Inverters & equipment

Voltage drop calculator

Enter your circuit's current, one-way run length, wire gauge, and system voltage. Get the resistive voltage drop in volts and as a percentage of your system voltage.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Circuit
Combined string current from the parallel strings current calculator, or inverter output current.
Nominal circuit voltage, used to express the drop as a percentage.
3 Wire run
Measure one-way along the actual routed path, not straight-line distance.
Start with the minimum from the wire gauge sizing calculator, then check if it needs upsizing here.

How it works

Three steps to your voltage drop

Step 1

Start from a wire gauge that already clears ampacity

Run the wire gauge sizing calculator first so you're checking voltage drop on a gauge that's already safe from an overheating standpoint.

Step 2

Measure the actual one-way run

Walk the routed path, not the straight-line distance between endpoints. Conduit that jogs around a roof edge or foundation wall adds real feet the calculator needs to know about.

Step 3

Upsize if you're past your target percentage

If the result lands above 2-3%, try the next gauge up in the dropdown, or re-check the run against the string length voltage calculator if your system voltage itself varies with temperature. Doubling the circular mil area roughly halves the voltage drop for the same run.

Understanding voltage drop

Why resistance turns distance into lost voltage

Every foot of wire has a small amount of electrical resistance, and pushing current through that resistance dissipates some voltage as heat along the way, rather than delivering it to the load. This is separate from the ampacity limit checked in the wire gauge sizing calculator, which only asks whether the wire will overheat. Voltage drop asks a different question: how much of your system's voltage never makes it to the far end of the run. The wiring loss output calculator covers this same resistive loss from the annual production side rather than the voltage-percentage side.

Why the formula counts the wire twice

For a single-phase or DC circuit, current has to travel out to the load and back to the source to complete the circuit, so the effective resistance is double the one-way wire length. That's the factor of 2 in the standard Vdrop = (2 x K x I x D) / CM formula. Three-phase circuits use the square root of 3 instead, since the phase currents share the return path differently. Getting this factor right matters. Using one-way distance without doubling it for a single-phase run understates the actual drop by half. Wire resistivity assumptions here follow NEC Table 310.16.

Why long DC home runs are the usual culprit

A rooftop array feeding a ground-mounted inverter, or a detached garage system running back to the main panel, which the system size for detached garage calculator covers from the array-planning side, can easily rack up 100 feet or more of one-way conductor length. Combined string circuits sized with the parallel strings current calculator carry enough current that even a modest run length pushes voltage drop past the usual comfort zone on a minimum-ampacity gauge. It's these longer runs, not typically the short jumper cables between array components, where voltage drop actually forces a gauge upsize.

Why 2-3% is the number installers target

There's no single NEC-mandated voltage drop percentage for most branch circuits, but 2% is the common target and 3% the usual outer limit in solar design guides, chosen because losses beyond that start eating into system efficiency and, on some circuits, can affect how well downstream equipment performs at its rated voltage. This calculator reports the exact percentage so you can decide whether your run needs to land under 2%, is acceptable up to 3%, or needs a larger gauge to get there. NREL's PVWatts calculator can help confirm how a given voltage drop percentage translates into your site's actual annual production loss.

What this tool doesn't cover

This calculator handles the resistive voltage drop along a single wire run, using the standard copper and aluminum resistivity values at 75C. Termination and connector resistance are not included here, though that resistance is usually small, and this tool doesn't replace the ampacity check from the wire gauge sizing calculator, which can independently require a larger gauge than voltage drop alone would. Always run both checks and use whichever gauge is larger.

Questions

Frequently asked

A 25A circuit run 75 feet on 10 AWG copper drops roughly 4.66V, about 1.9% at 240V. Multiply the circuit current by twice the one-way distance in feet and a resistivity constant (about 12.9 for copper), then divide by the wire's circular mil area.

Most solar designers target 2% or less voltage drop per circuit segment, with 3% as a common outer limit before efficiency losses and inverter performance become a real concern. The NEC doesn't mandate a specific percentage for most branch circuits, but it's the industry rule of thumb for keeping energy losses reasonable.

Voltage drop scales directly with distance, so a home run from a rooftop array to a ground-mounted inverter can lose several times more voltage than a short run carrying the same current, simply because the wire is longer. This is why ground-mount and long-conduit installations often need a larger gauge than the ampacity minimum alone would suggest.

No, they're separate checks that both need to pass. Ampacity sizing, covered in the wire gauge sizing calculator, keeps the wire from overheating. Voltage drop sizing keeps the circuit efficient over distance. A wire gauge that clears the ampacity minimum can still need to be upsized further if the run is long enough to push voltage drop past your target percentage.

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