Voltage Drop Calculator

Last updated: Jul 22, 2026 | Reviewed by Calkulator Team | 2 min read

Science

Check if your electrical wiring can handle the load

Long wire runs cause voltage drop — a 20A load on 30 metres of 2.5mm² cable at 230V drops about 5.2V (2.3%). NEC standards recommend keeping voltage drop below 3% for branch circuits and 5% total.

Use a thicker wire gauge for long runs — doubling the wire cross-section halves the voltage drop.

Wire Gauge (AWG)

Wire Length (meters)

Load Current (Amperes)

Supply Voltage (V)

Phase

Voltage Drop Analysis

Voltage Drop

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Voltage at Load

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Drop Percentage —

Wire Resistance (Ω/km) —

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Live Result Illustration

Visual summary — updates instantly as you enter values above

LIVE

Real-Life Guide to Using the Voltage Drop Calculator

Wire voltage drop by length. Use the examples and checks below to turn the number into a practical decision.

When this calculator is useful

Used by electricians and DIYers sizing wire gauge for long cable runs, to confirm that equipment at the far end of the run still receives enough voltage to operate correctly.

For most people, the best way to use the Voltage Drop Calculator is to try the real case first, then change one input at a time. That makes the trade-off visible. For example, with a loan calculator you can change tenure while keeping the same rate; with an investment calculator you can change return assumption while keeping the same monthly contribution; with a health, education or measurement calculator you can check how much one input changes the final category.

The result should answer a practical question: Can I afford this? How much should I save? Is this score enough? Is this measurement within range? What is the safer or cheaper option? If the output does not answer the decision clearly, adjust the inputs until the scenario matches your real situation.

Real-Life Example

Sizing cable for a water pump: A contractor is running 230V single-phase power 40 metres to a motor that draws 10 A.

1Using 2.5 mm² copper cable (about 7.41 mΩ per metre per conductor) over a round-trip length of 80 m: R = 80 × 0.00741 = 0.593 Ω. Voltage drop = I × R = 10 × 0.593 = 5.93 V, which is 5.93 / 230 × 100 ≈ 2.58% of supply voltage.

2Now change one input, such as rate, time, quantity, unit or score, and compare the new result with the first one.

At 2.58% the drop is within the commonly accepted 3% limit, but a longer run or a higher current draw would push it over and require stepping up to a thicker cable.

Practical Advice

Use the Voltage Drop Calculator as a planning tool, not just a number generator. Write down the inputs you used, because the final answer is meaningful only when you remember the assumptions behind it.

If the decision affects money, health, tax, safety, academics or legal compliance, keep a second check ready. That second check may be a bank quote, payslip, official rule, prescription, site measurement, mark sheet or invoice.

Common Mistakes

Entering the one-way cable length instead of the full circuit length — current travels out to the load and back through the return conductor, so a 40 m run actually uses 80 m of wire.
Using copper resistance values for what is actually an aluminium conductor, which has roughly 60% higher resistivity and will understate the real voltage drop.
Ignoring that conductor resistance rises with temperature, so a cable running hot under sustained load will drop slightly more voltage than the cold-resistance figure suggests.
Applying the plain resistive voltage-drop formula to an AC circuit with a poor power factor without adjusting for the reactive component of the load.
Calculating the drop percentage against the wrong reference voltage — dividing by the drop itself instead of the supply voltage inflates or shrinks the reported percentage.

How to Interpret Results

Compare the percentage drop to standard guidance of keeping voltage drop under roughly 3–5% for a branch circuit — anything higher means the wire gauge should be increased before installation.

A good interpretation looks at both the main result and the supporting values. If a page shows totals, ratios, categories, schedules or warnings, read those together instead of focusing only on the biggest number.

Voltage Drop Calculator FAQs

Useful answers for interpreting the output, avoiding mistakes and using the result responsibly.

What is voltage drop and why does it matter?

It is the voltage lost across a wire's own resistance as current flows through it, calculated as V_drop = I × R_wire. Too much drop means equipment at the far end receives less voltage than it needs to run properly.

Why do I need to double the cable length in the calculation?

Current flows out to the load through one conductor and returns through another, so the electrical path is twice the physical distance between source and load — a 40 m run means 80 m of total conductor length.

Does it matter if the wire is copper or aluminium?

Yes — aluminium has a resistivity about 1.6 times higher than copper, so an aluminium cable of the same size will show a noticeably larger voltage drop for the same current and length.

What voltage drop percentage is considered acceptable?

A commonly used guideline keeps total voltage drop under about 3% for lighting circuits and up to 5% for power circuits, though local electrical codes may specify their own limits.

How does temperature affect the result?

Conductor resistance increases with temperature, so a cable that heats up under sustained heavy load will have a slightly higher actual resistance — and therefore a slightly higher drop — than its rated resistance at 20°C.

Does wire gauge or cross-sectional area change the outcome?

Yes, significantly — doubling the cross-sectional area roughly halves the resistance per metre, which directly halves the voltage drop for the same current and length.

Can this be used for AC circuits with motors?

It gives a good resistive estimate, but motors and other inductive loads have a power factor less than 1, meaning the true drop under AC conditions can be somewhat higher than the simple resistive calculation shows.

What should I do if the drop percentage is too high?

Increase the wire's cross-sectional area (a thicker gauge), shorten the cable run if possible, or split the load across a dedicated circuit closer to the source.

Voltage Drop Explained

Voltage drop occurs when current flows through the resistance of a wire. NEC (National Electrical Code) recommends maximum 3% drop for branch circuits and 5% total (feeder + branch). Excessive voltage drop causes equipment to run hotter, reduces efficiency, and can damage motors.

For single-phase: VD = 2 × L × R × I (factor of 2 for live + neutral). For three-phase: VD = √3 × L × R × I. Use larger wire gauges for long runs or high-current loads. Going from 14 AWG to 12 AWG reduces resistance by ~37%.

Example

12 AWG, 30m run, 20A, 230V single-phase:

1R = 5.21 Ω/km → 0.00521 × 30 × 2 × 20

2VD = 6.25 V (2.7%)

✓ Within 3% NEC limit

Frequently Asked Questions

What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage along a wire due to its resistance. For every metre of wire carrying current, V_drop = I × R_wire. Excessive voltage drop causes equipment to run at reduced voltage — motors run hotter, lights dim, and electronics malfunction. NEC and IS standards limit voltage drop to 3% for branch circuits and 5% total (feeder + branch combined).

How do I choose the correct wire gauge?

Use a larger wire (lower gauge number = larger diameter) for longer runs or higher current loads. If the calculator shows voltage drop exceeding 3%, upsize the wire (e.g., from 2.5 mm² to 4 mm²). In India, IS 694 specifies standard copper conductor sizes: 1, 1.5, 2.5, 4, 6, 10, 16, 25 mm² are the common options.

What is the difference between single-phase and three-phase voltage drop?

For single-phase: V_drop = 2 × I × R × L (current travels to the load and back). For three-phase: V_drop = √3 × I × R × L. Three-phase distribution is more efficient — for the same power and voltage drop, three-phase requires less copper than single-phase. This is why industrial and commercial installations use three-phase power.