Short Circuit Rating 120/240 Service Panels 3

[csparks]

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?

 

[EEJaime]

According to Southern California Edison's Electrical Service Requirements, (ESR), handbook, the Company's contribution to the available short-circuit current at the service entrance will not exceed 10,000 AIC for single-family dwellings, duplexes or individually metered mobile homes which utilize self-contained type Company meters, (standard ratings of 125 Amps and 200 Amps). This goes up to 42 kAIC for 120/240, 1-ph < 600 Amps and 120/208 or 240V, 3ph services to 800 Amps, and 30 kAIC for 480v, 3ph, < 1200 Amps. (See

http://www.sce.com,

ESR-1, pp1-23 table 1-5:Short Circuit current).

Specific installations with possible higher available fault currents will need to be individually evaluated to properly plan for the possible higher equipment bracing requirements.

 

[csparks]

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. Some utilities state that they make sure there is enough service lead to drop the short circuit to a safe level (not always practical). I have found that Square D offers an interrupter rated at 22,000 amps sc symetrical for 200-amp services. Short of the above is the best remedy to cold sequence the equipment that includes a current limiting device or is there a better way?

 

[alehman]

I think 10K is fairly standard. I have seen some 5K panels.

 

[wareagle]

csparks
"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?

 

[davidbeach]

I think that any utility should ensure that they do not provide a service capable of supplying more that 10kA to any residential service that the homeowner is likely to service. I've seen utility design standards that include looping the service lateral out and back to provide enough conductor impedance to keep the fault level at 10kA or below. Way too much risk of someone going to the hardware store and buying a 10kA breakers (5kA is fortunately long gone history) because that's what the hardware stores sell to allow services above 10kA. Electricians should understand the difference, but it would be too much to expect homeowners to understand why they would need to purchase a special order, much higher cost, breaker when they can use the cheaper one available in the store. 30kA to a residential service would be criminally negligent IMHO.

 

[ScottyUK]

wareagle,

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 . 10kA at a UK domestic panel would be rather surprising - 2 kA is more typical.


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[LionelHutz]

Federal Pioneer has series ratings for their panels. It's a published brochure called Series Rating Guide. Basically, with the right main breaker in front of the branch circuit breakers you can get a very high fault rating. The Stab-lok panels can be applied on a system with 200kA available fault current if you use the right main breaker. The minimum series rating is 22kA. I'm sure others have the same type of series ratings too so you'd need to check with them.

Just trying to say that you don't necessarily need 30kA branch circuit breakers to have a 30kA rated panel.

 

[csparks]

All of the comments I have been seeing are very good. True, the short circuit for a 50kva, 2% transformer is 10,400 amps. However, we are finding anymore due to increases in AC useage that a 50kva is just not big enough. 2% Z is typical for a padmount but we have legacy overhead installations. New construction often puts the first panel within 20 feet of the transformer. We use low-loss 1.3% transformers: 75kva, 100kva, and sometimes 167kva, 1.6% transformers to serve residential developments. The 1/2 winding short circuits with these impedances range from 13,000 amps for a 25 to 69000 amps for a 167. The 240-volt short circuit ranges from 12,000 amps for a 37.5 to 43,000 amps for a 167. The primary impedance can only reduce the short circuit about 5% to 8% if it is considered. The solutions I have heard are: 1) make the secondary run longer to increase the impedance, 2) increase the impedance of the secondary, 3) use a higher impedance transformer (not always practical), 4)cold sequence with a currentliminting device. How common is 4 and how have people done it?

 

[stevenal]

10,400 amps is for the full winding fault. A half winding line to ground fault is approximately 1.5 times that figure. And which type of fault do you expect is more common? A more precise multiplier can be derived if X and R are both known. See the equivalent circuit in the Westinghouse Distribution Systems book, page 223. Most software simply uses 1.5 times. The reduced source voltage for the half winding fault means the current drops more quickly with cable distance. If sufficient cable is used, the line to line fault will exceed the line to ground value. Please always check both cases.

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.

 

[csparks]

The 1.5 times is a good rule of thumb. I modify the R and X and perform the calculation but how to modify the R and the X is also a rule of thumb. True, the 1/2 winding short circuit drops off rapidly with the impedance of the secondary. Usually at 15 to 20 feet the full winding short circuit is larger. We check the 1/2 winding calculation when our service panel is within 20 feet of the tranformer. As you can see from previous discussion we have cases where the fault current can exceed the 10,000 amps typical of residential panels and circuit breakers. We are wondering what practical solutions people have tried to limit the short circuit besides replacing the transformer with a higher impedance one or looping the secondary to increase its impedance. Replacing all the panels with higher rated panels is not practical.

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?

 

[cuky2000]

A single-family house with 200 A service, 120/240, may require a the 50 kVA transformer capacity at the most without considering diversity factor.

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.

 

[csparks]

Cuky2000: Your data and graphs are very interesting. Thanks

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.

 

[stevenal]

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?

.

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.

 

[stevenal]

csparks,
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.

 

[csparks]

Thanks everyone. This has been a great forum for finding information I have been trying to get for some time.

It was too practical to find the answers using the Internet search method.