70°C flat twin & earth mV/A/m values (Table 4D5). Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This free voltage drop calculator works out the volt drop across a run of flat twin and earth cable and checks it against the BS 7671 limits. It is built for UK electricians who need a quick answer on site or at the design stage: will this cable size, at this design current, over this length, stay within the permitted drop?
The tool uses the mV/A/m values for 70°C thermoplastic flat twin and earth from Table 4D5 of BS 7671. You pick the cable size, enter the design current and circuit length, and choose whether the circuit is lighting or power. The calculator returns the drop in volts, the drop as a percentage of the 230V nominal supply, the applicable limit and a clear pass or fail.
Excessive voltage drop causes dim lighting, poor appliance performance and nuisance problems that are expensive to fix after first fix. Checking it takes seconds here, so there is no reason to guess.
Vd = (mV/A/m × Ib × L) / 1000
Vd is the voltage drop in volts. mV/A/m is the tabulated voltage drop per amp per metre for the chosen conductor size, taken from Table 4D5 for 70°C thermoplastic twin and earth. Ib is the design current of the circuit in amps, and L is the circuit length in metres (the one-way route length — the tabulated value already accounts for the return conductor). Dividing by 1000 converts millivolts to volts. The result is compared with 3% of 230V (6.9V) for lighting circuits or 5% of 230V (11.5V) for other circuits.
Choose the twin and earth conductor size from 1mm² to 16mm². The calculator applies the matching mV/A/m value from Table 4D5 automatically.
Type in the design current Ib in amps — the current the circuit is expected to carry in normal service, not necessarily the device rating.
Enter the route length of the circuit in metres, from the board to the furthest point. Measure the actual cable route, not the straight-line distance.
Select Lighting (3%) or Power (5%). The tool shows the drop in volts, the percentage of 230V, the limit that applies and whether the circuit passes. On a fail, increase the cable size or shorten the run and recalculate.
The voltage drop limits used here follow BS 7671:2018+A2 for installations supplied directly from a public low-voltage network: 3% for lighting and 5% for other uses, which at 230V nominal works out to 6.9V and 11.5V respectively. The calculator applies whichever limit matches the circuit type you select.
The mV/A/m figures are for 70°C thermoplastic (PVC) flat twin and earth only, per Table 4D5. If you are running SWA, singles in conduit, or cable at a different conductor operating temperature, the tabulated values differ and this tool will not give the right answer. Long runs at high load can also justify a temperature-corrected voltage drop calculation, which this simple check does not perform.
Voltage drop is only one part of circuit design. A cable that passes on volt drop must still be checked for current-carrying capacity with correction factors, earth fault loop impedance and, where relevant, the adiabatic equation. Use this tool alongside the cable sizing and max Zs calculators rather than instead of a full design.
As with all these tools: guidance only. Verify the final design against BS 7671 and the On-Site Guide for the actual installation method.
70°C flat twin & earth mV/A/m values (Table 4D5). Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
For installations supplied from a public low-voltage network, the recommended limits are 3% for lighting circuits and 5% for other circuits. At 230V nominal that is 6.9V and 11.5V. These are the limits this calculator checks against.
Multiply the tabulated mV/A/m value for the cable by the design current in amps and the route length in metres, then divide by 1000. For example, 2.5mm² twin and earth at 18 mV/A/m carrying 26A over 18m drops (18 × 26 × 18) / 1000 = 8.42V, which passes the 5% power limit but would fail the 3% lighting limit.
Use the one-way route length of the circuit. The tabulated mV/A/m values already account for the voltage drop in both the line and return conductors, so doubling the length would give a result twice as large as it should be.
The usual fixes are to increase the conductor size, shorten the cable route, or split the load across more circuits. Symptoms of excessive drop include dim or flickering lights and motors or appliances running poorly at the far end of long runs.
No. It uses the Table 4D5 values for single-phase 70°C thermoplastic flat twin and earth only. SWA, singles in containment and three-phase circuits have different tabulated mV/A/m values in BS 7671, so use the appropriate table for those cables.
BS 7671 works from the nominal UK supply voltage of 230V, and this calculator expresses the drop as a percentage of 230V. Actual site voltage is often higher, but design checks are done against nominal voltage.
Required tabulated capacity It ≥ In / (Ca·Cg·Ci·Cf), with a T&E suggestion.
Maximum earth fault loop impedance for BS EN 60898/61009 devices.
Expected conductor resistance for a run from OSG Table I1 values.
Convert between power and current for single or three phase.
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