BS 7671 Regulation 525: Voltage Drop Limits

Regulation 525 of BS 7671:2018+A2 sets the maximum permissible voltage drop between the origin of an electrical installation and any point of utilisation. Excessive voltage drop causes reduced equipment performance, dimming of lighting, overheating of motors, and potential nuisance tripping of sensitive electronics.

The regulation applies to all circuits in a low-voltage installation and requires the designer to verify that the voltage at the terminals of every item of current-using equipment is sufficient for it to operate safely and effectively.

Voltage drop verification is the final step in cable sizing: after selecting a cable based on current-carrying capacity and derating factors, the designer must confirm that the selected cable does not exceed the voltage drop limits specified in Table 4Ab.

Table 4Ab in BS 7671 Appendix 4 specifies the maximum voltage drop for different supply types and load categories:

For a standard 230 V single-phase supply from the public network:

• Lighting circuits: Maximum 3% = 6.9 V
• Power circuits: Maximum 5% = 11.5 V

For a 400 V three-phase supply:

• Lighting circuits: Maximum 3% = 12 V
• Power circuits: Maximum 5% = 20 V

The higher limits for private supplies recognise that the generator or UPS voltage can be adjusted at the source to compensate for voltage drop in the distribution system.

BS 7671 uses the mV/A/m method (millivolts per ampere per metre) for voltage drop calculations. Tables 4E1A through 4E4A provide voltage drop values in mV/A/m for each cable type and size:

The voltage drop calculation formula is:

Single-phase:  VD = (mV/A/m × Ib × L) / 1000

Three-phase:   VD = (mV/A/m × Ib × L) / 1000

where:

• mV/A/m = voltage drop value from the appropriate Table 4E
• I_b = design current of the circuit in amperes
• L = route length of the cable in metres (one way, not return)
• VD = voltage drop in volts

The percentage voltage drop is then: VD% = (VD / U₀) × 100, where U₀ is the nominal supply voltage (230 V single-phase or 400 V three-phase).

A 32 A single-phase power circuit uses 6 mm² multicore PVC copper cable (Method C, clipped direct) with a cable run length of 35 m.

Step 1: Look up mV/A/m from Table 4E2A
  6 mm² multicore PVC copper = 7.3 mV/A/m

Step 2: Calculate voltage drop
  VD = (7.3 × 32 × 35) / 1000
  VD = 8,176 / 1000
  VD = 8.18 V

Step 3: Calculate percentage
  VD% = (8.18 / 230) × 100 = 3.56%

Step 4: Compare with limit
  Power circuit limit = 5%
  3.56% ≤ 5%  √ PASS

  If this were a lighting circuit:
  3.56% > 3%  × FAIL — would need to increase cable to 10 mm²

This example shows why voltage drop can sometimes be the governing factor in cable selection, especially for long cable runs. The 6 mm² cable is adequate for current capacity (46 A in Method C vs 32 A required) but is marginal on voltage drop for lighting circuits.

In installations with distribution boards fed by submains, the total voltage drop is the sum of the submain drop and the final circuit drop. Both must be within the overall limits of Table 4Ab.

There are two common approaches to allocating the voltage drop budget:

• Even split: Allocate roughly equal percentages to the submain and final circuit. For a 5% limit: 2.5% each.
• Proportional: Allocate based on expected cable lengths. Longer submains may need 3% while shorter final circuits take 2%.

Example: 400 V three-phase submain + 230 V single-phase final circuit

  Submain: 70 mm² XLPE, 50 m, 200 A design current
    mV/A/m = 0.55 (Table 4E3A, three-phase)
    VD = (0.55 × 200 × 50) / 1000 = 5.5 V
    VD% = (5.5 / 400) × 100 = 1.38%

  Final circuit: 6 mm² PVC, 25 m, 32 A
    mV/A/m = 7.3 (Table 4E2A, single-phase)
    VD = (7.3 × 32 × 25) / 1000 = 5.84 V
    VD% = (5.84 / 230) × 100 = 2.54%

  Total VD% = 1.38% + 2.54% = 3.92% ≤ 5%  √ PASS

For cable sizes above approximately 25 mm², the inductive reactance of the cable becomes significant and cannot be ignored. Tables 4E1A–4E4A provide separate columns for the resistive (r) and reactive (x) components of voltage drop.

For circuits with a power factor less than unity, the voltage drop is calculated using:

VD = Ib × L × (r × cosφ + x × sinφ) / 1000

where cosφ is the circuit power factor. For resistive loads (power factor = 1.0), the reactive term drops out and the calculation simplifies to using only the ‘r’ column. For motor loads with power factor 0.8, the reactive component can add 10–20% to the voltage drop for large cables.

For cables 16 mm² and below, the reactance is negligible and the single mV/A/m value from the tables is sufficiently accurate for most installations.