Voltage Drop Formula and Calculation Table: Quick Reference Guide

If you just need the voltage drop formula and calculation table without wading through a full guide this page gets straight to the point. You'll find the exact formulas for single phase, three phase and DC circuits, a clear breakdown of what each variable means and a ready to use resistance table for quick estimates without running the full calculation every time.

For the complete step by step method see our full guide on how to calculate voltage drop. This page focuses specifically on the formulas and tables you'll come back to again and again.

Key Takeaways

One Formula Covers Most Circuits

DC and single phase circuits use the same basic resistance based approach while balanced three phase circuits use a √3 multiplier. AC circuits with significant reactance may require impedance and power factor to be considered as well.

The Table Gives You a Fast Estimate

A voltage drop calculation table lets you look up approximate resistance values by wire size instantly without searching for a full resistance chart every time.

Always Confirm With the Full Calculation

A quick table estimate is useful for planning but the actual formula should always be used to confirm the final result before finalizing a cable size.

What Is the Voltage Drop Formula?

The voltage drop formula is the equation used to calculate exactly how much voltage is lost as current travels through a conductor, based on the current, the conductor's resistance and the cable length. It's built directly on Ohm's Law which states that voltage equals current multiplied by resistance.

Electricians and engineers use this formula constantly when sizing cables since it turns a physical property of the wire resistance into a concrete number they can compare against acceptable limits.

Voltage Drop Equation

Single Phase / DC: VD = 2 × I × R × L

Three Phase: VD = √3 × I × R × L

Where VD is the voltage drop in volts I is the current in amps R is the conductor's resistance per unit length and L is the one way cable length.

Single Phase vs Three Phase Formula

The single phase and DC version of the voltage drop formula calculation doubles the cable length since current has to travel out to the load and back through a return conductor.

The three phase version replaces that multiplier with √3 approximately 1.732 instead reflecting how current is shared across three conductors rather than two.

Understanding Each Variable in the Formula

Each part of the formula for voltage drop calculation represents a specific measurable property of your circuit and getting any one of them wrong will throw off the final result.

Current, Resistance and Cable Length Explained

Current I is the load's expected draw in amps usually taken from the equipment's rating or a load calculation. Resistance R depends on the wire's material and cross sectional size and is normally given as ohms per meter or ohms per foot.

Cable length L is measured as the one way distance between the source and the load which the formula itself accounts for when doubling or applying the √3 multiplier. For AC circuits with significant inductive or capacitive loads voltage drop can depend on both resistance and reactance with power factor affecting the result.

The simplified resistance based formula is suitable for many basic calculations but larger or more complex installations may require a full impedance based calculation. For most residential and general purpose calculations though the standard formula above gives an accurate result without needing to factor in power factor separately.

For more detailed examples involving branch circuits, see our guide on how to calculate voltage drop in a parallel circuit .

Voltage Drop Calculation Table

A voltage drop calculation table lets you skip looking up individual resistance values every time by giving you approximate figures for common copper cable sizes at a glance.

Wire Resistance Table by Size

How to Use the Table for a Fast Estimate?

Find your cable's size in the table, take the resistance value shown and plug it straight into the formula alongside your current and cable length.

This gives you a workable estimate in seconds though it's worth confirming with the manufacturer's exact resistance rating before finalizing a larger or more critical installation.

How to Calculate Voltage Drop Using the Formula?

Once you have your resistance value from the table, applying the formula takes just a few steps.

Worked Example Using the Formula

Suppose you have a 230V circuit carrying 15 amps through 30 meters of 2.5mm² copper cable. Using the table above 2.5mm² cable has a resistance of roughly 0.0074 ohms per meter.

Applying the formula voltage drop equals 2 multiplied by 15 amps multiplied by 0.0074 ohms per meter multiplied by 30 meters giving a voltage drop of 6.66 volts or about 2.9% of the 230V supply just within the recommended 3% limit for branch circuits.

Three Phase Formula Example

For a three phase example take a 400V circuit carrying 40 amps through 60 meters of 6mm² copper cable with a resistance of roughly 0.0031 ohms per meter.

Using the three phase voltage drop calculation formula voltage drop equals 1.732 multiplied by 40 amps multiplied by 0.0031 ohms per meter multiplied by 60 meters giving a voltage drop of about 12.9 volts or roughly 3.2% of the 400V supply.

For circuits where components are connected in a single path, see our guide on how to calculate voltage drop in a series circuit .

Common Mistakes When Using the Voltage Drop Formula

A couple of small errors show up repeatedly when people apply the formula or table incorrectly.

Using Table Values Without Checking the Cable Rating

Table values are approximate for standard copper conductors at room temperature so relying on them alone for a critical or high current installation without confirming the manufacturer's exact rating can produce a slightly inaccurate result.

Forgetting Which Formula Applies

Using the single phase multiplier of 2 on a three phase circuit or vice versa produces a noticeably wrong voltage drop calculation so always confirm the circuit type before applying the equation.

Ignoring Power Factor on Heavily Loaded AC Circuits

For most calculations the standard formula is accurate but on heavily loaded three phase industrial circuits ignoring power factor entirely can slightly understate the real voltage drop so it's worth checking with a qualified engineer on large commercial installations.

Conclusion

Having the voltage drop formula and a quick resistance table on hand makes cable planning significantly faster whether you're sizing a single circuit or comparing several options.

Just remember that table values are estimates for anything beyond a rough check run the full calculation with your cable's exact resistance rating.

If you'd rather skip the manual lookup entirely our free voltage drop calculator applies the correct formula and resistance values automatically giving you an instant accurate result.

FAQs

What is the formula for voltage drop calculation?

The standard formula is VD = 2 × I × R × L for single phase and DC circuits, or VD = √3 × I × R × L for three phase circuits where I is current, R is resistance per unit length and L is the one way cable length.

How do I use a voltage drop calculation table?

Look up your cable's size in the table to find its approximate resistance per meter or foot then use that value directly in the voltage drop formula along with your circuit's current and cable length.

Is the voltage drop formula the same for AC and DC?

DC and single phase AC circuits use the same basic formula but three phase AC circuits use a different multiplier √3 to account for how current is distributed across three conductors.

Are voltage drop tables accurate enough to use directly?

Voltage drop tables are accurate enough for planning and quick estimates but for critical installations it's best to confirm with the exact resistance rating from your cable manufacturer.

Can I get the voltage drop formula and table as a PDF?

We don't currently offer a separate PDF download but this page's formula and table are always kept up to date and you can print or save it directly from your browser for offline reference.

Does this formula work for 12V DC systems?

Yes the same VD = 2 × I × R × L formula applies to 12V DC circuits since 12V systems run at a lower voltage the same voltage loss represents a larger percentage so see our dedicated DC voltage drop calculator guide for 12V specific examples and limits.