Max Zs Values Table - BS 7671 Table 41.3
Complete reference table of maximum earth fault loop impedance (Zs) values for UK electrical installations. All values from BS 7671:2018+A2:2022 with 0.8 correction factor applied for ambient temperature measurements.
Understanding the Values
Table values: BS 7671 Table 41.3 values at 70°C conductor temperature.
Measured values (×0.8): Maximum values when testing at ambient temperature (~20°C).
Always use the ×0.8 corrected values when comparing against loop impedance tester readings.
Type B MCB - Max Zs Values (0.4s Disconnection)
Type B MCBs trip at 3-5 times rated current. Used for most domestic and commercial circuits including lighting, socket outlets, and general loads.
| Rating (A) | Table 41.3 (Ω) | ×0.8 Measured (Ω) | Common Uses |
|---|---|---|---|
| 6A | 7.67 | 6.13 | Lighting circuits |
| 10A | 4.60 | 3.68 | Lighting, small appliances |
| 16A | 2.87 | 2.30 | Immersion heaters, radial circuits |
| 20A | 2.30 | 1.84 | Radial socket circuits, water heaters |
| 32A | 1.44 | 1.15 | Ring finals, cookers, showers |
| 40A | 1.15 | 0.92 | Large cookers, EV chargers |
| 50A | 0.92 | 0.74 | Submains, large loads |
| 63A | 0.73 | 0.58 | Submains, distribution |
Type C MCB - Max Zs Values (0.4s Disconnection)
Type C MCBs trip at 5-10 times rated current. Used for circuits with moderate inrush currents such as fluorescent lighting, small motors, and some commercial equipment.
| Rating (A) | Table 41.3 (Ω) | ×0.8 Measured (Ω) | Common Uses |
|---|---|---|---|
| 6A | 3.83 | 3.07 | Fluorescent lighting |
| 10A | 2.30 | 1.84 | Small motors, LED drivers |
| 16A | 1.44 | 1.15 | Air conditioning, commercial equipment |
| 20A | 1.15 | 0.92 | Medium motors, transformers |
| 32A | 0.72 | 0.57 | Larger motors, commercial loads |
| 40A | 0.57 | 0.46 | Industrial equipment |
| 50A | 0.46 | 0.37 | Large industrial loads |
| 63A | 0.36 | 0.29 | Industrial distribution |
Type D MCB - Max Zs Values (0.4s Disconnection)
Type D MCBs trip at 10-20 times rated current. Used for circuits with high inrush currents such as transformers, X-ray equipment, and large motors.
| Rating (A) | Table 41.3 (Ω) | ×0.8 Measured (Ω) | Common Uses |
|---|---|---|---|
| 6A | 1.92 | 1.53 | Small transformers |
| 10A | 1.15 | 0.92 | Welders, X-ray equipment |
| 16A | 0.72 | 0.57 | Medium transformers, compressors |
| 20A | 0.57 | 0.46 | Large motors |
| 32A | 0.36 | 0.29 | Industrial motors |
| 40A | 0.29 | 0.23 | Large industrial equipment |
| 50A | 0.23 | 0.18 | Heavy industrial |
BS 88 Fuses - Max Zs Values (5s Disconnection)
BS 88 (BS EN 60269) fuses are typically used for distribution circuits where 5-second disconnection time is acceptable. Values shown are for distribution circuit protection.
| Rating (A) | Table 41.4 (Ω) | ×0.8 Measured (Ω) | Common Uses |
|---|---|---|---|
| 16A | 3.43 | 2.74 | Small distribution |
| 20A | 2.55 | 2.04 | Submains protection |
| 32A | 1.41 | 1.13 | Distribution boards |
| 40A | 1.04 | 0.83 | Main switch fuses |
| 50A | 0.78 | 0.62 | Industrial supply |
| 63A | 0.55 | 0.44 | Main incomer protection |
| 80A | 0.39 | 0.31 | Large supply protection |
| 100A | 0.27 | 0.22 | Main supply fuse |
Quick Reference - Common Circuit Types
Ring Final Circuit (32A Type B)
Max Zs: 1.44Ω (table) / 1.15Ω (measured)
R1+R2: Typically 0.6-1.0Ω for 2.5mm² T&E
Lighting Circuit (6A Type B)
Max Zs: 7.67Ω (table) / 6.13Ω (measured)
R1+R2: Typically 1.5-3.0Ω for 1.5mm² T&E
Shower Circuit (40A Type B)
Max Zs: 1.15Ω (table) / 0.92Ω (measured)
R1+R2: Typically 0.3-0.5Ω for 10mm² T&E
Cooker Circuit (32A Type B)
Max Zs: 1.44Ω (table) / 1.15Ω (measured)
R1+R2: Typically 0.4-0.7Ω for 6mm² T&E
How to Use the Max Zs Table
Identify the protective device
Determine the MCB type (B, C, or D) and rating (Amps) protecting the circuit.
Find the ×0.8 corrected value
Look up the corrected value (right column) - this is what your loop impedance tester reading must not exceed.
Measure and compare
Use a calibrated loop impedance tester. If measured Zs ≤ table value × 0.8, the circuit complies.
Record results
Document the measured Zs value on the Electrical Installation Certificate (Schedule of Test Results).
Need to Calculate Max Zs?
Use our free Max Zs calculator to instantly check compliance for any circuit.
Open Max Zs Calculator →