Technical Articles
My capacitor name plate shows three ratings at three different voltages. What is the relation between capacitor applied voltage and kVAr rating
kVAr rating of the capacitor is directly proportional to square of the applied voltage. This is evident from the below formula:
kVAr = V2/XC
As capacitance (C in μF) remains same for a capacitor, XC will remain constant.
For example, if a capacitor is rated for 30 kVAr at 480 V, and if the applied voltage is 440 V, the kVAr output can be calculated as follows:
kVAr output = Rated kVAr * (Applied voltage)2 / (Rated voltage)2
= 30 * (440)2/ (480)2
= 25.2 kVAr at 440 V
Similarly, if we apply 500 V for the same capacitor, the kVAr output will be 32.5 kVAr. In this case, we are applying voltage more than the rated value and the capacitor will deliver kVAr output more than its rated value. Hence the life of the capacitor will reduce drastically, because of over-voltage and over-current.
To summarize, the name plate of a capacitor (for e.g. 30 kVAr, 480V) has the following details:
Similarly, for 25 kVAr and 440 V capacitor, the name plate contains the respective kVAr ratings and current ratings at 440 V, 415 V and 400 V.
Before deciding the voltage of the capacitor, it is important to understand about the percentage impedance of the transformer (%Z). The percentage impedance is the voltage drop on full load due to the winding resistance and leakage reactance of the transformer. This is expressed as a percentage of the rated voltage. For example, if the secondary of the transformer is rated for 433 V and %Z as 4%, the voltage available at the load end, during full load conditions, would be 415 V only. When the load decreases, the voltage drop decreases and hence the voltage at the load end increases. During no-load conditions, the voltage can reach a maximum of 433 V.
If a capacitor is selected with 415 V (in the above case), it would be subjected to over-voltage during partial load or no-load conditions. This would impact the capacitor life drastically. For a normal capacitor, following are the over-voltage limits permitted as per IS:
Hence, the capacitor should be rated for 440 V, even though the voltage at the load end is measured as 415 V. In general, it is a better practice to select capacitor voltage greater than the rated secondary voltage of the transformer and hence, avoid the prolonged over-voltage conditions.
Abhinav L Purkar,
Manager Product Management & Marketing (PMM)Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.
Assistance Required?
Select an option to Contact Us
Contact Us 
