Use this EV charging amps calculator to convert charger kW, voltage, and current, and size the minimum circuit breaker using the NEC 125% continuous-load rule.
EV Charging Amps Formula
An EV charger’s continuous current is found with Ohm’s law, dividing charging power by voltage. Power is entered in kilowatts, so it is multiplied by 1000 to convert to watts:
I = P * 1000 / V
Because an EV charger runs for hours at a time, the NEC treats it as a continuous load. The circuit and its breaker must be rated for at least 125 percent of that current, then rounded up to the next standard breaker size:
Breaker >= 1.25 * I
Rearranging the same rule shows the most a given breaker can deliver. A continuous load may use only 80 percent of the breaker rating, so the maximum current and power are:
I(max) = 0.8 * Breaker P = V * I / 1000
Variables:
- I is the continuous charging current, in amps (A)
- P is the charger power, in kilowatts (kW)
- V is the supply voltage, in volts (typically 120, 208, or 240 V)
- Breaker is the circuit breaker rating, rounded up to a standard size (15, 20, 30, 40, 50, 60, 70, 80, 90, or 100 A)
- 1.25 is the NEC continuous-load factor; its inverse, 0.8, is the 80 percent of the breaker rating a charger may draw
Pick what you want to find at the top of the calculator. “Amps & breaker from charger power” turns a charger’s kW rating and voltage into the continuous current and the minimum breaker. “Max charging power from breaker size” works backward from an existing breaker to the largest charger it can run. “Breaker size from charger amps” sizes the circuit from a charger’s rated current.
Common Level 2 EV Charger Circuits
This table lists typical Level 2 home charging setups at 240 volts, matching each charger’s continuous current to its power output and the minimum breaker required under the 125 percent rule. Use it as a quick reference before you size a circuit.
| Charger current | Voltage | Charger power | Minimum breaker |
|---|---|---|---|
| 16 A | 240 V | 3.8 kW | 20 A |
| 24 A | 240 V | 5.8 kW | 30 A |
| 32 A | 240 V | 7.7 kW | 40 A |
| 40 A | 240 V | 9.6 kW | 50 A |
| 48 A | 240 V | 11.5 kW | 60 A |
Example Problems
Example 1: Size the circuit for a charger by power.
You have an 11.5 kW charger on a 240 V home circuit. First find the continuous current:
I = 11.5 * 1000 / 240 = 47.9 A, which rounds to a 48 A charger. Applying the continuous-load rule, Breaker = 1.25 * 47.9 = 59.9 A, which rounds up to the next standard size, a 60 A breaker.
Example 2: Find the maximum charger for an existing breaker.
You already have a 50 A breaker on a 240 V circuit and want to know the biggest charger it can run. A continuous load may use 80 percent of the breaker:
I(max) = 0.8 * 50 = 40 A, and P = 240 * 40 / 1000 = 9.6 kW. So the 50 A breaker supports a charger drawing up to 40 A, or about 9.6 kW.
Frequently Asked Questions
What size breaker do I need for a 48-amp EV charger?
A 48 A charger is a continuous load, so its breaker must be rated for 125 percent of that current: 1.25 * 48 = 60 A. A 48 A charger therefore needs a 60 A breaker, which is why most 11.5 kW home units are installed on a 60 A circuit with appropriately sized wire.
Why can a charger only use 80 percent of the breaker rating?
The National Electrical Code defines any load running for three hours or more as continuous, and requires the circuit to be sized at 125 percent of that load. The inverse of 1.25 is 0.8, so in practice a continuous load such as an EV charger may draw no more than 80 percent of the breaker’s rating. This margin keeps the breaker and conductors from overheating during long charging sessions.
What voltage do home EV chargers use?
Most home Level 2 chargers run on a 240 V circuit, the same type used for an electric range or dryer. Basic Level 1 charging uses a standard 120 V outlet but is much slower. Commercial and multi-family installations often use 208 V, which delivers slightly less power than 240 V at the same current.
