Originally posted by Open Neutral
View Post
Announcement
Collapse
No announcement yet.
PSE
Collapse
X


Originally posted by Open Neutral View PostGiven the dedicated 100KVA transformer PSE has spec'ed, the AIC is so large the existing four panel boards would have to be ripped out and replaced.
Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
Comment

A good rule of thumb is that the available fault current at the secondary terminals of a transformer can be estimated by taking the full load current of the secondary side and dividing by the "per unit" impedance of the secondary windings. That later value is often on the order of 5.75%, so the value you divide by is 0.0575. Different transformers will have different values. In the example I gave earlier, the full load current for a 120/240V single phase 100 KVA transformer is 100,000/240, or 417 amps. Dividing that by 0.0575 give an AIC of 7,246 amps.
Most panels have a minimum AIC of 10,000 amps. To get that from the transformer, it would need to have an impedance of 4.17% or lower. It is possible that the utility transformer's impedance could be lower than that. Even still, the impedance of the secondary conductors will significantly reduce the available fault current at the building's main breaker.
So don't take the EC's statement at face value. Ask for the exact number that PSE provided to him (or her). Then you can do a calculation of the impact of the secondary conductor's impedance on the value PSE gives you. I am willing to predict that the final result is below 10,000 amps, and that you therefore do not have a problem in need of a solution.Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
Comment

Thank you.
We do have a substantial secondary connection  dual 350MCM of about 80 ft in length.
But I will find out the exact details.
And a side question: Does Washington State L&I require a PE signoff on utility designs?
After the Merrimack Valley Gas disaster, the NTSB came out swinging against utility exemptions to the PE requirement.
Last edited by Open Neutral; 110619, 09:55 AM.
Comment

Originally posted by Open Neutral View PostAnd a side question: Does Washington State L&I require a PE signoff on utility designs?
Charles E. Beck, P.E., Seattle
Comments based on 2017 NEC unless otherwise noted.
Comment

Originally posted by charlie b View PostA good rule of thumb is that the available fault current at the secondary terminals of a transformer can be estimated by taking the full load current of the secondary side and dividing by the "per unit" impedance of the secondary windings. That later value is often on the order of 5.75%, so the value you divide by is 0.0575. Different transformers will have different values. In the example I gave earlier, the full load current for a 120/240V single phase 100 KVA transformer is 100,000/240, or 417 amps. Dividing that by 0.0575 give an AIC of 7,246 amps.
Most panels have a minimum AIC of 10,000 amps. To get that from the transformer, it would need to have an impedance of 4.17% or lower. It is possible that the utility transformer's impedance could be lower than that. Even still, the impedance of the secondary conductors will significantly reduce the available fault current at the building's main breaker.
So don't take the EC's statement at face value. Ask for the exact number that PSE provided to him (or her). Then you can do a calculation of the impact of the secondary conductor's impedance on the value PSE gives you. I am willing to predict that the final result is below 10,000 amps, and that you therefore do not have a problem in need of a solution.
https://www.pse.com//media/PDFs/Com...13E4DC06D07E63
Note also that LN faults are higher than LL (as they show in their tables).Ethan Brush  East West Electric. NY, WA. MA
"You can't generalize"
Comment

If I recall most 'Public Utility Commissions' or NESC regulations set a maximum available fault current for any residential service point. I might be mistaken though.Comments based on 2017 NEC unless otherwise noted.
Comment
Comment