# Electrical Equipment Size Calculation

**URL:** <https://forumautomation.com/t/electrical-equipment-size-calculation/12827>\
**Category:** Electrical\
**Tags:** calibration\
**Created:** [November 14, 2024, 1:49pm UTC](https://forumautomation.com/t/electrical-equipment-size-calculation/12827 "2024-11-14T13:49:21Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [November 14, 2024, 1:49pm UTC](https://forumautomation.com/t/electrical-equipment-size-calculation/12827/1 "2024-11-14T13:49:21Z")

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> # Thumb rules with Size Calculations for Electrical Equipment

 ![IMG_20241114_191758](https://cdn.forumautomation.com/original/2X/a/a8137fffc446c19f59f6823d64e6f6bb28bd1d5e.jpeg)

> # 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

> # Click Here for more Calculators

> **[100+ Online Electrical Calculators collection](https://forumelectrical.com/electrical-calculator/)**
>
> Use our extensive collection of over 100+ accurate online electrical calculators to simplify complicated engineering tasks such as voltage, wire gauge, circuit analysis,  & more. Reduce complexity and maximize productivity in...

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