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Cable Sizing

Proper cable sizing is crucial for safety, efficiency, equipment protection, cost savings, and effective fault handling. Undersized cables can lead to overheating, high voltage drop, fire risks, and equipment damage, while oversized cables incur unnecessary costs and space issues. Validation of cable sizing involves checking ampacity ratings, voltage drop limits, short-circuit withstand capabilities, and mechanical suitability.

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Agbojo Abayomi
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5 views2 pages

Cable Sizing

Proper cable sizing is crucial for safety, efficiency, equipment protection, cost savings, and effective fault handling. Undersized cables can lead to overheating, high voltage drop, fire risks, and equipment damage, while oversized cables incur unnecessary costs and space issues. Validation of cable sizing involves checking ampacity ratings, voltage drop limits, short-circuit withstand capabilities, and mechanical suitability.

Uploaded by

Agbojo Abayomi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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CABLE SIZING

1. IMPORTANCE OF PROPER CABLE SIZING


• Safety: Correct cable sizing prevents overheating and reduces the risk of fire.
• Efficiency: Minimizes power loss and keeps voltage within acceptable limits.
• Equipment Protection: Ensures machines receive stable voltage and operate reliably.
• Cost Saving: Avoids overspending or undersizing—optimal cable size for the task.
• Fault Handling: Capable of carrying short-circuit current until protective devices
operate.

2. CONSEQUENCES OF UNDERSIZED CABLES


• Overheating
• High Voltage Drop
• Fire Risk
• Equipment Damage

3. CONSEQUENCES OF OVERSIZED CABLES


• Unnecessarily high cost of cable and installation
• Wasted space
• Larger accessories may be required (e.g., lugs, glands)

4. EQUATIONS USED FOR CABLE SIZING (3-PHASE)


𝐑𝐚𝐭𝐞𝐝 𝐏𝐨𝐰𝐞𝐫 × 𝟏𝟎𝟎𝟎
Full Load Current (IFL) =
√𝟑 × 𝐕𝐨𝐥𝐭𝐚𝐠𝐞 × 𝐄𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲 × 𝐏𝐨𝐰𝐞𝐫 𝐅𝐚𝐜𝐭𝐨𝐫

𝐅𝐮𝐥𝐥 𝐋𝐨𝐚𝐝 𝐂𝐮𝐫𝐫𝐞𝐧𝐭


Number of Runs =
𝐂𝐚𝐛𝐥𝐞 𝐀𝐦𝐩𝐚𝐜𝐢𝐭𝐲 × 𝐃𝐞𝐫𝐚𝐭𝐢𝐧𝐠 𝐅𝐚𝐜𝐭𝐨𝐫

Overall Derating Factor = Gf × Df × Rf × Tf


Where:
• Gf – Grouping factor: Reduces cable capacity when multiple cables are laid together
due to heat buildup.
• Df – Depth of laying factor: Applies to underground cables. Deeper laying reduces heat
dissipation and cable capacity.
• Rf – Soil thermal resistivity: Indicates soil's heat conduction capability. High resistivity =
more derating.
• Tf – Ambient temperature factor: Higher temperature reduces cable capacity.
Corrected Ampacity = Cable Ampacity × Derating Factor
√𝟑 × 𝐈_𝐅𝐋 × (𝐑 𝐜𝐨𝐬𝛗 + 𝐗 𝐬𝐢𝐧𝛗) × 𝐋
Voltage Drop (Vd) =
𝟏𝟎𝟎𝟎 × 𝐍𝐨.𝐨𝐟 𝐑𝐮𝐧𝐬

𝐕𝐝
Voltage Drop (%) = × 100
𝐕_𝐫𝐚𝐭𝐞𝐝
5. HOW TO VALIDATE CABLE SIZING
• Ampacity Rating: Derated capacity should be ≥ load current.
• Voltage Drop: Should remain within permissible limits.
• Short-circuit Withstand: Cable must endure fault current for the required duration.
• Mechanical Suitability: Should be installable Considering bending radius, space, proper
glands, etc.

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