Cable Size Calculator BS 7671 (IET Wiring Regulations)
This Cable Sizing Calculator calculates minimum active (line conductor), neutral, and earth (protective earth) cable sizes in compliance with BS 7671 Requirements for Electrical Installations (IET Wiring Regulations). It covers all cable types, installation methods, and correction 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 BS 7671 and the rules related to cable sizing, which are discussed below in detail.
Assumed conditions:
- Ambient air temperature: 30°C
- Ambient ground temperature: 20°C
- Soil thermal resistivity: 2.5 K·m/W
- Reference depth of laying: 0.7 metre (700 mm)
The tables' current-carrying capacities are based on this standard burial depth. If the actual depth differs, a correction factor for depth of laying must be applied. - Installation and grouping: Based on typical UK installation practices and conditions
Cable Sizing Overview
The minimum size of a low-voltage power cable is determined for a particular installation based on its ability to satisfy current-carrying capacity, voltage drop, and short-circuit performance/rating.
The minimum cable size will be the smallest cable that satisfies the three requirements.
⚠️ There is also a minimum cable conductor size to meet mechanical strength requirements.
1. Current-carrying Capacity
Current-carrying capacity, often called ampacity, 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, ambient temperature, and external factors. Constant current (100% load factor) and constant ambient conditions are assumed.
BS 7671 Appendix 4 provides current-carrying capacity tables (Tables 4D1A through 4J4A) for cables installed in various methods under the assumed standard conditions.
ℹ️ The current-carrying capacity dictates the active cable size for large load currents.
2. Voltage Drop
Voltage drop is the reduction in voltage that occurs as current travels through a cable due to the conductor's inherent impedance. This drop is influenced by factors such as the phases of the supply, the length of the cable, the cross-sectional area, the material, the operating temperature of the conductor, the amount of current flowing, and the power factor of the load it supplies.
ℹ️ The voltage drop dictates the active cable size for long route lengths.
3. Short-circuit Rating
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 — the fault clearing time of the protection, obtained from the time-current curve.
ℹ️ The short-circuit rating (rarely) dictates the active size for short cable runs carrying a small load current where the supply's fault level is high.
Minimum Conductor Size for Mechanical Strength
Conductors must have sufficient mechanical integrity to withstand handling, installation stresses, and environmental conditions without sustaining damage.
BS 7671 Table 52.3 provides guidelines for the nominal minimum cross-sectional area of conductors based on their intended application, which is outlined below.
Cable Sizing Coordination with Protective Device
A protective device's primary function is to protect cables from thermal damage due to overload and/or short circuits. Below is a summary of the required coordination between cables and circuit protection as defined in BS 7671 Regulation 433.
The following parameters are defined:
| Requirements | Notes | |
|---|---|---|
| ZONE 'A' | The protective device (PD) rating (IN) must be ≥ design current (IB) and ≤ current-carrying capacity (IZ) of the cable. |
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| ZONE 'B' | The protective device's conventional tripping current (I2) must be ≤ 1.45IZ for circuit breakers and 0.725IZ for fuses. |
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| ZONE 'C' | The protective device fault breaking capacity (ISCB) must be ≥ ISC. |
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Cable Temperature Limits by Insulation Type
The current carried by a power cable heats the conductor. The conductor temperature is limited to avoid overheating the adjacent insulation layers.
BS 7671 Appendix 4 specifies the maximum conductor operating temperatures for common insulation types used in UK installations:
- PVC (thermoplastic, 70°C): up to 70°C continuous, 160°C short circuit
- XLPE and EPR (thermosetting, 90°C): up to 90°C continuous, 250°C short circuit
- Thermosetting (60°C): up to 60°C continuous, 200°C short circuit
- Mineral insulated (bare, exposed to touch, 70°C sheath): up to 70°C continuous, 160°C short circuit
The continuous temperature limit of cable insulation determines a cable's current-carrying capacity by setting the maximum allowable temperature rise during operation. Higher temperature-rated insulation generally permits greater current-carrying capacity for the same conductor cross-section.
⚠️ The current-carrying capacity tables in BS 7671 Appendix 4 are based on the maximum continuous operating temperatures for each insulation type under the standard assumed conditions.
Correction Factors
BS 7671 Appendix 4 provides correction factors that adjust the tabulated current-carrying capacity for installation conditions that differ from the assumed standard conditions.
It is straightforward to apply correction factors:
- Obtain the tabulated current rating based on the standard conditions from the Appendix 4 tables.
- Determine the applicable correction factors from the Appendix 4 tables for your specific installation conditions.
- Multiply all correction factors together, then multiply this combined factor by the cable's tabulated current rating to obtain the corrected cable current rating.
The Correction Wizard of this calculator includes all correction factors from BS 7671 Appendix 4, including:
- Table 4B1: Ambient temperature correction for cables installed in air
- Table 4B2: Ambient temperature correction for cables buried in the ground
- Table 4B3: Correction factor for soil thermal resistivity
- Table 4B2: Correction factor for depth of laying
- Tables 4C1–4C5: Grouping correction factors for cables installed in various arrangements (bunched, on trays, in conduit)
Voltage Drop Calculations
Excessive voltage drop can cause equipment malfunction and failures. Therefore, the standards 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.
Accurate Voltage Drop Equation
Accurately calculating voltage drops results in lower voltage drops, which leads to smaller cable sizes and saves money. The accurate voltage drop equation for three-phase systems is shown below.
ℹ️ For DC voltage drop calculations, the value for reactance X is zero.
Voltage Drop Limits
The voltage drop between the origin of a low-voltage electrical installation and any load point in that installation should not exceed the applicable percentage of the nominal voltage of the installation.
For low-voltage installations supplied directly from a public low-voltage distribution system, the permissible voltage drop is 3% for lighting and 5% for other uses.
Where the low-voltage installation is supplied from a private LV supply, the permissible voltage drop may be increased to 6% for lighting and 8% for other uses.
The calculated voltage drop should include any effects due to harmonic currents. Refer to Table 4Ab for the voltage drop limits.
The following voltage drop limits can be used as a guide to assist with design.
| Consumers mains | allow 0.5% voltage drop |
|---|---|
| Sub-mains | allow 1.5-2% voltage drop |
| Final subcircuits | allow 2.5% or up to the prescribed limit, considering the upstream circuits |
Power Factor Effect on Cable Sizes
Power factor can significantly affect voltage drop and cable sizes, especially for large cables with high reactance. For a given load current and route length, a lower power factor increases the reactive component of voltage drop, which can require a larger cable to remain within the permissible limit even though the resistive component decreases.
This effect becomes most pronounced for large conductor sizes (above 95 mm²) where cable reactance is relatively significant compared to resistance. Improving power factor — for example through capacitor banks — can reduce the required cable size and lower installation costs.
ℹ️ BS 7671 recommends that the installation power factor be maintained at 0.9 lagging or above where practicable.
Unbalanced Multiphase Circuits
In an unbalanced system, current will be flowing through the neutral conductor as illustrated by the phasor diagram in the figure below:
There are two ways of dealing with voltage drop for unbalanced three-phase circuits:
| Where: | Voltage drop calculation method | |
|---|---|---|
| 1 | Out-of-balance conditions are intermittent | Assume balanced three-phase load conditions and use the heaviest loaded phase current. |
| 2 | Out-of-balance conditions are consistent | Calculate voltage drop on a single-phase basis by summing the voltage drop in the heaviest loaded phase and the neutral conductor. |
Conductors in Parallel
Running conductors in parallel is a standard method of increasing a circuit's current-carrying capacity without resorting to excessively large conductors. This technique involves using two or more conductors in parallel to share the electrical load, but requires careful consideration to ensure safety, efficiency, and compliance with BS 7671.
Using multiple smaller conductors in parallel significantly boosts the circuit's overall current-carrying capacity. Smaller conductors are easier to install, route through conduit, and navigate around obstacles. Parallel conductors also provide redundancy; if one conductor fails, the others can continue to carry the load, albeit at a reduced capacity.
⚠️ Parallel conductors require more cables and incur higher installation costs than using fewer or larger single cables.
Requirements
To avoid overheating of one or more parallel cables, it is essential to ensure that the grouping of cables does not impair the cooling of each cable, and that load current is shared equally between them. This is achieved as follows:
- Conductors are made of the same material and have the same cross-sectional area.
- Cables follow the same route and have the same length.
- Each parallel cable or group's conductors are effectively bonded at both ends.
- The relative position of phase and neutral conductors in and between parallel groups considers mutual impedances (i.e., separation is symmetrical and consistent, and phase arrangement is optimal).
Section 523.7 of BS 7671 provides guidelines and specifications for the use of conductors in parallel.
Neutral Cable Sizing
The neutral conductor is connected to the system's neutral point and carries current under normal operating conditions. BS 7671 Regulation 524.2 specifies the requirements for neutral conductor sizing.
In single-phase two-wire circuits, the neutral conductor must have a current-carrying capacity not less than that of the phase conductor.
⚠️ When there are substantial harmonic currents — particularly 3rd-order harmonics and their multiples (triple-N harmonics) — the neutral conductor size may need to be larger than the phase conductors, as these harmonic currents add arithmetically in the neutral rather than cancelling.
For three-phase circuits feeding predominantly balanced three-phase loads with negligible harmonic content, the neutral conductor may be sized smaller than the phase conductors, provided it carries the maximum out-of-balance current, including any harmonic component.
| Type of Circuit | Neutral conductor size requirement | |
|---|---|---|
Single-phase two-wire circuits
Multiphase circuits
| Neutral conductor cross-section not less than phase conductor cross-section. | Sneutral = Sphase |
Multiphase circuits
| Neutral conductors may be required to be larger than the associated phase conductors. Refer to BS 7671 Regulation 524.2 and IEC 60364-5-52. | Sneutral ≥ Sphase |
Multiphase circuits
| The neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. | Sneutral ≤ Sphase |
Earth Cable Sizing
In low-voltage (LV) electrical installations, the protective earth (PE) conductor is critical for ensuring safety and compliance with BS 7671. Its primary purpose is to provide a low-impedance path to earth, protecting both people and equipment from electrical faults. When a fault occurs — such as a short circuit or insulation failure — the earth conductor directs the fault current safely to earth, facilitating the immediate operation of protective devices such as circuit breakers or fuses, reducing the risk of electric shock, fire, and equipment damage.
How to Size the Earth Conductor
The earth conductor size must be sufficient to ensure:
- Appropriate earth fault loop impedance (Zs) to ensure the protective device operates within the required disconnection time (typically 0.4 s for final circuits, 5 s for distribution circuits).
- Adequate current-carrying capacity for prospective earth fault currents for a duration at least equal to the tripping time of the associated circuit protection, determined by the adiabatic equation.
- Adequate mechanical strength.
According to BS 7671 Regulation 543, the protective conductor cross-sectional area is determined by either:
(a) The adiabatic equation (Regulation 543.1.3):
S = √(I² × t) / k
Where S is the minimum cross-sectional area (mm²), I is the fault current (A rms), t is the operating time of the protective device (s), and k is a factor from Tables 54.4–54.6 depending on conductor material and insulation.
(b) Table 54.7 of BS 7671, which provides conservative protective conductor sizes based on the phase conductor size, for use when protective device details are not fully known.
The earth conductor size should be determined by calculation using the adiabatic equation because it provides a more accurate and often more economical result than Table 54.7.
Table 54.7 of BS 7671
The simplified look-up Table 54.7 provides minimum protective conductor cross-sections based on the phase conductor size. This table is commonly used in industry but may result in oversizing compared with the adiabatic equation method.
Cable Sizing Calculation Examples
Understanding the fundamentals of performing cable sizing calculations in accordance with BS 7671 is important for electrical designers and engineers working on UK electrical installations.
You should understand the cable selection process outlined in BS 7671 Appendix 4, including the correct application of current-carrying capacity tables, installation method references, and correction factors.
You should also understand how to perform neutral and earth conductor sizing, covered by BS 7671 Regulations 524 and 543 respectively.