Sizing a 160 m Chiller Feeder Through a 45 °C Boiler Room in a Shared Tray
A 320 A continuous, 480 V, three-phase, three-wire chiller (pf 0.90) is to be fed from an MCC 160 m away on a covered cable tray that already carries two other three-phase circuits, so the chiller's conductors will be part of a six current-carrying-conductor group. The route passes through a boiler room where the ambient is 45 °C. Terminations are rated 75 °C and the feeder breaker is 400 A. Available fault current at the MCC is 35 kA rms symmetrical, and the breaker's published let-through at that current is I²t = 1.4 × 10⁶ A²s with a 0.017 s clearing time. Data you may use: 500 kcmil Cu = 380 A, 350 kcmil Cu = 310 A, 4/0 Cu = 230 A (all at 75 °C before adjustment); ambient correction 0.82 at 45 °C; grouping adjustment 0.80 for six current-carrying conductors; 350 kcmil Cu R = 0.126 Ω/km and X = 0.08 Ω/km per conductor at operating temperature; voltage drop limit 3%.
1. Determine the minimum conductor configuration that satisfies the ampacity requirement, showing the adjustments applied.
Read the worked answer
Required ampacity: the load is continuous, so 320 A × 1.25 = 400 A, matching the 400 A breaker.
Combined adjustment: 0.82 (45 °C) × 0.80 (six current-carrying conductors) = 0.656.
- Single 500 kcmil: 380 × 0.656 = 249 A - fails.
- Two 4/0 per phase: 2 × 230 × 0.656 = 302 A - fails.
- Two 350 kcmil per phase: 2 × 310 × 0.656 = 407 A - ≥ 400 A, acceptable.
Two points worth making in the calculation note. First, the ambient correction alone is not the problem: even at 30 °C the grouping factor would leave 380 × 0.80 = 304 A, so parallel conductors are needed on either count. Second, the neutral is not counted as a current-carrying conductor for a balanced three-wire load, but the grouping factor must be rechecked if a further circuit is added to that tray. The 407 A result has only 7 A of margin - any change to ambient, grouping or conductor count reopens the selection.
2. Verify voltage drop for the selected configuration and state whether an upsize is required.
Read the worked answer
Parallel 350 kcmil halves both impedance terms: R = 0.126/2 = 0.063 Ω/km, X = 0.08/2 = 0.040 Ω/km. Over 0.16 km:
- R = 0.063 × 0.16 = 0.0101 Ω
- X = 0.040 × 0.16 = 0.0064 Ω
- sin φ for pf 0.90 = 0.436
Vd = √3 × I × (R cos φ + X sin φ) = 1.732 × 320 × (0.0101 × 0.90 + 0.0064 × 0.436) = 554 × (0.00909 + 0.00279) = 554 × 0.01188 ≈ 6.6 V
6.6 / 480 = 1.4%, comfortably inside the 3% limit. At the 400 A design basis it would be 8.2 V (1.7%), still acceptable. No upsize for voltage drop is required; the conductor is governed by ampacity after adjustment, not by drop. Note that the source impedance and any upstream feeder drop were not included - if this is the last leg of a long service, the cumulative drop should be checked against the 3% budget allocated to the whole path.
3. Beyond ampacity and voltage drop, identify the checks and compliance items you would close out before releasing this feeder for construction.
Read the worked answer
Terminations and conductor application. With 75 °C terminations, the conductors must be applied at their 75 °C ampacity - do not take credit for the 90 °C column. Confirm lugs are listed for the conductor class and size, and that both parallel conductors land on the same terminal block with equal length and routing so the current splits evenly.
Equipment grounding conductor. Size the EGC for the 400 A overcurrent device and run a full-size EGC in each parallel raceway - the EGC rating is not divided between the two. Bond the tray and any metallic enclosures.
Short-circuit withstand. Compare the breaker's published let-through (1.4 × 10⁶ A²s in 0.017 s) against the conductor damage curve and the tray's bracing; check the physical support of parallel conductors under fault magnetic forces.
Tray fill and grouping. Confirm the six-conductor assumption once the neutral, EGC and any spare circuits are counted, and that tray fill and cover requirements are met.
Ambient basis. Verify 45 °C is a genuine maximum, including radiant heat from boiler surfaces; if it can reach 50 °C the correction drops further and the 407 A selection fails.
Constructability and code. Fire-rated penetration through the boiler-room wall, thermal expansion provisions, working clearances at the MCC and chiller, and confirmation that the chiller terminals accept parallel conductors.