# Calculate the wire size, MCB (Miniature Circuit Breaker) and ELCB (Earth Leakage Circuit Breaker) for a 20kW load

**URL:** <https://forumautomation.com/t/calculate-the-wire-size-mcb-miniature-circuit-breaker-and-elcb-earth-leakage-circuit-breaker-for-a-20kw-load/13863>\
**Category:** Electrical\
**Tags:** switchgear, calculator\
**Created:** [January 30, 2026, 6:50pm UTC](https://forumautomation.com/t/calculate-the-wire-size-mcb-miniature-circuit-breaker-and-elcb-earth-leakage-circuit-breaker-for-a-20kw-load/13863 "2026-01-30T18:50:40Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [January 30, 2026, 6:50pm UTC](https://forumautomation.com/t/calculate-the-wire-size-mcb-miniature-circuit-breaker-and-elcb-earth-leakage-circuit-breaker-for-a-20kw-load/13863/1 "2026-01-30T18:50:40Z")

</div>

# Wire Size Calculation

## Step-1: Calculate Current

**[Single phase:](https://forumelectrical.com/how-to-test-a-single-phase-motor/)** I = P / (V x PF) = 20000 / (230 x 0.8) ≈ 109A

**[Three phase:](https://forumelectrical.com/three-phase-induction-motor-interview-questions/)** I = P / (√3 x V x PF) = 20000 / (1.732 x 400 x 0.8) ≈ 36A

## Step-2: Apply Derating Factors

Ambient temperature correction.

Grouping factor (if multiple cables).

Installation method.

## Step-3: Select Cable

For three phase 36A (more common for 20kW):

After applying derating (assume 0.8 factor): 36/0.8 = 45A required current capacity.

Cable size: 10mm² copper (rated ~57A in typical conditions).

For **[aluminium](https://forumelectrical.com/11-types-of-electrical-conduits-and-their-applications/)**: 16mm²

For single-phase 109A:

Cable size: 35mm² copper or 50mm² aluminum.

# MCB Rating

Select MCB rating ≥ full load current but ≤ cable capacity

Three phase: 40A (or) 50A MCB (Type C / D for motors)

Single phase: 125A MCB

Use Type B for resistive loads, Type C for general use, Type D for motors

 ![image](https://cdn.forumautomation.com/original/2X/7/72b114e67023222b1a5c50c862c443c7e676d270.jpeg)

# ELCB/RCCB Rating

Current rating should be equal to or higher than MCB

Three phase: 40A (or) 63A, 4-pole RCCB

Single phase: 125A, 2-pole RCCB

Sensitivity: 30mA for personnel protection, 100-300mA for equipment protection.

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