Voltage Drop Calculator (IEC 60364 & NEC NFPA 70)
Voltage drop calculator with a single page covering BOTH IEC 60364-5-52 Annex G (metric mm² cables, A/B limits) and NEC NFPA 70 Chapter 9 Tables 8–9 (AWG / kcmil, 3% branch / 5% feeder FPNs). Copper / aluminium, 1-phase / 3-phase / DC, temperature correction, parallel conductors, conduit material (NEC), reactance X and ampacity cross-check.
Switch between IEC metric (mm², metres, Annex G A/B limits) and NEC imperial (AWG / kcmil, feet, 3%/5% FPN recommendations).
Voltage Drop Calculator
Metric cable sizes. Enter the circuit, get the drop in volts and percent, checked against the IEC limit.
Line to neutral · typical 1φ 230 V
IB, not the breaker rating
≈ 12 AWG
One way, not the loop
0.8 if you don't know it
Advanced
0.08 is the default when no cable data is available
Derated value from IEC 60364-5-52 Annex B or the cable datasheet
- Drop
- 5.46 V
- Voltage at load
- 224.5 V
- Permitted
- 5.00 %
- Power lost
- 135 W
Calculated with the Annex G method: resistivity in service taken as 1.25 × the value at 20 °C. Ampacity, protection and fault withstand are separate checks.
Overview
Excessive voltage drop can cause motors to overheat, lights to dim, and protective devices to misbehave. Most standards (IEC 60364-5-52, NEC NFPA 70, HD 60364…) cap the drop at around 3 to 5 percent of the nominal voltage depending on whether the circuit is a feeder or a final sub-circuit.
Use the IEC / NEC toggle at the top of the tool to switch between the two standard families: the IEC 60364 (Annex G) tab works with metric mm² cross-sections, lengths in metres, and the classic A / B drop limits (with a distance bonus for runs longer than 100 m). The NEC NFPA 70 (Chapter 9) tab uses AWG / kcmil conductor sizes, lengths in feet, Chapter 9 Tables 8–9 impedance at your power factor, and the traditional 3% branch / 5% feeder FPN benchmarks for guidance.
Before specifying a cable, always run this check against the cross-section chosen by ampacity. Very often the cable sizing tool will recommend a larger size than ampacity alone because of long distance.
How it works
- 1. Pick the standard familyUse the IEC 60364 / NEC NFPA 70 toggle at the top of the tool to switch between metric (mm², metres) and North-American (AWG/kcmil, feet) input units.
- 2. Define the systemDC, single-phase or three-phase and the nominal voltage (230 V, 120/240 V, 277/480 V… whatever applies to your circuit).
- 3. Set line and loadCurrent A, length, cross-section or AWG size, cos φ for AC, and material (Cu/Al). In NEC mode also pick conduit and coating; in IEC mode set the Annex G installation class.
- 4. Read drop % vs the allowed limitIEC result is compared against Annex G classes A/B with optional >100 m allowance. NEC result is compared against the 3% branch / 5% feeder FPN guidance. Iterate the size if needed.
FAQ
What is the allowed voltage drop?+
IEC 60364-5-52 Annex G uses classes A (3% lighting / 5% other) and B (6% / 8%), with a distance bonus for runs >100 m. NEC FPNs (non-enforceable guidance) recommend ≤ 3% on branch circuits and ≤ 5% total feeder + branch.
Do you include reactance?+
The IEC tab lets you set X mΩ/km (default 0.08) and the NEC tab reads effective impedance (R · cos φ + X · sin φ) from Chapter 9 Table 9 at the actual power factor. For very long or HV cables reactance matters most.
Should I use line-neutral or line-line voltage?+
Enter the nominal as labelled. For three-phase systems (both IEC and NEC) the calculator applies the √3 factor internally to derive the line-drop formula.
Are NEC voltage-drop limits enforceable?+
No — voltage drop in NEC is Fine-Print Note (FPN) guidance, not enforceable text. Local jurisdictions or project specifications often tighten or enforce them anyway.
Why metres vs feet / mm² vs AWG?+
IEC is metric (mm², metres) and is used in EU, Africa, ME, APAC. NEC uses AWG / kcmil and feet (North America). The toggle at the top switches the input units and the whole calculation recipe.
Why feet instead of metres in NEC?+
NEC NFPA 70 is written in imperial; lengths are traditionally entered in one-way feet and the per-run 2·L or √3·L factor is applied to obtain the full loop or three-phase drop.