center in Jurong East, Singapore, deployed a new 1250 kVA double-conversion UPS. Engineers needed to size the downstream 400 V AC output feeders to two adjacent IT distribution boards (IDBs). Critical constraints included tropical ambient temperatures (up to 45°C), dense cable tray stacking (6+ circuits per tray), and strict PUE < 1.3 requirements mandating high power factor correction.

Given Data

• Voltage = 400 V
• Power Factor = 0.92 (achieved via integrated active PFC and harmonic filters)
• Active Power = 1020 kW (design load at 81.6% UPS utilization)
• Apparent Power = 1108.7 kVA (verified during factory acceptance test)

Calculation

Using the Power Conversion Calculator:

1. Current from kW:
   ( I = \frac{1020 \times 1000}{1.732 \times 400 \times 0.92} = \frac{1{,}020{,}000}{637.7} \approx 1599.5,\text{A} )

2. Apparent Power from kW and PF:
   ( S_{\text{kVA}} = \frac{1020}{0.92} \approx 1108.7,\text{kVA} ) (matches FAT report)

3. Current from kVA:
   ( I = \frac{1108.7 \times 1000}{1.732 \times 400} = \frac{1{,}108{,}700}{692.8} \approx 1600.2,\text{A} )

Discrepancy < 0.5 A — well within rounding tolerance.

Result and Decision

The calculated ~1600 A current drove selection of dual parallel runs of 3×300 mm² Cu single-core XLPE cables (each rated 650 A @ 45°C, 3-cable group derating = 0.82 → 533 A per run; two runs = 1066 A). This fell short. Final design used three parallel runs (3 × 533 A = 1599 A), meeting NEC 310.10(H) and Singapore SS 530:2018 requirements for redundancy and thermal margin. Harmonic mitigation ensured PF stability under dynamic IT load — avoiding reactive current penalties.

Lesson

In high-density, thermally challenged environments, derated ampacity — not catalog ratings — governs cable selection. Always apply ambient and grouping derating before comparing to calculated current; skipping this step would have resulted in 20% thermal overload and premature insulation failure.