Voltage sag is a short-duration reduction in supply voltage below its normal level, usually caused by a sudden increase in electrical demand, a fault on the network, or the start-up of large equipment. In EV charging infrastructure, voltage sag can affect charger performance, site stability, and the operation of other connected electrical systems.
What Is Voltage Sag?
Voltage sag, sometimes called a voltage dip, occurs when the voltage temporarily drops for a brief period but does not disappear completely. These events may last from fractions of a second to several seconds, depending on the cause and the strength of the electrical network.
In EV charging environments, voltage sag can happen when chargers switch on, when multiple vehicles begin charging at the same time, or when the site experiences wider grid disturbances.
Why Voltage Sag Matters in EV Infrastructure
Voltage sag matters because EV chargers, payment terminals, network devices, and building systems all depend on a stable power supply. If voltage drops too far, chargers may reduce output, interrupt charging sessions, trigger protective shutdowns, or create communication faults.
For site owners, fleet operators, and installers, repeated voltage sag can indicate that the electrical infrastructure is under strain or that the system has not been properly designed for the connected load.
How Voltage Sag Happens
A large electrical load starts suddenly or increases rapidly
The electrical network experiences a temporary drop in voltage
The drop may be caused by EV chargers, motors, HVAC systems, lifts, or external grid events
Sensitive equipment on the same supply experiences reduced voltage during the disturbance
Once the fault clears or the system stabilises, voltage returns to normal
Voltage sag is more likely on sites with limited transformer capacity, long cable runs, weak grid connections, or poor load coordination.
Common Causes of Voltage Sag in EV Charging
Typical causes include:
– Simultaneous start-up of multiple EV chargers
– Sudden increases in charging load
– Insufficient transformer or connection capacity
– Large site loads switching on at the same time
– Grid faults or disturbances upstream of the site
– Poorly managed load balancing
– Undersized cabling or overloaded internal distribution systems
These issues can affect both the charging equipment and other building loads connected to the same network.
Key Impacts of Voltage Sag
– Reduced charger performance or slower charging power
– Interrupted charging sessions
– Resets or faults in sensitive electronics
– Communication issues in networked charging systems
– Disturbance to lighting, controls, or building systems
– Indication that the site may need electrical upgrades or better load management
Even short voltage sag events can create operational issues if they occur frequently.
How Voltage Sag Is Managed
– Performing proper site assessment before installation
– Using load management to prevent sudden demand spikes
– Staggering charger start times
– Sizing transformers, switchgear, and cables correctly
– Monitoring the site with power quality equipment
– Separating sensitive loads where appropriate
– Upgrading site infrastructure if demand exceeds existing capacity
Good electrical design helps reduce voltage sag risk and supports more reliable charger operation.
Limitations to Consider
– Voltage sag may originate from the wider grid, not only from the charging system
– Some sites experience sag only during specific operating conditions
– Software controls can reduce risk, but may not solve underlying capacity problems
– Frequent sag events can be difficult to diagnose without power quality monitoring
– Mitigation may require electrical upgrades, which increase project cost
Because of this, voltage sag should be evaluated as part of the full site power environment.
Voltage Sag vs Voltage Flicker
Voltage sag is a temporary drop in voltage below normal operating level
Voltage flicker is a repeated fluctuation in voltage that often causes visible lighting changes
A site can experience voltage sag without flicker, but recurring sag-like changes may contribute to flicker effects
Both are important power quality concerns in EV charging design.
Where Voltage Sag Is Most Relevant
Voltage sag is especially relevant in:
– Commercial charging sites
– Fleet depots
– Residential developments with limited capacity
– Public charging hubs
– Buildings with shared electrical infrastructure
– Sites with multiple high-load devices operating together
In these environments, controlling voltage sag helps improve charging reliability, user experience, and electrical system stability.
Related Glossary Terms
Voltage flicker
Voltage drop
Power quality
Load balancing
Transformer sizing
Site power limit
Three-phase power
Power analyzer
Grid capacity
Short-circuit level