Limited Time Offer: Save 15% on Pro Plan with discount coupon: MX15. Study specialized LV/MV/HV technical articles and studies.

# Quick estimation of the short circuit current at the end of a feeder

Home / Technical Articles / Quick estimation of the short circuit current at the end of a feeder

## Rule of thumb for quick estimation

If we consider a branch circuit going away from the point of supply and follow it radially along all the connections and branches, we shall find that further away we are from the transformer, lower is the value of the maximum possible short-circuit current.

Each length of conductor or each device in the circuit provides an impedance which helps to reduce the short-circuit current.

### Example of Isc at the end of feeder

A prospective short-circuit current of 50 kA at the secondary terminals of a transformer at 400 V, will be limited to about 10 kA at the end of a connecting lead with a length of 10 m and cross-section of 10 mm2. If the same feeder has a cross-section of 25 mm2, the length of the wire is to be 25 m for reducing the current down to 10 kA.

### Specimen example //

The expected short-circuit current at the end of the feeder is expressed by the relation :

Where:

• IK” – Short-circuit current [kA]
• UNTrafo – Rated voltage of the supply transformer on the low-tension side [V]
• ZTrafo – Impedance of the transformer
• ZConducting – Impedance of the conducting lead

However, short-circuit at the point 5 is according to the above mentioned formula:

The value calculated above is rather conservative and includes a factor of safety as other sources of current reduction like the arc voltage, the contact resistance and the internal resistances of the different devices in the path of the short-circuit were not considered.

Reference: Basics of circuit breakers – Rockwell

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.

### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. Andy
Apr 10, 2018

Why is the I_shortcircuit for transformer different when calculated using kVA and different while using this relation you mention?
For example, Xfmr_FLA=kVA/(1.73 x kVsec) = 630/(1.73×0.4) = 909A then Xfmr_I_shortcirc=Xfmr_FLA/0.06=15.15kA
While I_sc_xfmr = kV_sec/(1.73 x Zxfmr) = 0.4/(1.73 x (3.1^2+13.5^2)^0.5) = 16.694kA
Why are these two giving mismatch? what am I missing?

2. Mustafa
Nov 26, 2014

Good Example but I dont understand some values. How are they calculated Uk , all R/mohm , X/mohm and all cable length and cross section best regards.

could you help me pls

best regards

3. T.N.Ramesh
Nov 25, 2014

Yes I’m an Electrical Engineer

4. Girish
Nov 22, 2014

Nice article. The procedure is similar to IEC 60909 method, only the correction factor in case of the transformer and voltage is not considered. Nevertheless, it is a good way of estimating the short circuit current.

5. Vicente Martinez
Nov 21, 2014

Those are very instructive articles, they help me a lot Thank you

6. LALO EL HIPPIE
Nov 21, 2014

La simpleza hace hermoso los desarrollos matemáticos. Calculo con Melshort de Mitsubishi dio el mismo resultado

### Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).