What Size Breaker for an EV Charger?

How to size a breaker for an EV charger in five steps

Every version of this calculation, in either direction, is the same five steps. The first two come from the car and your habits; the last three come from the code and the panel.

• 01Turn the energy you need into kilowatts. Wall kilowatt-hours divided by the hours you are willing to wait. A 200-mile charge for an electric pickup doing 60 MPGe is 112.3 kWh, so a twelve-hour window needs 9.36 kW and an eight-hour window needs 14.0 kW.
• 02Turn kilowatts into continuous amps. Divide by the supply voltage: 9.36 kW at 240 V is 39.0 A, 14.0 kW is 58.5 A. This is the current the charger will hold for hours, not a momentary surge.
• 03Multiply by 1.25 and round up to a standard size. 39.0 × 1.25 = 48.8, which is a 50 A breaker. 58.5 × 1.25 = 73.1, which is an 80 A breaker. NEC 240.6 keeps you honest here: the list is 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, and there is no 55 A or 120 A device to round into.
• 04Size the conductor at the same 125 percent, before the breaker is anybody's problem. 48.8 A of required conductor is 6 AWG copper NM-B (55 A); 73.1 A is 3 AWG (85 A). Doing step 3 and skipping step 4 is how a 50 A breaker ends up on 8 AWG wire.
• 05Check the panel and the outlet. Spare double-pole slots and the rating on the main breaker decide whether this is a new circuit or a service upgrade, and the amp figure decides whether the charger plugs into a NEMA 14-50 or is hardwired.

From the rating on the box to the breaker in the panel

Chargers are sold by their output current, so this is the table most people need first. The required-conductor column is 125 percent of the charger's own current, because that is the load the wire has to carry continuously, and the wire sizes are for NM-B cable — the sheathed household cable, which NEC 334.80 limits to the 60°C column of Table 310.16.

Miles an hour are for a car doing 115 MPGe, or 29.3 kWh per 100 miles at the wall, and kilowatts are calculated at 240 V throughout — the two smallest rows are normally sold as 120 V Level 1 cables, where the same amps are 0.96 kW and 1.44 kW. The plug column follows the EPA's statement that “For chargers rated at 48-Amps or higher, they are usually required to be hardwired instead of plugging them into an outlet”, and its note that “NEMA 14-50 outlets are a common choice for Level 2 EV chargers”. Nothing here is a substitute for a permit and an inspection; it is the arithmetic to read the quote against.

The row that surprises most people is 30 A. A charger that outputs 30 A does not sit on a 30 A breaker: 30 × 1.25 = 37.5, and the next standard size up is 40 A. It is the same arithmetic as the EPA's example one step further along, and it is why an installer quoted “a 40 A circuit for a 30 A charger” is doing the code rather than padding the job.

From the breaker you already have to the amps you may use

Read the same rule in the other direction and it becomes the number that matters at 2 a.m.: how much current a circuit may carry for hours. A breaker's rating is not usable current; 80 percent of it is. Conductor requirements are then calculated on the load, not on the breaker.

A 15 A or 20 A double-pole breaker is not a Level 1 circuit — Level 1 is 120 V, and the two 120 V rows on the charging speed page are 1.44 kW and 1.92 kW, half these figures. The hours column is an 11.7 kWh session (40 miles at 115 MPGe) divided by the kilowatts in the row, at 240 V.

Why 125 percent, and where the 80 percent comes from

Both numbers hang off one definition. NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for three hours or more, and a charging session is exactly that: an EVSE pulling the same amps from ten at night to six in the morning. Article 625 then takes the argument out of anyone's hands. In the 2023 code, §625.42 states that “Electric vehicle charging loads shall be considered to be continuous loads”, and §625.41 sizes the overcurrent protection at not less than 125 percent of that continuous current.

The 80 percent ceiling is not a second rule. It is the first rule inverted: if a breaker has to be at least 125 percent of a load, then a breaker can be the source of at most 80 percent of its own rating in current. Both halves of the pair appear in plain English on the EPA's home-charging page, along with the example that a 40-amp Level 2 charger “would require a dedicated 50-amp electrical circuit”. Connecticut's own electrician continuing-education material works the identical arithmetic coming in the other way — “32 A x 1.25 = 40 A minimum OCPD” — which is why a charger that outputs 32 amps is sold as a 40-amp unit.

Two honest caveats about the citation. Section numbers move between NEC editions and between the amendments your state adopts, so if you are arguing with an inspector, quote Article 625 and the sentence rather than relying on the number alone. And the 2023 code also recognises listed energy-management systems and labelled adjustable means that let a charger be sized below its nameplate output — a real product category that reduces the circuit you need, and one an electrician has to specify rather than a homeowner improvise with a dial on the box.

The cable behind the breaker, and why you cannot just fit a bigger one

The breaker is not there to protect the car. It is there to protect the copper in the wall, which is why a breaker swap is a different job from a breaker installation. Three provisions do the work, and they divide the conductor range in half:

• 01NEC 240.4(D) covers the small conductors only. It limits overcurrent protection to 15 amps on 14 AWG copper, 20 amps on 12 AWG and 30 amps on 10 AWG, and its scope stops at 10 AWG. Those three pairings are absolute: they apply whatever the wire's own ampacity column says, which is why a 14 AWG circuit cannot be re-rated to 20 amps by swapping the breaker.
• 028 AWG and thicker are protected at their ampacity, in Table 310.16. The same table gives three columns — 60°C, 75°C and 90°C — and the column you may use is decided by the cable, not by the wire's print.
• 03NEC 334.80 limits NM-B to the 60°C column. Sheathed household cable, whatever it says on its own jacket, is treated as 60°C: 12 AWG at 20 A, 10 AWG at 30 A, 8 AWG at 40 A, 6 AWG at 55 A, 4 AWG at 70 A. That single rule is why the 40 A row above lands on 6 AWG rather than 8.
• 04Terminals decide the rest. Under NEC 110.14(C) a conductor's usable ampacity is capped by the temperature rating of the terminals it lands on. THHN in conduit at 75°C makes 8 AWG a 50-amp wire, which is why the same 40 A charger can be wired two different ways in two different buildings — and why the choice belongs to the person holding the permit.

Now the sentence that matters for anybody planning to upgrade: a 40 A charger needs 50 A of conductor capacity, because 40 amps of continuous load is sized at 125 percent. 8 AWG NM-B is a 40-amp wire. So putting a 50 A breaker on an existing 8 AWG run does not create a 50 A circuit — it creates a circuit whose wire can reach its limit while the breaker still thinks everything is normal, which is a fire risk and an inspection failure rather than a shortcut. Replacing the run is the job. Adding a 40 A continuous load to a 100 or 125 A service, meanwhile, is not a breaker question at all: it is the panel calculation in the next section.

For the pairing in the other direction — 12 AWG on a 15 A breaker is fine, 14 AWG on a 20 A breaker is not — the site's circuit wattage table works breaker, wire and watts for ordinary appliances as well as chargers, and the amps to watts calculator handles the volts-times-amps step.

Sizing backwards from the hours you have

The tables above start from a charger you have already chosen. The more useful version starts from the drive you have to do and the window you have to do it in, which is the conversation to have before buying anything. The example is the heaviest case in the calculator: a 200-mile recharge for an electric pickup at 60 MPGe, which is 112.3 kWh at the wall.

Kilowatts are 112.3 kWh divided by the window; amps are kilowatts divided by 240 V; the breaker is 125 percent of that rounded up. Ask for the same charge in four hours and the arithmetic stops being a household circuit: 117 amps of continuous current is past NM-B, past a residential double-pole breaker and into parallel conductors in conduit, which is a commercial installation.

Read down the breaker column and the real cost step is between ten hours and eight. The first is a 60 A circuit and a mainstream charger; the second is an 80 A circuit, a heavier cable, usually a hardwired unit, and a service that has to be able to spare the load. That is the decision an installer is pricing when the quote doubles.

The panel is the other limit

A short, correctly sized cable does not create capacity that the service cannot supply. The EPA's screening test is written for exactly this moment: “If your home has at least 200-Amps and two empty slots for a double-pole circuit breaker in your panel, you likely have capacity for a Level 2 charger.” It softens that usefully — “Even 100-Amp service is often sufficient, depending on how much electricity your other appliances use” — and gives the demographic rough guide: older homes “usually have 60 or 100 Amps of power”, while homes “built around the 1980s or later tend to have 200 Amps or more”.

Its fallback when capacity is tight is the most practical line on the page: “A 16-Amp Level 2 charger might perfectly suit your needs if your electrical panel is tight on capacity.” That is 3.84 kW and 13.1 miles of range an hour, which is 105 miles in an eight-hour night — enough for almost every American commute and a great deal less than a panel upgrade.

The step that no calculator can do for you is the load calculation itself, which adds up what the house actually pulls and what the service can carry. The code has statistical methods for existing dwellings precisely so that a new large load does not automatically mean a new service, and the electrician who signs the permit is the person who runs that arithmetic. What this page gives you is the input they will ask for: the wall kilowatt-hours, the window, the resulting amps, the breaker and the cable.

Outlet, GFCI and hardwiring

Three code requirements sit behind the breaker question and none of them are amperage arithmetic:

• 01The circuit is dedicated. NEC 625.40 requires an individual branch circuit for EVSE outlets rated more than 16 amps or more than 120 volts; the EPA describes the same thing as a “dedicated 50-amp electrical circuit”.
• 02Ground-fault protection is part of the circuit. NEC 625.54 requires ground-fault circuit-interrupter protection for cord-and-plug-connected charging equipment, which is a device being specified rather than a box being added later. Read the section in the edition your state has adopted — the treatment of hardwired units has been tightened in the most recent codes.
• 03Plugs have a ceiling. Above 48 amps the EPA's statement is that chargers “are usually required to be hardwired instead of plugging them into an outlet”, and the 14-50 receptacle it calls “a common choice” is the end of the cord-and-plug world for most homes.

What this page deliberately does not tell you

• 01Prices. Installation cost is regional, depends on the run length, the panel and whether a wall is finished, and any single figure on this page would be a guess dressed as a fact. Two sources of a quote are worth more than one number.
• 02Your local amendment. States and municipalities adopt NEC editions on their own schedule and change text. Section numbers here are 2023; the sentences are what to check.
• 03Voltage drop and derating. A long run to a detached garage, conductors sharing a hot conduit, and ambient temperature all reduce what a cable can carry. The tables here are the base case.
• 04Which charger to buy. Anything from 16 A to 48 A will charge a car overnight; the differences are price, cable length, network features and how much future-proofing you want to pay for.

The EV charging cost calculator runs this arithmetic on your own numbers, including the breaker required to finish inside a chosen window, and the other two pages in the cluster cover what charging at home costs and which circuit is fast enough. The energy tools hub has the rest of the set.