ENGINEERING – ELECTRICAL ENGINEERING (POWER) CALCULATOR
Cable Rating Derating
A precise tool.
What is the Cable Rating Derating & How does it work?
The base current rating of a power cable is primarily a function of its crossβsectional area and the conductivity of the conductor material. Copper conducts roughly 6β―A per mmΒ² while aluminum conducts about 4β―A per mmΒ² under standard installation conditions.
Ambient temperature reduces the permissible current because higher temperatures increase the resistance of the conductor and the heat generated. A common practice is to linearly derate from a reference temperature (30β―Β°C) to a maximum allowable temperature (90β―Β°C).
When several cables share the same conduit or tray, mutual heating requires an additional derating factor. A simple approximation uses the inverse squareβroot of the number of cables, reflecting the reduced ability of each cable to dissipate heat.
I_{derated}=I_{base}times k_{T}times k_{G}
I_{base} = base current rating (A)
k_{T} = temperature derating factor
k_{G} = grouping derating factor
k_{T} = temperature derating factor
k_{G} = grouping derating factor
Parameters
Result
β
Frequently Asked Questions
How do I calculate the derated current for a copper cable at 40Β°C?
To derate a copper cable at 40Β°C, use the formula: Derated Current = Base Current Γ (Reference Temperature + 273) / (Ambient Temperature + 273). For example, if the base current is 6 A/mmΒ² at 30Β°C, the derated current would be 6 Γ (30+273) / (40+273) β 5.1 A/mmΒ².
What is the difference between copper and aluminum cable ratings?
Copper cables generally have higher current ratings per mmΒ² compared to aluminum cables due to better conductivity. Copper conducts about 6 A/mmΒ², while aluminum conducts about 4 A/mmΒ² under standard conditions.
How does ambient temperature affect cable current ratings?
Higher ambient temperatures reduce the permissible current rating of a cable because they increase conductor resistance and heat generation. The derating factor is applied linearly from a reference temperature (usually 30Β°C) to the actual ambient temperature.
Can I use this calculator for both copper and aluminum cables?
Yes, you can use this calculator for both copper and aluminum cables. Simply input the appropriate current rating per mmΒ² for the conductor material (6 A/mmΒ² for copper, 4 A/mmΒ² for aluminum) when performing the calculation.
What is the reference temperature used in cable derating calculations?
The reference temperature commonly used in cable derating calculations is 30Β°C. Derating factors are typically calculated based on this temperature to account for changes in conductor resistance at higher ambient temperatures.
How do I determine the cross-sectional area of a cable?
The cross-sectional area of a cable can be determined by measuring its diameter and using the formula: Area = Ο Γ (Diameter/2)Β². Ensure you measure the outer diameter if insulation is included, or the inner conductor diameter for bare wires.
What should I consider when choosing a derating factor for cables?
When choosing a derating factor, consider factors such as cable type (insulated vs. bare), installation environment (indoor vs. outdoor), and potential for future upgrades or increased loads. Always err on the side of caution to ensure safe operation.
Results are for informational purposes only and do not constitute professional advice.