Long Runs and Voltage Drop

Voltage drop is the part of an EV charger install that never fails an inspection and quietly costs you for twenty years. Ampacity, the number everybody looks up, answers a question about heat per foot and does not change with distance: a 6 AWG copper conductor is good for the same 65 amps at 10 feet and at 300. Resistance accumulates the whole way, so the voltage arriving at a distant charger is measurably lower than the voltage leaving the panel, and the difference is permanent.

The number to work to is three percent on the branch circuit. Run your own distance through the voltage drop calculator before anyone buys wire, because the answer decides the conductor and the conductor is the second largest material line on the job.

What is the three percent target, and is it actually a rule?

It is a recommendation, and it is worth being precise about that because people argue about it in both directions. The National Electrical Code addresses voltage drop in informational notes, suggesting a branch circuit be designed for no more than three percent drop and that the total across a feeder plus the branch circuit stay near five percent. Informational notes are explanatory material, not enforceable requirements, so an inspector will not normally fail a circuit for drop alone.

What makes the number stick anyway is that it is a good engineering compromise and that it is not fixable later. A conductor is chosen once. If it is undersized for the distance, the charger sees reduced voltage every session for the life of the house, and the only remedy is to pull new conductors. A designer who ignores drop has not broken a rule; they have handed the homeowner a permanent inefficiency the homeowner will never notice and can never cheaply undo.

Two secondary reasons matter for charging in particular. This is a high-current load that runs for hours, so the loss is continuous rather than momentary. And chargers are electronics with supply voltage expectations, which means low voltage under load produces derating or fault codes rather than simply proportional slowness.

How far can a 48 amp circuit run before the conductor has to grow?

Forty-eight amps is the continuous draw of the 60 amp circuit most people are planning, so this is the table that decides real jobs. Each cell is the percentage drop at 240 volts for a one-way run of that length, calculated from published conductor resistance. The last column is the longest one-way run that stays inside three percent.

Read the 6 AWG row across and the whole design question appears. At 50 feet the drop is trivial. At 100 feet it is still comfortable. At 150 feet it reaches 2.9 percent and the charger receives roughly 232.9 volts instead of 240, which is inside the target with nothing spare. The honest limit for 6 AWG copper at 48 amps is about 152 feet, and any estimate that lands within ten percent of that number should be treated as being over it, because the tape measure is optimistic and the route is longer than it looks.

One size up changes the picture completely. Four AWG copper takes the same 150 foot run to 1.8 percent and roughly 235.6 volts at the charger, and pushes the three percent distance out to about 243 feet. That is the real decision on a long run: not whether the conductor is safe, because 6 AWG is thermally fine at any residential distance, but whether you want to live with a charger at the edge of the target.

What do the same runs look like at 40 amps?

A 40 amp charger on a 50 amp circuit draws a sixth less current, and drop scales directly with current, so every figure improves by the same sixth. This is the quietest argument for the 40 amp install that nobody makes: on a long run, the smaller charger needs less copper to reach the same design target, and on a car whose onboard limit is near 11 kW it gives up roughly one mile of range per hour of charging.

Six AWG copper at 40 amps reaches about 183 feet inside the target, against 152 feet at 48 amps. On a detached garage at 175 feet, that difference is the difference between one conductor size and the next, across three conductors, for the whole distance. It is frequently a larger saving than the price gap between the two chargers.

What does the lost voltage actually do?

Power is current multiplied by voltage. A charger set to 48 amps draws 48 amps whether it sees 240 volts or 233, so what falls is the power delivered to the car. At 232.9 volts the same 48 amp session delivers around three percent less energy per hour, which turns a nine hour charge into a nine and a quarter hour charge. On its own, nobody would notice.

The second effect is less benign. Chargers monitor supply voltage, and some reduce output or report an undervoltage condition when the supply sags under load. A circuit at the edge of the target on a warm day with a soft utility supply can dip enough to trigger that behaviour, and the symptom the owner sees is not "three percent slower" but "sometimes charges at 32 amps for no reason". That is a genuinely hard fault to diagnose after the fact, and it is why margin matters more than the target itself.

The third effect is heat. The energy that vanished did not vanish; it warmed the conductor along its whole length. That is thermally harmless in a correctly sized conductor and it is why voltage drop is not a safety issue. It is, however, the part you pay for.

What does the loss cost in electricity?

Worth quantifying, because the answer keeps the argument honest. The figures below assume 12,000 miles a year at 3.5 miles per kWh, which is about 3,429 kWh into the car and roughly 331 hours of charging at 11.5 kW with the 90 percent efficiency figure this site uses throughout. Electricity is priced at the national average of 16.5 cents per kWh.

So a 150 foot run in 6 AWG copper throws away about 112 kWh a year, worth $18.54. Moving to 4 AWG cuts that to 70 kWh and $11.63, an annual saving of about $6.91. Over twenty years that is real money and it is nowhere near the cost of the extra copper, which is the point: you size up for the voltage the charger receives and for the margin, not for the energy bill. Anyone selling you a conductor upgrade on payback maths is doing the arithmetic backwards.

Notice also how steeply the loss falls with size. Three hundred and thirty-nine watts of continuous heat in a wall cavity for nine hours a night is not a hazard in a correctly sized conductor, but it is a reasonable thing to want less of, and it is the clearest physical illustration of what drop means.

Does aluminium fix a long run more cheaply?

This is where the material argument becomes genuinely interesting, because the thing that makes aluminium awkward on a short run makes it attractive on a long one. Aluminium carries less current per size, so it needs one or two sizes more than copper. It also costs a fraction as much per foot. When the fix for your distance is "go up two sizes", doing that in aluminium is cheap and doing it in copper is not.

Four AWG aluminium is the smallest size that carries a 60 amp circuit at 65 amps in the 75 degree column, and at 150 feet it sits exactly on the three percent line with nothing at all in reserve. Two AWG aluminium brings the same run comfortably inside the target at 1.9 percent. Two sizes up in aluminium is a small price change; two sizes up in copper, from 6 AWG to 3 AWG, is not.

The researched prices make the scale of it obvious. Six AWG copper THHN works out near $1.40 a foot on a 100 foot spool, and 6 AWG XHHW-2 aluminium works out near $0.80 a foot. A 150 foot run needs three conductors plus a ground, so the three current-carrying conductors alone come to roughly $630 in copper against $360 in aluminium at the same gauge, a gap of about $270 before you have even accounted for the fact that the aluminium version needs to be larger. Prices were accurate when researched and change without notice, but the ratio between the two metals is stable and it is the ratio that decides the design.

None of that makes aluminium a homeowner purchase. It needs terminals and lugs rated AL/CU, an anti-oxidant compound at every termination, and connections torqued to specification rather than tightened by feel, because aluminium creeps under pressure and a hand-tight lug loosens over years. Those three conditions are the whole reason aluminium earned a bad name in branch circuits, and they are all termination practice rather than anything wrong with the metal. The complete comparison is in aluminium or copper wiring.

Is a subpanel a better answer than a bigger conductor?

Frequently, and this is the option people forget. On a long run to a detached garage you are already trenching, already pulling large conductors and already paying for the distance. A feeder to a small subpanel in the garage costs marginally more than a single branch circuit over the same route and gives you a 240 volt circuit for the charger plus lighting, receptacles and a future second charger.

The voltage drop arithmetic also changes shape. A feeder plus branch circuit is allowed a larger combined budget, near five percent, and the branch circuit inside the garage is short enough to contribute almost nothing. That means the feeder can use most of the allowance, and a feeder sized for drop rather than a branch circuit sized for drop is a more useful thing to own at the end of it. The trade-offs, including the separate grounding requirements at a second structure, are covered in detached garage EV charger.

Why cable length beats conductor length

Here is the cheapest fix on the page, and it is not electrical at all. Every foot of conductor in a wall or a trench costs conduit, fittings, labour and sometimes digging. Every foot of cable on the charger itself costs nothing extra to install, because the manufacturer already put it in the box.

A 25 foot charging cable reaches a long way around a two-car garage. Mounting the unit near the panel and letting the cable travel to the car turns a 100 foot conductor run into a 60 foot one, removes a size from the conductor decision and takes the voltage drop question off the table entirely. The charging speed is identical, because the drop across a manufacturer-supplied charging cable is part of the unit's design rather than part of your circuit.

Two cautions. Cable length is not the same as cable manageability: 25 feet of cold, stiff cable on a garage floor is a trip hazard and it deserves a holster and a hook. And an extension cable between the charger and the car is a different thing entirely, with its own connection resistance and its own problems, and it is not a substitute for a unit with the reach you need. Units with genuinely long cables are compared in long cable EV chargers.

How to measure the run properly

Almost every voltage drop surprise comes from measuring the wrong distance. The number the calculation needs is the length of the conductor, not the distance between the two boxes, and conductors do not travel in straight lines. They go up the wall, along a joist bay, around a duct, across to the other side of a stud cavity, down another wall and into an enclosure.

• Walk the actual route with a tape rather than measuring the diagonal across the garage.
• Add the vertical legs at both ends: down from the panel and up to the charger, both counted.
• Add a working allowance inside each enclosure, because conductors are not cut to the millimetre.
• Add the depth of the trench twice if the run goes underground, once down and once back up.
• Round up. A route measured optimistically is the most common cause of a spool that finishes ten feet short.

A useful sanity check: a run that measures 50 feet on the diagonal is usually 70 to 80 feet of conductor once it is actually installed. If the drop calculation is close to the target at the measured figure, it is over the target at the real one. Conductor sizing itself, including the ampacity and derating side that has to be satisfied first, is in wire size for an EV charger.

What about 208 volts?

In some multi-family and converted commercial buildings the supply is 208 volts rather than 240. Drop in volts is unchanged, because it depends on current and resistance, but drop as a percentage is worse, because the same voltage lost is a larger share of a smaller supply. Three percent of 208 volts is 6.2 volts against 7.2 volts at 240, so a run that just satisfied the target at 240 volts does not satisfy it at 208.

The power arithmetic compounds it. A 40 amp charger on 208 volts delivers 8.3 kW rather than 9.6, so charging is already 13 percent slower before any drop is counted. If your building is 208 volts, work the calculation at 208 and expect one conductor size more than the tables on this page suggest.

Where this calculation stops

Everything here is calculated from published conductor resistance values at 240 volts, single phase, across a two-conductor run, and reproduced so you can plan and question a quote. A precise calculation accounts for conductor temperature under load, which raises resistance, for alternating current effects that matter at larger sizes, and for the actual measured length of the installed conductor rather than an estimate from a drawing.

Voltage drop is also the second question, never the first. A conductor has to satisfy ampacity, the terminal temperature column and every applicable derating adjustment before drop is even considered, and a conductor chosen for drop alone can still be the wrong conductor. If you want the whole matrix in one place, the voltage drop chart lays out percentage drop by gauge, amperage and distance, and the conductor products themselves are the 6 AWG THHN copper and 6 AWG XHHW-2 aluminium priced above.

Common questions

Is the three percent voltage drop limit a code requirement?

No. It appears in the National Electrical Code as an informational note recommending that a branch circuit be designed for no more than three percent drop, with a combined feeder and branch limit near five percent. Informational notes are guidance rather than enforceable text, so an inspector will not normally fail a circuit on drop alone. It is still the number every competent designer works to, because the consequences are permanent.

How far can 6 AWG copper carry 48 amps?

About 152 feet one way before the drop passes three percent at 240 volts. At 150 feet the drop is 2.9 percent and the charger sees roughly 232.9 volts. Four AWG copper brings the same 150 foot run down to 1.8 percent and pushes the three percent distance out to about 243 feet. Ampacity does not change with length; only the drop does.

What actually happens if the drop is too high?

Three things, none of them dramatic. The charger delivers less power because power falls with voltage at a fixed current, so charging takes longer. Some units derate deliberately or report an undervoltage fault when supply sags under load. And the energy that disappeared became heat in the conductor, every session, for the life of the installation.

Does going up a wire size pay for itself in saved electricity?

Almost never on its own. Moving a 150 foot run from 6 AWG to 4 AWG copper saves roughly $6.91 a year at 16.5 cents per kWh and 12,000 miles of driving. You size up for the voltage the charger receives and for the margin, not for the energy bill. Treating the saving as the reason gets the decision right for the wrong reason.

Is aluminium a sensible way to fix a long run?

Often yes, and it is what feeders normally use. Aluminium needs one or two sizes more than copper for the same ampacity, but it costs a fraction as much per foot, so sizing up for voltage drop is far cheaper in aluminium. It requires AL/CU rated terminals, an anti-oxidant compound at every termination and torqued connections, which is exactly why it belongs to the electrician.

Is it cheaper to move the charger or to upsize the wire?

Moving the charger, nearly always. A 25 foot cable on the unit costs nothing per foot to install, while conductor in the wall costs conduit, fishing, trenching and labour for every foot. Mounting the charger nearer the panel and letting the cable reach the car turns a 100 foot conductor problem into a 60 foot one, and the charging speed is identical.

Getting your own panel and load numbers ready for an electrician? The EV Home Charging Install Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.