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.
This tool calculates the maximum symmetrical short-circuit current at the output of a transformer substation, based on IEC 60865 and IEEE C37.100 standards. Results are essential for selecting circuit breakers, fuses, busbars, and cables, as well as verifying equipment’s short-circuit withstand capability.
1.Power Net Fault (MVA): Short-circuit power of upstream network, indicating source strength. Higher values lead to higher fault currents.
2.Primary Voltage (kV): Rated voltage on the high-voltage side of the transformer (e.g., 10 kV, 20 kV, 35 kV).
3.Secondary Voltage (V): Rated voltage on the low-voltage side (typically 400 V or 220 V).
4.Transformer Power (kVA): Apparent power rating of the transformer.
5.Voltage Fault (%): Short-circuit impedance percentage (Uk%), provided by manufacturer. Key factor in determining fault current.
6.Joule Effect Losses (%): Load loss as a percentage of rated power (Pc%), used to estimate equivalent resistance.
7.Medium Voltage Line Length: Length of MV feeder from transformer to load (in m, ft, or yd), affecting line impedance.
8.Line Type: Choose conductor configuration:
Overhead line
Unipolar cable
Multipolar cable
9.Medium Voltage Wire Size: Conductor cross-section, selectable in mm² or AWG, with Copper or Aluminum material options.
10.Medium Voltage Conductors in Parallel: Number of identical conductors connected in parallel; reduces total impedance.
11.Conductor Material: Copper or Aluminum, affecting resistivity.
12.Low Voltage Line Length: Length of LV circuit (m/ft/yd), usually short but significant.
13.Low Voltage Wire Size: Cross-sectional area of LV conductor (mm² or AWG).
14.Low Voltage Conductors in Parallel: Number of parallel conductors on the LV side.
1.Three-phase short-circuit current (Isc, kA)
2.Single-phase short-circuit current (Isc1, kA)
3.Peak short-circuit current (Ip, kA)
4.Equivalent impedance (Zeq, Ω)
5.Short-circuit power (Ssc, MVA)
6.Reference Standards: IEC 60865, IEEE C37.100
Designed for electrical engineers, power system designers, and safety assessors conducting short-circuit analysis and equipment selection in low-voltage distribution systems.
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.
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.
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.
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.
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.
Login
