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⚡ Circuit Parameters
⚡ Design Currents (A)
L1
L2
L3
Ring Final Circuit (halves effective length for Zs) In Thermal Insulation (×0.5 derating)
🗂 Distribution Boards
📋 Circuit Schedule
RefDescriptionBkrIn (A)Bk (kA) PhL1 (A)L2 (A)L3 (A) Div%Cgmm²Len(m)Method Ib (A)CCC(A)Vd(V)Vd% Zs(Ω)PSCC(A)DiscCCPVD✓BK✓
📊 Board Summary
🔍 Automatic Cable Sizer
How it worksEnter your load requirements and constraints. The auto-sizer will test every cable size from 1.5mm² upwards and recommend the minimum cable that satisfies ALL conditions: current carrying capacity, voltage drop ≤3%, and earth fault disconnection.
⚖ Phase Load Balancer
BS 7909 NoteFor temporary 3-phase installations, phase imbalance should be minimised. Significant imbalance (>10%) can cause neutral overcurrent and voltage unbalance. Use the balancer below to optimise your circuit assignment.
✅ BS 7909 / BS 7671 Compliance Checker
AboutThis checker reviews your entire project against key requirements of BS 7909:2023 (Code of Practice for Temporary Electrical Systems) and BS 7671:2018+A2:2022. Click "Run Full Check" to scan all boards.
📊 Maximum Zs Values — BS 7671 Table 41.3 & 41.4
Maximum earth fault loop impedance values for MCBs at 230V per BS7671 Tables 41.3 & 41.4. Values include the 0.8 conductor temperature correction factor (BS7671 Appendix 14). Ensures disconnection within 0.4s for final circuits ≤32A and 5s for distribution circuits. Measured Zs must not exceed these values.
TYPE B MCBs — Table 41.3
Rating (A)Max Zs (Ω) — 0.4sMax Zs (Ω) — 5s
TYPE C MCBs — Table 41.4
Rating (A)Max Zs (Ω) — 0.4sMax Zs (Ω) — 5s
TYPE D MCBs
Rating (A)Max Zs (Ω) — 0.4sMax Zs (Ω) — 5s
Zs CALCULATOR — Check your circuit
📐 HO7 Cable Data — CCC & mV/A/m
Size (mm²)Method C (A)Method E (A)Method B (A) mV/A/m 1ØmV/A/m 3ØR1+R2 (mΩ/m)Weight (kg/m)
📐 Grouping Factors (Cg) — Table 4C1
No. CircuitsCgEff. CCC (% of single)
📐 Temperature Correction — Table 4B1
Ambient (°C)Ca FactorNotes
📐 Formula Reference
Actual Design Current (3Ø)
Ib = max(L1, L2, L3) × (1 − D/100)
D = diversity %. Use maximum phase for worst case.
Derated CCC
It = It(table) × Ca × Cg × Ci
Ca = temp factor, Cg = grouping, Ci = insulation (0.5 if buried in insulation)
Voltage Drop
Vd = (mV/A/m × Ib × L) / 1000
BS 7671 Appendix 4. Max 3% for temp installs (BS 7909).
Earth Fault Loop Impedance
Zs = Ze + (R1+R2) × L / 1000
R1+R2 from cable tables (mΩ/m), corrected to 70°C conductor temp. ×1.2 factor applies.
PSCC
PSCC = U0 / Zs = 230 / Zs
Prospective short-circuit current. Must exceed breaker instantaneous trip threshold.
Disconnection Check
PSCC ≥ In × (trip multiplier)
B = 5×, C = 10×, D = 20× rated current for instantaneous magnetic trip.