250 kW Indoor Unit: Conductor Derating and Field Installation Deficiencies
A 250 kW / 500 kWh indoor cabinet is being installed in the mechanical room of a light-industrial building. The inverter output is 480 V three-phase, 250 kW continuous at unity power factor, with terminals listed for 75 °C only. The feeder runs 180 ft in rigid metal conduit sharing a single raceway with three other current-carrying conductors, in a room that reaches 40 °C in summer. The installing electrician proposes one set of 500 kcmil Cu THHN, citing its 90 °C ampacity of 430 A. On the pre-energization walkdown the inspector records: conduit support spans of 12 ft, a feeder bend radius measured at 8× the cable diameter, field-made lugs torqued by feel, a DC section with a 4 in clearance to a stored pallet of cardboard, an AC disconnect with its handle 7 ft 6 in above the floor, and no torque marking on any termination. The listing requires 3 ft of working clearance in front and a lockable disconnect within sight.
1. Size the AC feeder and overcurrent protection. Explain why the proposed 500 kcmil Cu is inadequate and recommend a compliant design.
Read the worked answer
Load current: 250,000 ÷ (480 × √3) = 301 A. Continuous source, so 125% = 376 A minimum conductor ampacity and a 400 A OCPD.
Why the proposal fails. With 75 °C terminations, 110.14(C) makes the 75 °C column govern regardless of the 90 °C insulation. 500 kcmil Cu at 75 °C = 380 A. Then apply the factors the electrician ignored: four current-carrying conductors in one raceway = 80%, and 40 °C ambient on the 75 °C column = 0.88. Adjusted ampacity = 380 × 0.80 × 0.88 = 267 A, far below the 376 A required and below the 301 A load. The conductor would overheat and the OCPD would not be protecting it.
Required 75 °C ampacity before adjustment: 376 ÷ (0.80 × 0.88) = 534 A.
Recommendation: two sets of 350 kcmil Cu (310 A each at 75 °C = 620 A; × 0.704 = 436 A ≥ 376 A) with a 400 A device and a 3 AWG Cu equipment grounding conductor per Table 250.122. A single 1,000 kcmil gives 545 × 0.704 = 384 A and is marginal. Splitting the four conductors into two raceways removes the 80% factor and may allow a smaller feeder - verify fill and derating for each raceway. Also check voltage drop over the 180 ft run; at these sizes it is only a few tenths of a percent, so ampacity governs.
2. Identify every installation deficiency noted in the walkdown, cite the basis for each, and state what must be corrected before energizing.
Read the worked answer
Deficiencies:
- 12 ft conduit support spans. RMC must be supported within 3 ft of terminations and at intervals not exceeding 10 ft (NEC 344.30). Add supports.
- Bend radius of 8× cable diameter. Below the cable's listed minimum and likely below the Chapter 9 Table 2 minimum for the raceway. Re-pull with the correct radius; a tight bend damages insulation and later appears as a DC ground fault or an AC arcing failure.
- Lugs torqued by feel. 110.14(D) requires a calibrated torque tool and the manufacturer's value, with the torque recorded. Loose terminations are a leading cause of ESS AC failures.
- 4 in clearance to combustible storage. Violates the listing and NFPA 855 housekeeping provisions. Relocate the storage and mark the clear zone.
- Disconnect handle at 7 ft 6 in. 404.8(A) limits the operating handle to 6 ft 7 in above the floor. Lower the disconnect.
- Working clearance. The listing requires 3 ft; 110.26 governs the actual depth by condition and may require more.
Hold point. Do not energize until torque records, insulation-resistance and ground-fault test results, and corrected clearances are documented and signed off by the installer and the commissioning agent.