This Cable Sizing Calculator can calculate minimum line, neutral, and protective conductor (earth) cable sizes in accordance with the cable-sizing principles of BS 7671:2018 (IET Wiring Regulations, 18th Edition). It covers all cable types, installation methods, and rating factors referenced in the standard.
This cable sizing standard applies to circuits up to 1,000 V AC or 1,500 V DC.
The calculator follows the cable sizing procedure outlined in Appendix 4 (informative) of BS 7671:2018 and the regulations related to cable sizing in Chapter 52 and Chapter 43, which are discussed below in detail.
Assumed conditions:
The tables' current-carrying capacities are based on the stated reference depth. If the actual depth differs, a rating factor for depth of laying (Table 4B4) should be applied.
Installation and grouping: Based on the Reference Methods defined in Table 4A2 of Appendix 4.
The minimum size of a low-voltage power cable is determined for a particular installation through cable sizing, which a cable calculator uses to select the minimum cross-sectional area for an electrical cable based on its ability to satisfy:
The minimum cable size will be the smallest cable that satisfies the three requirements, but the user must first determine the total circuit power or current, with design current being the maximum normal electrical current drawn by the connected load. This helps determine a safe minimum cable size to avoid overheating and excessive voltage drop.
⚠️ There is also a minimum cable conductor size to meet mechanical strength requirements (Table 52.3).
Cable sizing is influenced by several factors, including:
Current-carrying capacity is the maximum amount of electric current a cable can safely conduct without exceeding its temperature rating. This limit is determined by the cable’s conductor material, insulation type, installation method (Reference Method), ambient temperature, and external factors; the maximum allowable temperature rise depends on the insulation type and surrounding conditions, and environmental conditions affect how well the cable dissipates heat and therefore its current rating.
The tabulated current-carrying capacities are based on specific reference methods and installation conditions, so the designer should refer to BS 7671 and the relevant Apprendix 4 tables to check suitability, relevant correction factors, and compliance for the whole range of cases, including electrical installation layouts with copper conductors, insulated cables, flexible cords, twin and earth, clipped direct, conduit, grouping factor adjustments, and other different installation methods as installed. Constant current (100% load factor) and constant ambient conditions are assumed.
Per Regulation 523.1, the current carried by any conductor for sustained periods during normal operation shall not cause the appropriate temperature limit specified in Table 52.1 to be exceeded.
ℹ️ The current-carrying capacity dictates the line cable size for large load currents.
Voltage drop is the reduction in voltage that occurs as current travels along a conductor or wiring system due to the conductor's inherent impedance. This drop is influenced by factors such as whether the circuit is single-phase or three-phase, the route length of the cable, the cross-sectional area, the conductor material, the operating temperature of the conductor, the amount of current flowing, and the power factor of the load it supplies.
Per Regulation 525 and Appendix 4 section 6, voltage drop is expressed in mV/A/m values tabulated for each cable size and installation method.
ℹ️ The voltage drop dictates the line cable size for long route lengths.
Short-circuit rating is the maximum current a cable can safely carry for a very short duration during a fault condition, such as a short circuit, without sustaining damage. The minimum cable size to withstand the short-circuit current is determined based on the actual short-circuit current and the specified period (fault clearing time of the protection, obtained from its time–current curve).
Per Regulation 434 and Chapter 43, conductors shall be protected against fault current before such current could cause danger due to thermal or mechanical effects.
ℹ️ The short-circuit rating (rarely) dictates the line size for short cable runs carrying a small load current where the supply's fault level is high.
Conductors must have sufficient mechanical integrity to withstand handling, installation stresses, and environmental conditions without sustaining damage.
Table 52.3 of BS 7671:2018 provides the minimum cross-sectional areas of conductors:
A protective device's primary function is to protect cables from thermal damage due to overload and/or short circuits.
Per Regulation 433.1.1, the operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:
| Requirements | Notes | |
|---|---|---|
| Condition (i) | The rated current or current setting of the protective device (In) shall not be less than the design current (Ib) of the circuit, and | Ensures the device does not nuisance-trip under normal load. |
| Condition (ii) | In shall not exceed the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit, and | Ensures the cable is not continuously overloaded. |
| Condition (iii) | The current (I₂) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities (Iz) of any conductor. | I₂ ≤ 1.45 Iz for circuit-breakers. |
ℹ️ Note for semi-enclosed (rewirable) fuses to BS 3036: Compliance with condition (iii) is afforded if In does not exceed 0.725 times Iz (Regulation 433.1.202).
ℹ️ Note for buried cables: Where the tabulated current-carrying capacity is based on an ambient temperature of 20 °C, compliance with condition (iii) is afforded if In does not exceed 0.9 times Iz (Regulation 433.1.203).
Current carried by a conductor produces heat. The conductor temperature is limited to avoid overheating the adjacent insulation layers.
Table 52.1 of BS 7671:2018 specifies maximum operating temperatures:
| Type of insulation | Maximum operating temperature |
|---|---|
| Thermoplastic (PVC) | 70 °C at the conductor |
| Thermosetting (XLPE/EPR) | 90 °C at the conductor |
| Mineral insulated (thermoplastic covered or bare, exposed to touch) | 70 °C at the sheath |
| Mineral insulated (bare, not exposed to touch and not in contact with combustible material) | 105 °C at the sheath |
⚠️ Where a conductor operates at a temperature exceeding 70 °C, it shall be confirmed that the equipment connected to the conductor is suitable for the resulting temperature at the connection point.
Appendix 4 of BS 7671:2018 provides rating factors in Tables 4B1 to 4C6, and installation conditions such as ambient temperature and cable grouping affect the cable size required when conditions differ from the reference conditions.
ℹ️ For semi-enclosed (rewirable) fuses to BS 3036, an additional rating factor Cf = 0.725 is applied (see Appendix 4, section 3).
Ambient Temperature Correction Factor (Ca)
Ground Temperature Correction Factor (Ca)
Soil Thermal Resistivity
Depth of Laying Correction Factor (Cd)
Loaded Cores Correction Factor
Grouping Factor (Cg)
Installation and grouping are based on the Reference Methods defined in Table 4A2 of Appendix 4. Each method affects the cable's current-carrying capacity.
Reference Method A
Description and application details for Reference Method A.
Reference Method B
Description and application details for Reference Method B.
Reference Method C
Description and application details for Reference Method C.
Reference Method D
Description and application details for Reference Method D.
Reference Method E
Description and application details for Reference Method E.
Excessive voltage drop can cause equipment malfunction and failures. Regulation 525 and Appendix 4 section 6 of BS 7671:2018 dictate maximum permissible voltage drop limits. Voltage drop often significantly impacts the minimum cable size and must be calculated accurately.
✅ Accurate voltage drop calculations will result in smaller cable sizes.
BS 7671:2018 uses tabulated mV/A/m values from Appendix 4 for calculating voltage drop. The voltage drop is calculated as:
Voltage drop (V) = (mV/A/m value × design current × route length) / 1000
For cables larger than 16 mm², separate resistive (r) and reactive (x) components are tabulated, allowing for a more accurate calculation that accounts for load power factor.
Per Table 4Ab (Appendix 4, section 6.4), the voltage drop between the origin of an installation and any load point should not exceed:
| Installation type | Lighting | Other uses |
|---|---|---|
| (i) LV installation supplied directly from a public LV distribution system | 3% | 5% |
| (ii) LV installation supplied from a private LV supply | 6% | 8% |
⚠️ The voltage drop within each final circuit should not exceed the values given in row (i), even for installations supplied from a private LV supply.
ℹ️ Where wiring systems are longer than 100 m, the voltage drops above may be increased by 0.005% per metre of wiring system beyond 100 m, without this increase exceeding 0.5%.
The following voltage drop limits can be used as a guide to assist with design:
ℹ️ A greater voltage drop than stated may be accepted for a motor circuit during starting and for other equipment with high inrush currents, provided that voltage variations remain within the limits specified in the relevant equipment standard.
Power factor can significantly affect voltage drop and cable sizes, especially for larger cables with higher reactance. The mV/A/m tables in Appendix 4 for cables larger than 16 mm² separate the resistive (mV/A/m)r and reactive (mV/A/m)x components, enabling correction for both operating temperature and load power factor.
Correction for operating temperature (Appendix 4, section 6.1): For cables with conductors larger than 16 mm², the (mV/A/m)r value is corrected using factor Ct based on the actual conductor operating temperature.
Correction for load power factor (Appendix 4, section 6.2): For cables with conductors larger than 16 mm², the voltage drop is determined from: Ct(mV/A/m)r × cos φ + (mV/A/m)x × sin φ
ℹ️ For cables 16 mm² and below, reactance is negligible and the simplified formula applies.
In an unbalanced system, current will flow through the neutral conductor. Per Regulation 523.6, two approaches apply:
| Where: | Voltage drop calculation method | |
|---|---|---|
| 1 | Out-of-balance conditions are intermittent | Assume balanced three-phase load and use the heaviest loaded phase current. |
| 2 | Out-of-balance conditions are consistent | Calculate on a single-phase basis by considering both the phase conductor and the neutral conductor voltage drops. |
Per Regulation 523.7, where two or more live conductors or PEN conductors are connected in parallel, equal load current sharing shall be achieved, or special consideration given.
Equal load current sharing is considered to be achieved if the conductors:
When correctly arranged, parallel conductors can increase the available current-carrying capacity by sharing load current.
Where adequate current sharing is not possible, or where four or more conductors must be connected in parallel, consideration shall be given to the use of busbar trunking systems.
⚠️ There is no specified minimum conductor size for parallel operation stated in Regulation 523.7 itself, but the general minimum conductor sizes in Table 52.3 apply.
Per Regulation 524.2 of BS 7671:2018, the neutral conductor shall have a cross-sectional area not less than that of the line conductor:
| Type of circuit | Neutral conductor size requirement |
|---|---|
| Single-phase two-wire circuits (any CSA) | Neutral ≥ line conductor size |
| Polyphase and single-phase three-wire circuits where line conductors ≤ 16 mm² (copper) or ≤ 25 mm² (aluminium) | Neutral ≥ line conductor size |
| Circuits where third harmonic distortion > 15% of fundamental line current (Reg. 523.6.3) | Neutral ≥ line conductor size (must be treated as a loaded conductor) |
| Polyphase circuits feeding predominantly balanced multiphase loads, where line conductors > 16 mm² (copper) or > 25 mm² (aluminium) | Neutral may be reduced, subject to conditions in Reg. 524.2.3, but not less than 16 mm² copper or 25 mm² aluminium |
| Circuits where total harmonic content due to triplen harmonics > 33% of fundamental line current | Neutral conductor may need to be larger than line conductors — see Regulation 523.6.3 |
In low-voltage (LV) electrical installations, the circuit protective conductor (cpc) plays a critical role in ensuring safety and compliance with BS 7671:2018. Its primary purpose is to provide a low-impedance path to earth, directing fault current safely to facilitate rapid operation of protective devices, thereby reducing the risk of electric shock, fire, and equipment damage.
Per Regulation 543, the cross-sectional area of every protective conductor shall be:
Table 54.7 — Minimum cross-sectional area of protective conductor in relation to the cross-sectional area of associated line conductor:
| Cross-sectional area of line conductor S (mm²) | If the protective conductor is of the same material as the line conductor (mm²) | If the protective conductor is not of the same material as the line conductor (mm²) |
|---|---|---|
| S ≤ 16 | S | (k₁/k₂) × S |
| 16 < S ≤ 35 | 16 | (k₁/k₂) × 16 |
| S > 35 | S/2 | (k₁/k₂) × S/2 |
Where k₁ is the k value for the line conductor (Table 43.1) and k₂ is the k value for the protective conductor (Tables 54.2 to 54.6), both selected according to conductor material and insulation type.
ℹ️ The adiabatic equation (Regulation 543.1.3) provides a more accurate result and may yield a smaller (more economical) conductor size than Table 54.7.
ℹ️ Where Table 54.7 produces a non-standard size, a conductor having the next larger standard cross-sectional area shall be used.