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Voltage Drop Calculator

Calculate voltage drop percentage and verify compliance with permissible limits across 4 international standards.

AS/NZSBSIECNEC
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Voltage Drop Limits

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Voltage drop is the reduction in electrical potential along a conductor caused by its impedance when current flows through it. IEC 60364-5-52 Clause 525 limits permissible voltage drop to ensure equipment operates within rated tolerances. It is calculated using the formula Vd equals mV per amp per metre multiplied by design current and cable length.

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How to Calculate Voltage Drop

  1. 1
    Gather circuit parameters — Record the design current in amperes, cable length in metres, supply voltage, number of phases, and the power factor of the load. These are the inputs required for the voltage drop formula.
  2. 2
    Find the mV/A/m value — Look up the millivolt drop per ampere per metre from the cable manufacturer's data or standard tables. This value depends on conductor size, material, and whether the circuit is AC or DC.[IEC 60364-5-52 Clause 525]
  3. 3
    Apply the voltage drop formula — Calculate Vd = (mV/A/m x Ib x L) / 1000 for single-phase, using the single-phase mV/A/m value. For three-phase balanced loads, use the three-phase mV/A/m value with the same formula (Vd = mV/A/m x Ib x L / 1000) — these values already account for the phase relationship. The result is in volts.
  4. 4
    Convert to percentage — Express the voltage drop as a percentage of the nominal supply voltage: Vd% = (Vd / Vnom) x 100. This allows direct comparison against the allowable limit for the installation.
  5. 5
    Compare against allowable limit — Check the result against the applicable standard limit. BS 7671 and IEC 60364 typically allow 5% total, while NEC recommends 3% for branch circuits and 5% total for feeder plus branch combined.[BS 7671 Regulation 525.1]
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How Voltage Drop Works

The voltage drop calculator determines the voltage loss along a cable run and verifies it falls within the permissible limits defined by the applicable standard.

Voltage drop is calculated using the formula Vd = (mV/A/m x Ib x L) / 1000, where mV/A/m is the millivolt drop per ampere per metre (from cable manufacturer data or standard tables), Ib is the design current in amperes, and L is the one-way cable route length in metres. For three-phase circuits, the formula uses three-phase mV/A/m values directly; for single-phase circuits, the single-phase mV/A/m values apply.

The mV/A/m values account for both resistive and reactive components of the cable impedance. At higher power factors, the resistive component dominates, while at lower power factors the reactive component becomes significant — particularly for larger conductor sizes. AS/NZS 3008.1.1:2017 provides these values in Tables 35-42 for various conductor and insulation types.

Each standard specifies different permissible limits. BS 7671:2018+A2 Regulation 525.1 recommends a maximum of 3% for lighting circuits and 5% for other circuits from the origin of the installation. IEC 60364-5-52 Clause 525 provides similar guidance. NEC/NFPA 70:2023 Section 210.19(A) Informational Note No. 4 recommends 3% for branch circuits and 5% total (feeder plus branch circuit), though these are advisory rather than mandatory. AS/NZS 3008.1.1:2017 Clause 4.5 limits the total voltage drop to 5% from the point of supply.

Where voltage drop exceeds limits, the calculator recommends the next cable size up. The results display the voltage at the load end, percentage drop, comparison against the standard limit, and a breakdown of resistive versus reactive voltage drop components. For long runs or heavily loaded circuits, voltage drop often governs cable selection over current carrying capacity alone.

Maximum Permissible Voltage Drop by Standard

StandardLightingOther CircuitsTotal Limit
BS 76713%5%From origin
IEC 603644%5%From distribution board
NEC (NFPA 70)3% (recommended)3% branch5% total
AS/NZS 30085%5%From point of supply

Source: BS 7671 Appendix 12, IEC 60364-5-52 Clause 525, NEC 210.19(A) Note 4, AS/NZS 3008.1.1 Clause 4.5

Frequently Asked Questions

What is the maximum voltage drop allowed per BS 7671?
BS 7671:2018+A2 Regulation 525.1 and Appendix 12 recommend maximum voltage drop limits of 3% for lighting circuits and 5% for other circuits, measured from the origin of the installation to the load terminals. These limits apply to the installation wiring only and do not include the voltage drop in the distributor's supply cable. For public supply installations, a total of 3% or 5% respectively from the meter is the guideline.
How do you calculate voltage drop using the mV/A/m method?
The mV/A/m method uses tabulated values from standard tables (AS/NZS 3008.1.1 Table 35-42, or BS 7671 Table 4Ab-4Db). Voltage drop in millivolts is calculated as VD = (mV/A/m) x Ib x L / 1000 for single-phase, and VD = (mV/A/m) x Ib x L x 0.577 / 1000 for three-phase circuits, where Ib is the design current in amperes and L is the cable route length in metres. The tabulated mV/A/m value already accounts for both conductor resistance and reactance at the given power factor.
What voltage drop limit does AS/NZS 3008.1.1 specify?
AS/NZS 3008.1.1:2017 Clause 4.5 states that the total voltage drop from the point of supply to any point in the installation shall not exceed 5% of the nominal supply voltage. For a 230V single-phase supply this means a maximum drop of 11.5V, and for a 400V three-phase supply the limit is 20V line-to-line. Subcircuit limits may be more restrictive per AS/NZS 3000:2018 Table C7.
How does power factor affect voltage drop calculations?
Power factor significantly affects voltage drop because cables have both resistance (R) and reactance (X) components. The effective impedance per unit length is Zeff = R cos(phi) + X sin(phi), where phi is the phase angle. At low power factors (e.g., 0.8 lagging for motor loads), the reactive component contribution increases, and for larger cables where X becomes proportionally significant, the voltage drop can be substantially higher than what resistance alone would predict. NEC Chapter 9 Table 9 provides both R and X values for this purpose.
What is the NEC voltage drop recommendation?
Unlike BS 7671 and AS/NZS 3008, the NEC does not mandate a specific voltage drop limit as a code requirement. However, NEC 210.19(A) Informational Note No. 4 and NEC 215.2(A) Informational Note No. 2 recommend that voltage drop for branch circuits should not exceed 3%, and the total drop from feeder plus branch circuit should not exceed 5%. While informational notes are not enforceable, most jurisdictions and engineers treat these as design targets.
How do you account for cable length in a voltage drop calculation?
Cable length used in voltage drop calculations must be the actual route length of the cable, not the straight-line distance. This includes vertical risers, horizontal runs, bends, and spare length at termination points. For single-phase circuits, the total conductor length is twice the route length (active + neutral). For three-phase balanced loads, the route length is used directly with the three-phase mV/A/m values, since neutral current is zero and only line conductors contribute to voltage drop.
Is it true that at high power factor, a larger cable can produce more voltage drop than a smaller one on the same circuit?
No, this is a persistent myth. The mV/A/m values in AS/NZS 3008.1.1:2017 Table 42 have separate resistance (r) and reactance (x) components. As cable size increases, r decreases but x stays roughly constant (around 0.08 ohm/km). The reactive component acts as a floor that no amount of upsizing can reduce. For a 400 A, 200 m three-phase circuit at 0.95 power factor, 185 mm2 Cu gives 10.89 V drop while 240 mm2 Cu gives 8.82 V. The larger cable is still better, but the improvement is only 19% rather than the 30% the resistance reduction alone would suggest. At unity power factor the reactive term vanishes, so upsizing is always beneficial, but returns diminish rapidly above 150 mm2 where x approaches r in magnitude.
AS/NZS 3008 allows 5% voltage drop from origin to final subcircuit, but BS 7671 gives 3% for lighting and 5% for other uses. What happens when both standards apply on the same international project?
The hidden trap is in the definition of 'origin.' Under BS 7671, the origin is typically the supply terminals, excluding the supply authority's cable. Under AS/NZS 3008, the 'point of supply' includes the supply authority connection point per AS/NZS 3000 Clause 1.4.108. For a building with a 150 m submain plus a 30 m final subcircuit, AS/NZS gives a single 5% budget for the entire 180 m path. BS 7671 might split the budget as 1.5% for the submain and 1.5% for a lighting final circuit within the 3% lighting budget. A 20 A single-phase lighting circuit at 30 m using 2.5 mm2 cable gives 10.86 V (passes AS/NZS at 5%), but the same cable fails BS 7671 at 3% (allowable 6.9 V). You need 4 mm2 to comply under BS 7671.
Why does the voltage drop formula give incorrect results for long single-phase circuits with power-factor-corrected loads, and what adjustment is needed?
The standard mV/A/m tables assume a fixed conductor temperature (typically 75 degrees C). When a circuit carries less than its rated current, the actual temperature is lower and the resistance is lower than the tabulated value. The more subtle issue arises with power-factor-corrected loads. When capacitors correct PF from 0.8 to 0.95, the current drops to 84.2% of original, but since Vd is proportional to I x (r cos phi + x sin phi), the r cos phi term increases when cos phi rises, partially offsetting the current reduction. In a worked example with 100 A at 0.8 pf over 100 m on 35 mm2 Cu, the current drops by 15.8% after PFC but voltage drop improves by only 4.9%. For long runs where voltage drop governs, PFC at the load provides surprisingly little benefit compared to upsizing the cable.
How does the diversity factor interact with voltage drop calculations for sub-distribution boards, and why does ignoring it cause oversized mains?
Many engineers calculate voltage drop on the submain using full maximum demand and then separately calculate each final subcircuit at its individual full load, effectively double-counting the submain drop. The correct approach under AS/NZS 3008 Clause 4.3 is to determine total voltage drop from source to the most remote outlet. For a submain carrying a diversified load of 200 A, the submain voltage drop attributable to a specific 20 A lighting circuit is only 20/200 multiplied by the submain drop. Using the full 200 A submain drop plus the full 20 A subcircuit drop is conservative but can push you to unnecessarily large submain cables. IEC 60364-5-52 Annex G acknowledges this by permitting relaxation where diversity is applied. On projects with 100 m submains, this can mean the difference between 95 mm2 and 70 mm2.
At what cable length does the voltage drop constraint, rather than current capacity, start governing the cable size, and why does this crossover point shift dramatically between standards?
The crossover point is calculated directly: L_max = (Vd_allow x 1000) / (I x mV_per_A_per_m). For a 32 A single-phase circuit at 230 V using 6 mm2 Cu with BS 7671's 3% lighting limit: L_max = 29.5 m. Under AS/NZS 3008 with a 5% limit: L_max = 49.2 m. The crossover point almost doubles because of the more generous voltage drop budget. For three-phase 400 V circuits the effect is even more pronounced because the higher voltage gives a larger absolute drop allowance. A 100 A three-phase circuit using 25 mm2 Cu at 5% gives L_max = 111 m, while at 3% for lighting L_max = 66.7 m. The 2% difference between BS 7671 and AS/NZS limits matters enormously for project cable costs. On a typical 20-storey commercial building, the more restrictive BS 7671 lighting limit can add 10-15% to total cable cost for vertical risers.

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Standards Reference

  • AS/NZS 3008.1.1:2017 — Clause 4.5, Tables 35-42
  • BS 7671:2018+A2 — Regulation 525.1, Appendix 12
  • IEC 60364-5-52 — Clause 525
  • NEC/NFPA 70:2023 — Section 210.19(A), Chapter 9 Table 9