Does BS 7671 require voltage drop calculation for every circuit?

Regulation 525.1 requires that voltage drop limits are met, but the standard does not mandate that a formal calculation is documented for every circuit. However, under the Electricity at Work Regulations 1989 and Part P of the Building Regulations, the designer must be able to demonstrate compliance. In practice, a voltage drop calculation should be performed for all circuits except very short runs where compliance is obviously satisfied.

How do I handle voltage drop for circuits with mixed lighting and power loads?

If a circuit supplies both lighting and power loads (which is uncommon in UK installations due to separate circuit conventions), the stricter 3% lighting limit should be applied to the entire circuit. In practice, UK installations typically separate lighting and power onto different circuits, each checked against their respective limits. If a circuit genuinely serves both, apply 3% to be safe.

Voltage Drop Calculator — BS 7671 🇬🇧

United KingdomEdition 2018+A2:2022 (IET Wiring Regulations 18th Edition)Free Online Tool

Voltage drop calculations for UK electrical installations are governed by BS 7671:2018+A2:2022, the IET Wiring Regulations (18th Edition). Regulation 525.1 sets the fundamental requirement: the voltage drop between the origin of the installation and any load point must be within specified limits to ensure equipment operates correctly and safely.

BS 7671 distinguishes between lighting circuits (3% maximum) and other circuits (5% maximum). These percentages apply from the origin of the installation (typically the incoming supply terminals) to the point of utilisation. The standard provides comprehensive voltage drop tables in Appendix 4, giving mV/A/m values for copper and aluminium conductors with thermoplastic (70°C and 90°C) and thermosetting (90°C) insulation.

A key feature of BS 7671 is the C_t correction factor in Appendix 4, which allows engineers to adjust the tabulated voltage drop values when the actual conductor operating temperature is below its rated maximum. Since the tables assume maximum conductor temperature, lightly loaded cables will have lower resistance and therefore lower real voltage drop than the tabulated values suggest.

Open Voltage Drop Calculator

How Voltage Drop Works Under BS 7671

Voltage Drop Methodology per BS 7671:2018+A2

BS 7671 Appendix 4 provides a tabulated method for voltage drop calculation. The tables give millivolts per ampere per metre (mV/A/m) values that include both resistive and reactive components for conductors at their maximum operating temperature.

Step 1: Determine the Circuit Type and Limit

Regulation 525.1 specifies two voltage drop limits measured from the origin of the installation (usually the supply terminals):

Lighting circuits: 3% of nominal voltage (6.9V at 230V)
Other circuits (power): 5% of nominal voltage (11.5V at 230V)

For three-phase 400V supplies, these limits are 12V (3%) and 20V (5%) respectively. Note that Regulation 525.201 allows increased limits where the supply is from a private LV supply (e.g., generator): 6% for lighting and 8% for other uses.

Step 2: Select the Correct Appendix 4 Table

Appendix 4 organises voltage drop tables by conductor material and insulation type:

Table 4Ab — Single-phase and three-phase, copper, thermoplastic (PVC, 70°C)
Table 4Bb — Single-phase and three-phase, copper, thermosetting (XLPE, 90°C)
Table 4Db — Single-phase and three-phase, copper, thermoplastic (PVC, 90°C)
Table 4Hb — Single-phase and three-phase, aluminium, thermoplastic (PVC, 70°C)

Each table provides separate columns for (r) resistive mV/A/m, (x) reactive mV/A/m, and (z) combined mV/A/m at conductor sizes from 1mm² to 1000mm².

Step 3: Calculate Voltage Drop

The basic formula mirrors the approach in other mV/A/m-based standards:

VD (volts) = (mV/A/m × I_b × L) / 1000

Where I_b is the design current in amperes and L is the cable route length in metres (one-way). The single-phase mV/A/m values already include the go-and-return factor; three-phase values include the √3 factor.

Step 4: Apply the C_t Conductor Temperature Correction

BS 7671 Appendix 4 provides a correction factor C_t to reduce the tabulated voltage drop when the conductor operates below its rated temperature. The actual conductor temperature depends on the ratio of actual current to the cable's current-carrying capacity:

C_t = (230 + t_p) / (230 + t_r)

Where t_p is the actual conductor operating temperature and t_r is the rated maximum conductor temperature. The actual temperature is found from: t_p = t_a + (t_r − t_a) × (I_b / I_z)², where t_a is ambient and I_z is the cable's current rating.

Step 5: Special Consideration for Ring Final Circuits

For ring final circuits (common in UK domestic installations), the effective cable length is half the total ring length when the load is balanced. The voltage drop is calculated using L = total ring length / 2 for the worst-case mid-point loading. Appendix 15 of the On-Site Guide provides detailed guidance on ring circuit voltage drop.

Step 6: Verify Compliance

Calculate VD% = (VD / V_nominal) × 100 and compare against the applicable limit. For circuits fed from a distribution board (not the origin), account for the upstream voltage drop from the origin to the DB. The cumulative total must remain within the 3% or 5% limit.

Key Reference Tables

Appendix 4 Table 4Ab — Voltage Drop, Copper, Thermoplastic 70°C

Provides mV/A/m values for copper conductors with 70°C thermoplastic (PVC) insulation, the most common cable type in UK installations. Covers single-phase and three-phase circuits from 1mm² to 1000mm² with separate r, x, and z columns.

Use for standard PVC/PVC (6242Y twin & earth, 6491X singles) cables. The z column gives the combined voltage drop at unity power factor; use r and x separately for circuits with significant reactive loads.

Appendix 4 Table 4Bb — Voltage Drop, Copper, Thermosetting 90°C

Provides mV/A/m values for copper conductors with 90°C thermosetting (XLPE or EPR) insulation. These cables have higher current ratings but also higher resistive voltage drop due to the elevated operating temperature.

Use for XLPE-insulated cables (e.g., BS 5467 armoured). Note that although XLPE cables carry more current, their higher operating temperature means higher mV/A/m values compared to 70°C PVC for the same cross-section.

Regulation 525.1 — Voltage Drop Limits

Defines the maximum allowable voltage drop from the origin of the installation: 3% for lighting circuits (6.9V at 230V) and 5% for other circuits (11.5V at 230V). These are mandatory limits, not recommendations.

The primary compliance check. Determine whether your circuit is lighting or power, then verify the total voltage drop from origin to load point does not exceed the applicable percentage. Regulation 525.201 permits higher limits for private supplies.

Table G1 (Appendix G) — Conductor Impedance per Metre

Provides conductor impedance values (Ω/m) for earth fault loop impedance calculations. While primarily used for fault loop verification, these values can cross-check voltage drop calculations since voltage drop is fundamentally V = I × Z.

Use to verify voltage drop results by alternative calculation method, or when you need impedance values for combined voltage drop and fault loop analysis.

Appendix 4 C<sub>t</sub> Correction Factor

Provides the method to correct tabulated voltage drop values for conductors operating below their rated maximum temperature. This yields a more accurate (and favourable) voltage drop when the circuit is not fully loaded relative to the cable's current capacity.

Apply when the design current is significantly less than the cable's derated current rating to obtain a less conservative voltage drop result. Particularly useful when voltage drop is marginal and a more precise calculation might avoid upsizing the cable.

Table 4Hb — Voltage Drop, Aluminium, Thermoplastic 70°C

Provides mV/A/m values for aluminium conductors with 70°C thermoplastic insulation. Aluminium cables are used for larger sub-mains and distribution circuits in the UK, starting from 16mm².

Use for aluminium-conductor cables such as BS 6346 or aluminium SWA. Values are approximately 1.6× higher than copper equivalents due to aluminium's higher resistivity.

Worked Example — BS 7671 Voltage Drop

Scenario

A 230V single-phase ring final circuit in a domestic installation uses 2.5mm² copper cable with 70°C thermoplastic (PVC) insulation. The total ring length is 40m. The design current at the most loaded point is 20A. Check voltage drop compliance for a power circuit.

Identify circuit parameters

Ring final circuit: total ring length = 40m, effective length for VD = 40/2 = 20m (worst-case mid-point). Design current I_b = 20A. Supply = 230V single-phase. Cable = 2.5mm² Cu, PVC 70°C insulation. Circuit type = power (5% limit applies).

Look up mV/A/m from BS 7671 Appendix 4 Table 4Ab

For 2.5mm² copper conductor with 70°C thermoplastic insulation, single-phase: the tabulated (z) value is 18 mV/A/m (r = 18, x = 0.145). At this cable size, the reactive component is negligible.

mV/A/m = 18 (from Table 4Ab, 2.5mm² row, single-phase z column)

18 mV/A/m

Calculate voltage drop for the ring circuit

Use the effective half-ring length for balanced loading.

VD = (mV/A/m × I_b × L) / 1000 = (18 × 20 × 20) / 1000

VD = 7.20V

Calculate percentage voltage drop

Express as a percentage of the 230V nominal supply.

VD% = (7.20 / 230) × 100

VD% = 3.13%

Check compliance with Regulation 525.1

This is a power circuit (ring final circuit supplying socket outlets), so the 5% limit applies. The calculated 3.13% is well within the 5% limit. Even accounting for upstream voltage drop from the consumer unit to the origin, there is ample margin.

3.13% < 5% limit — PASS

Apply C_t correction (optional refinement)

The 2.5mm² cable has a current rating of 27A (Table 4D1A, Reference Method A). With I_b = 20A and ambient 30°C: t_p = 30 + (70 - 30) × (20/27)² = 30 + 21.9 = 51.9°C. C_t = (230 + 51.9) / (230 + 70) = 281.9 / 300 = 0.94. Corrected VD = 7.20 × 0.94 = 6.77V (2.94%).

C_t = (230 + 51.9) / (230 + 70) = 0.94; VD_corrected = 7.20 × 0.94 = 6.77V

Corrected VD% = 2.94% — PASS with additional margin

The 2.5mm² ring final circuit produces a voltage drop of 7.20V (3.13%) at the uncorrected tabulated values, or 6.77V (2.94%) with the C_t temperature correction. Both values are well within the 5% limit for power circuits per Regulation 525.1. The C_t correction provides a useful margin when the calculation is borderline.

Common Mistakes When Using BS 7671

1
Not distinguishing between lighting (3%) and power (5%) voltage drop limits — BS 7671 Regulation 525.1 mandates different limits for each. Applying the 5% power limit to a lighting circuit means the installation does not comply. Always classify the circuit type before checking compliance.
2
Forgetting the C_t correction factor for conductor operating temperature — the mV/A/m values in Appendix 4 assume the conductor is at its maximum rated temperature. For lightly loaded cables, the actual temperature and resistance are lower. While ignoring C_t is conservative (safe), it can lead to unnecessary cable upsizing when VD is marginal.
3
Ignoring the reactive component for cables 25mm² and above — for small cables the resistive voltage drop dominates, but for larger cross-sections the inductive reactance contributes significantly. Using only the (r) column instead of the (z) column for a 95mm² cable at 0.8 power factor could underestimate VD by 10-15%.
4
Not accounting for ring final circuit effective length — a ring circuit's voltage drop is calculated using half the total ring length (L/2) for balanced loading at the mid-point. Using the full ring length doubles the calculated voltage drop and may lead to unnecessary cable upgrades.
5
Treating the VD limit as applying only to the final circuit — the 3% or 5% limit applies from the origin of the installation to the point of utilisation. The upstream drop from the supply to the distribution board must be added to the final circuit drop to check overall compliance.

How Does BS 7671 Compare?

BS 7671 is distinctive in applying separate voltage drop limits for lighting (3%) and power (5%) circuits, whereas AS/NZS 3000 uses a single 5% limit for all circuits. BS 7671 also provides the C_t correction factor for temperature adjustment, which is a refinement not explicitly provided in the same form in AS/NZS 3008. The mV/A/m tabulated approach is shared with AS/NZS 3008, but the reference conditions differ: BS 7671 tables assume 70°C for PVC conductors while AS/NZS uses 75°C for V-75 insulation.

Frequently Asked Questions

Regulation 525.1 specifies 3% for lighting circuits and 5% for other circuits, measured from the origin of the installation. At 230V single-phase, this equates to 6.9V for lighting and 11.5V for power. Regulation 525.201 allows higher limits (6% and 8% respectively) where the supply is from a private LV source such as a generator.

For a balanced ring final circuit, the effective cable length for voltage drop is half the total ring length. This is because current flows in both directions around the ring to reach the load point. For a 50m total ring length, use L = 25m in the voltage drop formula. If the ring is significantly unbalanced (e.g., spur connections), the worst-case loading scenario should be assessed separately.

The C_t factor corrects tabulated voltage drop values for the actual conductor operating temperature. The formula is C_t = (230 + t_p) / (230 + t_r), where t_p is the actual temperature and t_r is the rated maximum. Use it when the design current is significantly below the cable's current rating and the voltage drop calculation is borderline. It can reduce the calculated VD by 5-15% depending on loading.

Voltage Drop Calculator for Other Standards

🇦🇺AS/NZS 3008.1.1

🌍IEC 60364-5-52

🇺🇸NEC/NFPA 70

Voltage Drop Calculator — BS 7671 🇬🇧

How Voltage Drop Works Under BS 7671

Voltage Drop Methodology per BS 7671:2018+A2

Step 1: Determine the Circuit Type and Limit

Step 2: Select the Correct Appendix 4 Table

Step 3: Calculate Voltage Drop

Step 4: Apply the C_t Conductor Temperature Correction

Step 5: Special Consideration for Ring Final Circuits

Step 6: Verify Compliance

Key Reference Tables

Appendix 4 Table 4Ab — Voltage Drop, Copper, Thermoplastic 70°C

Appendix 4 Table 4Bb — Voltage Drop, Copper, Thermosetting 90°C

Regulation 525.1 — Voltage Drop Limits

Table G1 (Appendix G) — Conductor Impedance per Metre

Appendix 4 C<sub>t</sub> Correction Factor

Table 4Hb — Voltage Drop, Aluminium, Thermoplastic 70°C

Worked Example — BS 7671 Voltage Drop

Scenario

Common Mistakes When Using BS 7671

How Does BS 7671 Compare?

Frequently Asked Questions

Voltage Drop Calculator for Other Standards

Related Resources

Voltage Drop Calculator — AS/NZS 3008

Voltage Drop Calculator — IEC 60364

Voltage Drop Calculator — NEC/NFPA 70

Cable Sizing Calculator — BS 7671

BS 7671 Wiring Regulations Overview

How Voltage Drop Works Under BS 7671

Voltage Drop Methodology per BS 7671:2018+A2

Step 1: Determine the Circuit Type and Limit

Step 2: Select the Correct Appendix 4 Table

Step 3: Calculate Voltage Drop

Step 4: Apply the Ct Conductor Temperature Correction

Step 5: Special Consideration for Ring Final Circuits

Step 6: Verify Compliance

Key Reference Tables

Appendix 4 Table 4Ab — Voltage Drop, Copper, Thermoplastic 70°C

Appendix 4 Table 4Bb — Voltage Drop, Copper, Thermosetting 90°C

Regulation 525.1 — Voltage Drop Limits

Table G1 (Appendix G) — Conductor Impedance per Metre

Appendix 4 C<sub>t</sub> Correction Factor

Table 4Hb — Voltage Drop, Aluminium, Thermoplastic 70°C

Worked Example — BS 7671 Voltage Drop

Scenario

Common Mistakes When Using BS 7671

How Does BS 7671 Compare?

Frequently Asked Questions

Voltage Drop Calculator for Other Standards

Related Resources

Voltage Drop Calculator — AS/NZS 3008

Voltage Drop Calculator — IEC 60364

Voltage Drop Calculator — NEC/NFPA 70

Cable Sizing Calculator — BS 7671

BS 7671 Wiring Regulations Overview

Step 4: Apply the C_t Conductor Temperature Correction