UPS Sizing and Cable Derating in a Branch IT Room
An RBI-serviced urban branch is upgrading its IT room. The existing UPS is rated 15 kVA / 12 kW at 0.9 pf, with a 192 V, 40 Ah battery bank (C10 rating) and an inverter efficiency of 90%. The UPS-backed load is: 40 teller thin clients at 120 W each (diversity 0.8), 8 servers at 600 W each, 4 core switches at 150 W each, and one firewall at 100 W. Printers and a 2.5 kW split AC are currently on raw mains, but the branch manager proposes to move the AC to the UPS so that the IT room stays cool during outages. The battery is at end of life, so only 80% of its rated capacity can be used. The UPS output cable is a 4-core 16 mm² copper cable with a free-air rating of 76 A at 30°C; it runs in conduit with three other loaded circuits, and the UPS room ambient is 42°C. Derating factors are 0.87 for 42°C, 0.80 for grouping, and 0.95 for conduit.
1. Compute the UPS-backed load, the available battery energy, and the backup time at end-of-life. Does the existing battery meet the branch's 30-minute backup requirement?
Read the worked answer
Load calculation
- Teller thin clients: 40 × 120 W × 0.8 diversity = 3,840 W
- Servers: 8 × 600 W = 4,800 W
- Core switches: 4 × 150 W = 600 W
- Firewall: 100 W
- Total UPS load = 9,340 W = 9.34 kW
The UPS real-power rating is 12 kW, so the existing load is 9.34/12 = 77.8% of rating - acceptable but with limited spare capacity.
Battery energy Nominal energy = 192 V × 40 Ah = 7,680 Wh. Usable energy at end of life = 7,680 × 0.80 × 0.90 = 5,529.6 Wh ≈ 5.53 kWh.
Backup time 5,529.6 Wh ÷ 9,340 W = 0.592 h ≈ 35.5 minutes.
Conclusion: The battery nominally meets the 30-minute requirement, but only by about 5.5 minutes. Because the calculation already assumes end-of-life capacity, any additional ageing, high temperature, or load growth would erode that margin. The branch should conduct a discharge test and record the actual runtime rather than rely on the nameplate calculation alone.
2. Evaluate the proposal to add the 2.5 kW AC to the UPS. Include UPS capacity, runtime, and cable ampacity. What should the branch do?
Read the worked answer
New UPS load = 9,340 + 2,500 = 11,840 W = 11.84 kW.
UPS capacity: 11.84/12 = 98.7% of the kW rating, leaving only 160 W. This is not acceptable design practice because it leaves no margin for start-up surges, future IT load, or battery recharge. It also keeps the UPS near its limit, which shortens component life.
Runtime: 5,529.6 Wh ÷ 11,840 W = 0.467 h ≈ 28.0 minutes, below the 30-minute requirement.
Cable check: At 230 V single-phase output and load pf 0.9, current = P/(V × pf).
- Existing load: 9,340/(230 × 0.9) = 45.1 A. Derated cable ampacity = 76 × 0.87 × 0.80 × 0.95 = 50.3 A. The existing cable is adequate.
- With the AC: 11,840/(230 × 0.9) = 57.2 A, which exceeds 50.3 A. The cable would be overloaded.
Recommendation: Do not move the AC to the UPS. Keep the AC on raw mains with a dedicated MCB/RCCB and voltage stabiliser, and consider a separate small inverter only if cooling during outages is genuinely essential. If the branch insists on UPS-backed cooling, it must upgrade to a UPS of at least 15 kW, replace the output cable with 25 mm² (or an equivalent verified arrangement), and add battery capacity to restore at least 30 minutes at the new load. Recalculate neutral sizing, earthing, and selectivity after the upgrade.