Voltage drop is one of the most common compliance checks an electrician performs, yet it is widely misunderstood. Whenever current flows through a conductor, some of the supply voltage is “lost” along the cable before it ever reaches the load. That lost voltage is converted to heat in the conductor's own resistance. This guide explains exactly what voltage drop is, why it matters, how to calculate it under both the NEC and BS 7671, and the limits you need to design to.
What voltage drop actually is
Every conductor has resistance. When you push current through that resistance, Ohm's law tells us a voltage develops across it: V = I × R. In a circuit, that voltage appears as the difference between the voltage at the origin (the board) and the voltage delivered at the far end (the load). The longer the cable run, the thinner the conductor, and the higher the current, the larger the drop.
Two factors set a cable's resistance: its cross-sectional area (a bigger conductor has lower resistance) and its length (resistance is proportional to length). Because current flows out to the load and back, the effective length is the round-trip distance - which is why the formulas you will see below include a factor of two for single-phase circuits.
Why voltage drop matters
Voltage drop is not merely an efficiency concern. Equipment is rated to operate within a band of its nominal voltage, and excessive drop pushes it outside that band:
- Motors draw heavy starting current. If voltage sags too far, the motor may fail to start, stall under load, or overheat - torque falls with the square of the voltage.
- Lighting dims and colour temperature shifts; LED drivers can flicker or shut down when their input falls below the minimum working voltage.
- Heating elements deliver less power, because power varies with the square of voltage (
P = V² ÷ R). - Energy lost to conductor heating is wasted and, on long high-current runs, can be a genuine running cost.
The voltage drop formula (BS 7671 / AS/NZS)
In the UK and Australia/New Zealand, voltage drop is calculated from the cable's tabulated mV/A/m value (millivolts of drop per ampere of current per metre of run). These values are published in BS 7671 Appendix 4 and AS/NZS 3008 for every conductor size and installation method:
Voltage Drop (V) = (mV/A/m × Ib × L) ÷ 1000
Here Ib is the design current in amps and L is the one-way route length in metres. Division by 1000 converts millivolts to volts. For a three-phase circuit, use the line-to-line mV/A/m figure; the tables already account for the √3 relationship, so you do not add an extra factor.
The voltage drop formula (NEC)
The NEC approach uses the conductor's resistivity constant K and the conductor area in circular mils:
Voltage Drop (V) = (2 × K × I × D) ÷ CM
Where K= 12.9 for copper and 21.2 for aluminium (ohm-circular-mils per foot at 75 °C), I is the current in amps, D is the one-way distance in feet, and CM is the conductor area in circular mils (from NEC Chapter 9, Table 8). The factor of 2 represents the out-and-back path of a single-phase circuit; for a balanced three-phase circuit, replace the 2 with 1.732 (√3).
Worked example (BS 7671)
A 32 A radial circuit runs 45 metres in 6 mm² twin-and-earth copper cable on a 230 V single-phase supply. The tabulated value for 6 mm² flat twin copper is 7.3 mV/A/m:
- Voltage Drop = (7.3 × 32 × 45) ÷ 1000 = 10.51 V
- Percentage = 10.51 ÷ 230 × 100 = 4.57%
Result:within the 5% limit for a power circuit, but with little margin. A longer run, a sensitive load, or a future extension would push it over - so 10 mm² (4.4 mV/A/m) would be the safer specification.
Worked example (NEC)
A 20 A circuit feeds a load 120 feet away on a 120 V single-phase supply using 12 AWG copper (6530 circular mils):
- Voltage Drop = (2 × 12.9 × 20 × 120) ÷ 6530 = 9.48 V
- Percentage = 9.48 ÷ 120 × 100 = 7.9%
Result:nearly 8% - well above the recommended 3% for a branch circuit. Upgrading to 10 AWG (10380 circular mils) cuts the drop to about 5.0%, and 8 AWG brings it under 3.2%.
Acceptable voltage drop limits
NEC (United States)
The NEC does not mandate a hard voltage-drop limit in its enforceable text. Instead, informational notes to 210.19(A) (branch circuits) and 215.2(A) (feeders) recommend a maximum of 3% on a branch circuit or feeder, and 5% for the feeder and branch circuit combined, for reasonable efficiency of operation. Some local jurisdictions adopt these figures as enforceable requirements.
BS 7671 (United Kingdom)
BS 7671 Appendix 4 recommends a maximum voltage drop, between the origin of the installation and the load, of 3% for lighting circuits and 5%for all other circuits, on a public low-voltage supply (230 V nominal). That equates to 6.9 V for lighting and 11.5 V for power.
AS/NZS (Australia & New Zealand)
AS/NZS 3000 recommends a total voltage drop of 5% from the point of supply to any point in the installation, with consumer mains, submains and final subcircuits sharing that budget.
How to reduce voltage drop
- Increase the conductor size - the single most effective change, since resistance falls roughly in proportion to area.
- Shorten the run - relocate the board or take a more direct route.
- Split the load across multiple circuits so each carries less current.
- Raise the supply voltage where feasible (e.g. a three-phase distribution rather than single-phase).
Run the numbers instantly
Rather than working through these formulas by hand, use the free Voltix voltage drop calculator, which supports NEC, BS 7671 and AS/NZS and returns both volts and percentage. If the drop is too high, the right fix is usually a larger conductor - check it with the wire size calculator (NEC) or the cable sizing calculator for BS 7671 and AS/NZS. For a deeper dive into sizing methodology, read our NEC wire sizing guide and our BS 7671 cable sizing guide.