EV Charger Voltage Drop Calculator

Voltage drop is the calculation that catches people out after the wire is already in the wall. Conductor sizing by ampacity is a question about heat and does not care how far the run goes. Voltage drop is a question about resistance and cares about nothing else. Past a certain distance the second one takes over, and the wire that is thermally perfect is electrically inadequate.

The number to remember: a 48 amp load on 6 AWG copper reaches three percent drop at about 75 feet one way. Most garages are inside that. Detached garages and long basement runs frequently are not.

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Voltage drop

2 x L x I x R

Single-phase drop across a two-conductor run, using published conductor resistance. Distance is one way; the calculator doubles it because current travels out and back.

Conductor size 10 AWG 8 AWG 6 AWG 4 AWG 3 AWG 2 AWG 1 AWG 1/0 AWG 2/0 AWG

Material Copper Aluminium

Continuous load

The charger output, not the breaker size.

amps

One-way run length

feet

Supply voltage 240 V 208 V, some multi-family 120 V, Level 1

Voltage drop

2.4 percent

Target is 3 percent or less

Volts lost in the run

5.7 V

Voltage at the charger

234 V

Charging power lost

0.27 kW

Longest run at this size

75 ft

Inside target

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The formula, and the mistake almost everyone makes

For a single-phase circuit the drop is straightforward: two, multiplied by the one-way length, multiplied by the current, multiplied by the conductor resistance per foot.

Voltage drop = 2 x one-way length x amps x (ohms per 1,000 ft / 1,000)

That leading two is where people go wrong. Current has to travel out to the charger and back to the panel, so a charger 50 feet away sits at the end of 100 feet of conductor. Calculate with the one-way distance and you get an answer exactly half as large as the real one, which is the difference between comfortably inside the target and clearly outside it.

Worked example. A 48 amp load on 6 AWG copper over a 60 foot one-way run. The resistance of 6 AWG copper is 0.491 ohms per 1,000 feet. So 2 times 60 times 48 times 0.000491 gives 2.83 volts, which is 1.2 percent of 240 volts. Comfortable. Move the same circuit to 150 feet and the drop becomes 7.07 volts, or 2.9 percent, which is right on the target line.

Distance limits at 48 amps, by conductor size

This is the table worth screenshotting before you buy wire. Each cell is the voltage drop percentage for a 48 amp continuous load at 240 volts, copper.

Two things to take from it. First, 6 AWG runs out of road somewhere between 75 and 100 feet at this load, which covers most attached garages and fails for most detached ones. Second, each step up in conductor size buys roughly 50 percent more distance, so the fix escalates quickly: 4 AWG gets you to about 110 feet, 2 AWG to about 175.

What the drop actually costs you

A charger at reduced voltage draws the same current and therefore delivers less power. The relationship is close to proportional, so three percent less voltage is about three percent less charging power.

On a 48 amp circuit at 11.5 kW, three percent is about 0.35 kW. At 3.5 miles per kWh that is roughly a mile and a quarter of range per hour of charging. Over an eight hour night, ten miles. It will not ruin your life, and you will probably never notice it.

The reason to care is that it is permanent and free to avoid at install time. One extra wire size on a 60 foot run costs perhaps 60 dollars in copper and nothing in labour, because the electrician is pulling a conductor either way. The same fix after the fact means opening the wall again and paying the labour block a second time, which the buildouts show is the majority of the project cost.

Three ways to fix a run that is too long

1. Size up the conductor

The default answer, and the right one up to about 100 feet. Cheap in materials, free in labour if done during the original install. See what your circuit needs on the wire gauge calculator, and note that if you specified conduit rather than cable, as the hardwired 48 amp buildout does, this fix stays available later.

2. Reduce the charger output

Voltage drop scales with current, so a 32 amp charger on the same conductor drops a third less than a 48 amp one. If your car accepts 7.7 kW anyway, this costs you nothing at all and solves the problem for free. Check the onboard limit on the battery capacity chart before spending money on copper you may not need.

3. Move the panel closer, with a subpanel

Past roughly 100 feet this usually wins. One appropriately sized feeder out to a garage subpanel carries capacity for the charger plus everything else you will ever want out there, and every subsequent circuit becomes a ten foot local run. A feeder can also be aluminium, which at these sizes and lengths saves hundreds of dollars. This is the core of the panel upgrade buildout and the standard answer for a detached garage.

Aluminium, and why long runs favour it

Aluminium has higher resistance per size than copper, so for the same drop it needs a larger conductor. But it costs dramatically less per foot, and on a long feeder the larger aluminium conductor is still cheaper than the smaller copper one.

Roughly, aluminium 4 AWG matches copper 6 AWG for resistance, and aluminium 2/0 matches copper 1/0. On a 150 foot feeder that trade is strongly in aluminium's favour. The conditions are the ones covered in aluminium or copper wiring: lugs rated for aluminium, an anti-oxidant compound at every termination, and correct torque.

Where the three percent figure comes from

Three percent on a branch circuit, and five percent total across feeder plus branch circuit, appear as informational recommendations in the code rather than as enforceable requirements. An inspector will not normally fail a circuit for voltage drop alone.

They are still worth respecting, and for EV charging more than for most loads. A lighting circuit sees its full load for a few hours a day at partial current. A charger circuit runs at close to its design limit for hours every night, which is exactly the condition under which drop matters and under which heat in terminations accumulates. Treating the recommendation as a rule costs you very little and buys a circuit that performs the way its nameplate says.

Where to go next

Confirm the conductor on the wire gauge calculator, check the breaker on the breaker size calculator, and price the run on the installation cost calculator. The full drop tables are on the voltage drop chart, and the practical discussion including what long runs cost is in long runs and voltage drop.

Common questions

What is an acceptable voltage drop for an EV charger circuit?

Three percent on the branch circuit is the widely used design target, which is 7.2 volts at 240 volts. It is a recommendation rather than a hard code requirement, but it matters more for EV charging than for most loads because the circuit runs near its limit for hours at a time, so any drop translates directly into reduced charging power on every session.

What does voltage drop actually cost me?

Roughly proportional charging power. A three percent voltage drop means the charger sees about 233 volts instead of 240, and at the same current that is about three percent less power. On a 48 amp circuit that is about 0.35 kW, or a little over a mile of range per hour. Small, but permanent, and it is why the fix belongs before the conductor is buried.

Why is the distance doubled in the formula?

Because current travels out along one conductor and back along the other, so the total length of conductor in the circuit is twice the one-way run. A charger 50 feet from the panel sits at the end of 100 feet of copper. This is why people who calculate with the one-way distance get an answer half the size of the real one.

Is it cheaper to size up the wire or add a subpanel?

Up to roughly 100 feet, sizing up the conductor is nearly always cheaper. Past that, and especially for a detached garage, a feeder to a subpanel usually wins, because one heavy feeder carries capacity for several future circuits and turns each of them into a short local run rather than another trip across the property.

Does voltage drop affect safety?

Not directly. A conductor sized for ampacity is thermally safe regardless of the drop across it, and the breaker still protects it. Excessive drop is a performance and equipment-longevity issue: motors and electronics run warmer at reduced voltage. For a charger the practical effect is slower charging rather than a hazard.

Should I use 208 volts in the calculation?

Only if your building actually supplies 208 volts, which happens in some multi-family and converted commercial buildings. The absolute drop in volts is the same, but it is a larger percentage of a smaller supply, so a run that is comfortable at 240 volts can exceed the target at 208. Check what your panel actually delivers before assuming.

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.