Voltage Drop Calculator

Estimate voltage drop, end voltage, and circuit resistance for single-phase/DC or three-phase AC runs based on conductor size, material, current, and distance.

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Practical Guide

Voltage Drop Calculator: Design, Limits, and Real-World Use

Use the Voltage Drop Calculator like an engineer: select the right system type and conductor, enter realistic load and distance values, interpret percent voltage drop, and decide when to upsize cable, change layouts, or adjust system voltage.

8–10 min read Updated 2025

Quick Start with the Voltage Drop Calculator

This section mirrors how the Voltage Drop Calculator on this page is set up. Follow these steps in order to avoid the most common mistakes.

  1. 1 At the top of the calculator, choose the correct Phase / System type: DC, single-phase 2-wire (line and neutral), single-phase 3-wire, or three-phase (3-wire or 4-wire). This determines which base voltage and equation the tool uses.
  2. 2 Immediately under that, select the conductor material (typically copper or aluminum). This changes the resistivity \( \rho \) used in the resistance calculation and directly affects voltage drop.
  3. 3 Enter the source voltage (line-to-line or line-to-neutral, as appropriate for the system type), the load current or circuit ampere rating, and the one-way run length from the source to the load. The calculator automatically applies the correct path length factor (for example \( 2L \) for single-phase).
  4. 4 Specify the conductor size using the input format the calculator expects (for example, cross-sectional area in mm² or an equivalent value). Make sure your entry matches an actual cable size you could purchase and install.
  5. 5 Hit Calculate. The calculator computes: the absolute voltage drop \( \Delta V \), the percent voltage drop \( \%VD \), and the voltage at the load \( V_{\text{load}} = V_s – \Delta V \).
  6. 6 Compare the percent voltage drop to your criteria. Many guides use about 3 % maximum for branch circuits and 5 % total from service to the farthest load. Adjust cable size, length, or system voltage until the result meets your project limits.
  7. 7 Expand the Quick Stats and Calculation Steps panels under the calculator to see the equations, substitutions, and unit conversions used. This is helpful for design notes and peer review.

Tip: Treat the Voltage Drop Calculator like a design knob: keep everything fixed and vary one input (current, length, or conductor size) at a time. This shows you which lever gives the biggest improvement.

Warning: Make sure the system voltage in the calculator matches the actual configuration. Using a 480 V line-to-line value when the load is 277 V line-to-neutral will give misleadingly low percent drop.

Choosing Your Voltage Drop Method

The underlying math in the Voltage Drop Calculator is based on conductor resistance and (optionally) impedance. Different design situations favor different ways of applying the same physics.

Method A — Physics-Based Resistance from Conductor Size

This is the default approach: the calculator computes resistance from length, cross-sectional area, and material resistivity, then applies a single-phase or three-phase voltage drop equation.

  • Works in both metric and imperial units, for any reasonable conductor size.
  • Transparent: you can show every step from \( \rho \) to \( R \) to \( \Delta V \).
  • Good for early design, when you are still deciding between copper and aluminum or different sizes.
  • By default, it considers resistance only; reactance is ignored unless you explicitly model it.
  • Requires that the entered cross-sectional area actually corresponds to an available cable size.
\( R = \rho \dfrac{L}{A} \Rightarrow \Delta V_{1\phi} = 2 I R,\quad \Delta V_{3\phi} = \sqrt{3} I R \)

Method B — Using Manufacturer Impedance Tables

Here, instead of computing resistance from geometry, you use tabulated \( R \) or \( Z \) per unit length from the cable manufacturer.

  • Automatically includes temperature, stranding, and sometimes reactance \( X \) for specific cable types.
  • Excellent for detailed design when a particular cable family is specified.
  • Requires you to have the exact table for the exact cable construction and installation condition.
  • Harder to see how a small change in area or material affects the result without rechecking tables.
\( \Delta V = I \cdot Z_{\text{per length}} \cdot L_{\text{equiv}} \), where \( Z = \sqrt{R^2 + X^2} \) if reactance matters.

Method C — Back-Solving from Allowed Percent Drop

Often you know the limit on voltage drop before you know the cable. In that case, you can back-solve for the maximum length or minimum cable size.

  • Directly aligned with code or client requirements like “keep feeders < 3 % drop”.
  • Useful for rough routing decisions: “How far can I go at this size?”
  • Typically requires iteration: pick a size, check drop, adjust, and repeat.
  • Can tempt you to optimize for voltage drop alone and forget thermal ampacity or short-circuit limits.
\[ \Delta V_{\max} = \dfrac{\%VD_{\max}}{100}\,V_s \Rightarrow \text{choose } A \text{ or } L \text{ so } \Delta V \le \Delta V_{\max}. \]

What Moves the Voltage Drop the Most

If a voltage drop result seems surprising, these are the levers to check first. The calculator makes them explicit so you can see which one is limiting your design.

Run length \( L \)

Voltage drop is directly proportional to conductor length. In the resistance formula \( R = \rho \dfrac{L}{A} \), doubling \( L \) doubles \( R \) and doubles \( \Delta V \). The calculator uses one-way length and applies the appropriate factor for the circuit type.

Load current \( I \)

Drop scales linearly with current: \( \Delta V \propto I \). High inrush currents (motors, transformers, LED drivers) can cause a momentary voltage dip even when steady-state drop looks acceptable on paper.

Conductor cross-sectional area \( A \)

Larger area reduces resistance: \( R = \rho \dfrac{L}{A} \). Upsizing from a smaller cable to the next standard size is often the simplest way to bring percent drop within limits, especially on long runs.

Material resistivity \( \rho \)

Copper has lower resistivity than aluminum. For the same length and cross-section, aluminum will have more voltage drop: \( \rho_{\text{Al}} > \rho_{\text{Cu}} \). The material selector in the calculator swaps the resistivity used in the computation.

System voltage \( V_s \)

Percent voltage drop is defined as \[ \%VD = \dfrac{\Delta V}{V_s} \times 100. \] For the same power and conductor, higher system voltage means lower current and smaller percent drop, which is why many large loads are served at 480 V or higher rather than 208 V.

Reactance and power factor

For short building runs of typical cable, resistance dominates. For long three-phase feeders or underground circuits with large motors, reactance \( X \) and power factor can affect the effective impedance \( Z = \sqrt{R^2 + X^2} \) and therefore voltage drop.

Worked Examples Using the Voltage Drop Calculator

These examples follow the exact computation path that the Voltage Drop Calculator uses. You can plug the same values into the tool and compare the results, allowing for minor differences due to rounding and unit conversions.

Example 1 — Single-Phase 120 V Branch Circuit (Copper)

  • System: single-phase, 2-wire, 120 V
  • Material: copper, \( \rho \approx 1.724 \times 10^{-8}~\Omega\cdot\text{m} \)
  • One-way length: 30 m (≈ 98 ft)
  • Conductor area: 10 mm²
  • Load current: 18 A
1
Compute the resistance of one conductor leg:
\[ R_{\text{leg}} = \rho \dfrac{L}{A} = 1.724\times10^{-8} \dfrac{30}{10\times10^{-6}} \approx 0.0517~\Omega. \]
2
For a 2-wire single-phase circuit, current goes out and back, so:
\[ R_{\text{circuit}} = 2 R_{\text{leg}} \approx 2 \times 0.0517 = 0.1034~\Omega. \]
3
Compute the voltage drop at 18 A:
\[ \Delta V = I R_{\text{circuit}} = 18 \times 0.1034 \approx 1.86~\text{V}. \]
4
Convert to percent and find the load voltage:
\[ \%VD = \dfrac{1.86}{120} \times 100 \approx 1.6\%. \] \[ V_{\text{load}} = 120 – 1.86 \approx 118.1~\text{V}. \]

In the calculator, select single-phase 120 V, copper, enter 18 A, 30 m one-way, and 10 mm² conductor area. You should see a percent voltage drop around 1.5–1.6 % and a load voltage around 118 V, indicating plenty of margin for a typical branch circuit.

Example 2 — Long Three-Phase 400 V Feeder (Aluminum)

  • System: three-phase, 400 V (line-to-line)
  • Material: aluminum, \( \rho \approx 2.82 \times 10^{-8}~\Omega\cdot\text{m} \)
  • One-way length: 120 m
  • Conductor area: 35 mm²
  • Load current: 75 A
1
Resistance of one phase conductor:
\[ R_{\text{leg}} = \rho \dfrac{L}{A} = 2.82\times10^{-8} \dfrac{120}{35\times10^{-6}} \approx 0.0966~\Omega. \]
2
For a balanced three-phase system, the voltage drop between lines is approximated by:
\[ \Delta V = \sqrt{3} I R_{\text{leg}}. \]
3
Substitute the numbers:
\[ \Delta V \approx \sqrt{3} \times 75 \times 0.0966 \approx 12.6~\text{V}. \]
4
Percent voltage drop and load voltage:
\[ \%VD = \dfrac{12.6}{400} \times 100 \approx 3.2\%. \] \[ V_{\text{load}} = 400 – 12.6 \approx 387.4~\text{V}. \]

A 3.2 % drop may or may not be acceptable depending on project standards and motor sensitivity. In the calculator, you could test a 50 mm² aluminum conductor or switch to copper to see how much the percent drop improves for the same run length.

Common Layouts & Variations

Real projects usually fall into a small set of circuit patterns. The table below summarizes how each pattern behaves from a voltage drop perspective and what to watch for when you use the calculator.

ConfigurationTypical UseProsVoltage Drop Considerations
Short 120 V single-phase branch circuitsLighting, receptacles, small appliances in one roomUsually well under 3 % drop when sized by ampacity rules High inrush loads can still cause noticeable flicker. Use the calculator to verify worst-case runs at the end of the circuit.
Long 120/240 V feeders to outbuildingsDetached garages, barns, small shopsSimple topology, common cable sizes Length dominates. The calculator often shows that you must upsize the cable or increase system voltage to keep percent drop within limits.
480 V three-phase motor feedersIndustrial pumps, fans, compressorsHigher voltage reduces current and drop for the same power Motor starting torque is very sensitive to voltage. Use the calculator not only at full load, but also at locked-rotor current to check that temporary drop is acceptable.
Low-voltage DC (12–48 V)Controls, battery systems, LED strips, telecomSafe voltage levels; flexible routing Even small absolute drops are large in percent. The calculator will often show that voltage drop, not ampacity, controls conductor size.
Aluminum feeders and service lateralsCost-sensitive building feeders and utility servicesLower material cost and weight than copper Higher resistivity drives higher voltage drop. In the calculator, toggle between copper and aluminum while keeping length and current fixed to see how much larger aluminum cable you need.
Multi-segment distribution (main feeder + subpanels)Large commercial buildings and campusesFlexible system expansion and sectionalizing Total voltage drop is cumulative. Run the calculator on each segment and track both individual segment drop and end-to-end drop at the critical load.
  • Always check both volts of drop and percent drop against your criteria.
  • Consider the most distant and most heavily loaded circuits; those usually set the design.
  • For motors and sensitive electronics, verify that the minimum load voltage remains within manufacturer limits.
  • Document assumptions: ambient temperature, conductor material, and system voltage used in the calculator.

Specs, Logistics & Sanity Checks

The Voltage Drop Calculator gives you clear numbers. Turning those numbers into a safe, code-compliant design requires a few additional checks in the field and in your documentation.

Key Specification Items

  • Define the maximum allowable percent drop for feeders and branch circuits in your design basis.
  • Record the system voltage and configuration used in all calculations (e.g., 480 V 3-phase, 4-wire).
  • Confirm that conductor ampacity and short-circuit ratings are satisfied independently of voltage drop.
  • For long runs, note any temperature or grouping correction factors applied alongside voltage drop checks.

Field & Installation Notes

  • Long, heavy conductors may require larger conduits, stronger supports, and more pulling effort.
  • Routing changes (extra bends, detours around obstacles) increase actual length and therefore real voltage drop.
  • Measure as-built routes when possible and compare to the calculator assumptions.
  • For aluminum conductors, pay attention to termination hardware, anti-oxidant compounds, and approved lugs.

Sanity Checks Before Issuing Drawings

  • Compare your conductor sizes and drops against a similar past project; large unexplained differences are a red flag.
  • Use the calculator to run a “+20 % load” scenario and confirm you still have acceptable margin.
  • Check a few random circuits at different locations (near source, mid-run, far end) to ensure consistent assumptions.
  • Where possible, have another engineer quickly re-run critical circuits in the calculator as a peer review step.

In practice, you will iterate: pick a conductor size, run the Voltage Drop Calculator, compare to your drop limits and ampacity, adjust either size or routing, and repeat. The goal is not zero voltage drop, but a level that is technically sound, economical, and aligned with code and equipment requirements.

Frequently Asked Questions

What is voltage drop in an electrical circuit?
Voltage drop is the reduction in voltage between the source and the load caused by the resistance and impedance of the conductors. As current flows through a cable with resistance \( R \), a portion of the source voltage is lost as heat: \( \Delta V = I R_{\text{path}} \). If the drop is too high, motors may fail to start, lights may dim, and sensitive electronics may misbehave or shut down.
What is an acceptable percent voltage drop for feeders and branch circuits?
Acceptable percent voltage drop depends on local code and project requirements, but a common design practice is to limit individual branch circuits to around 3 % and total drop from service to the farthest load to around 5 %. Critical loads, long feeders, or low-voltage DC systems may need tighter limits. Always compare the calculator’s percent drop to the specific limits defined in your standard or specification.
How do I reduce voltage drop if my circuit fails the check?
The most effective ways to reduce voltage drop are to increase conductor size, shorten the run length, or increase system voltage so current is lower for the same power. In the Voltage Drop Calculator, try upsizing the cable, moving the source closer to the load, or changing to a higher voltage distribution level and re-checking the results until the percent drop meets your limit.
Is it okay to ignore reactance and power factor in voltage drop calculations?
For short building runs using standard cable, a resistance-only approximation is often adequate, and the calculator’s \( \Delta V = I R \) based methods are appropriate. For long three-phase feeders, underground circuits, or systems with large motors, reactance and power factor can significantly affect voltage drop. In those cases, you may need to use impedance values from manufacturer data or more detailed modeling that includes both \( R \) and \( X \).
Can I use this Voltage Drop Calculator for DC and low-voltage systems?
Yes. As long as the calculator is set to a DC mode and uses the correct two-wire path length \( \Delta V = 2 I R_{\text{leg}} \), it can be used for 12 V, 24 V, or 48 V systems. Be aware that in low-voltage DC applications, even a small absolute drop can be a large percentage of the nominal voltage, so conductor sizing is often dominated by voltage drop rather than ampacity.
Does voltage drop affect protection and short-circuit studies?
Voltage drop and protection are related but not identical. Breakers and fuses are primarily selected based on current, fault levels, and conductor ampacity. However, high impedance from long runs that cause voltage drop also reduce fault current at the far end of the circuit. When using the Voltage Drop Calculator for long feeders, it is good practice to confirm that minimum fault currents are still high enough for your protective devices to operate correctly and coordinate as intended.
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