EV charging projects are often discussed in terms of charger count, kW rating, and utility service capacity, but in real deployments, breaker sizing and protection strategy are some of the most frequent causes of redesigns, nuisance trips, and permitting delays.
Because EVSE behaves differently than many conventional loads (high utilization uncertainty, long duty cycles, large step loads, and often power electronics–driven harmonics), breaker selection needs to be treated as a power systems engineering problem, not a simple amp rating exercise.
Below is a structured summary of what we’ve found to be the most important considerations.
1) EV Chargers Are Continuous Loads (and That Changes Everything)
For most EVSE, especially DC fast chargers and depot charging, load behavior commonly meets the definition of a continuous load (operating at maximum current for 3 hours or more). In the U.S., that drives conductor and OCPD sizing requirements (commonly the “125% rule,” depending on the exact configuration and interpretation during permitting).
This is one of the biggest reasons designs fail review: teams size breakers for nameplate current but forget the continuous load multiplier, or don’t apply it consistently across feeders, taps, and upstream equipment.
2) Breaker Sizing Must Track Conductor Ampacity (and Correction Factors)
Even if the breaker is correctly sized for the EVSE, the design can still be wrong if the wire ampacity is overstated. Typical issues:
3) Fault Current and AIC Ratings Are Often Overlooked
As EV charging sites scale, they’re frequently served from larger transformers and closer to stronger distribution nodes ,which means higher available fault current.
A breaker that is properly sized in amps but has an insufficient interrupting rating (AIC) is a hard stop for AHJs and utilities.
Common failure mode: projects assume 10kAIC or 14kAIC by default, then interconnection review reveals much higher fault duty (especially near substations), forcing equipment replacement late in design.
4) Selective Coordination Is Not Optional for Uptime
At multi-port charging hubs, coordination matters because one fault should not drop the whole yard.
Poor coordination causes:
5) Demand Diversity and Load Management Should Inform Protection Strategy
Many sites will never run all chargers at nameplate simultaneously. EMS/load management may cap demand, stagger charging, or curtail based on site limits.
This creates a key design question:
Do you size breakers for full nameplate, or for controlled maximum demand?
This often depends on:
6) Voltage Drop Can Indirectly Affect Protection and Performance
Long feeder distances to dispensers can cause:
Takeaway
Breaker sizing for EV charging is not a checkbox. It’s a multi-variable problem involving:
Because EVSE behaves differently than many conventional loads (high utilization uncertainty, long duty cycles, large step loads, and often power electronics–driven harmonics), breaker selection needs to be treated as a power systems engineering problem, not a simple amp rating exercise.
Below is a structured summary of what we’ve found to be the most important considerations.
1) EV Chargers Are Continuous Loads (and That Changes Everything)
For most EVSE, especially DC fast chargers and depot charging, load behavior commonly meets the definition of a continuous load (operating at maximum current for 3 hours or more). In the U.S., that drives conductor and OCPD sizing requirements (commonly the “125% rule,” depending on the exact configuration and interpretation during permitting).
This is one of the biggest reasons designs fail review: teams size breakers for nameplate current but forget the continuous load multiplier, or don’t apply it consistently across feeders, taps, and upstream equipment.
2) Breaker Sizing Must Track Conductor Ampacity (and Correction Factors)
Even if the breaker is correctly sized for the EVSE, the design can still be wrong if the wire ampacity is overstated. Typical issues:
- Ambient temperature correction not applied (especially in hot regions or rooftop raceways)
- Conduit fill/derating ignored when multiple circuits are bundled
- Terminal temperature ratings (75°C vs 90°C) not matched to the breaker/lug rating
- Long feeder runs causing higher losses and heat
3) Fault Current and AIC Ratings Are Often Overlooked
As EV charging sites scale, they’re frequently served from larger transformers and closer to stronger distribution nodes ,which means higher available fault current.
A breaker that is properly sized in amps but has an insufficient interrupting rating (AIC) is a hard stop for AHJs and utilities.
Common failure mode: projects assume 10kAIC or 14kAIC by default, then interconnection review reveals much higher fault duty (especially near substations), forcing equipment replacement late in design.
4) Selective Coordination Is Not Optional for Uptime
At multi-port charging hubs, coordination matters because one fault should not drop the whole yard.
Poor coordination causes:
- One downstream fault → upstream breaker trips → entire site goes down
- Nuisance trips during start-up/inrush or transient grid events
- Hard-to-diagnose outages that look like “random charger issues”
- Coordination study (or at least time-current curve validation)
- Appropriate breaker types and trip unit settings
- Ground-fault protection design that doesn’t fight with charger electronics
5) Demand Diversity and Load Management Should Inform Protection Strategy
Many sites will never run all chargers at nameplate simultaneously. EMS/load management may cap demand, stagger charging, or curtail based on site limits.
This creates a key design question:
Do you size breakers for full nameplate, or for controlled maximum demand?
This often depends on:
- How the load management is enforced (hardware vs software)
- Utility and AHJ acceptance
- Risk tolerance (what happens if controls fail or are bypassed)
- Future expansion plans
6) Voltage Drop Can Indirectly Affect Protection and Performance
Long feeder distances to dispensers can cause:
- Charger derating at low voltage
- Higher current draw under some control strategies
- Increased thermal stress in conductors and terminations
Takeaway
Breaker sizing for EV charging is not a checkbox. It’s a multi-variable problem involving:
- Continuous load requirements
- Ampacity + correction factors
- Fault current + AIC
- Coordination strategy
- Demand management assumptions
- Real field conditions (temperature, length, aggregation)
