Key takeaways
- EV charging is a continuous load, so the code requires the circuit and breaker to be rated at not less than 125 percent of the charger's maximum load.
- A 48 amp charger therefore needs a 60 amp circuit, which is most of what a 100 amp service has to give once the house is accounted for.
- A load calculation, not a guess about panel slots, is what determines whether your panel can carry a charger.
- A listed energy management system can let a charger share a constrained service instead of forcing a full upgrade, and that path is worth pricing before you assume an upgrade.
Most EV charger quotes in San Antonio start the same way. The homeowner has picked a charger, usually the fastest one the car will accept, and wants to know what it costs to put it on the garage wall. The answer depends almost entirely on a number nobody has looked at yet: how much capacity the electrical service has left after the house is done using it.
This is the part that turns a charger quote into a panel quote, and it is worth understanding before you buy hardware.
The 125 percent rule, and why your breaker is bigger than your charger
EV charging is a continuous load. The car plugs in and pulls close to its maximum for hours, which is different from a dryer or an oven that cycles. The National Electrical Code treats continuous loads differently for exactly that reason: conductors and breakers sized precisely to the load would sit at their limit for the whole duration, and heat is cumulative.
So Article 625 requires the overcurrent protection for EV supply equipment to be rated at not less than 125 percent of the equipment's maximum load. That produces the numbers people find confusing on a quote.
- A 32 amp charger needs a 40 amp circuit.
- A 40 amp charger needs a 50 amp circuit.
- A 48 amp charger needs a 60 amp circuit.
Read that last line again with your service in mind. A 48 amp charger asks your panel to reserve 60 amps. On a 200 amp service in a newer home, that is usually workable. On a 100 amp service, you have just asked for more than half the house.
Slots are not capacity
The most common misconception we run into is that available breaker slots mean available capacity. They do not. Two empty spaces in the panel tell you there is room to land a breaker. They tell you nothing about whether the service can carry what that breaker feeds.
The actual question is answered by a load calculation. The code provides a standard method for this, and it works from the square footage of the home, the fixed appliances, the heating and cooling equipment, and the other loads, to produce a realistic figure for demand rather than a worst-case sum of every nameplate. Subtract that from the service rating and what remains is what an EV charger can have.
It is a short piece of work and it is the only honest basis for a quote. An installer who tells you the charger will be fine without doing it is guessing with your service.
What happens when the number comes up short
There are three real paths, and one of them gets skipped far too often.
The first is to install a smaller charger. This is underrated. Charging speed at home matters much less than people expect, because the constraint is not how fast the car fills, it is whether it is full when you leave. A 32 amp charger left plugged in overnight covers a normal San Antonio commute with room to spare. Buying the biggest charger available and then paying for a service upgrade to feed it is sometimes solving a problem you do not have.
The second is a listed energy management system. These monitor the home's total draw and throttle or pause EV charging when demand approaches a limit, so the service is never asked for more than it has. Because the code recognizes load management of this kind, it can allow a substantial charger on a service that could not otherwise support one. It is not the right answer everywhere, and it adds a device that has to be maintained and understood, but it deserves to be priced rather than skipped.
The third is a service upgrade. Sometimes this is simply the correct answer, particularly if the panel is also full, obsolete, or already struggling, or if solar or a second EV is on the horizon. Doing it once, sized for what is coming, beats doing it twice.
We run the calculation before quoting on every EV charger installation, and if the answer is a panel upgrade we say so with the numbers in front of you rather than after the hole is in the wall.
The San Antonio wrinkle: air conditioning
Load calculations account for cooling, but there is a practical point specific to this climate. Your service sees its hardest weeks in July and August, when the condenser runs nearly continuously and everything else in the house runs on top of it. An EV that charges overnight overlaps with an air conditioner that does not stop at night here.
It is manageable, and it is a reason to be conservative rather than optimistic when the calculation lands close to the line. A charger that works fine in March and browns the house out in August is a bad install that passed inspection.
Why summer exposes a marginal service is covered in why summer is hardest on your electrical panel, and the broader install decisions, plug-in against hardwired and where the unit goes, are in the San Antonio guide to installing a home EV charger.
What to have ready when you call
- The service rating on your main breaker, which is printed on its handle.
- A photo of the open panel door showing the breaker layout and the panel label.
- The charger you are considering, or the maximum charge rate your car accepts.
- Where you park, and where the panel is relative to it.
- Anything else coming: a second EV, a pool, solar, an addition.
That is enough for us to tell you over the phone whether this is a straightforward circuit or a capacity conversation, before anyone drives out.
We install EV circuits across the metro, and the split runs about how you would expect: newer all-electric homes in Alamo Ranch and Stone Oak usually have the capacity, while older services closer in more often need a look first, including in Schertz. Tell us your car and your panel and we will size it right.