Cable derating factors are adjustment multipliers used in electrical design to reduce a cable’s allowable current-carrying capacity (ampacity) when real installation conditions make the cable run hotter than standard reference conditions. In EV charging installations—where loads are often continuous and high-power—derating factors are critical to prevent overheating, nuisance trips, insulation damage, and long-term reliability issues.
What Are Cable Derating Factors?
Cable ampacity tables assume standard conditions (ambient temperature, installation method, spacing, and ventilation). Derating factors modify those base values to reflect actual conditions, such as:
– Higher ambient temperature
– Multiple loaded cables grouped together
– Installation in thermal insulation or poorly ventilated spaces
– Cable buried in soil with poor thermal conductivity
– Cables in conduit or trunking with limited heat dissipation
– Harmonic currents increasing conductor heating (especially neutral)
The result is a reduced permissible current for the same cable size, or the need to select a larger cable.
Why Cable Derating Factors Matter in EV Charging
EV chargers commonly operate for hours at near-rated current. If cables are not derated correctly:
– Conductors can overheat under continuous charging load
– Insulation aging accelerates, reducing cable lifetime
– Voltage drop can increase and affect charger performance
– Protective devices may trip unexpectedly
– Fire risk increases in worst-case conditions
– Site availability rate suffers due to faults and shutdowns
Correct derating supports safe, compliant, and reliable charging infrastructure—especially in multi-charger sites and long cable runs.
How Cable Derating Is Applied
A typical design approach looks like this:
– Start with base ampacity from applicable standards for the cable type and installation method
– Apply derating factors for the real installation conditions (temperature, grouping, soil, insulation, etc.)
– Confirm the resulting allowable current is ≥ the design current of the EV charger load
– Verify voltage drop limits and adjust cable size if needed
– Confirm protective device sizing and coordination (breaker, RCD, selectivity)
For multi-charger sites, derating is often combined with load management assumptions and diversity factors.
Common Derating Factors in Practice
Typical contributors to derating include:
– Ambient temperature correction
– Higher temperatures reduce heat dissipation and lower allowable current
– Grouping/bundling (mutual heating)
– Multiple loaded circuits in a tray, conduit, or trunking raise conductor temperature
– Installation method
– In conduit vs free air, buried vs surface-mounted, or enclosed trunking changes cooling
– Thermal insulation
– Cables surrounded by insulation (walls/ceilings) can require significant derating
– Soil conditions for buried cables
– Soil thermal resistivity, moisture level, and burial depth affect heat transfer
– Harmonics and neutral loading
– Non-linear loads can increase heating, requiring neutral sizing and additional derating
– Continuous load assumptions
– EV charging is often treated as a continuous load; design current selection must reflect this
Typical Use Cases
– Sizing feeder cables to a row of bollard chargers in an outdoor car park
– Selecting cables for long runs through conduit in a parking garage
– Multi-charger trunking routes with many circuits grouped together
– Depot charging where high simultaneous load creates sustained heating
– Retrofits where cables pass through insulated building elements
Key Benefits of Correct Derating
– Reduced overheating risk and longer cable service life
– Stable charging performance with fewer faults and shutdowns
– Better compliance with electrical standards and inspection requirements
– Correct coordination between cables and protective devices
– Lower total lifecycle cost through fewer service calls and failures
Limitations to Consider
– Derating rules vary by country, standard, and cable type (PVC/XLPE, copper/aluminum)
– Real site conditions can differ from assumptions (ventilation changes, added circuits later)
– Future expansion can increase grouping and require re-evaluation
– Load management does not eliminate thermal risk if worst-case operation is still possible
– Incorrect assumptions about ambient temperature or soil conditions can lead to under-sizing
Related Glossary Terms
Branch Circuit
Cable Ampacity
Continuous Load
Voltage Drop
Load Management
Dynamic Load Balancing
Circuit Breaker
Protection Coordination
Available Import Capacity
Charging Station Installation