Cable & load
Conditions, fault and your cable rating
The derating factor combines ambient temperature, grouping and other factors from your wiring code (1 means none). Enter the rating your own cable table gives for the recommended size to check it against the load.
How it's calculated
Vd = 2 × I × L × (R cos φ + X sin φ)Vd = √3 × I × L × (R cos φ + X sin φ)S = I × √t / kI_required = I / CI_rated × C ≥ IR_t = R_ref × [1 + α (t − t_ref)]The cable has to pass three checks, and the recommended size is the smallest one that passes all of them. The voltage drop check is the same calculation as the voltage drop calculator: resistance at the operating temperature (70 °C for PVC, 90 °C for XLPE or EPR, or 75 °C for NEC PVC types) and reactance, over the one-way length. The short-circuit check uses the adiabatic equation: S is the minimum cross-section in mm², I the fault current in A, t the time the fault lasts in s and k a constant for the conductor and insulation. It is valid for fault times up to 5 s. The current rating check does not look anything up. It divides the load current by your derating factor C, the product of the ambient temperature, grouping and other factors from your wiring code, to give the tabulated rating the cable must have. If you enter the rating your own table gives for the recommended size, it checks that rating, multiplied by C, against the load. These k values are for live conductors, not for protective (earth) conductors, which start from a lower temperature and are not sized here.
| k (A·√s/mm²) | PVC, up to 300 mm² | PVC, over 300 mm² | XLPE / EPR |
|---|---|---|---|
| Copper | 115 | 103 | 143 |
| Aluminium | 76 | 68 | 94 |
The final conductor temperature is 160 °C for PVC (140 °C above 300 mm²) and 250 °C for XLPE and EPR. The values are the commonly published figures, and each can be derived from the heat capacity, resistivity and temperature coefficient of the metal using the adiabatic relation behind IEC 60949.
Worked examples
| Case | Working | Result |
|---|---|---|
| Single-phase 230 V, 20 A, 30 m, copper PVC 70 °C, 3% limit | 4 mm²: 2 × 20 × 0.030 × 4.61 × 1.1965 | 4 mm², 6.62 V (2.88%) |
| Rating it needs | I_required = 20 / 1 | 20 A |
| The same load with a derating factor of 0.8 | I_required = 20 / 0.8 | 25 A, size unchanged |
| Fault 3000 A for 0.1 s, copper PVC | S = 3000 × √0.1 / 115 | 8.25 mm², so 10 mm² |
| Fault 3000 A for 0.1 s, copper XLPE | S = 3000 × √0.1 / 143 | 6.63 mm², so 10 mm² |
| Fault 3000 A for 0.1 s, aluminium XLPE | S = 3000 × √0.1 / 94 | 10.09 mm², so 16 mm² |
| Voltage drop and fault together | larger of 4 mm² and 10 mm² | 10 mm², decided by the fault |
| Your table gives 30 A, derating 0.8, load 20 A | 30 × 0.8 = 24 A, 24 − 20 | covers the load by 4 A (16.7%) |
Assumptions
- There is no table of cable current ratings here. Ratings depend on the cable construction, the installation method and your wiring code, so you take the required rating from this page and check it in your cable maker's data or your code. The page can only check a rating you type in.
- The length is the one-way route length from the supply to the load.
- The conductor is taken at the maximum temperature for the insulation. That is conservative for the voltage drop.
- Conductor resistances are the standard maximum values (IEC 60228 Class 2 stranded, NEC Chapter 9 Table 8). Reactance defaults to 0.08 Ω/km and can be edited.
- The short-circuit check is adiabatic: it ignores heat lost to the insulation during the fault. That is conservative for times between 0.2 s and 5 s, and it is not valid for longer or very short faults. It sizes live conductors only, not earth or protective conductors, and does not check the protective device or the cable's mechanical strength.
- The derating factor is the single number you enter. The page does not give ambient or grouping factors, which depend on the code and the installation.
- Balanced, steady loads. Motor starting current, harmonics, buried cables, soil thermal resistivity and DC solar strings are not covered.
- Metric (IEC mm²) and imperial (NEC AWG and kcmil) sizes are different series.
Frequently asked questions
How do I choose a cable size?
Find the load current, choose the cable's insulation and material, and then meet three limits together: the voltage drop at the full length, the current rating after derating, and the short-circuit withstand if the fault level is known. The largest size demanded by any of them is the one to use. This calculator does the first and last and tells you the second, so you can look it up.
What size cable for 20 A?
It depends on the length, the installation and the code, so there is no single answer. With a 230 V single-phase supply, 30 m of copper PVC cable and a 3% drop limit, the voltage drop alone needs 4 mm², and the cable must also be rated for at least 20 A in your installation (more after derating). Change the length and the answer changes.
What size cable for a 3-phase motor?
Set the system to three-phase, enter the line-to-line voltage, the current and the power factor, and the one-way length. For a motor, use the full-load current from the nameplate or from the power calculator. Starting current is not checked, so a long cable to a motor started direct-on-line needs a separate check.
What is a derating factor?
A multiplier below 1 that lowers a cable's tabulated rating for conditions worse than the table assumes, such as a high ambient temperature or several circuits bundled together. Your wiring code gives the factors. Enter the combined factor, and the calculator shows the rating the cable would need in the table: load current divided by the factor.
Is voltage drop or current rating the deciding factor?
Short cables are usually decided by the current rating, because the drop is small. Long cables are usually decided by the voltage drop, because the cable has to be larger than its rating requires to keep the volts up. Compare the "Voltage drop" size with the rating your table needs, and take the larger.
How do I find a cable's current rating?
From the datasheet for the cable you will buy, or from the current-carrying capacity tables in your wiring code (for example BS 7671 Appendix 4, IEC 60364-5-52 or NEC 310.16). The table is chosen by the cable type, how many loaded conductors it has and how it is installed.
Does it size protective (earth) conductors?
No. The short-circuit constants here are for live conductors, which start at their operating temperature. Protective conductors use different constants and rules.
Standards and references
- IEC 60364-5-52: selection and erection of wiring systems, including current-carrying capacity and voltage drop guidance.
- IEC 60364-4-43 and IEC 60364-5-54: protection against overcurrent, and the short-circuit constant k for conductors.
- BS 7671 (IET Wiring Regulations), Appendix 4 and Chapter 43.
- IEC 60228: conductors of insulated cables, including the maximum resistance of each size.
- IEC 60949: calculation of thermally permissible short-circuit currents.
- NFPA 70 (NEC) Chapter 9 Table 8, and Table 310.16 for ampacity (not included here).
- Free to read: NBS Handbook 100, Copper Wire Tables (US National Bureau of Standards), for the resistivity and temperature coefficient of copper.
This calculator estimates conductor size for voltage drop and short circuit. It does not size cables for current rating, protection or earthing, and it does not check any code requirement. Have the design checked by a qualified engineer.