Whole network design, checked against BS 7671.
Calculated to BS 7671:2018+A4:2026, with the table and equation behind every number, so the working stands up when it's checked.
In the subscription
Infrastructure. Whole network design.
Draw the network, supply to final circuit, and have every circuit checked against BS 7671 with the result sourced to the regulation it came from. One subscription, no limit on the size of the design.
Circuit
Full circuit design: OCPD selection, fault current, and disconnection times checked against Chapter 41.
The free calculators
Volt Drop
Voltage drop for every Appendix 4 cable type, with temperature correction (Ct), power factor, and a pass or fail against Table 4Ab.
Maximum Demand & Diversity
Maximum demand with diversity applied load by load, using the IET Design Guide allowances that Reg 311.1 permits.
Load
Build a load schedule and check total equipment load against supply capacity.
Every result is traceable.
Table-driven lookups
Tabulated current values (It), correction factors (Ca, Cg, Cd), and volt drop coefficients are all pulled directly from BS 7671:2018+A4:2026 Appendix 4 A-series and B-series tables.
Tables 4D1A–4J4B · Table 4B1 · Table 4B3
The right equation for the conductor
Up to 16 mm², the tabulated mV/A/m with power factor and Ct (Eq 6 to 12). From 25 mm², the resistive and reactive parts are combined with the power factor (Eq 9 to 13) instead of using the z value as an approximation. Every result names the equation it used.
Eq 6 to Eq 13 · Appendix 4 section 6
Ct from Appendix 4 Equation 6, applied when it is valid
Ct is applied whenever the tabulated current is known and the design current sits below the corrected rating, and set to 1.0 for a BS 3036 fuse. The working shows the Appendix 4 Equation 6 calculation behind it.
BS 7671:2018+A4:2026 · Appendix 4 Equation 6
Automatic limit checking
Checked against Table 4Ab: 3% lighting and 5% other uses on a public supply, 6% and 8% on a private one with each final circuit still held to 3% and 5%, and the 100 m relaxation applied where the run is longer.
Table 4Ab · BS 7671 Appendix 4
Example — Volt Drop calculation working
Results you can document and defend.
Layer 1
The result
A clear pass/fail compliance status, the voltage drop in volts, and the percentage of nominal voltage. Immediately actionable.
Layer 2
The working
Every intermediate value shown — correction factors (Ca, Cg, Ct), the effective current-carrying capacity, and the voltage drop components. Nothing hidden.
Layer 3
The reference
Every value cited to its exact BS 7671 table or equation number. If a checker or client asks where a number comes from, the answer is right there.
Replace the spreadsheet.
A spreadsheet can do the arithmetic. It can't pick the right table, calculate Ct per Appendix 4 Equation 6, or cite the reference for each value. CurrentProof does all three on every calculation.
| Feature | CurrentProof | Spreadsheet | Manual calc |
|---|---|---|---|
| Standards traceability — every value cited to BS 7671 | ✓ | ✗ | Partial |
| Automatic correction factors (Ca, Cg, Ct) | ✓ | Manual entry | ✗ |
| Table 4Ab compliance check on every result | ✓ | If built in | ✗ |
| Ct calculated per BS 7671 Appendix 4 Equation 6, not assumed | ✓ | ✗ | If known |
| The right voltage drop equation for the conductor size | ✓ | ✗ | Rarely |
| PDF export with full calculation working | ✓ Professional | Manual format | ✗ |
| Calculation history and saved projects | ✓ | File management | ✗ |
| Updated to current BS 7671:2018+A4:2026 | ✓ | If maintained | If known |
Start with the Volt Drop calculator. Free.
No account needed to calculate. Create a free one to save and export, and move to Professional for Circuit and Infrastructure.