Voltage Drop on Long Wire Runs: Why Distance Kills Power

Voltage drop is the reduction in voltage that occurs as electricity travels through a conductor, caused by the wire's own resistance working against the current flowing through it. Every conductor has some resistance, and every foot of wire between a panel and the equipment it feeds subtracts a small amount of voltage along the way. Over a short run that loss is negligible. Over a long run, especially one feeding a motor, it can be the difference between equipment that runs for years and equipment that fails early for reasons nobody connects back to the wire size until someone actually measures it.

What Is Actually Happening in the Wire

Current flowing through a conductor encounters resistance, and that resistance converts some of the electrical energy into heat rather than delivering it to the equipment at the far end. The voltage available at the load is always somewhat lower than the voltage at the source, and the amount lost depends on three things: how much current is flowing, how far the current has to travel, and how much resistance the conductor itself presents per foot.

A thicker conductor has less resistance per foot than a thinner one of the same material, which is why increasing wire size is the standard remedy for excessive voltage drop on a long run. Distance works the other way: double the length of a run and you roughly double the voltage lost, all else being equal, which is exactly why a circuit that performs fine at thirty feet can perform poorly at a hundred and fifty feet using the same wire size and carrying the same load.

Why Motors Suffer More Than Lights or Simple Resistive Loads

A light bulb or a simple heating element does not particularly care about a few percent less voltage than expected. It simply produces slightly less light or heat, which is a mild, largely invisible inefficiency rather than a functional failure. A motor is an entirely different story.

A motor running on lower voltage than it was designed for draws more current to produce the same torque, and that extra current generates additional heat inside the motor windings. Sustained undervoltage operation is one of the more common, least understood causes of premature motor failure, because the motor does not necessarily fail immediately. It runs hotter than it should for months or years, degrading its winding insulation a little more with every cycle, until it eventually fails in a way that looks unrelated to the wiring that fed it.

Motors also draw a substantial inrush current the moment they start, well above their steady running current, and that starting surge makes voltage drop worse at exactly the moment the motor most needs adequate voltage to develop starting torque. A motor struggling to start on a long, undersized run can stall, hum, overheat, and trip its internal protection repeatedly, wearing down components with every failed start attempt even if it eventually gets going.

The Practical Percentage Guidance

Industry guidance recommends keeping voltage drop within roughly 3 percent on a branch circuit and within roughly 5 percent for the combined branch circuit and feeder together. These are not absolute legal limits in the way an overcurrent protection requirement is, but they are longstanding, widely followed guidance because equipment reliability and efficiency both degrade measurably once voltage drop exceeds that range.

Three percent sounds small, and on a short run it is trivial to stay well under it with ordinary wire sizing. On a long run, particularly one feeding a motor load, staying inside that guidance is what actually determines the correct conductor size, sometimes requiring a conductor noticeably larger than what the load's current alone would otherwise call for. The load determines the minimum safe wire size for carrying current without overheating. The distance, separately, can push the required wire size up further to keep voltage drop within a reasonable range. Both calculations have to be checked, and the larger of the two results is the wire size that actually goes in the ground or in the conduit.

How Distance and Wire Size Actually Interact

The relationship is straightforward in concept even though the arithmetic behind it involves a few variables. For a given load, voltage drop increases in direct proportion to the length of the run, and it decreases as the conductor's cross-sectional area increases. Doubling the distance to a piece of equipment while keeping the same wire size roughly doubles the voltage drop. Moving up one or two standard wire sizes at the same distance can cut voltage drop meaningfully, often enough to bring a marginal run back within the recommended range.

This is why two projects that look similar on paper, same equipment, same amperage, can call for genuinely different conductor sizes once the actual distance from the panel is accounted for. A well pump forty feet from a service panel and the same pump three hundred feet away are not the same electrical project, even though the pump itself is identical. We calculate voltage drop specifically for the distance involved on every long run rather than defaulting to a wire size based on amperage alone, because amperage alone only tells part of the story.

Copper Versus Aluminum on Long Feeder Runs

Conductor material factors directly into voltage drop, since copper has meaningfully lower resistance per unit of cross-sectional area than aluminum. On a long feeder, this sometimes makes copper the more practical choice even where its higher material cost would otherwise favor aluminum, because reaching an acceptable voltage drop with aluminum can require jumping up to a conductor size large enough to offset its higher resistance, which affects conduit sizing, termination hardware, and the physical practicality of the installation.

This is a completely different conversation from the branch circuit aluminum wiring issues found in some 1960s and 1970s homes. Modern aluminum feeder cable, using a different alloy engineered for building wire, is a normal and accepted choice for long runs today when it is sized correctly and terminated with connectors and technique suited to aluminum. The decision between copper and aluminum on a given long run comes down to the specific distance, load, and installation conditions rather than a blanket preference for one material over the other.

Power Factor and Why Some Equipment Drops Voltage Worse Than Others

Not all loads of the same amperage affect voltage drop identically, because motors and other inductive loads draw current that is slightly out of step with the voltage waveform, a characteristic described by a load's power factor. A load with a poor power factor draws more total current from the source than its actual useful power output would suggest, and that additional current contributes to voltage drop along the conductor just as any other current would.

This is part of why a calculation based purely on a nameplate wattage rating can understate the actual voltage drop a motor load produces in practice. We calculate long runs using the equipment's actual current rating rather than working backward from a wattage figure alone, since the current rating already accounts for the load's real electrical behavior rather than an idealized version of it.

Detached Garages and Workshops

A detached garage or workshop set back from the house is one of the most common long-run situations we encounter in South Florida, where larger lots and separate structures are common. The feeder to a detached structure has to be sized for both its total connected load and the distance it travels, and it is common for that distance alone to push the required conductor size up a step or two beyond what the load current would suggest on its own.

This matters even more once the garage itself has real equipment in it: power tools, a compressor, a second refrigerator or freezer, or a mini split system. Undersizing the feeder because the garage "only needs a few outlets" is a frequent mistake, since the actual load a workshop ends up carrying is often much higher than what it was originally wired for, and a marginal feeder that was fine for a couple of light fixtures becomes a real voltage drop problem the moment real tools get plugged in.

Well Pumps

Well pumps are a textbook case for why this matters, because a pump motor experiencing chronic undervoltage does not always fail outright. It can run for years while working harder than it should, drawing more current than its nameplate rating suggests, and running measurably hotter, all while still delivering water and giving no obvious sign that anything is wrong until the motor finally burns out well before its expected service life.

Rural and semi-rural properties with a well located a significant distance from the electrical service are exactly where voltage drop calculations earn their keep. We size the conductor to the well pump for the actual distance involved and the pump's actual starting and running current, not just a generic wire size based on the breaker rating, because a pump motor is precisely the kind of load that quietly punishes an undersized run for years before it fails.

Barns and Outbuildings

Barns, stables, and other outbuildings on larger properties often sit even farther from the main service than a detached garage, and they frequently carry a mix of loads that grows significantly over time: lighting, ventilation fans, water heating for livestock, power tools, and sometimes refrigeration. A feeder sized correctly for the initial, modest load at installation can become inadequate years later once the building's actual use expands, and voltage drop is usually the first symptom that shows up, in the form of dimming lights when a fan or a heater kicks on.

Planning ahead for a barn or outbuilding's likely future load, not just its current load, is worth the conversation before a feeder goes in the ground, since upsizing a conductor during initial installation is a far smaller expense in effort than digging up and replacing an undersized feeder years later once the building's electrical demand has grown past what it can comfortably carry.

Gate Operators

Automatic gate operators sit at the end of what is often the longest, most awkward run on an entire property, frequently a few hundred feet from the nearest panel, run underground along a driveway or property line. Gate operators are motor-driven equipment, which makes them just as sensitive to voltage drop as any other motor load, and an operator straining against low voltage tends to move slowly, struggle against normal resistance like wind load or a slightly out-of-adjustment track, and wear out its motor and gearing well ahead of schedule.

These circuits also commonly involve low voltage control wiring for sensors, keypads and safety edges running alongside the power feed, and voltage drop on the control wiring itself can cause its own separate set of intermittent, hard-to-diagnose problems if that wiring was not sized with the same care as the power conductor. We treat a gate operator installation as a voltage drop calculation from the start rather than an afterthought, given how far these runs typically travel and how completely a marginal power feed can undermine an otherwise well-installed gate system.

Parking Lot and Site Lighting

Commercial site lighting brings voltage drop into an entirely different scale of project, since a parking lot or a large commercial site can involve pole fixtures spread across hundreds or thousands of feet of underground feeder, all fed from one or a small number of panels. Voltage drop across a lighting circuit affects light output directly, since fixtures operating below their rated voltage produce less light than their specification promises, which matters both for basic visibility and for meeting a property's lighting design requirements at every point across the lot, not just near the panel.

LED fixtures, now standard for most new and retrofit site lighting, are somewhat more tolerant of voltage variation than older lighting technology, but they are not immune to the problem, and a poorly planned feeder layout can still leave fixtures at the far end of a run noticeably dimmer than fixtures near the panel. We design these feeder layouts with voltage drop calculated for every segment, sometimes feeding a large lot from more than one panel location specifically to keep every run within a reasonable distance rather than running one enormous single feeder to the farthest corner of the property. Our parking lot and site lighting work and our broader commercial parking lot lighting services are both built around getting this calculation right at the design stage, before conduit goes in the ground.

Three-Phase Equipment and Longer Industrial Runs

Three-phase motor loads in commercial and industrial settings face the same underlying physics as any other motor, but often at higher horsepower and over longer distances across a larger property or building. A three-phase feeder run to a piece of equipment at the far end of a warehouse, or to an outbuilding on a commercial site, needs the same voltage drop analysis as a residential well pump, scaled up to match the equipment's actual starting and running current.

Undersized feeders on three-phase equipment show up as the same symptoms described throughout this article: motors running hotter than they should, nuisance tripping of thermal overload protection, and premature failure that often gets blamed on the equipment itself rather than the conductor feeding it. Our three-phase power work includes this calculation as standard practice on any run of meaningful length, and we would rather explain why a conductor needs to be larger than the bare minimum than have a client discover the reason the hard way after a motor burns out.

What We Actually Do Differently on a Long Run

Rather than sizing a conductor purely off the load's amperage and calling it done, we calculate voltage drop for the specific distance involved on any run long enough for it to plausibly matter, checking both the branch circuit and any feeder segment upstream of it. Where the calculation shows the load-based wire size would exceed the recommended voltage drop range at that distance, we size up the conductor rather than leaving the equipment to run on marginal voltage for its entire service life.

We also account for future load growth on runs feeding a structure or area likely to add equipment over time, since a conductor sized exactly for today's load and distance often becomes marginal again the moment anything else gets added downstream. Getting this right at installation is a straightforward calculation. Correcting it after the fact means digging up or re-pulling a conductor that is already in service, which is a far more disruptive project than getting the sizing right the first time.

How to Tell Voltage Drop Is the Actual Problem

A few practical signs point toward voltage drop rather than some other electrical issue. Lights that dim briefly and predictably every time a specific motor starts, at the far end of a long run rather than throughout the whole property, is a strong indicator. A motor that hums, struggles, or takes noticeably longer to reach normal running speed than an identical piece of equipment closer to the panel points the same direction.

The reliable way to confirm it is measuring actual voltage at the equipment while it is running under real load, then comparing that reading against the voltage at the panel taken at the same time. A meaningful gap between the two, especially one that grows worse as the load increases, confirms voltage drop as the cause rather than a coincidence. This measurement has to be taken with the equipment actually operating and drawing its normal current, since a reading taken with no load connected will not reveal a problem that only appears under real operating conditions. We take these measurements as part of any long-run evaluation, rather than assuming a wire size is adequate just because the equipment technically turns on.

When to Call

If you have equipment on a long run from your panel, a well pump, a gate operator, a detached structure, or site lighting spread across a large property, and you are seeing dimming, sluggish motor starts, or equipment that seems to run hot or fail earlier than expected, voltage drop is worth investigating as a real possibility rather than an afterthought. We work on both residential and commercial properties throughout our service area and we calculate this properly rather than guessing at a wire size.

Call (954) 602-0050 and describe the run, the equipment, and roughly how far it sits from the panel, and we will explain what we would want to measure and calculate before recommending a fix. We answer the phone around the clock at (954) 602-0050.