Bare conductor Current Capacity Calculator
This tool calculates the maximum continuous current-carrying capacity of mineral-insulated bare conductors rated at 750V, based on Tables B.52.6 to B.52.9 of IEC 60364-5-52. It supports copper or aluminum conductors under various installation conditions and environmental corrections.
How Current-Carrying Capacity Is Determined
The calculator implements the following formula derived directly from IEC 60364-5-52:
Imax = Ibase × Ktemp × Kcircuit × Kparallel
- Imax: Maximum continuous current (A)
- Ibase: Base current from IEC 60364-5-52 Tables B.52.6–B.52.9
- Ktemp: Ambient temperature correction factor
- Air temperature: Table B.52.14
- Ground temperature: Table B.52.15
- Soil thermal resistivity: Table B.52.16
- Kcircuit: Reduction factor for multiple circuits in the same conduit (Table B.52.17)
- Kparallel: Parallel conductors factor (identical conductors share load; total current equals sum of individual ratings)
Note: All values are derived directly from IEC 60364-5-52 tables. No empirical assumptions or approximations are used.
Input Parameters
- Method of Installation: As defined in IEC 60364-5-52 Table A.52.3 (e.g., open air, buried, in conduit). Not all methods are recognized in every country’s regulations.
- Conductor Material: Copper (Cu) or Aluminum (Al)
- Type:
- PVC-covered or bare exposed to touch (metallic sheath temperature limit: 70 °C)
- Bare not exposed to touch and not in contact with combustible material (sheath temperature limit: 105 °C)
- Wire Size (mm²): Cross-sectional area of the conductor
- Phase Conductors in Parallel: Identical conductors can be connected in parallel; total permissible current is the sum of individual core ratings
- Ambient Temperature: Temperature of surrounding medium when unloaded
- Circuits in Same Conduit: Number of independent circuits sharing one duct (e.g., two lines for two motors)
Output Results
- Maximum continuous current (A)
- Corrected value for ambient temperature
- Reduction factor for multiple circuits
Industry-Specific Applications
| Field | Use Case | Why It Matters |
|---|---|---|
| Industrial Plants | Motor feeders, high-temperature zones | MI cables withstand fire and mechanical stress; accurate ampacity ensures safety |
| Data Centers | Critical power distribution | Reliability under fault conditions requires precise thermal rating |
| Commercial Buildings | Emergency lighting, fire pumps | Compliance with fire safety codes (e.g., BS 7671, IEC 60364) |
| Transportation | Railway signaling, tunnel power | High reliability in confined, high-risk environments |
Who Should Use This Tool?
- Electrical design engineers selecting MI cable sizes for industrial projects
- Project managers verifying compliance with IEC 60364 for international tenders
- Inspectors and authorities validating installations against code requirements
- Contractors ensuring safe and efficient field installations
- Students and educators studying real-world application of IEC derating principles
Frequently Asked Questions
What is the 125% rule in electrical wiring?
The NEC 125% rule requires that conductors supplying continuous loads (operating more than 3 hours) be rated at least 125% of the load current. For example, a 40 A continuous load requires a conductor rated for at least 50 A. This calculator provides the conductor’s ampacity; circuit protection must still comply with local code rules like this one.
Can a 4 mm² cable carry 40 A?
Under ideal conditions (open air, 30 °C, copper), a 4 mm² bare conductor has a base ampacity of approximately 32–35 A. After derating for temperature or conduit installation, it may drop to 25–28 A. Therefore, 4 mm² is insufficient for a 40 A load. Use 6 mm² or larger.
What is the current carrying capacity of an ACSR conductor?
ACSR (Aluminum Conductor Steel Reinforced) is used in overhead transmission lines, not low-voltage MI systems. Its ampacity ranges from 120 A (#2) to 240 A (2/0) depending on weather and spacing. This calculator does not apply to ACSR conductors.
Is this calculator valid for underground installations?
Yes. Select “buried” or “direct earth” installation method and input soil thermal resistivity. The tool applies correction factors from Tables B.52.15 and B.52.16 to account for reduced heat dissipation.
What’s the difference between “exposed to touch” and “not exposed”?
Exposed to touch: sheath temperature limited to 70 °C for safety.
Not exposed and not in contact with combustible material: sheath can reach 105 °C, allowing higher current. Always verify local regulations before using the higher rating.
Reference Standards
- IEC 60364-5-52: Selection and erection of electrical equipment — Wiring systems
- IEC 60364-5-52 Annex B: Tables B.52.6 to B.52.17
- BS 7671: Requirements for electrical installations (UK)
- NEC Article 310: Conductors for general wiring (US equivalent principles)
