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Derating Factors Deep Dive: Ambient Temperature, Grouping, Burial Depth, and Soil Resistivity

๐Ÿ“˜ Definition

Selection and sizing of conductors based on load, derating factors, voltage drop, and installation environment.

๐ŸŽจ Concept Diagram

Ambient TempGroupingBurial DepthSoil Resistivityโ†’ Derating Multiplierโ†’ Conductor AmpacityFinalAmpacity

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๐Ÿ“– Detailed Explanation

Content for: Derating Factors Deep Dive: Ambient Temperature, Grouping, Burial Depth, and Soil Resistivity. Selection and sizing of conductors based on load, derating factors, voltage drop, and installation environment.

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๐Ÿ“‹ Real Project Case

Industrial Plant Power Design: 250 MW Steel Mill Substation Upgrade

A 250 MW integrated steel mill in Gary, Indiana, required a complete substation upgrade to support new electric arc furnace (EAF) loads and expanded rolling mill operations. The project involved replacing aging 138 kV GIS switchgear and upgrading the 138/13.8 kV main step-down transformer, necessitating full re-engineering of medium-voltage (13.8 kV) feeder cables from the substation to six critical process buildings.

Challenge: Existing 13.8 kV copper cables were undersized and thermally overloaded during peak EAF cycling (dut...
Industrial Plant Power Design: 250 MW Steel Mill Substation Upgrade CHALLENGE โ€ข 13.8 kV Cu cables overloaded โ€ข T > 90ยฐC (IEEE limit) โ€ข Ambient soil: 35ยฐC โ€ข 4 circuits in trench (k=0.15) โ€ข No excavation permitted DESIGN APPROACH โœ“ Soil ฯ = 0.95 Kยทm/W โœ“ SCADA RMS & peak load โœ“ Transient EAF thermal model โœ“ Cable options evaluated โœ“ Harmonic derating (THD=8.2%) RESULT I_adj = 1024 A D_f = 0.87 I_allowed = 978 A โœ“ n=4 circuits โ€ข k=0.15 โ€ข ฯ„=1800 s t_on/t_cycle = 12/20 min โ€ข ฮธ_max=90ยฐC THD=8.2% โ†’ โˆ’0.34% derating
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