Electrical Equipment Size Calculation

Electrical
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Thumb rules with Size Calculations for Electrical Equipment

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Power Cable Selection

  1. Cable size (m * m ^ 2) = (Load current x Distance) / (Voltage drop x 1000)

Example: 3-phase, 400V, 100A, 50m cable length Cable size = (100A x 50m) / (3% x 400V x 1000) ≈ 35m * m ^ 2

  1. Cable size (m * m ^ 2) = Load current (A) / 2.5 (for 3-phase) or 1.5 (for 1-phase)

Example: 3-phase, 100A load

Cable size 100A/2.5≈ 35mm²

Earthing Cable Selection

  1. Earthing cable size (m * m ^ 2) = Phase conductor size (m * m ^ 2) * 0.5 to 0.67

Example: Phase conductor size = 35m * m ^ 2

Earthing cable size approx 35m * m ^ 2 * 0.5 = 17.5m * m ^ 2

Motor Selection

  1. Motor HP = (Load kW x 1000) / 0.746 (efficiency)

Example: Load kW = 50kW Motor HP ≈ (50kW x 1000) / 0.746 ≈ 67HP

Transformer Selection

  1. Transformer kVA = Load kW / 0.8 (efficiency)

Example: Load kW = 500kW Transformer kVA ≈ 500kW / 0.8≈625kVA

UPS Selection

  1. UPS KVA = Load kW / 0.8 (efficiency)

Example: Ioad kW = 200kW

UPS KVA ≈ 200kW / 0.8 ≈ 250kVA

Inverter Selection

  1. Inverter kW = Load kW x 1.2 (derating)

Example: Load kW = 100kW Inverter kW ≈ 100kW x 1.2 ≈ 120kW

Circuit Breaker (CB) Selection

  1. CB rating (A) = Load current (A) x 1.2 to 1.5 Example: Load current = 100A CB rating ≈ 100A x 1.2 ≈ 120A

Busbar Selection

  1. Busbar size (mm²) = (Load current x Distance) / (Voltage drop x 1000)

Example: 3-phase, 400V, 1000A, 10m busbar length Busbar size ≈ (1000A x 10m) / (3% x 400V x 1000) ≈ 83mm²

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Note: These calculations serve as guidelines and may vary depending on specific application, industry standards, and local regulations.