Why Is My EV Charger Not Charging at Full Speed?

If your EV charger is delivering far less power than it should, the cause is almost always one of three things: the car or the charger is set to a lower amperage than the circuit can supply, the charger is plugged into a 120-volt outlet instead of a dedicated 240-volt circuit, or the circuit itself was installed at a smaller size than the charger’s maximum. Temperature, cable length, and a nearly full battery also slow charging down, and those are normal.

This is the single most useful thing to understand about slow charging: your charger does not choose the speed on its own. The car and the charging equipment negotiate a current level every time you plug in, and both sides can say no. If your car’s onboard charger accepts a maximum of 32 amps, a 60-amp circuit will not make it charge faster. If your circuit is 20 amps, a car capable of 48 amps still gets 16 amps. The lowest number wins, every time.

Amperage, by the way, is just the rate of electrical flow, like the width of a pipe. Voltage is the pressure. Multiply them and you get power in watts, which is what actually fills the battery.

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What is usually causing it

Here are the causes worth checking in roughly the order they turn up, starting with the ones that need no tools at all.

You are on 120 volts, not 240. If you are seeing roughly 1.4 to 1.9 kW, you are almost certainly using the portable cord that came with the car, plugged into a standard household outlet. That is Level 1 charging, and it is working exactly as designed. It adds only a few miles of range per hour. Level 2 needs a 240-volt dedicated circuit.

Why am I only getting 2 kW? This is the same story told a slightly different way. About 2 kW usually means 120 volts at roughly 16 amps, or 240 volts at a very low current setting. If you know you are on a 240-volt circuit and still seeing 2 kW, something has limited the current, and that is a setting, not a fault.

The current is set low in the app or on the charger. Most hardwired chargers have a configurable maximum, set during installation to match the circuit. Many also have a schedule, a “reduce power during peak hours” mode, or a shared-load setting for two vehicles. Some cars let you cap charging current per location, and that cap sticks after you set it once.

The circuit is smaller than the charger. A 48-amp charger installed on a 40-amp circuit has to be configured down to 32 amps. That is correct and safe, not a mistake, but it explains the speed.

The car’s onboard charger is the limit. Onboard charger capacity varies widely between models and trim levels, and some vehicles have an optional upgrade. Plenty of EVs top out at 7.7 kW no matter what you plug them into.

The battery is nearly full, or cold. Charging tapers sharply once the battery is most of the way full on many vehicles. In a Wisconsin January, a cold battery pack will also accept less current until it warms, and some of the energy you are paying for goes into heating the pack rather than storing charge.

Voltage sag from a long or undersized run. A long circuit run to a detached garage, sized tightly, loses voltage under load. The charger still works, just slower.

Typical Level 2 circuit sizes and what they deliver

A continuous load like EV charging is limited to 80% of the breaker rating, which is why the numbers below look the way they do.

Circuit breaker	Maximum continuous charging current	Approximate power at 240 V
20 A	16 A	about 3.8 kW
30 A	24 A	about 5.8 kW
40 A	32 A	about 7.7 kW
50 A	40 A	about 9.6 kW
60 A	48 A	about 11.5 kW

A charger drawing 32 amps on a 40-amp breaker is a correctly matched pair and should not trip. Breakers trip above their rating, following a time-delay curve. If yours trips at a load it should carry, that is a fault to investigate, not a reason to fit a bigger breaker. The breaker protects the wire, and a larger one on the same wire is a fire risk.

What is safe to check yourself, and what is not

Safe to check: the charger’s app or display for a current limit or schedule, the car’s charging settings for a per-location amperage cap, whether the plug is a 240-volt outlet or a standard 120-volt receptacle, and the state of charge and outside temperature. Also worth checking: whether the cord and connector are seated fully and whether the connector faces are clean and undamaged.

Not safe, and not legal for an unlicensed person in most Wisconsin municipalities: anything behind the panel cover, any work on the service conductors between the meter and the main breaker, or altering the circuit feeding the charger. Turning the main breaker off does not make the panel safe. The service conductors and the main breaker’s line-side lugs stay energized after the main is off. A homeowner cannot de-energize them. Only the utility can, usually by pulling the meter, or a separate service disconnect ahead of the panel if one is fitted.

Stop and call an electrician immediately if you notice a burning or hot-plastic smell, discoloration or scorch marks around the outlet, charger, or panel, or buzzing or crackling. Stop using that circuit, leave the breaker off, and do not touch the device to judge how hot it is. Smell, sight, and sound tell you what you need to know.

If a breaker has tripped, reset it once at most: push firmly to OFF, then to ON. If it trips again, leave it off and have it inspected. The same rule applies to a GFCI. A ground-fault circuit interrupter, or GFCI, trips when as little as 4 to 6 milliamps of current leaks out of the circuit by any path, including through a person or through damp material. No metal contact is needed. A GFCI that will not hold is telling you there is a real leakage fault or a failed device. It is not “being sensitive.”

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When it points to a bigger problem

Slow charging that started suddenly, on a circuit that used to be fast, is different from slow charging that has always been slow. A charger that has quietly throttled itself, or that trips partway through a session, may be responding to a genuine problem: a loose termination heating up, moisture in an outdoor enclosure, a failing connector, or a neutral or ground fault upstream.

Some chargers also reduce current automatically when their internal temperature climbs or when the supply voltage sags, and they may not tell you clearly that they have done it. On paper that looks like a slow charger. In practice it can be a connection problem that is generating heat.

Repeated trips of the charger’s own ground-fault protection are worth taking seriously. EV charging equipment uses a charging-circuit interrupting device rated at 20 milliamps under UL 2231, which is a higher threshold than a standard household GFCI. If that keeps tripping, the leakage is real.

A Level 2 charger must be on its own dedicated circuit with nothing else on it, per NEC 625.40. Wisconsin SPS 316, which adopts the 2023 NEC (in effect since September 1, 2026), governs the work, and your local municipality, whether Milwaukee, Wauwatosa, Waukesha, or another community in the area, issues the permit and inspects it. Existing installations permitted before that date were done under the 2017 NEC and do not have to be brought up to the newer edition unless they are altered.

What an electrician does about it

The first job is measurement, not guessing. An electrician confirms the voltage at the charger, the actual current being drawn during a session, and the voltage drop under load. That alone separates “your car is the limit” from “this circuit has a problem.”

From there the work depends on what the numbers show. A charger configured below the circuit’s capacity gets reconfigured. A loose or heat-damaged termination gets repaired. A circuit that was sized for a smaller charger may be able to carry more, or may need a new run. And before any of that, a load calculation, which is a formal tally of the home’s existing electrical demand against the capacity of the service entrance where power arrives from the utility, determines how much current the panel can safely give a charger. That calculation is why nobody honest can promise you 48 amps over the phone. Spare capacity varies house to house.

If you want a rough sense of what your vehicle and home could support before you call, the EV charging speed estimator is a reasonable starting point. For Tesla owners, a Wall Connector installation has its own configuration steps that affect final charge rate.

Lehmann Electrical & Design is led by master electrician Jacob Lehmann and holds an A+ rating with the BBB and 4.9 stars on Google. Our work carries a 12-month workmanship warranty.

If your charger is slower than it should be and you would rather have it measured than guessed at, request an estimate for charger diagnosis or installation or call 414-207-0109. We cover Milwaukee and Waukesha County.

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Frequently asked questions