EV Charger Circuit Sizing: Breaker, Wire and Voltage Drop in 5 Steps
Size a Level 2 EV charger circuit the NEC way: apply the 125% continuous-load rule, pick the breaker, check wire ampacity and voltage drop, and avoid the common NM-B trap.
Step-by-Step Procedure
Start with the charger current, not the breaker
Read the charger's maximum charging current from its nameplate or settings, for example 32 A, 40 A or 48 A. EV charging is a continuous load, so the circuit must be sized above that current, not equal to it.
Apply the 125% continuous-load rule
Multiply the charger current by 1.25 to get the minimum breaker size, then round up to the next standard breaker size. This gives 32 A to 40 A, 40 A to 50 A, and 48 A to 60 A.
Pick wire by ampacity and temperature column
Check the wire ampacity in the correct temperature column for its wiring method. This is where the NM-B trap matters: 6 AWG copper as NM-B cable is limited by the 60 C column, while the same copper as THHN in conduit can use the 75 C column. A 48 A charger on 6 AWG NM-B is therefore not the same as 48 A on 6 AWG THHN.
Check voltage drop for the real run length
Measure the one-way distance from the panel to the charger, then check voltage drop as well as ampacity. A wire that passes ampacity can still fail on a long garage run, and the fix is to upsize the wire, not to ignore the drop.
Check panel capacity and local rules before installing
Confirm the panel has spare capacity for the new continuous load, and check the locally adopted NEC edition and your authority having jurisdiction. This guide is a planning estimate only; final design and installation should be verified by a licensed electrician.
Worked Practical Example
48 A charger on a 60 ft run
A 48 A Level 2 charger, 240 V, with a 60 ft one-way run from the panel.
- •Continuous-load rule: 48 A x 1.25 = 60 A minimum breaker
- •Breaker: 60 A (next standard size at or above 60 A)
- •Wire check: do not use 6 AWG NM-B for this job, because its 60 C limit does not support a 60 A breaker for a 48 A continuous load
- •Use copper THHN in conduit at the 75 C column, or a larger NM-B size, and verify ampacity and derating for the actual installation
- •Voltage drop: run the voltage-drop calculator with 48 A, 240 V and 60 ft, and upsize if the drop exceeds 3% on the branch circuit
Common Mistakes to Avoid
- ✕Sizing the breaker equal to the charger current and forgetting the 125% continuous-load rule.
- ✕Using the 75 C ampacity column for NM-B cable, which is limited to the 60 C column.
- ✕Checking ampacity but never checking voltage drop on a long run.
- ✕Assuming a 100 A panel automatically has spare capacity for a new 60 A EV circuit.
Frequently Asked Questions
What size breaker do I need for a 48 A EV charger?
As a planning rule, 48 A x 1.25 = 60 A, so a 60 A breaker. Always check the wire temperature column, the panel capacity and the locally adopted NEC edition.
Can I use 6 AWG wire for a 48 A charger?
It depends on the wiring method. 6 AWG copper THHN in conduit can be different from 6 AWG NM-B cable, because NM-B is limited to the 60 C column. Do not assume they are interchangeable.