kW to Cable Size Calculator
Determine the correct conductor cross-section size (mm²) based on load in kilowatts (kW), operating voltage, installation conditions, and cable length.
Power Load to Wire Size Finder
How to Use the kW to Cable Size Calculator
Estimating conductor size is key to prevent overheating and fire hazards. Follow these steps to size your cable:
- 1Load Power (kW): Enter the total power consumption of the equipment in kilowatts.
- 2Sizing System Phase: Select "Single Phase" for typical residential applications or "Three Phase" for commercial/industrial layouts.
- 3Configure Parameters: Input the line voltage (V), load power factor (PF), and conductor length in meters.
- 4Select Materials & Methods: Choose Copper or Aluminum conductors, insulation type (XLPE/PVC), and installation configuration.
- 5Click Calculate: The calculator displays recommended cross-sectional area (mm²) along with active and adjusted currents.
How to Calculate Cable Size
Electrical wire sizing is governed by physical conductivity limits and thermal dissipation conditions. In electrical design, sizing a cable requires calculating the full-load current, applying environmental derating multipliers (accounting for insulation materials, conduit grouping, and ambient heat), and confirming that voltage drop over the wire length remains within the statutory 5% limit.
General Cable Sizing Formulas
Real-Life Sizing Scenarios
Scenario 1: Sizing a Single-Phase Shower Heater
A residential instantaneous electric shower heater rated at 9.5 kW runs on a 230V single-phase circuit (PF=1.0). The cable runs through a conduit in a thermal wall (Method A) for a distance of 15 meters at 30°C. Sizing the required copper cable:
- 1. Compute Load Current: Use the single-phase load current formula:
I = (9.5 kW × 1000) ÷ (230V × 1.0 PF) = 41.30 Amps - 2. Conductor Sizing Match: Applying Method A installation limits under IEC 60364-5-52, a 6 mm² copper cable is rated for 34A (which is undersized). Sizing up to the next rating, a 10 mm² copper cable handles up to 46A under Method A conditions. Therefore, 10 mm² copper is selected.
- 3. Check Voltage Drop: Sizing voltage drop using a standard 10 mm² copper factor of 4.4 mV/A/m:
Vdrop = (4.4 mV/A/m × 41.30A × 15m) ÷ 1000 = 2.73V
Vdrop % = (2.73V ÷ 230V) × 100% = 1.18%(Compliant, since 1.18% ≤ 5% limit)
Scenario 2: Sizing a Three-Phase Industrial Compressor
An industrial site installs a 30 kW cooling compressor operating on a 400V three-phase grid (PF=0.85). The cable runs in an open tray (Method C) grouped with two other cables (grouping factor = 0.80) at 40°C ambient temperature (derating factor = 0.87). The run is 60 meters long using copper XLPE cables. Sizing the conductor:
- 1. Compute Load Current: Use the three-phase load current formula:
I = (30 kW × 1000) ÷ (√3 × 400V × 0.85 PF) = 50.94 Amps - 2. Compute Adjusted Current: Sizing with ambient and grouping derating factors:
Iadjusted = 50.94A ÷ (0.80 × 0.87) = 73.19 Amps - 3. Conductor Sizing Match: Under IEC current capacity tables, a 16 mm² copper XLPE conductor handles up to 76A under Method C conditions, which exceeds the adjusted current of 73.19A. Therefore, 16 mm² copper XLPE is selected.
- 4. Check Voltage Drop: Sizing voltage drop using a standard 16 mm² copper factor of 2.8 mV/A/m:
Vdrop = (2.8 mV/A/m × 50.94A × 60m × 0.866) ÷ 1000 = 7.41V
Vdrop % = (7.41V ÷ 400V) × 100% = 1.85%(Compliant, since 1.85% ≤ 5% limit)
Step-by-Step Manual Sizing Guide
- 1Compute Circuit Current: Settle the load current from power rating using standard phase equations.
- 2Compute Adjusted Current: Divide the nominal current by correction multipliers:
I_adjusted = I ÷ (C_temp × C_group × C_install). - 3Match Conductor Cross-Section: Pick a standard cross-section area (mm²) that matches or exceeds the adjusted current.
- 4Check Voltage Drop: Verify the drop remains below 5% using standard millivolt-drop tables.
kW to Cable Size Sizing Chart
The table below outlines copper and aluminum cable recommendations under standard 400V 3-phase conditions at 30°C ambient temperature (PF=0.80) running over a distance of 30 meters:
| Load Power (kW) | Full Load Current (A) | Copper Conductor (XLPE) | Aluminum Conductor (XLPE) | Voltage Drop % (Copper) |
|---|---|---|---|---|
| 5 kW | 9.0 A | 1.5 mm² | 2.5 mm² | 0.34% |
| 10 kW | 18.0 A | 2.5 mm² | 4.0 mm² | 0.70% |
| 15 kW | 27.1 A | 4.0 mm² | 6.0 mm² | 0.65% |
| 20 kW | 36.1 A | 6.0 mm² | 10.0 mm² | 0.58% |
| 30 kW | 54.1 A | 10.0 mm² | 16.0 mm² | 0.45% |
| 45 kW | 81.2 A | 16.0 mm² | 25.0 mm² | 0.31% |
| 75 kW | 135.3 A | 35.0 mm² | 50.0 mm² | 0.27% |
3 Phase Cable Size Calculation
Three-phase load circuits balance power delivery over three distinct phase lines. Sizing three-phase cables requires determining the line current: I = kW × 1000 ÷ (√3 × V × PF). Because motor start-up draws inrush currents, design engineers size conductors with an extra 125% continuous-load safety factor before looking up matching cross-section ratings in standard tables.
kW to Cable Size Chart 3 Phase
A reference table for standard three-phase motor/load ratings at 400V helps electrical technicians speed up layout designs. Assuming standard copper XLPE conductors installed in open trays at 30°C:
• 10 kW load: 2.5 mm² copper cable (handles up to 24A)
• 15 kW load: 4.0 mm² copper cable (handles up to 32A)
• 20 kW load: 6.0 mm² copper cable (handles up to 42A)
• 30 kW load: 10.0 mm² copper cable (handles up to 60A)
1 Phase Cable Size
Single-phase sizing is typical for residential utility layouts. Computing single-phase load current uses the standard equation: I = kW × 1000 ÷ (V × PF). Because residential walls contain thermal insulation, single-phase cables dissipate heat slowly, often requiring larger wire gauges (e.g. 6 mm² or 10 mm² copper) to safely handle high-power kitchen ranges or shower loads without raising thermal fatigue.
How to Calculate Cable Size in Sq Mm Formula
Sizing metric cable cross-sections in square millimeters (mm²) manually requires computing load current and analyzing voltage drop over distance. For long cable runs, conductor area is sized using the voltage drop formula: Area (mm²) = (2 × L × I × ρ) ÷ V_drop, where L is length, I is load current, ρ is resistivity (0.0172 for copper), and V_drop is the maximum allowable voltage drop (5% of source voltage).
Frequently Asked Questions (FAQs)
First, calculate current using: I = (kW × 1000) ÷ (V × PF) (for single-phase) or I = (kW × 1000) ÷ (1.732 × V × PF) (for three-phase). Divide this current by ambient and grouping derating factors, then match the adjusted current with standard ampacity tables and verify that voltage drop is within 5% limits.
At a standard 230V single-phase supply (PF=0.90), a 10 kW load draws 48.3A, requiring a 10 mm² copper cable. On a 400V three-phase grid (PF=0.80), the load draws 18A, which is safely handled by a 2.5 mm² copper cable.
For a 230V single-phase layout, a 15 kW load draws 72.5A, requiring a 16 mm² copper conductor. For a 400V three-phase setup, a 15 kW load draws 27.1A, requiring a 4 mm² copper conductor.
Assuming a 400V three-phase system and a standard 0.80 power factor, a 30 kW load draws 54.1A. Over a typical run length of up to 30 meters, this requires a 10 mm² copper cable.
Determine the maximum electrical load, calculate circuit current, divide by derating factors (temperature and grouping limits), choose a wire cross-section size from engineering standards tables, and verify the voltage drop does not exceed 5% over distance.
For a 400V three-phase layout at 0.80 PF, a 20 kW load draws 36.1A, which requires a 6 mm² copper cable. In a single-phase 230V layout, the load draws 96.6A, requiring a 25 mm² copper cable.
At 230V single-phase (PF=0.90), a 12 kW load draws 58A, requiring a 10 mm² copper cable. At 400V three-phase (PF=0.80), a 12 kW load draws 21.7A, requiring a 4 mm² copper cable.
A 9 kW load at 230V single-phase draws 43.5A. Sizing rules require at least a 10 mm² copper cable to avoid voltage drop issues and allow for thermal margin under continuous operation.
A 6 kW load on 230V single-phase draws 29A, which requires a 4 mm² copper cable. Sizing up to 6 mm² is recommended for long runs to prevent voltage drop.
A 9.5 kW instant shower drawing 41.3A from a 230V single-phase supply must be wired using a 10 mm² copper cable. This is required due to continuous heat build-up under high load and to comply with domestic building wiring codes.
An 8 kW load drawing 34.8A from a 230V single-phase supply requires a 6 mm² copper cable for typical installations. Sizing up to 10 mm² is required if running through thermal wall insulation.