Specifying the Appropriate Short Circuit Rating Based on Utility Service
Costly mistakes can be made when specifying short circuit ratings without verifying the utility fault current. Best practice is to contact the utility early in the design however some educated assumptions can be made prior to receiving the utility fault letter. This presentation, given by Andrew Legro at ABB, explains the variation in fault currents based on the type of utility service and then discusses considerations in specifying the short circuit rating of connected equipment.
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Answers From Q&A Following Live Webinar
| Question | Answer (Andrew Legro at ABB) |
| We often get told to assume an infinite bus by the Utility. Do you have any thoughts? | The difference in short circuit between an infinite bus vs actual source impedance is small when the transformer is less than 1000kVA. With larger transformers, the difference becomes more pronounced. Refer to slide 20 of the presentation. If you want to be very conservative with the calculation, use infinite bus. |
| Please clarify why 50% of the NEC calculated demand was used to estimate the kVA rating of the utility transformer. Why does the utility provide a smaller transformer than project demand load? | The 50% of calculated demand is not a hard and fast rule but an assumption based on utility practice. Utilities do not use NEC calculation methods to size transformers. They typically use watts per square foot tables for various occupancy types / loads. These tables are based on historical real-world data. Note that the 50% assumption is only valid for occupancy / load types defined as “in scope” on slide 8. For explanation of why a transformer can be applied to a demand that exceeds its kVA rating, refer to slide 14. |
| My question is if I have (3x) 1-ph transformers connected in YG-Yg, Do I have to add impedance of each 1-ph transformer or I have to consider impedance of 1-ph transformer. In my case, do I have to consider 4.7% impedance or 1.5% impedance for a 3-Phase bolted fault analysis? | The impedance of banked single-phase transformers can get tricky. That’s why I included them in the exception category on slide 8. However, in your case of a Wye secondary connection, the single-phase impedance remains the same when 3 equal size transformers are connected in Wye. You multiply the kVA rating by three but keep the %Z the same, do not add them. Banked single phase transformers tend to have a lower impedance versus an equal kVA rated 3 phase transformer. |
| Often, it is very difficult or impossible to get accurate utility source (Thevenin) impedance data that includes maximum & minimum contribution in order to calculate the worst-case AF energy downstream levels. What is the best assumption that can be made? | This is an important question, and I don’t have a good answer. I partially addressed this on slide 25 where I stated that the utility’s short circuit information is conservative on the high side and is only valid for determining equipment interrupt & withstand ratings. As you stated, a complete arc fault analysis requires maximum and minimum contribution. The best advice I have is to make sure any information provided by the utility is fully documented in the arc flash study and (for the case of arc-reduction maintenance switches a.k.a RELT) set the RELT value as low as practical so that lower than expected arcing faults are cleared on instantaneous trip. This helps account for the missing minimum short circuit value. |
| Is it still a good estimate to use IEEE standard impedances when the utility standards quote a Min %Z of 1.5 to 2.5 on a 500kVA? I commonly see this in utility standards. | These impedance values seem unusually low however, I recommend always using utility provided information when available. |
| Does the AHJ accept this 50% KVA de-rating for the service size as justification? | No, for the purpose of sizing the service entrance conductors and protective device, NEC service calculations must be used. The 50% assumption only applies to estimating the size of the utility provided transformer for purposes of establishing the available short circuit current prior to receiving a utility fault current letter. |
| Is every building or service supplied by a dedicated utility transformer? Or is the utility transformer shared by multiple different buildings/services/customers? | No, often a single transformer or bank of transformers is used to provide multiple services. This is especially the case where a large building like a shopping center is divided up into multiple tenants. In these cases, the total connected load(s) must be accounted for when estimating the size of the utility transformer. Refer to slide 8. |
| What X/R value was used for the utility primary? | The calculations in the presentation used an X/R value of 10. A value of between 10 to 12 is typically a valid assumption due to rating limits of utility protective devices such as fuse cutouts. |