EV Charger Circuit & Load Calculator (NEC 625)

Why an EV charger is sized differently from a dryer

A clothes dryer runs for forty minutes. An EV charger runs for six hours at full output, and the Code treats those two situations differently. A continuous load is one expected to run at its maximum current for three hours or more, and NEC 625.41 puts electric vehicle supply equipment squarely in that category. The consequence is a single number that drives the whole installation: both the overcurrent device and the branch-circuit conductors have to be rated at 125% of the charger's maximum output current.

That 25% adder is not a safety margin against surprise. It exists because a breaker's thermal element and a conductor's insulation are both rated on the assumption that the load has quiet periods in which heat can dissipate. Remove the quiet periods and the same current runs the parts hotter. Sizing at 125% restores the thermal margin the ratings assume.

The second thing that makes EVSE distinctive is how it enters the service calculation. NEC 220.57 says the EVSE load is the greater of 7,200 VA or the nameplate rating. A 16 A charger on a 240 V circuit draws only 3,840 VA, but you still add 7,200 VA to the service load — the Code assumes you will eventually swap it for something bigger. That floor is why a modest charger can still fail a panel capacity check.

Everything else on this page follows from those two rules plus ordinary conductor sizing. If you want the underlying ampacity and derating mechanics on their own, the wire size and ampacity calculator and the ampacity derating calculator handle conduit fill and ambient corrections in more depth than this page does.

The four calculations behind the result

Breaker size. Multiply the nameplate output current by 1.25, then round up to the next standard rating in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 and so on. A 48 A charger gives 60 A exactly, which is why 48 A and 60 A appear together on so many product pages. A 40 A charger gives 50 A. A 32 A charger gives 40 A.

Conductor size. The conductor's allowable ampacity, taken from the correct temperature column of NEC Table 310.16, must be at least as large as the breaker. Which column you may use is set by NEC 110.14(C): you size from the lowest-rated termination in the circuit, and breakers and EVSE lugs are normally marked 75 °C at most. The 90 °C column is for derating starting points, not for final selection. Two more constraints then apply — the small-conductor rule in 240.4(D) caps 14 AWG copper at a 15 A device, 12 AWG at 20 A and 10 AWG at 30 A regardless of what the table says, and your own voltage-drop target may force a size up.

Voltage drop. For a two-wire circuit the drop is V = 2·ρ·L·I ÷ cmil, where L is the one-way run in feet and ρ is 12.9 for copper or 21.2 for aluminium. The factor of two is there because current travels out and back. The Code does not mandate a drop limit on branch circuits, but its informational note recommends 3%, and long garage-to-panel runs are exactly where that becomes the governing constraint rather than ampacity. The voltage drop calculator covers the three-phase and parallel-set variants.

Service load. Add max(7,200 VA, V × I) ÷ V amps to whatever your existing calculated load is, and compare the total to the main breaker rating. The existing load must itself come from a real calculation — see the dwelling load calculation calculator — because adding breaker handle ratings together will overstate it badly and adding meter readings will usually understate it.

Reading the result before you buy anything

The breaker size is a minimum, and also a maximum. You may not fit a larger breaker to give yourself room, because the breaker also protects the conductor. If you want a 100 A circuit later, install the 100 A conductor now and the correct breaker for today's charger.

Negative remaining capacity is not automatically a service upgrade. NEC 625.42 permits EVSE with an adjustable output setting to be treated at its configured maximum, and it permits an energy management system under 750.30 to limit the total. Both are far cheaper than a 400 A service. Most modern chargers let the installer dial output down to 40 A, 32 A or 24 A, and the calculator will show you exactly what that buys in headroom.

Compare charging speed to your actual commute, not to the spec sheet. A 48 A charger adds roughly 36 miles per hour of charging in the default case; a 32 A charger adds about 24. If you drive 40 miles a day, both refill the car overnight several times over, and the extra copper for the 48 A circuit buys nothing but the option value of a future vehicle. Cold weather is the real variable — efficiency near freezing commonly falls by a fifth or more against the mild-weather figure the trip computer shows in summer, and the honest way to handle that is to re-run this page with your own winter mi/kWh reading rather than trust a rule of thumb.

Voltage drop below about 2% is not worth chasing. The EVSE compensates: it is a constant-power device over its input range, so a 2% drop costs you roughly 2% in charging time, not in energy. The reason to control drop is heat in the conductor and voltage at the far end of a shared feeder, not efficiency.

Standard EVSE circuits at 240 V, copper, 75 °C terminations

Range per hour assumes 90% onboard charger efficiency, so miles per hour = kW × 0.90 × 3.5 = kW × 3.15. Ampacities are the 75 °C copper column of NEC Table 310.16, with the 240.4(D) small-conductor limits applied to 14, 12 and 10 AWG.

Mistakes that fail an EVSE inspection

• Sizing from the breaker instead of the nameplate. A 60 A breaker does not mean a 60 A charger. Enter the EVSE's continuous output rating; the breaker is an output of this calculation, not an input to it.
• Using the 90 °C column. NEC 110.14(C) ties you to the lowest-rated termination. The 90 °C column exists so you have room to derate for ambient temperature and conduit fill, not so you can install smaller wire.
• Skipping the 7,200 VA floor in 220.57. Small chargers still add 7,200 VA to the service calculation. This is the single most common reason a load calculation submitted with a permit gets kicked back.
• Putting a receptacle on the circuit. NEC 625.40 requires an individual branch circuit for each EVSE outlet. A shared circuit with a garage receptacle is a violation even when the arithmetic works.
• Forgetting GFCI protection on a plug-in unit. A 240 V receptacle serving EVSE requires GFCI protection under NEC 625.54, and that changes the breaker you buy.
• Estimating the existing load from breaker handles. Adding up handle ratings on a 200 A panel routinely produces 400 A or more. Run the standard or optional dwelling calculation instead.
• Ignoring the disconnect requirement. EVSE rated over 60 A or over 150 V to ground needs a lockable disconnecting means in sight of the equipment under NEC 625.43.

When a different tool fits better

If the panel is genuinely full, your options are a load-management device, a smaller charger setting, or a service upgrade — in that order of cost. Load management under NEC 750 and 625.42 lets the EVSE shed to zero when the rest of the house peaks, and it is permitted to be counted at its managed maximum in the service calculation. That single sentence has saved more service upgrades than any other change in recent code cycles.

If you are wiring a multi-unit building or a workplace, the branch-circuit arithmetic here still holds per charger, but the feeder and service sizing follow Article 220 Part IV and can take demand factors that a single dwelling cannot. Do not apply a residential diversity assumption to a bank of chargers.

If you are checking equipment ratings rather than sizing a circuit, the interrupting rating of the breaker matters as much as its trip rating; the available short circuit current calculator gives you the fault current the panel must survive. And if the question is running cost rather than capacity, the appliance energy cost calculator converts kWh delivered into a monthly bill at your own tariff.

If you are comparing Level 2 against DC fast charging, note that everything on this page is AC. A DC fast charger converts externally and its site requirements are an order of magnitude larger — three-phase service, a dedicated transformer, and a demand charge that dominates the economics. Level 2 is the right answer for anything that sits still overnight.

Key terms

EVSE

Electric vehicle supply equipment — the wall unit, cable and connector. It is not a charger in the technical sense; the charger is inside the vehicle, which is why onboard charger efficiency appears in the speed calculation.

Continuous load

A load whose maximum current is expected to continue for three hours or more. NEC 625.41 classifies EVSE as continuous, which triggers the 125% sizing factor on both conductors and overcurrent devices.

Ampacity

The current a conductor can carry continuously without exceeding its temperature rating, taken from NEC Table 310.16 and then corrected for ambient temperature and the number of current-carrying conductors bundled together.

Circular mil

The area of a circle one thousandth of an inch in diameter. Conductor areas are quoted in circular mils because the voltage-drop formula divides by them directly: 6 AWG is 26,240 cmil, 4/0 is 211,600 cmil.

Calculated load

The service load produced by the Article 220 procedure, in amps. It is deliberately smaller than the sum of connected equipment because it applies demand factors that reflect what actually runs at the same time.