Breaker and wire size are the two numbers that make an EV charger circuit safe or unsafe, and they are where online advice gets dangerously casual. This guide explains how an electrician gets from the charger's output to the breaker and then to the wire, using the 2020 National Electrical Code that Florida's 8th Edition building code adopts (Florida Building Commission). It is a guide for understanding a quote, not a do-it-yourself chart. Your building department and the charger's installation manual have the final word.
Step 1: the breaker follows the charger
EV charging is treated as a continuous load, and the branch circuit's overcurrent protection must be at least 125% of the charger's maximum load. A 40 A charger therefore needs at least a 50 A breaker (NEC 625.41; Mike Holt, Article 625 summary). Turned around, a charger may draw no more than 80% of its breaker's rating.
Every charger also gets its own branch circuit with no other outlets on it (NEC 625.40; Leviton code reference). A charger tapped off a range or dryer circuit is not a shortcut; it is a code violation.
Step 2: the wire must carry the breaker
The conductors have to be rated for at least the breaker's current, and that rating depends on the wire's size, material and temperature column. NEC Table 310.16 lists allowable ampacity at 60°C, 75°C and 90°C, based on an ambient of 30°C (86°F) and no more than three current-carrying conductors together (ExpertCE, how to use Table 310.16). For copper:
| Copper size | 60°C column | 75°C column | 90°C column |
|---|---|---|---|
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
Which column you may use is the catch. A conductor's ampacity cannot exceed the lowest temperature rating of anything it connects to, and for circuits of 100 A or less the 60°C column applies unless the conductor and the equipment terminals are both rated 75°C (ExpertCE on terminations). Sheathed NM cable, the common "Romex" used inside walls, is used at its 60°C ampacity (NM-B ampacity reference). Individual THHN or THWN-2 conductors in conduit can use the 75°C column when the breaker and the charger terminals are 75°C-rated.
Putting it together
This is how the common EV circuits usually come out in copper. It is a starting point, not a substitute for a site-specific design.
| Charger output | Minimum breaker | NM cable (60°C) | Conductors in conduit, 75°C terminations |
|---|---|---|---|
| 24 A | 30 A | 10 AWG | 10 AWG |
| 32 A | 40 A | 8 AWG | 8 AWG |
| 40 A | 50 A | 6 AWG | 8 AWG |
| 48 A | 60 A | 4 AWG | 6 AWG |
Tesla's Wall Connector manual lines up with the last row: at maximum output on a 60 A circuit it calls for minimum 4 AWG copper with a 60°C-rated breaker, or minimum 6 AWG with a 75°C-rated breaker, both in 90°C THWN-2 wire, and it accepts copper terminations only (Tesla Wall Connector wire guidance).
What makes a correct size wrong
Florida heat
The table assumes 86°F surroundings. A conduit run across a sun-baked exterior wall, or cable through a South Florida attic in July, sits well above that, and the ambient-temperature correction factors that go with the ampacity table reduce allowable ampacity as the surroundings get hotter (Thomas & Betts reprint of the table and correction factors). A wire that is fine in a conditioned wall may need to be upsized in an attic. This is one of the most common reasons two electricians quote different wire sizes for the same charger.
Long runs
A charger at the far end of a driveway or a detached carport may be 100 feet or more from the panel. Voltage drop over that distance wastes energy as heat in the wire. Upsizing the wire on long runs is normal practice; ask your electrician whether they calculated it.
Aluminum
Aluminum conductors need a larger size for the same ampacity, and some chargers, including Tesla's Wall Connector, accept copper only at their terminals. Never assume an aluminum feeder can land on a charger.
Bundling
More than three current-carrying conductors in one raceway reduces ampacity. It matters when an EV circuit shares conduit with others on the way out of the panel.
GFCI and the breaker type
If the charger plugs into a receptacle, that receptacle needs GFCI protection for personnel (NEC 625.54; Leviton code reference), usually a GFCI breaker in the panel. Hardwired chargers are not covered by that receptacle rule. For the trade-offs, see hardwired vs plug-in chargers and our page on NEMA 14-50 outlet installation.
Signs a circuit was undersized
- A breaker that trips after an hour or two of charging (see why an EV charger keeps tripping).
- A warm or discolored receptacle, plug or breaker.
- A charger set higher than its breaker allows, for example a 48 A setting on a 50 A breaker.
- A 14-50 on 10 AWG wire, which the 60°C column rates at 30 A.
If any of these apply, stop charging and have the circuit checked. Our electrical safety inspections cover this.
What to ask your electrician
- What is the charger's maximum output, and what breaker are you pairing it with?
- What wire type and size, and which temperature column did you use?
- Does the route pass through an attic or across a sunny wall?
- How long is the run, and did you check voltage drop?
- Will you set and lock the charger's current to match the breaker?
Questions homeowners ask
Can I use 8 AWG wire on a 50 A breaker?
Only where the 75°C column applies: individual conductors in conduit, with 75°C-rated terminations at both ends. Not with NM cable, which is limited to 40 A in 8 AWG.
Why not just install the biggest wire?
Larger wire costs more and may not fit the charger's terminals. The goal is the right size for the breaker, the route and the temperature, with room for a future upgrade if you want it.
Does the breaker brand matter?
Yes. Breakers must be the type listed for your panel. Mixing brands is a common problem in older South Florida panels.
My old 14-50 was on a 40 A breaker. Is that wrong?
Not necessarily, but whatever plugs into it must be set to no more than 32 A. Have the wire size checked too.
Have it sized by someone who signs the permit
KRW Electric (EC13012832) designs and installs EV charger circuits across Miami-Dade, Broward and Palm Beach and meets the inspector on each one. Start with EV charger installation, see how many amps your charger needs, browse the Florida Home EV Charging Guide, or send us your charger model and a panel photo.
Reviewed by Kyle White, KRW Electric Company LLC — licensed electrical contractor EC13012832.
