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Voltage Drop

Voltage drop limits, mV/A/m values, and calculation formula from BS 7671 Appendix 4.

Voltage Drop Limits

Maximum Permitted Voltage Drop

From the origin of the installation: • Lighting circuits: 3% (6.9V at 230V) • Other circuits: 5% (11.5V at 230V)

Note: These limits apply from the supply terminals to the most distant point of the circuit. Where the supply is from a public distribution system, an additional 0.5% is allowed.

BS 7671: Appendix 4

Flat Twin & Earth (PVC) mV/A/m

Flat Twin & Earth (PVC 70°C) mV/A/m

Size (mm²)	1Φ 2-core	3Φ 3-core
1.0	44	38
1.5	29	25
2.5	18	15
4.0	11	9.5
6.0	7.3	6.4
10.0	4.4	3.8
16.0	2.8	2.4

PVC 70°C copper conductors. For sizes ≤16mm², values are purely resistive and identical across installation methods.

BS 7671: Table 4D1B

Singles in Conduit (PVC) — Table 4D1B

Singles in Conduit (PVC 70°C) — Table 4D1B

Size (mm²)	1Φ 2-cable	3Φ 3-cable
1.0	44	38
1.5	29	25
2.5	18	15
4.0	11	9.5
6.0	7.3	6.4
10.0	4.4	3.8
16.0	2.8	2.4

Reference Methods A & B (enclosed in conduit/trunking). Values in mV/A/m.

BS 7671: Table 4D1B

Voltage Drop Formula

Voltage Drop

VD = (mV/A/m × Ib × L) / 1000

VD Voltage drop (Volts)

mV/A/m Table value (mV/A/m)

Ib Design current (Amperes)

L Cable length (metres)

Use 1Φ (2-core) values for single-phase circuits. Use 3Φ (3-core) values for three-phase circuits. The formula is the same — only the mV/A/m value changes.

BS 7671: Appendix 4 Open Calculator →

Correction Factors

Conductor Resistance

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