Voltage Drop Calculator

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Free voltage drop calculator with NEC and IEC modes: get percent drop, wire size in AWG or mm², power loss and a 3% branch / 5% total check in seconds.

Inputs

Code / units

System

Current (A)

System voltage (V)

One-way length (m)

Conductor cross-section (mm²)

Conductor material

Conductor operating temperature (°C)

Permitted drop (%)

Parallel runs

How to use this calculator: Voltage Drop Calculator

Free voltage drop calculator with NEC and IEC modes: get percent drop, wire size in AWG or mm², power loss and a 3% branch / 5% total check in seconds. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Voltage Drop Calculator matches the quantity or design check you need.
2Enter Current, System voltage, and One-way length using the units printed beside each field.
3Select the applicable Code / units, System, and Conductor material options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Code / units	IEC / IS (metric) — mm², metres	Select the option that matches the real installation or scenario.
System	AC three phase	Select the option that matches the real installation or scenario.
Current	50 A	Measured or known current used by the calculation engine.
System voltage	415 V	Measured or known system voltage used by the calculation engine.
One-way length	80 m	Measured or known one-way length used by the calculation engine.
Conductor cross-section	25 mm²	Measured or known conductor cross-section used by the calculation engine.
Conductor material	Copper	Select the option that matches the real installation or scenario.
Conductor operating temperature	70 °C	Measured or known conductor operating temperature used by the calculation engine.
Permitted drop	3 %	Measured or known permitted drop used by the calculation engine.
Parallel runs	1	Measured or known parallel runs used by the calculation engine.

Formula inputs & variables for Voltage Drop Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Code / units	—	Select the option that matches the real installation or scenario.
System	—	Select the option that matches the real installation or scenario.
Load current	A	Measured or known load current used by the calculation engine.
System voltage	—	Select the option that matches the real installation or scenario.
One-way run length	ft	Measured or known one-way run length used by the calculation engine.
Wire size	—	Select the option that matches the real installation or scenario.
Conductor	—	Select the option that matches the real installation or scenario.
Insulation / termination rating	—	Select the option that matches the real installation or scenario.

Formula, derivation and worked example

Voltage drop is the volts you lose in the wire between the panel and the load, and it turns into heat. In NEC mode this calculator works the way US electricians do: pick the wire in AWG or kcmil, enter the one-way run in feet, and it checks both the Table 310.16 ampacity (after ambient and bundling corrections) and the 3% branch-circuit / 5% total voltage drop recommendation from the NEC informational notes. Metric mode uses IEC 60364 resistivity at operating temperature for mm² cable. In both modes long runs are usually governed by voltage drop, not by heating.

NEC: Vd = 2 × I × R (1-phase/DC) or √3 × I × R (3-phase), R = Ω/kft × ft ÷ 1000 ÷ sets

NEC ampacity = Table 310.16 value × ambient correction × bundling adjustment

IEC: ρ_T = ρ₂₀ × (1 + α(T − 20))

IEC: ΔV = 2 × ρ_T × L × I / A (DC / 1-ph), √3 × ρ_T × L × I / A (3-ph)

Loss = k × I² × R, ΔV% = ΔV / V × 100

Substitution steps

1. Resistivity at temperature
ρ₂₀ × (1 + α(T − 20))
= 0.02058 Ω·mm²/m
2. Effective area
A × parallel runs
= 25 mm²
3. Voltage drop
√3 × ρ × L × I / A
= 5.703 V
4. Percentage drop
ΔV / V × 100
= 1.374%
5. Power loss
3 I²R
= 493.92 W

Computed example results

Voltage drop: 5.7 V; 1.37% of 415 V
Status: ✅ Within 3% limit
Voltage at the load: 409.3 V
Conductor resistance: 65.86 mΩ; ρ at 70 °C = 0.02058 Ω·mm²/m
Power loss in cable: 493.9 W; 4,327 kWh/year at full load
Minimum size for the limit: 11.45 mm² → use 16 mm²

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Voltage Drop Calculator

Do not mix units for Current (A), System voltage (V), One-way length (m). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Code / units on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed NEC: Vd = 2 × I × R (1-phase/DC) or √3 × I × R (3-phase), R = Ω/kft × ft ÷ 1000 ÷ sets relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Voltage drop = 5.7 V from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Voltage Drop Calculator is useful

Voltage Drop Calculator is designed for cases where Code / units, System, Current, System voltage are known and you need Voltage drop, Status, Voltage at the load. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is NEC: Vd = 2 × I × R (1-phase/DC) or √3 × I × R (3-phase), R = Ω/kft × ft ÷ 1000 ÷ sets. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Code / units and System: what changes the answer

The worked example uses Code / units = IEC / IS (metric) — mm², metres, System = AC three phase, Current = 50 A, System voltage = 415 V. With those values, Voltage drop is 5.7 V. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Code / units: Select the option that matches the real installation or scenario. Available choices include NEC (US) — AWG, feet, 120/240/480 V, IEC / IS (metric) — mm², metres. System: Select the option that matches the real installation or scenario. Available choices include DC (2-wire), AC single phase, AC three phase. Current (A): Measured or known current used by the calculation engine. System voltage (V): Measured or known system voltage used by the calculation engine.

How to sanity-check a Voltage Drop Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Voltage drop, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Wire Size Calculator (AWG & mm²)

Useful next check because both tools use System voltage and Load current, while Wire Size Calculator (AWG & mm²) answers a different part of the same workflow.

Open Wire Size Calculator (AWG & mm²)

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Formula

NEC: Vd = 2 × I × R (1-phase/DC) or √3 × I × R (3-phase), R = Ω/kft × ft ÷ 1000 ÷ sets
NEC ampacity = Table 310.16 value × ambient correction × bundling adjustment
IEC: ρ_T = ρ₂₀ × (1 + α(T − 20))
IEC: ΔV = 2 × ρ_T × L × I / A (DC / 1-ph), √3 × ρ_T × L × I / A (3-ph)
Loss = k × I² × R, ΔV% = ΔV / V × 100

Voltage Drop Formula Guide

Learn the voltage-drop equation, variable units, percentage-drop method, worked examples and common cable-calculation mistakes.

Worked example

1NEC: 30 A at 240 V, 100 ft one-way, copper THWN, 75 °C column, 86 °F ambient.
210 AWG is 1.21 Ω per 1000 ft → R = 1.21 × 100 / 1000 = 0.121 Ω.
3Vd = 2 × 30 × 0.121 = 7.26 V = 3.03% — just over 3%, so step up to 8 AWG (4.58 V, 1.9%).

Assumptions

NEC resistance from Chapter 9 Table 8 (uncoated stranded conductors at 75 °C); metric mode uses resistivity corrected to the entered operating temperature.
Resistive drop only; conductor reactance is neglected, valid up to about 4/0 AWG / 120 mm².
Balanced three-phase loading and uniform conductor temperature.

Tips

Going up one AWG size drops the voltage drop by roughly 20%; going up two sizes cuts it by about 37%.
Doubling the run length doubles the drop — measure the actual wire path, not the straight-line distance.
Aluminum needs roughly two sizes larger than copper for the same drop.

Warnings

The 3% and 5% voltage drop figures are NEC informational notes, not enforceable code — local amendments can be stricter.
Check terminal ratings under NEC 110.14(C): the 90 °C column is for correction factors only unless every termination is listed for 90 °C.
For conductors above 4/0 AWG on long runs, include reactance — the drop can be 10–20% higher than the resistive value.

Standards & references

NEC 310.16 (ampacity)
NEC 310.15(B)(1) & (C)(1) (corrections)
NEC 240.4(D)
NEC 210.19(A) / 215.2(A)
NEC Chapter 9 Table 8
IEC 60364-5-52 Annex G
IS 732:2019

Frequently asked questions

What is the maximum voltage drop allowed by the NEC?

The NEC does not enforce a limit, but informational notes to 210.19(A) and 215.2(A) recommend a maximum of 3% on a branch circuit and 5% for the feeder and branch circuit combined. This calculator checks against whichever target you select.

What size wire do I need for a 100 ft run at 30 amps?

At 240 V single phase, 10 AWG copper gives about 3.0% drop — right at the limit — so most electricians use 8 AWG, which comes in near 1.9%. At 120 V the same 30 A load over 100 ft needs 6 AWG to stay under 3%.

How do I calculate voltage drop in AWG wire?

Vd = 2 × I × R for single-phase or DC, and √3 × I × R for three-phase, where R is the NEC Chapter 9 Table 8 resistance in ohms per 1000 ft multiplied by the one-way run in feet divided by 1000. This tool does it, plus the ampacity check.

Does the NEC allow the 90 °C column for sizing?

Only for applying ambient and bundling correction factors. Under 110.14(C) the final ampacity is limited to the 60 °C column for circuits at 100 A or less, and the 75 °C column above that, unless every termination is rated 90 °C.

What is an acceptable voltage drop under IEC rules?

3% for lighting circuits and 5% for power circuits measured from the origin of the installation is the widely accepted limit in IEC-based codes such as IEC 60364 and IS 732.

Does voltage drop waste energy?

Yes. The dropped voltage times the current is dissipated as heat in the wire — this calculator reports that loss in watts and kWh per year.

What inputs does the Voltage Drop Calculator use?

It uses Code / units, System, Current, System voltage, One-way length, Conductor cross-section, Conductor material, Conductor operating temperature, Permitted drop, and Parallel runs. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Voltage Drop Calculator calculate?

It calculates Voltage drop, Status, Voltage at the load, Conductor resistance, Power loss in cable, and Minimum size for the limit. With the displayed default inputs, Voltage drop is 5.7 V.

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