Short Circuit Rating 120/240 Service Panels
Short Circuit Rating 120/240 Service Panels
(OP)
A practical question:
What is the most common short circuit rating of 120/240 volt service panels for 100 amp, 150 amp and 200 amp services? What should be done if the available short circuit exceeds this rating?
What is the most common short circuit rating of 120/240 volt service panels for 100 amp, 150 amp and 200 amp services? What should be done if the available short circuit exceeds this rating?






RE: Short Circuit Rating 120/240 Service Panels
Specific installations with possible higher available fault currents will need to be individually evaluated to properly plan for the possible higher equipment bracing requirements.
RE: Short Circuit Rating 120/240 Service Panels
RE: Short Circuit Rating 120/240 Service Panels
RE: Short Circuit Rating 120/240 Service Panels
"We are an electric distribution entity where in some instances the fault current could exceed 10,000 amps and could be as high as 30,000 amps."
If you take a 50 kva 120/240 volt transformer with 2% impedance you max fault at the terminals of the transformer would be 10400 amps. It would be much less when you add the primary impedance and the service impedance. This would apply to a residence. I don't think you would exceed the 10ka. If you are speaking of a commercial or industrial customer the fault current would be higher. I don't see how you can deliver 30ka even to these type of customers. What is the problem you are working on?
RE: Short Circuit Rating 120/240 Service Panels
RE: Short Circuit Rating 120/240 Service Panels
UK practice is to use larger distribution transformers with longer distribution runs serving a broader area. One of the benefits of our 240V system over 120V is that the volt-drop per kW load is lower, so this type of design is practical. Distribution transformers up to 1.5MVA, occasionally up to 2.5MVA, are not unusual. Fault level on these transformers is often in the 50kA region, but the fault level drops away very quickly with distance. I agree with the opinion that 30kA available at a low current panel is unlikely unless that panel is fed directly from, or incorporated in, the main LV switchgear for the transformer. This would not normally apply to a residential application, at least not in the UK
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RE: Short Circuit Rating 120/240 Service Panels
Just trying to say that you don't necessarily need 30kA branch circuit breakers to have a 30kA rated panel.
RE: Short Circuit Rating 120/240 Service Panels
RE: Short Circuit Rating 120/240 Service Panels
Distribution transformer impedances in the USA are not standardized by either IEEE or NEMA. The fault current you have today may not be same tomorrow after the transformer feeding your location fails and is replaced.
RE: Short Circuit Rating 120/240 Service Panels
A concern is this: If we upgrade the transformer from a 50kva to 100kva due to load increases, how are the existing services coordinated with the new available short circuit?
RE: Short Circuit Rating 120/240 Service Panels
Utilities may require a larger transformer to supply several houses with feeder length 20 ft and longer.
For reference, let us consider a 225 kVA transformer, 120/208 as describe in the enclose figure. This is close equivalent to a transformer bank of 3x75 kVA units.
The SC at the customer side will decrease dramatically with the feeder length. This made reasonable to think that the SC available in the house may be less than 10 kA for transformer size similar to the enclose example.
Beware that in area with interconnected network transformer typically found in large cities downtown, the SC may be a lot larger.
I hope this help.
RE: Short Circuit Rating 120/240 Service Panels
Our transformer sizing guides will allow as many as 10 small residences on a 50kva and as many as 7 larger residences on a 50kva. We can put 10 large homes on a 100kva [at 2% Z, 240-volts, the short circuit at the trasformer is 21,000 amps].
On a 50kva the residences more than say 50 feet from the transformer are not a problem. We have overhead transformers with 1.3% and padmount transformers this size with 1.75% impedance [this is consistent with your table].
The short circuit to be concerned about, then, is that available at the residence that is say 15 to 40 feet from the transformer. This is likely with padmounted transformers. With overhead it would be unusual to find one closer than 30 feet.
RE: Short Circuit Rating 120/240 Service Panels
Depends. Are there multiple services off transformer? If so, the customer with the load increase should be separated so the increased available fault current does not affect the others. In either case, a letter explaining the increased available fault current and consequences and code issues should be mailed to the customer with a copy filed away. Utility rules probably require this customer to notify you prior to the load increase. This gives some time to discuss all the issues and costs. Gets a little trickier if the load increase is sneaked in, and the upgrade occurs in response to a burnout after hours.
RE: Short Circuit Rating 120/240 Service Panels
I didn't see your previous response prior to posting mine. If the change out is due to the utility wanting to add large residence number 8 to the same transformer location, I'd advise them to add another transformer instead. No changes to existing customer equipment required, and all around better service.
RE: Short Circuit Rating 120/240 Service Panels
It was too practical to find the answers using the Internet search method.