# Cable Selection

**URL:** <https://forumautomation.com/t/cable-selection/14183>\
**Category:** Electrical\
**Tags:** cable-selction\
**Created:** [June 8, 2026, 7:24am UTC](https://forumautomation.com/t/cable-selection/14183 "2026-06-08T07:24:27Z")\
**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:** [June 8, 2026, 7:24am UTC](https://forumautomation.com/t/cable-selection/14183/1 "2026-06-08T07:24:28Z")

</div>

# Cable Selection

Cable Selection includes 8 Step Procedure

Step 1: Load Current

Step 2: Derating Factors

Step 3: Cable Capacity Chart

Step 4: 125% Rule

Step 5: Voltage Drop Check

Step 6: Short Circuit Rating

Step 7: Copper vs Aluminum

Step 8: Standard Cable Sizes (Cu / Al)

## Step 1: Load Current

**[Single Phase:](https://forumelectrical.com/how-to-test-a-single-phase-motor/)** I = P / (V × PF)

**[Three Phase:](https://forumelectrical.com/220v-three-phase-electrical-wiring-purpose-features-and-practical-uses/)** I = P / (√3 × V × PF)

 ![1](https://cdn.forumautomation.com/original/2X/8/8118ac83b421474621e12388e3ac340db7aa1b08.jpeg)

## Step 2: Derating Factors

Ambient temperature, grouping, installation, soil resistivity, copper/aluminum, voltage level

Step 3: Cable Capacity Chart

**[IEC 60364](https://forumelectrical.com/iec-standards-chart/)** , NEC, VDE, BS 7671

Step 4: 125% Rule

Cable current ≥ Load × 1.25

Step 5: Voltage Drop Check

Lighting ≤3%, Power ≤5%

## Step 6: Short Circuit Rating

Isc = k × A × √t

## Step 7: Copper vs Aluminum

- Copper: high conductivity, small size, expensive, heavy
- Aluminum: medium conductivity, bigger, cheaper, light

Step 8: Standard Cable Sizes (Cu / Al)

Refer charts in guide

## Example

40 kW, 400 V, 3Φ, PF 0.85 → Load current = 68 A → 125% margin = 85 A → select 25 mm² Cu