Cable Sizing and Selection Calculator
Configure your system parameters below to calculate the optimal cable size.
Recommended Cable
Safety Verification
Calculation Details
How to Use the Calculator
1. System Parameters
- Enter Load (kW, kVA, or Amps)
- Select Voltage (1-Phase / 3-Phase)
- Define Cable Length & Material
- Input Fault Current (kA) for safety check
2. Derating Factors
- Environment: Ambient Temp, Soil Resistivity
- Installation: Method, Grouping, Insulation coverage
- Load: Harmonics, Future Margin
3. Results & Selection
- Calculates Design Current (Ib) and Future Load
- Applies all 7 Derating Factors for Total Factor (K)
- Selects Cable where Ampacity >= Iz
- Verifies Voltage Drop & Short Circuit compliance
How to Select Cable As Per IEC, IEEE and NEC Standards
Understanding the key factors in proper cable sizing.
1. Calculate Design Current (Ib)
First, determine the full load current of your circuit. This is based on the connected load power (kW) and voltage.
Ib = P / (√3 × V × PF)
For 3-phase systems, use the formula above. The cable must be able to carry this current continuously.
2. Apply Derating Factors
Cables are rarely installed in ideal conditions. You must apply correction factors for temperature, grouping, and method.
Iz = Ib / (K1 × K2 × K3...)
This "Corrected Current" (Iz) dictates the minimum Ampacity the cable must have in standard tables.
3. Voltage Drop & Short Circuit
Safety checks are critical. Ensure the voltage at the load end doesn't drop below limits (usually 3-5%).
VD = (mV/A/m) × Ib × L
Also confirm the cable can withstand the fault current energy (k²S² ≥ I²t) without insulation damage.
Real Life Example: 30kW Motor Calculation
Scenario: Select a cable for a 30 kW, 415V 3-Phase motor (PF 0.85) installed on a cable tray at an ambient temperature of 35°C. The cable is grouped with 2 other circuits (Total 3). Run length is 40 meters.
Step 1: Calculate Design Current (Ib)
Ib = 30000 / (1.732 × 415 × 0.85) = 49.1 A
Using standard power formula: P / (√3 × V × PF)
Step 2: Apply Derating Factors
- • Temperature (35°C Air): 0.96
- • Grouping (3 circuits): 0.70
Total Factor (K) = 0.96 × 0.70 = 0.672
Step 3: Determine Required Current (Iz)
Iz = 49.1 A / 0.672 = 73.1 A
Step 4: Select Cable Size
Looking at the standard table for XLPE Copper Cable (In Air):
- • 6mm² = 58 A (Too small)
- • 10mm² = 80 A (Passes, > 73.1 A)
Selection: 10 mm² Copper XLPE
Step 5: Voltage Drop Check
VD = (4.0 × 49.1 × 40) / 1000 = 7.86 V
Max Allowed (5% of 415V) = 20.75 V.
Since 7.86 V < 20.75 V, the 10 mm² cable is suitable.
Cable Selection Chart
Standard Current Carrying Capacities (Approximate for Cu/XLPE/Air).
Derating Factor Reference Tables
IEC / NEC / IEEE Standard Correction Factors
Formula Reminder:
Iz >= Ib ÷ (Ca × Cg × Ci × Ct × Cs × Ch × Cf)
Final cable size must satisfy current, voltage drop, short-circuit withstand, and protection coordination.
Table 1: Ambient Temperature Correction Factor (Ca)
| Ambient Temp (°C) | Copper XLPE | Aluminum XLPE |
|---|---|---|
| 30°C | 1.00 | 1.00 |
| 35°C | 0.97 | 0.97 |
| 40°C | 0.94 | 0.94 |
| 45°C | 0.87 | 0.87 |
| 50°C | 0.82 | 0.82 |
| 55°C | 0.76 | 0.76 |
Ref: IEC 60364 / NEC 310
Table 2: Cable Grouping Factor (Cg)
| No. of Loaded Circuits | Touching | Spaced |
|---|---|---|
| 1 | 1.00 | 1.00 |
| 2 | 0.80 | 0.85 |
| 3 | 0.70 | 0.75 |
| 4 | 0.65 | 0.70 |
| >= 6 | 0.57 | 0.60 |
Ref: IEC 60364 / NEC 310.15(C)
Table 3: Installation Method Factor (Ci)
| Installation Method | Correction Factor |
|---|---|
| Free air | 1.00 |
| Perforated tray | 0.95 |
| Ladder tray | 1.00 |
| Conduit in air | 0.90 |
| Direct buried | 0.95 |
Ref: IEC 60287
Table 4: Thermal Insulation Factor (Ct)
| Insulation Thickness (mm) | Factor |
|---|---|
| None | 1.00 |
| <= 25 mm | 0.89 |
| 25-50 mm | 0.81 |
| > 50 mm | 0.70 |
Ref: IEC 60364-5-52
Table 5: Soil Thermal Resistivity Factor (Cs)
| Soil Resistivity (K·m/W) | Factor |
|---|---|
| 1.0 | 1.00 |
| 1.5 | 0.95 |
| 2.0 | 0.89 |
| 2.5 | 0.84 |
| 3.0 | 0.80 |
Ref: IEC 60287-2-1
Table 6: Harmonic Current Derating Factor (Ch)
| THD (%) | Factor |
|---|---|
| <= 10% | 1.00 |
| 10-20% | 0.95 |
| 20-30% | 0.90 |
| 30-40% | 0.85 |
| > 40% | 0.80 |
Ref: IEC 61000 / IEEE 519
Table 7: Future Load / Safety Margin Factor (Cf)
| Design Margin | Factor |
|---|---|
| None | 1.00 |
| 10% | 1.10 |
| 15% | 1.15 |
| 20% | 1.20 |
Ref: Engineering best practice (IEC / IEEE)
Cable Selection FAQs
What size cable for 3kW load?
For a 3kW load at 230V, the current is approx 13 Amps. A 1.5 mm² or 2.5 mm² copper cable is typically sufficient, depending on length and installation method.
How much power can a 2.5 mm cable carry?
A 2.5 mm² copper cable can typically carry between 20A to 27A depending on installation. This equates to approximately 4.6 kW to 6.2 kW on a single-phase 230V system.
How to decide what cable size to pick?
Cable size is decided by calculating the Design Current (Ib), applying Derating Factors (temp, grouping) to find Corrected Current (Iz), and checking for Voltage Drop and Short Circuit capacity using a calculator like this one.
Can I use a 2.5mm cable on a 32amp?
No. A 2.5mm² cable is typically rated around 20-27A. Putting it on a 32A breaker is dangerous as the cable could overheat before the breaker trips. You typically need a 4 mm² or 6 mm² cable for 32A.
What size cable for a 7 kW?
For 7 kW (approx 30-32A at 230V), you typically need a 6 mm² cable to be safe, especially if there is any voltage drop considerations. 4 mm² might differ marginally based on installation.
How to calculate cable size?
The basic formula is Ib = P / V. Then select a cable where It > Ib. However, for accuracy, you must use It > Ib / Derating Factors and verify that voltage drop is < 5%.
How do I calculate what size wire I need?
Use the load current (Amps). Match it against standard tables (like IEC 60364-5-52). Ensure the wire's rated capacity is higher than your circuit breaker rating.
Can 6mm cable carry 40A?
Yes, in most installation methods (clipped direct or tray), 6mm² copper cable can handle approx 46A - 53A. However, if buried or grouped, its capacity may drop below 40A.
Can 6mm take 50 amps?
It is borderline. A 6mm² cable is rated ~47A in conduit and ~53A clipped direct. For a continuous 50A load, it is safer practice to upgrade to 10 mm² to avoid overheating.
Can I put a 9.5 kW shower on a 6mm cable?
A 9.5 kW shower draws ~41A. A 6mm² cable (rated ~46-47A enclosed) is usually fine for this, provided the run is not too long or covered in thermal insulation. If in insulation, use 10 mm².
What is the cable size for a 7.5 kW motor?
A 7.5 kW 3-phase motor draws approx 14-15A. A 2.5 mm² or 4 mm² cable is usually sufficient, accounting for starting currents and voltage drop.
What size cable do I need for a 32amp supply?
For a standard 32A ring or radial circuit, 4 mm² or 6 mm² is the standard choice. 2.5mm is too small for a 32A radial.
What size cable for 50 amps?
For 50 Amps, you generally need a 10 mm² cable (rated ~60-70A). 6mm² is often insufficient/borderline.
How many kW can a 6mm cable handle?
A 6mm² cable (approx 47A capacity) can handle around 10.8 kW at 230V single phase.
What size mcb for a 9.5 kW shower?
9.5 kW at 230V is ~41.3A. You should use a 45A or 50A MCB, wired with at least 6mm² (preferably 10mm²) cable.
Can I run a 10.8 kW shower on 6mm cable?
No/Risky. 10.8 kW draws ~47A. This is at the very limit of 6mm² cable capacity. You must use 10 mm² cable for a 10.5kW or 10.8kW shower to ensure safety.
Related Power Calculators
Earthing System Calculator
Design safe and efficient grounding systems using our earthing system calculator for industrial and domestic applications.
Try CalculatorCable Cross Section Calculator
Determine cable cross-sectional area for current capacity.
Try Calculator