Voltage Drop Calculator
Calculate voltage drop across electrical conductors for any circuit. Select your electrical standard (NEC, BS 7671, or AS/NZS), enter your cable length, conductor size, and load current - and get instant voltage drop in volts and percentage. Free for any circuit type.
Voltage drop
10.51V
Percentage drop
4.57%
Acceptable (3–5%)
Acceptable for general circuits but close to the limit - consider a larger conductor for long runs or sensitive loads.
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How voltage drop works
Voltage drop occurs when current flows through a conductor - the longer or thinner the cable, the higher the drop. In a 230 V circuit, a 3% voltage drop represents 6.9 V lost before it reaches your load. This matters for motor starting, lighting performance, and compliance with your wiring standard.
The formula (BS 7671 / AS/NZS)
Voltage Drop (V) = (mV/A/m × Current × Length) ÷ 1000
The mV/A/mfigure (millivolts per amp per metre) comes from the cable manufacturer's tables and depends on the conductor material and cross-sectional area. Three-phase circuits use a factor of 0.866 (√3 ÷ 2).
The formula (NEC)
Voltage Drop (V) = (2 × K × Current × Distance) ÷ Circular Mils
Where K = 12.9 for copper and 21.2 for aluminium, distance is the one-way run in feet, and the factor of 2 accounts for the out-and-back path of a single-phase circuit (three-phase uses 1.732).
Worked example
A 32 A circuit runs 45 metres of 6 mm² twin-and-earth copper cable at 230 V single phase (BS 7671). The mV/A/m figure for 6 mm² copper is 7.3:
- Voltage Drop = (7.3 × 32 × 45) ÷ 1000 = 10.51 V
- Percentage = 10.51 ÷ 230 = 4.57%
Result:acceptable, but close to the 5% BS 7671 limit. For a longer run or sensitive load, consider upgrading to 10 mm².
Frequently Asked Questions
- What is the maximum voltage drop allowed by BS 7671?
- BS 7671:2018 recommends a maximum voltage drop of 3% for lighting circuits and 5% for all other circuits, measured between the origin of the installation and the load.
- What does the NEC say about voltage drop?
- The NEC does not mandate a specific voltage drop limit but recommends (as an informational note) a maximum of 3% for branch circuits and 5% combined for feeders and branch circuits, for efficiency reasons.
- Does voltage drop affect equipment performance?
- Yes. Motors may fail to start or overheat if voltage is too low. LED drivers and sensitive electronics can malfunction. Always check voltage drop for long cable runs or high-current loads.
- How do I reduce voltage drop?
- Use a larger conductor size, shorten the cable run, increase the supply voltage, or split the load across multiple circuits. The Voltix app lets you compare options side by side.
- Is voltage drop the same as voltage loss?
- They're used interchangeably. Technically, voltage drop refers to the potential difference across the conductor's resistance - this energy is converted to heat in the cable rather than delivered to the load.
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