Short Circuit Current Rating for Panel

The locked-rotor current of a motor can be obtained by taking the resistance of the windings, and dividing it into the supply voltage. As a rule-of-thumb, the locked-rotor current is around 5 times the rated full-load current.

The motor overload device should be set to the full load current figure, for adequate safety.

Dear Mr.PWSlack,

Thank you for your answer. From your answer Impedance(z) = R+jX, taking x as zero, and Z = R. Is it correct? Further, with this impedance, how to find the percentage impedance? Then, Shall we take the locked rotor current as short circuit current? Kindly please clarify sir.

thank you.

Regards,

Kailash

This is absolutely correct for your continuous locked rotor current. But I think he's only asking half of his question.

If the motors are in a cabinet they are still power users and not control so therefore don't require an interrupt rating. Only the control portion requires the rating to be posted on the exterior of the cabinet.

Most cage induction machines have LRC( Locked Rotor current) 6PU + 20% tol which makes it 7.2.In 2 Pole motors it is still worse LRC values as high as 6.7+tol can be noticed if motor manufacturers catalogues are referred to like ABB, WEG ,Siemens etc.So always it is better to consider 7.2 PU for such calculations as tol is always positive and limiting LRC during motor design is very tricky.Only for Boiler feed pumps normally we have lower LRC values limiting to5Pu which costs more.

Your motor has nothing to do with the interupt value of the cabinet unless they are physically inside the cabinet. This value isn't a calculated value it is assigned based on destructive testing. Therefore you must use either the rated interupt value or the minimum of 5ka. The UL508 has some fairly simple charts to use to calculate the rating of a cabinet and since it is required by law to be posted on the cabinet I would recommend breezing through a copy.

I am answering with the assumption you are asking for a method to determine the short circuit contribution from the motors you have connected to your bus that is under the short circuit study. When you analyze asymmetrical faults, the fault current will consist of two components, one is a DC offset which will decay exponentially and the other is an alternating current component which will also decay exponentially based on ther ratio of resistance to reactance commonly called the X/R ratio. The impedence of the motor is not really that importatnt, the motor load is. The motor contribution to fault current for the first one half cycle can be pretty accurately estimated at four times full load for induction motors and five times full load for synchronous motors. Thuis would be 25% and 20 % impedences respectively. Actually the impedence is not constant throughout the calculation, that is why we do subtransient, transient, etc. reactances. For your specific questions yes, the motors will all contribute to your fault current. Some of the contributions might be able to be ignored depending on the size and load. I find it a little bit hard to believe when you said that the motors are the source of short circuit current. Fault current primarily comes from your transformer, and when there is a fault this is where it wants to return (its source). Motors can be a significant contribution to available fault current and it is good that you are asking because some people ignore the contribution. Installed systems do not determine the SC rating of the panel, bus bars, or circuit breakers this is done at the design stage. When all the SC's are totalled, the minimum bus bar bracing is selected, and the conductors are verified to remain intact for the I squared T energy (Fault clearing times and conductor overheating / damage is beyond the scope of this discussion) At this time you can select the minimum circuit breaker interrupting capability, which is of course based on your fault current. When all that is done then the coordination study can be started. I am starting to ramble now, what I am saying is the SC rating of the bus bars and panel is a manufacturer's number which you can probably get off the label. It should have been properly selected long ago. It does not change when loads are added or removed.

If you ever get a chance to see a real faulted bus you should do so even if it is out of your way. The energy released can be tremendous, and the damage cannot be understated. Two of the busses move in one direction and the third moves in the opposite direction. It will give you an apprecciation for the SC studies and and the power of magnetism