VoltDrop

Voltage drop calculator

Check a circuit against the BS 7671 limits, find the maximum cable length for a load, or the smallest conductor that passes. Works for twin and earth, SWA, singles, flex and MICC in single-phase, three-phase and DC, plus thin-wall automotive cable for 12 V and 24 V systems. Results update as you type and the link in your address bar always reproduces them.

Supply
V

mm²
Circuit use (sets the limit)
Supply origin
Advanced optionspower factor, rings, parallel cables, Ct, custom limit

Used to convert kW to amps, and for r·cosφ + x·sinφ on cables of 25mm² and above.

Sub-main or tails drop, as a %. The final circuit gets what is left of the allowance.

Conductor operating temperature correction (Ct)

Appendix 4, 6.2. A lightly loaded cable runs cooler than its rated temperature, so its resistance and voltage drop are lower. Needs the tabulated current-carrying capacity It for the reference method, and the rating factors you applied (ambient Ca, grouping Cg, soil Cs and depth Cd for buried cables). Only applies to 16mm² and below.

Voltage drop

— V

— % of — V

—

Limit — % = — V

Voltage at load
— V
Design current Ib
— A
mV/A/m used
—
Max length at this load
—
Max current at this length
— A
Smallest size that passes
—
Power lost in cable
— W
Headroom
— V
Ct applied
—

Working

    Every size of this cable at the same load and length

    Click a row to select that size.

    SizemV/A/mDrop%Max lengthMax currentResult

    How the calculation works

    The drop is the tabulated millivolts per amp per metre for the cable, multiplied by the design current and the one-way route length:

    Vd (V) = mV/A/m × Ib × L ÷ 1000

    The mV/A/m values come from Appendix 4 of BS 7671 (Tables 4D1B to 4G2B, cross-checked against manufacturer reproductions). Aluminium cables are not included; enter the Table 4H value manually. For 25mm² and above the tables give resistive (r), reactive (x) and impedance (z) figures. The calculator uses z unless you enter a power factor, in which case it uses r·cosφ + x·sinφ.

    Three-phase values are line-to-line, so the percentage is taken against the 400 V line voltage. DC and extra-low-voltage cables that are not in the BS 7671 tables are worked from conductor resistance (BS EN 60228) at the chosen temperature, counting both legs of the run.

    The limits

    Table 4Ab of Appendix 4 gives the voltage drop that is deemed to satisfy Regulation 525.202, measured from the origin of the installation to the load:

    SupplyLightingOther uses
    Public low-voltage supply3 %5 %
    Private supply (generator, private transformer)6 %8 %

    For runs over 100 m the limit may be increased by 0.005 % per metre beyond 100 m, up to an extra 0.5 %. A final circuit fed from a sub-main shares the allowance with it, so enter the drop from the origin or check each leg against its share.

    Ring final circuits, evenly distributed loads, parallel conductors and the conductor operating temperature factor Ct are all under Advanced options.