Calculate cable sizes for 3-phase circuits in commercial and industrial installations per BS 7671:2018+A2:2022
Enter your 3-phase installation parameters to calculate cable sizes per BS 7671
Sizing cables for three-phase circuits requires different calculations to single-phase work. The key difference is the use of the square root of 3 (1.732) factor in current calculations and the use of 3-phase mV/A/m values from BS 7671 Appendix 4 for voltage drop.
For 3-phase circuits, the line current is calculated from the total power:
Where P is total power in watts, V is line voltage (400V), and pf is power factor. For example, a 30kW motor with pf of 0.85: I = 30000 / (1.732 x 400 x 0.85) = 50.9A
Calculate the required tabular current by dividing design current by the combined derating factor:
Use the 3-phase column from the appropriate table:
Select the smallest cable where tabular current rating is greater than or equal to your calculated It.
For 3-phase circuits, use the 3-phase mV/A/m values from BS 7671. These values already account for the square root of 3 factor:
The voltage drop limit is 5% of 400V = 20V for power circuits, or 3% = 12V for lighting circuits (BS 7671 Regulation 525).
Three-phase power delivers significantly more power than single-phase using less conductor material. Understanding the key differences is essential for correct cable sizing.
The factor 1.732 (square root of 3) appears because the three phase voltages are separated by 120 electrical degrees. The line-to-line voltage (400V) equals the phase voltage (230V) multiplied by sqrt(3). This same factor reduces the line current compared to single-phase for the same power output, which is why 3-phase systems are more efficient for high-power loads.
Same 30kW Load
1-phase: 130A at 230V
Same 30kW Load
3-phase: 43A at 400V
Conductor Saving
Up to 50% less copper
Typical cable sizes for common 3-phase installations. Always calculate for your specific cable length and installation conditions.
5.5kW (7.5HP): 2.5mm2 SWA, 16A MCCB
11kW (15HP): 4mm2 SWA, 25A MCCB
22kW (30HP): 6mm2 SWA, 50A MCCB
37kW (50HP): 16mm2 SWA, 80A MCCB
55kW (75HP): 25mm2 SWA, 100A MCCB
Use Type D MCBs or MPCB for motor circuits
Current: 32A per phase
Short run (<25m): 6mm2 4-core SWA
Long run (25-50m): 10mm2 4-core SWA
Protection: 40A Type B RCBO
IET Code of Practice for EV charging applies
32A DB: 6mm2 SWA
63A DB: 16mm2 SWA
100A DB: 35mm2 SWA
200A DB: 95mm2 SWA
4-core cable required for unbalanced loads
Combi Oven (18kW): 6mm2 SWA, 32A
Induction Range (15kW): 4mm2, 25A
Dishwasher (12kW): 4mm2, 20A
Apply CIBSE diversity factors for kitchens
Small AC (7kW): 2.5mm2, 16A
Chiller (30kW): 10mm2, 50A
Large AHU (45kW): 16mm2, 80A
Check starting current for voltage drop
Passenger Lift (15kW): 4mm2, 32A
Goods Lift (30kW): 10mm2, 50A
Escalator (15kW): 4mm2, 25A
Allow for regenerative braking loads
Important: These are typical values for short to medium cable runs with no grouping or elevated temperature derating. Always calculate for your specific installation. Use the calculator above for accurate BS 7671 compliant results.
Current ratings for common 3-phase cable types at 30 degrees C ambient. Values are for copper conductors. Use derating factors for non-standard conditions.
| Size (mm2) | Clipped/Tray | Buried Direct | Free Air | mV/A/m | Typical Use |
|---|---|---|---|---|---|
| 2.5 | 24A | 36A | 30A | 17 | Small motors, lighting DBs |
| 4.0 | 31A | 44A | 38A | 11 | Small motors, sub-DBs |
| 6.0 | 40A | 56A | 49A | 7.3 | 22kW EV chargers, small DBs |
| 10 | 54A | 75A | 67A | 4.4 | Medium motors, DBs |
| 16 | 71A | 97A | 88A | 2.8 | Large motors, sub-mains |
| 25 | 93A | 126A | 115A | 1.75 | 100A sub-mains |
| 35 | 114A | 153A | 141A | 1.25 | Large sub-mains |
| 50 | 137A | 181A | 170A | 0.93 | Main incoming, large DBs |
| 95 | 207A | 265A | 256A | 0.49 | Main distribution |
| 185 | 308A | 385A | 380A | 0.275 | Main incoming supply |
Reference: BS 7671 Table 4D4A (4-core armoured cable, copper conductors, 70 degrees C PVC)
Scenario:
Step 1 - Design Current (Ib):
Ib = 30000 / (1.732 x 400 x 0.85) = 50.9A
Step 2 - Protection Device (In):
Select 63A Type D MCCB (In = 63A, greater than Ib)
Step 3 - Derating Factors:
Step 4 - Required Tabular Current (It):
It = 63A / 0.70 = 90A
Step 5 - Cable Selection (Table 4D4A):
25mm2 4-core SWA = 93A on tray (greater than 90A required)
Step 6 - Voltage Drop Check:
From Table 4D4A: 25mm2 = 1.75 mV/A/m (3-phase)
VD = (1.75 x 50.9 x 40) / 1000 = 3.56V
Percentage = (3.56 / 400) x 100 = 0.89% (well within 5% limit)
Final Answer:
Use 25mm2 4-core SWA PVC cable with 63A Type D MCCB. Neutral: 25mm2 (balanced motor, but included in 4-core cable). CPC: Steel wire armour (verify with adiabatic equation) or separate 16mm2 earth.
In a perfectly balanced 3-phase system, the neutral carries zero current. However, modern non-linear loads create harmonic currents that can cause the neutral current to exceed the phase current.
| Condition | Harmonic Content | Neutral Size | Example |
|---|---|---|---|
| Balanced linear loads | <15% | Can be reduced (but full size recommended) | 3-phase motors, heaters |
| Mixed loads | 15-33% | Same as phase conductors | Office buildings, retail |
| Non-linear loads | >33% | Equal to or greater than phase | Data centres, LED arrays |
Fundamental currents (50Hz) at 120 degrees apart cancel in the neutral. Third harmonic currents (150Hz) are at 3 x 120 = 360 degrees apart, which equals 0 degrees - they are in phase. Instead of cancelling, they add together arithmetically. If each phase has 30% third harmonic content, the neutral can carry current equal to the phase current, even with perfectly balanced phases. This is why modern commercial installations often require full-size or even double-size neutrals.
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