Voltage Drop Chart

Ampacity answers whether a conductor gets too hot. Voltage drop answers whether enough voltage arrives at the far end. They are different questions with different answers, and on any run longer than a garage wall it is the second one that decides the conductor. A 6 AWG copper conductor carrying 48 amps is thermally comfortable at 150 feet and electrically poor, and nothing on an ampacity table will tell you that.

What is the voltage drop at 48 amps in copper?

Distances are one way, from the panel to the charger, measured along the actual cable route rather than across the floor. Anything marked over has passed the three percent design target.

The exact crossover distances, where each copper gauge passes three percent at 48 amps, are worth having as single numbers rather than reading off a grid.

What is the voltage drop at 40 amps in copper?

Less current means less drop for the same conductor, so a 40 amp charger on a 50 amp circuit reaches noticeably further than a 48 amp one before the target bites. This is one of the quieter arguments for the smaller circuit when the panel is a long way from the parking space.

What is the voltage drop at 48 amps in aluminium?

Aluminium has higher resistance per foot than copper for the same gauge, so every row shifts. The compensation is the same size increase that ampacity already requires, and on a long feeder the larger aluminium conductor is frequently still cheaper than the smaller copper one.

Aluminium is a legitimate conductor for a feeder and needs AL/CU rated lugs, an anti-oxidant compound such as Ideal Noalox, and torque to the manufacturer's specification rather than to feel. The full argument is in aluminium or copper wiring, and the ampacity side is on the wire gauge ampacity chart.

How is voltage drop actually calculated?

The formula for a single-phase two-conductor run is short. Voltage drop equals two, times the one-way length in feet, times the current in amps, times the conductor resistance in ohms per thousand feet, divided by one thousand. Express that as a percentage by dividing by the nominal supply voltage and multiplying by a hundred.

The factor of two is the part people leave out. Current does not travel one way and vanish. It goes out along one ungrounded conductor and returns along the other, so the circuit length is twice the distance you measured. A charger 100 feet from the panel sits on 200 feet of conductor as far as this arithmetic is concerned, which is why drop climbs so much faster than intuition suggests.

Resistance values come from published conductor property tables and are direct-current figures for stranded conductor at a reference temperature. Two effects push the real number up. Conductor resistance rises with temperature, so a conductor running warm under a continuous load has slightly more resistance than the table shows. And on large conductors, alternating current introduces skin and proximity effects that add a little more. Neither matters much at residential EV sizes, but both mean the table is a slightly optimistic floor rather than a ceiling.

Is three percent a rule or a suggestion?

A suggestion, and this is the single most important thing to understand about the whole subject. The three percent branch-circuit figure and the five percent combined feeder-plus-branch figure appear in informational notes, which are explicitly advisory rather than enforceable requirements. An inspector will not fail a circuit for exceeding them.

That is exactly why the number disappears from quotes. An electrician sizing purely to ampacity and overcurrent protection has produced a compliant installation, and on a short run the two calculations agree anyway so nothing is lost. On a long run they diverge sharply, and the homeowner is the only person with an incentive to notice, because the consequence lands on their electricity bill and their charging times rather than on the inspection.

So treat three percent as a design decision you are entitled to specify. Ask for it explicitly when you request a quote for a run over about 60 feet, and be prepared for the conductor to come back one size larger than the ampacity table alone would suggest. On a run you are only going to make once, that is a cheap upgrade.

What does an over-target run actually cost you?

Two things: charging speed and wasted energy. The wasted energy is the easier one to put a number on. The voltage lost across the conductor multiplied by the current is power dissipated as heat in the wall, and it runs for every hour of every session. Assuming 300 sessions a year at 4 hours each and the national average rate of 16.5 cents per kWh:

The dollar figures are modest, which is worth saying honestly rather than inflating. The stronger argument is the other one: an installation is a one-time labour cost and a conductor upgrade at install time is a material cost only. Paying twenty or forty dollars more for the next size up while the conduit is empty is cheap. Rerunning the same conduit in five years because charging got slow is not.

What does a charger do when it sees low voltage?

Three things, in escalating order. First, and always, it delivers less power. Power is voltage times current, and the current is fixed by the negotiated limit, so a supply sagging to 230 volts instead of 240 delivers roughly four percent less power. The session simply takes longer.

Second, some units monitor supply voltage and reduce their own output when it sags, on the reasonable theory that a sagging supply indicates a problem upstream. That turns a modest drop into a larger reduction in charging speed than the arithmetic alone predicts.

Third, at the extreme, a unit may fault out entirely with an undervoltage code and stop charging. That is rare on a merely long run and common on a run that is both long and undersized, or on a circuit with a developing connection problem. If a charger that used to work has started reporting voltage faults, the run length has not changed, so something else has, and that belongs in troubleshooting rather than in a table.

Practical decisions the chart supports

Under about 50 feet, ignore this page

At typical residential distances, ampacity governs and voltage drop is comfortably inside target for the conductor you were going to use anyway. Buy the conductor the ampacity chart calls for and move on.

Between 50 and 120 feet, check before you buy wire

This is the band where the two calculations disagree and where one size up is usually the right answer at 48 amps. It is also the band most detached-garage-adjacent installations fall into, and where reducing the charger to 40 amps genuinely buys back reach if the car cannot use 48 anyway.

Past about 120 feet, consider a subpanel

At that distance a single branch circuit sized for voltage drop is an expensive conductor doing one job. A feeder to a small load centre at the far end costs similar money and gives you circuits for lighting, receptacles and a second charger later. That is the argument in subpanel for an EV charger and detached garage EV charger.

Run your own distance and amperage in the voltage drop calculator, price the conductor difference in the installation cost calculator, and read long runs and voltage drop for the reasoning in prose rather than in grids.

Common questions

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

The widely used design target is three percent on a branch circuit, which at 240 volts is 7.2 volts. That figure comes from an informational note rather than an enforceable requirement, so a circuit exceeding it does not fail inspection. It is still worth meeting, because the difference is dissipated as heat in the wall for every hour of every charging session for the life of the installation.

How do you calculate voltage drop on a 240 volt circuit?

Multiply two times the one-way length in feet by the current in amps by the conductor resistance in ohms per thousand feet, then divide by one thousand. The factor of two accounts for the current travelling out along one conductor and back along the other. Divide the result by 240 and multiply by 100 to express it as a percentage of nominal supply voltage.

How far can I run 6 AWG copper for a 48 amp charger?

Roughly 153 feet one way before the drop passes three percent at 48 amps. Ampacity is not the constraint at that distance, since 6 AWG copper carries 65 amps in the governing 75 degree Celsius column. Beyond that distance the correct answer is a larger conductor rather than a longer run, and 4 AWG roughly doubles the reach.

What happens if the voltage drop is too high?

The charging station sees a lower supply voltage, so it delivers less power for the same current, and the charging session takes longer. Some units monitor supply voltage and reduce their output or fault out if it sags far enough. Meanwhile the missing voltage is dissipated as heat along the conductor run, which is wasted energy you pay for every session.

Does aluminium make voltage drop worse?

For the same gauge, yes, because aluminium has higher resistance per foot than copper. In practice you compensate by going up a size or two, which is the same adjustment ampacity already requires. On a long feeder the larger aluminium conductor is still frequently cheaper than the smaller copper one, which is exactly why aluminium is common for feeders and rare for short branch circuits.

Is voltage drop a code violation?

No, not on a branch circuit. The three and five percent figures appear in informational notes, which are advisory rather than enforceable, so an inspector will not fail a circuit for exceeding them. That is precisely why the number vanishes from quotes and why a homeowner should check it. Ampacity and overcurrent protection are enforceable, and those are separate calculations.

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.