I was referring IEC 61439-1 standard for the deriving the temperature rise of an assembly through comparison. We can derive the current rating of an assembly (low voltage switchboard) at any point (contacts such as busbar joints, SCPD joint, or any other contact) by using the formula of Copper Development Association, Publication No. 22:1996 formula no. 8. As per the standard clause,

To reduce the testing required to determine the rated current of a circuit \$I_1\$ at the maximum permissible temperature-rise \$\Delta T_1\$, the current rating may be calculated from the actual test current I2 if the measured temperature-rise \$\Delta T_2\$ of the current carrying parts (e.g. busbars and terminals) deviates from the permissible value by not more than \$\pm 5\text{K}\$, using the following formula: given in equation \eqref{1}

Let's examine this scenario by an example.


  1. Copper-copper joint Temperature rise limit \$\Delta T_1 = 105\text{K}\$
  2. Tested temperature \$\Delta T_2 = 102\text{K}\$
  3. Test current \$I_2 = 973\text{A}\$


  1. Maximum current rating


The maximum current rating can be determined as followed: $$\frac{I_1}{I_2}={\left[\frac{ΔT_1}{ΔT_2}\right]}^{0.61}\label{1}\tag{1}$$ Substituting above values in the equation, we get, $$I_1 = 990\text{A}\label{2}\tag{2}$$ Hence, we can deduce that the maximum current rating of the specified circuit is 990A which can be carried safely at 105K temperature rise which is also the limit. Now, here's the catch. This formula can be used if the tested temperature is less than ±5K than the permissible limit. If my tested temperature is 98K where, $$ \Delta T_1-\Delta T_2 > 5\text{K} $$ then the equation \eqref{1} is not applicable.

Now the question is, whether my inference is correct from the above statement ? Additionally, I have attached the snap from IEC 61439-1 for your reference.

IEC 61439-1, Clause - > General, Pg. No. 81


2 Answers 2


To start, copper-copper joint temperature rise limit ΔT1=105K is a gross mistake induced by the very bad and improper text of IEC61439 that do not write the limit values. Read this my article to understand. The limits for temperature rise of connections to - for example - circuit breakers are lower, which is equal to 75K, depending of bare or coated. It is given in IEC 62271-1 , IEC 60943 and all other IEC standards I know .The link for the article http://www.cognitor.com.br/IEC614391Table6.pdf

About the formula you mention, it has very limited application and only for long conductors without connections . All the switchgears and busways have many connections. Forget such formula and those old tables. Nowadays, with so many methods and tools you have better options. I suggest you to buy and read CIGRE Brochure 830 (2021) a Simulations for Temperature Rise Calculation. I am coauthor and assure that you will find good methods there. If you want to move faster do my training of the todays post. https://www.linkedin.com/posts/sergiofeitozacosta_iec62271-iec61641-switchboard-activity-7066369086955356160-5MVi?utm_source=share&utm_medium=member_desktop


Yes, I think your interpretation is correct.

They don’t want you extrapolating more than just a few degrees, but want to provide a formula so you don’t have to muck around forever getting the exact temperature delta.

  • \$\begingroup\$ thanks for your response. Any citation of actual implementation of the extrapolation could prove useful for me. \$\endgroup\$
    – DaSnipeKid
    May 16 at 3:22

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