Cable Voltage Drop Calculator: How to Check Your Circuits Comply

Summary

Voltage drop is the reduction in voltage along a cable caused by the resistance of the conductor itself. Every cable has resistance, and the longer the run or higher the current, the greater the drop. BS 7671:2018+A2:2022 Regulation 525 requires that the voltage at any load point does not fall below the limits set out in Appendix 12, Table 4Ab — 3% for lighting (6.9V on a 230V supply) and 5% for all other circuits (11.5V). The calculation uses millivolts per ampere per metre (mV/A/m) values from the voltage drop columns of the cable tables in Appendix 4. On longer domestic runs — particularly showers, cookers, EV chargers, and sub-mains — voltage drop rather than current-carrying capacity is often the factor that forces you to upsize the cable.

Key Facts

• Maximum voltage drop for lighting circuits: 3% of 230V = 6.9V (Appendix 12, Table 4Ab)
• Maximum voltage drop for all other circuits: 5% of 230V = 11.5V (Appendix 12, Table 4Ab)
• For three-phase 400V supplies: 3% = 12V (lighting), 5% = 20V (other)
• The mV/A/m values are taken from Table 4D5B (flat T&E cable, 6242Y) or Table 4D2B (single-core in conduit/trunking)
• For cables up to and including 16mm², the reactive component is negligible — use the tabulated (r) value only
• The mV/A/m values in BS 7671 are stated at the conductor's maximum operating temperature (70C for PVC), not ambient
• Voltage drop on a sub-main adds to the final circuit drop — the total from origin to load point must be within limits
• Ring final circuits: voltage drop is calculated on the design current per leg, not total circuit current, using the route length to the most distant point

Voltage Drop Table (mV/A/m) -- Copper Conductors, 70C Thermoplastic (PVC)

Table 4D5B -- Flat Twin & Earth Cable (6242Y), Single-Phase

Table 4D2B -- Single-Core PVC in Conduit/Trunking (Reference Method A), Single-Phase

Note: For cables up to 16mm2, the mV/A/m values for single-phase are effectively the same across Table 4D5B (flat T&E) and Table 4D2B (singles in conduit) because they are derived from the same copper resistivity. Differences appear at larger cross-sections where reactance becomes significant.

Three-Phase Circuits (Table 4D2B / 4D4B)

For three-phase balanced loads, use the three-phase mV/A/m column from the relevant table. As a rule of thumb, the three-phase value is approximately 0.866 x the single-phase value:

Worked Examples

Example 1: Ring Final Circuit (Socket Outlets)

Scenario: 32A ring final circuit, 2.5mm2 T&E, total ring length 50m, furthest socket at 25m from the CU.

Step 1 -- Determine design current. A ring circuit shares load between both legs. For voltage drop purposes, use the design current (Ib) at the most loaded point. Assume a worst-case design current of 20A at the midpoint.

Step 2 -- Determine effective route length. On a ring, the effective cable length to the midpoint is half the total ring length: L = 50 / 2 = 25m

Step 3 -- Look up mV/A/m. From Table 4D5B, 2.5mm2 = 18 mV/A/m

Step 4 -- Calculate voltage drop. Vd = (mV/A/m x Ib x L) / 1000 Vd = (18 x 20 x 25) / 1000 Vd = 9000 / 1000 Vd = 9.0V

Step 5 -- Check against limit. 5% of 230V = 11.5V. 9.0V < 11.5V. PASS.

Note: If this ring had a long spur, you would add the spur's voltage drop to the ring's drop at that point.

Example 2: Electric Shower -- When 6mm2 Is Not Enough

Scenario: 10.5kW electric shower, 18m cable run from CU to shower unit. Initially considering 6mm2 T&E on a 45A MCB.

Step 1 -- Determine design current. Ib = P / V = 10,500 / 230 = 45.7A

Problem: 6mm2 T&E clipped direct (Method C) is rated at 47A. That is tight -- and if the cable passes through any insulation, or grouping applies, 6mm2 will not carry 45.7A. You need 10mm2 on a 45A or 50A MCB.

Step 2 -- Calculate voltage drop for 6mm2 (to prove it fails). mV/A/m for 6mm2 = 7.3 Vd = (7.3 x 45.7 x 18) / 1000 Vd = 6004.6 / 1000 Vd = 6.0V (within 11.5V limit)

Voltage drop alone would pass -- but the current-carrying capacity does not support the load on 6mm2 once you apply any correction factors. This is why you check both current capacity and voltage drop.

Step 3 -- Calculate voltage drop for 10mm2 (the correct choice). mV/A/m for 10mm2 = 4.4 Vd = (4.4 x 45.7 x 18) / 1000 Vd = 3619.4 / 1000 Vd = 3.6V (well within 11.5V)

Result: Use 10mm2 T&E on a 45A Type B MCB. Voltage drop is 3.6V (1.6%). Current capacity is 64A with comfortable headroom.

Example 3: Cooker Circuit with Diversity

Scenario: 12kW cooker with a cooker control unit (with 13A socket), 6mm2 T&E, 14m cable run from CU.

Step 1 -- Determine full-load current. Full load = 12,000 / 230 = 52.2A

Step 2 -- Apply diversity (IET On-Site Guide, Table 2).

• First 10A at 100% = 10A
• Remaining: 52.2 - 10 = 42.2A at 30% = 12.7A
• Socket on cooker control unit: +5A

Design current with diversity: Ib = 10 + 12.7 + 5 = 27.7A

Step 3 -- Check cable capacity. 6mm2 clipped direct = 47A. 27.7A < 47A. Capacity OK on a 32A MCB.

Step 4 -- Calculate voltage drop. mV/A/m for 6mm2 = 7.3 Vd = (7.3 x 27.7 x 14) / 1000 Vd = 2830.9 / 1000 Vd = 2.8V

Step 5 -- Check against limit. 5% of 230V = 11.5V. 2.8V < 11.5V. PASS.

Result: 6mm2 T&E on a 32A MCB is adequate for this 12kW cooker on a 14m run. Voltage drop is 2.8V (1.2%).

Warning: Without diversity, the full-load current of 52.2A would exceed the 47A capacity of 6mm2 cable. Diversity is only applied to cooker circuits -- never to showers, immersion heaters, or other continuous loads.

Detailed Guidance

How do I calculate voltage drop?

Follow this step-by-step method for any single-phase circuit:

1. Determine the design current (Ib)

• For resistive loads: Ib = Watts / Volts (e.g., 7200W / 230V = 31.3A)
• For cookers: apply diversity per IET On-Site Guide Table 2
• For ring finals: use the design current at the most loaded point, not the MCB rating

2. Measure the cable route length (L)

• Measure the actual route the cable takes -- not the straight-line distance
• Include vertical runs through floors, across ceilings, and down walls
• For ring circuits: use half the total ring length to the midpoint

3. Look up the mV/A/m value

• Use Table 4D5B for flat twin and earth (6242Y) cable
• Use Table 4D2B for single-core cables in conduit or trunking
• Use Table 4D4B for multicore armoured (SWA) cables
• For single-phase circuits up to 16mm2, use the tabulated value directly

4. Apply the formula

Vd = (mV/A/m x Ib x L) / 1000

The result is in volts.

5. Check against the BS 7671 limits

• Lighting: Vd must not exceed 6.9V (3% of 230V)
• All other circuits: Vd must not exceed 11.5V (5% of 230V)
• For sub-main + final circuit combinations: the total drop from the origin of the installation to the load must be within limits

6. If the voltage drop exceeds the limit:

• Upsize the cable (e.g., 6mm2 to 10mm2)
• Shorten the cable route if the installation allows it
• Consider relocating the distribution board closer to the load
• For sub-mains: upsize the sub-main cable to leave more headroom for final circuits

What are the BS 7671 voltage drop limits?

BS 7671 Regulation 525.1 states that the voltage drop between the origin of the installation and any point of utilisation shall not exceed the values given in Appendix 12, Table 4Ab.

Key points:

• The 3%/5% limits apply to consumer installations (i.e., supplied from the DNO network), which covers virtually all domestic and most commercial work
• The higher 6%/8% limits apply to private generation (generators, off-grid solar, etc.)
• The percentages are based on the nominal voltage (230V single-phase, 400V three-phase), not the actual measured supply voltage
• Regulation 525.2 notes that a higher voltage drop may be accepted for motors during starting, or equipment with high inrush current, provided the voltage variations do not impair the correct functioning of the equipment

What if my voltage drop exceeds the limit?

If your calculated voltage drop exceeds 3% (lighting) or 5% (other), you have several options:

1. Upsize the cable -- the most common solution. Going from 6mm2 to 10mm2 nearly halves the mV/A/m value (7.3 to 4.4), which nearly halves the voltage drop.
2. Shorten the cable run -- route the cable more directly if possible. Every metre saved reduces the drop.
3. Relocate the distribution board -- on larger installations, fitting a sub-distribution board closer to the load can dramatically cut final circuit lengths.
4. Split the load -- where a single high-power circuit is causing problems, splitting it into two circuits (e.g., two lighting circuits instead of one long one) may resolve the issue.
5. Accept a higher supply voltage -- not something you can control, but if the measured supply voltage is above 230V (as it often is in the UK at 240V+), the actual percentage drop in practice is lower than the calculated figure.

How does voltage drop work with sub-mains?

When a sub-main feeds a distribution board, the voltage at that board is already reduced by the sub-main's own voltage drop. The final circuits then add further drop.

Example: A 25mm2 sub-main runs 20m to a garage distribution board, carrying 50A:

• Sub-main Vd = (1.75 x 50 x 20) / 1000 = 1.75V
• A 6mm2 shower circuit from the garage DB runs 8m at 45A:
  • Final circuit Vd = (7.3 x 45 x 8) / 1000 = 2.63V
• Total Vd = 1.75 + 2.63 = 4.38V (within 11.5V limit)

The designer can apportion the voltage drop budget between sub-main and final circuits as they see fit, provided the total does not exceed the limit at the load point.

Does temperature affect voltage drop?

Yes. The mV/A/m values in BS 7671 are given at the conductor's maximum operating temperature (70C for PVC, 90C for XLPE/LSF). If the cable operates at a lower temperature (because the load is less than the cable's rated capacity), the actual voltage drop will be slightly less than calculated.

For a more precise calculation, BS 7671 Appendix 4, Section 6 provides a correction method:

Actual Vd = Tabulated Vd x [230 + tp - (Ca2 x Cg2 x Ci2 x (tp - ta))] / [230 + tp]

Where tp = maximum operating temperature, ta = ambient temperature, and Ca/Cg/Ci are the correction factors applied.

In practice, most electricians use the tabulated values without temperature correction. This gives a conservative (worst-case) result, which is the safe approach.

Frequently Asked Questions

What table do I use for voltage drop on twin and earth cable?

Use Table 4D5B in BS 7671 Appendix 4 for flat twin and earth (6242Y/6242B) cables. Column 3 gives the single-phase mV/A/m value. For single-core cables in conduit or trunking, use Table 4D2B. For multicore armoured cable (SWA), use Table 4D4B.

Is voltage drop calculated on the MCB rating or the design current?

Use the design current (Ib) -- the actual current the circuit is expected to carry in normal service. For a shower, this is watts divided by volts. For a cooker, apply diversity first. Using the MCB rating would give an overly pessimistic result, although some electricians do this as a quick worst-case check.

How do I calculate voltage drop on a ring final circuit?

For a ring circuit, current flows in both directions around the ring. The voltage drop is greatest at the midpoint. Calculate using half the total ring length as L, and the design current at that point as Ib. If the ring has spurs, add the voltage drop of each spur to the ring voltage drop at the spur's takeoff point.

Can I ignore voltage drop on short runs?

On runs under about 10m with typical domestic loads, voltage drop is rarely an issue. But you must still verify -- a 10m run on a 45A shower circuit with 6mm2 cable gives Vd = (7.3 x 45 x 10) / 1000 = 3.3V, which is fine. The calculation takes seconds and should be part of every design.

What is the maximum cable run before voltage drop becomes a problem?

There is no single answer -- it depends on cable size, circuit current, and whether the circuit is lighting (6.9V limit) or power (11.5V limit). Use this quick formula to find the maximum run:

Max L (metres) = (Vd limit x 1000) / (mV/A/m x Ib)

For example, a 2.5mm2 radial at 20A: Max L = (11.5 x 1000) / (18 x 20) = 31.9m

Do I need to account for voltage drop on the meter tails?

Strictly, the voltage drop limits in Table 4Ab apply from the origin of the installation (the supply terminals). Meter tails are between the cutout and the consumer unit, which is effectively the origin. In practice, meter tails are typically short (under 3m) with large cable (25mm2), so the drop is negligible. However, on long meter tail runs (e.g., meter in a boundary box), you should include them in your calculation.

Regulations & Standards

• BS 7671:2018+A2:2022 -- Regulation 525.1: voltage drop limits

• BS 7671 Appendix 12, Table 4Ab -- maximum permissible voltage drop percentages

• BS 7671 Appendix 4, Table 4D5B -- voltage drop (mV/A/m) for flat thermoplastic (PVC) cables with protective conductor (twin and earth)

• BS 7671 Appendix 4, Table 4D2B -- voltage drop for single-core thermoplastic cables in conduit/trunking

• BS 7671 Appendix 4, Table 4D4B -- voltage drop for multicore thermoplastic armoured cables

• BS 7671 Appendix 4, Section 6 -- method for adjusting voltage drop for conductor operating temperature

• IET On-Site Guide (8th Edition) -- Table 2: diversity allowances for cooker circuits

• IET Guidance Note 1: Selection & Erection -- detailed worked examples of cable sizing and voltage drop

• IET: Appendix 4 of BS 7671

• NAPIT: How to Determine Voltage Drop Limits

• Total Skills UK: Volt Drop Calculations

• ECalPro: BS 7671 Regulation 525

• IET EngX: Volt Drop Discussion

• myCableEngineering: BS 7671 Voltage Drop

• the-Regs: Calculating Volt-Drop with the 18th Edition

• NICEIC: Calculating Voltage Drop

• cable sizing -- Full cable sizing guide covering current capacity, correction factors, and protective device selection

• consumer units -- Consumer unit standards and split-load board configurations