Voltage Drop Calculator

Understanding Voltage Drop in Real Wiring Runs

Voltage drop is what happens when the conductors between your source and your load behave like a small resistor. Every wire has resistance, so when current flows, part of the supply voltage is consumed in the cable itself. The load receives what is left. On a short run this may be negligible, but on longer runs or higher currents the drop can be large enough to change how equipment behaves.

People usually notice voltage drop in simple ways: lights get dimmer at the far end of a circuit, tools feel weaker on extension cords, motors run warmer, and chargers or power supplies may operate outside their preferred input range. In industrial systems, excessive drop can reduce torque in motors, increase current draw, and shorten component life due to heat. The goal of a voltage drop calculation is not just to produce a number. It is to support a decision: is the conductor size appropriate for the distance and the load?

What Causes Voltage Drop

The biggest drivers are current and distance. Double the current and the drop roughly doubles. Double the run length and the drop roughly doubles. The other major driver is conductor size: larger cross-sectional area means lower resistance and therefore less drop.

Material choice matters too. Copper generally has lower resistivity than aluminum, so the same physical size usually produces less resistance. That does not make aluminum “bad,” but it does mean that for the same drop target, an aluminum conductor typically needs a larger cross-section compared to copper.

Temperature also matters. Resistance increases as conductors get warmer. The difference between a cool cable and a cable operating at higher temperature can be noticeable in long-run drop calculations, especially when currents are high and the cable is already heating from I²R losses.

The Core Idea: Resistance, Current, and Length

A practical way to think about voltage drop is to separate the problem into two pieces. First, determine the electrical resistance of the cable run. Second, multiply that resistance by the current to find the voltage lost along the way. Resistance depends on the conductor material and the cross-sectional area, and it scales with length.

In its simplest form, conductor resistance can be estimated from resistivity:

Resistance ≈ (Resistivity × Length) ÷ Area

Once you have resistance, voltage drop follows. For DC and single-phase circuits, current typically goes out and back, so the loop length is roughly twice the one-way distance. Three-phase circuits use a line-to-line relationship, which introduces the √3 factor in the drop equation.

DC vs AC Voltage Drop

Many everyday voltage drop questions are effectively DC questions: batteries, solar strings, LED strips, automotive wiring, and DC power supplies. In these cases the resistance-only model is often very close to reality because reactance is not a factor.

AC circuits add complexity because conductors and cables have both resistance (R) and reactance (X). Reactance comes from magnetic fields around conductors and depends on cable construction and spacing. For short branch circuits at typical building frequencies, resistance often dominates, but for longer feeders or larger conductors, reactance can contribute. That is why some engineering methods use impedance and power factor in voltage drop calculations.

This calculator provides a resistance-based estimate by default and includes an optional reactance entry for AC. If you have a cable datasheet that lists X (Ω/km), you can include it. If you do not, leaving it as 0 gives you a clear resistance-only baseline that is still useful for planning.

Single-Phase vs Three-Phase: Why the Formulas Differ

In a single-phase circuit, the load current returns through the neutral (or the second conductor), so the effective length for resistance loss is commonly treated as two times the one-way distance. That is why voltage drop models often include a factor of 2 for DC and single-phase.

In a three-phase circuit, the geometry and the relationship between phase and line voltages lead to the √3 factor for line-to-line voltage drop estimates. The important takeaway is not the symbol itself. It is that three-phase drop is calculated differently even when current and conductor size are the same, because the voltage reference is different.

How Power, Voltage, and Power Factor Determine Current

Many people know their load power but not their current. Current is what drives voltage drop, so converting from power to current is a key step. For DC, the relationship is straightforward: power is voltage times current. For AC, power factor changes the effective current required to deliver a given real power.

If you choose Power (kW) as your input, the calculator estimates current from your voltage and power factor. For three-phase, it uses the standard three-phase power relationship. This allows you to run “what if” checks even when you only have equipment power ratings.

Choosing a Drop Target That Fits the Use Case

A small voltage drop is usually desirable, but “acceptable” depends on what you are powering. Some loads are tolerant. Others are sensitive. Long runs may be unavoidable, which shifts the decision toward a larger conductor or a different distribution approach (such as stepping up voltage and stepping it down near the load).

Designers often use a percent target because it scales naturally with system voltage. A 6 V drop may be huge on a 12 V system and trivial on a 400 V system. That is why a drop percent is often easier to interpret than volts alone.

Wire Size vs Voltage Drop: What the Sizing Tab Actually Solves

The Wire Size tab in this tool is a solver: it searches through a set of standard conductor sizes and picks the smallest one that keeps your computed drop at or below your target percent. That makes it fast to move from “what is the drop on this cable?” to “what cable do I need for this run?”

It is important to treat this as one constraint among several. Voltage drop is not the same as ampacity. A conductor can meet a drop target but still be undersized for current-carrying capacity depending on installation conditions. Conversely, a conductor can be safe for ampacity but still produce an undesirable voltage drop on a long run. Good design checks both.

Parallel Conductors and Why They Reduce Drop

Parallel runs are used when a single conductor size is impractical. Electrically, parallel conductors reduce resistance because current splits across multiple paths. In a simplified model, two identical conductors in parallel halve the resistance, three reduce it to one-third, and so on. This tool includes a “Parallel Runs” setting that applies that relationship directly to the resistance estimate.

Parallel conductors require careful installation practice so currents share evenly. In real systems, matching lengths and routing is important. The calculator’s role is to show the ideal electrical effect so you can evaluate whether parallel runs are even worth considering for drop control.

Power Loss and Heat: The Hidden Cost of Voltage Drop

Voltage drop is not just a performance issue. The voltage lost in the conductor represents power that turns into heat in the cable: I²R loss. This heat is why cables warm up under load and why resistance rises as temperature increases. On long runs, even a small percentage drop can correspond to a meaningful number of watts.

The Loss & Energy tab translates voltage drop into watts and then into energy over time. This is useful when you are comparing two conductor sizes: a larger cable may reduce both voltage drop and wasted energy. Even if energy cost is not your goal, lower loss usually means cooler conductors and more stable voltage at the load.

Step-by-Step: Using the Voltage Drop Tab

Start by selecting your system type: DC, AC single-phase, or AC three-phase. Enter the supply voltage and choose whether you will enter current directly or power in kW. If you use kW for AC, set a realistic power factor.

Next enter the one-way length and choose the unit. Then choose the conductor material and size. You can input size as AWG or directly as cross-sectional area in mm². If you have metric cable specifications, mm² is usually the most direct input.

If you want temperature adjustment, enter a conductor temperature that reflects your expected operating conditions. If you are doing a quick check and you do not know temperature, a modest value slightly above room temperature is a reasonable planning assumption. Finally, enter a target drop percent. The status card will show whether the calculated drop is within that target.

Step-by-Step: Using the Wire Size Tab

The Wire Size tab solves for the smallest conductor in a standard set that meets your target drop. Enter the same system settings, load inputs, length, material, parallel runs, temperature, and target drop percent. Then choose whether you want the solver to use an AWG set or a metric mm² set.

The result includes a minimum size plus a few next larger options. That helps when you want extra margin for future load growth, or when you need to match an available inventory size rather than the theoretical minimum.

Interpreting Results Without Guesswork

A good way to read the results is to focus on three numbers: drop percent, delivered voltage, and power loss. Drop percent tells you how “heavy” the wiring run is relative to your system. Delivered voltage tells you what the load actually sees. Power loss tells you how much energy is being dissipated in the conductors.

If the drop percent is too high, the remedy options are predictable: reduce current, reduce length, increase conductor cross-section, increase system voltage, or use more parallel conductors. In many cases the simplest fix is increasing conductor size. But if the length is extreme, increasing voltage (for example, moving distribution to a higher voltage and stepping down locally) can be the most effective engineering choice.

AWG vs mm² and What “Bigger” Really Means

AWG numbers run backward: a smaller AWG number means a larger wire. Metric mm² is more intuitive: larger mm² means a larger cross-sectional area. If you are comparing wires across standards, focus on cross-sectional area. Resistance is mostly driven by area and material.

The Reference tab helps you translate AWG into approximate mm², shows diameter, and estimates resistance per km or per 1000 ft. These values are meant for planning. If you are working from a cable datasheet, the datasheet resistance values are the best match for that exact product because strand count, compacting, and manufacturing tolerances can change the final resistance slightly.

Practical Tips for Better Voltage Drop Decisions

• Use the correct length. Most field measurements are one-way distance, but many formulas effectively use the loop length for DC and single-phase.
• Base current on realistic conditions. Continuous loads and motor starting conditions can differ; voltage drop under starting current may matter for motor performance.
• Check temperature assumptions. If cables run in hot environments, resistance is higher than room-temperature estimates.
• Validate with ampacity. Meeting a drop target does not guarantee the conductor is safe for current-carrying capacity.
• Use datasheet impedance when needed. For long feeders, including reactance can improve AC accuracy.

Limitations of Voltage Drop Calculations

A voltage drop calculator is only as accurate as its assumptions. This tool estimates resistance from fundamental geometry and resistivity and then adjusts for temperature. That is reliable for planning, but real installations include additional factors such as terminations, contact resistance, conduit fill, harmonics, cable spacing, and supply regulation.

For design-critical systems, treat this calculator as a fast screening step. Once you find a candidate conductor size, verify it using your cable manufacturer’s data, installation method, and any applicable electrical requirements. The strongest workflow is: estimate quickly, choose a reasonable size, then verify thoroughly.