fault current

[kwired]

25kA?

25000kV is 25,000,000 volts.

Is fairly common to have 22-25 kA main breakers in the average "loadcenter" and 10 kA branch breakers that are series rated with the main that is used.

Can't find fault current on what web site?

If you are looking for available fault current on your POCO's website - every installation will have a different value and only way you would find your installation is if they had a database with public access that had the available fault current of every customer they serve.

 

[wbdvt]

Most likely the first call to the utility will get you the customer service answer which is based on an infinite bus supplying the transformer with a representative txf impedance. This is usually given to size the equipment. To get more accurate information you have to get past the customer service rep to engineering department. Most utilities will have their systems modeled and be able to give you the actual fault current and X/R at your location. They are getting better at this due to the increase in arc flash studies where actual fault current is needed.

 

[bark]

a senior moment yep 25kv

 

[bark]

I've e-mailed them

 

[kwired]

a senior moment yep 25kv

Still having the moment?

I bet you mean 25kA:happyyes:

25 kV is more commonly seen at upper end of POCO distribution/lower end of transmission voltages.

 

[petersonra]

If you are looking for available fault current on your POCO's website - every installation will have a different value and only way you would find your installation is if they had a database with public access that had the available fault current of every customer they serve.

that's not true with at least a substantial number of POCOs. A bunch of them have a standard they use that says that for a certain service the available short circuit current will not exceed a certain amount. They don't generally provide this information on a case by case basis. I would guess it is a fairly conservative number.

I would also guess the practice varies widely from utility to utility. Some make the information easy to come by, others make it less so. Best bet is just to call them and ask.

 

[kwired]

that's not true with at least a substantial number of POCOs. A bunch of them have a standard they use that says that for a certain service the available short circuit current will not exceed a certain amount. They don't generally provide this information on a case by case basis. I would guess it is a fairly conservative number.

I would also guess the practice varies widely from utility to utility. Some make the information easy to come by, others make it less so. Best bet is just to call them and ask.

Such standard information would have to be what is available at transformer terminals and would assume infinite primary ability. This way they would be able to say 25 kVA transformer - XXXX amps available, 100 kVA transformer XXXXX amps available, etc. - you still will have variances at your service disconnect depending on what is between it and the transformer.

 

[bark]

Yep, I'm having more of the senior moments than the regular moments. I think the keys on the computer keyboard move around on me!

 

[hmspe]

Such standard information would have to be what is available at transformer terminals...

That is not the case in the Phoenix area. The contractor installs the conduits but the utilities install the service lateral conductors, and they typically are far smaller that the NEC would require. We are given a table of Isc at the service terminals. We are not allowed to calculate reduction due to the length of the service lateral.

 

[kwired]

That is not the case in the Phoenix area. The contractor installs the conduits but the utilities install the service lateral conductors, and they typically are far smaller that the NEC would require. We are given a table of Isc at the service terminals. We are not allowed to calculate reduction due to the length of the service lateral.

Unless they have same transformer at each service and service laterals are same size and length for every service - there is no way they all have same available fault current at customer end. If they give you a number it is probably what they expect to be worst case scenario.

If you did some calculating you may be surprised how much available fault current can drop just by adding 25 to 50 feet of length to the supply conductors in some instances.

 

[hmspe]

If you did some calculating...

I fully agree with what you posted, but it is not allowed. We MUST use the table values, applied at the SES. Even if we could use a value at the transformer there would be significant inaccuracies because the transformer and lateral sizes generally aren't determined by the utility until well after the plans are submitted to the AHJ. The utilities require a contract before they start design work, and then it takes several weeks. Both transformer and lateral sizes are based on the load the the utility expects, not on the NEC calcs or the ampacity of the SES. I don't agree with the approach, but it is what it is.

I just did a design in PG&E territory. They don't publish tables, but expect you to call and ask. The project manager contacted them and they declined to provide any information until they had a check in hand for their design.

I wish the NEC and NESC were coordinated, or at least that utilities were required to provide information required to produce an NEC compliant design. That's clearly not the case. Most utilities in Arizona are prohibited by the Corporation Commission from providing high demand data for a service without written releases from each meter holder, and they can be very picky about whose signature they accept. Trying to get a signature from a corporate officer for a tenant space for a national chain can be daunting. I fail to see how demand data is a privacy issue (what does it really tell anyone, and the data for the current period is in plain view on the meter), but again, it is what it is.

 

[electrofelon]

That is not the case in the Phoenix area..... We are not allowed to calculate reduction due to the length of the service lateral.

Says who? Maybe we are talking about different things, but for the purposes of 110.10 and 110.24, without a local amendment, you would use exactly the equipment that is in place. I understand that if the equipment is not in place you need another method.

 

[kwired]

I fully agree with what you posted, but it is not allowed. We MUST use the table values, applied at the SES. Even if we could use a value at the transformer there would be significant inaccuracies because the transformer and lateral sizes generally aren't determined by the utility until well after the plans are submitted to the AHJ. The utilities require a contract before they start design work, and then it takes several weeks. Both transformer and lateral sizes are based on the load the the utility expects, not on the NEC calcs or the ampacity of the SES. I don't agree with the approach, but it is what it is.

I just did a design in PG&E territory. They don't publish tables, but expect you to call and ask. The project manager contacted them and they declined to provide any information until they had a check in hand for their design.

I wish the NEC and NESC were coordinated, or at least that utilities were required to provide information required to produce an NEC compliant design. That's clearly not the case. Most utilities in Arizona are prohibited by the Corporation Commission from providing high demand data for a service without written releases from each meter holder, and they can be very picky about whose signature they accept. Trying to get a signature from a corporate officer for a tenant space for a national chain can be daunting. I fail to see how demand data is a privacy issue (what does it really tell anyone, and the data for the current period is in plain view on the meter), but again, it is what it is.

Sounds like a game the POCO wants to play, the physics don't change because the POCO wants to play their own game.

"We are not allowed to calculate reduction due to the length of the service lateral." Who is restricting that? POCO, AHJ? If you know the kVA of the transformer, you can probably apply worst case scenario for impedance and you can certainly come reasonably close on conductor length and come up with a reasonably accurate fault current at customer end of the run.

Right now I am dealing with a potential service upgrade in a school building and trying to plan things. POCO will be replacing transformer if we do this but I have no idea exactly what transformer they will use but still need to come up with a worst case fault current level when selecting the interrupt rating of equipment I am considering installing. If we do proceed with the project I will need to verify what I have hopefully overestimated at this point, or else I may have to purchase higher AIC equipment then I had estimated for and that may end up being something I have to eat cost difference on.

 

[hmspe]

Who is restricting that? POCO, AHJ?

POCO published rules, and every AHJ in the area requires that they be followed.

From the Arizona Public Service manual:
"Arc Flash hazard calculations should be performed by a Registered Professional Engineer. The dynamic electric utility system grid makes it impractical to provide any further information other than the fault current values noted in Table 800.2. APS will provide only maximum fault current values at the service entrance section. Impedance values, X/R Ratios, primary fault current data and coordination information will not be provided, except primary services see section 1000.2-1."

From the Salt River Project manual:
"The minimum interrupting rating of service equipment shall be the Breaker sizing value in the table."

 

[kwired]

POCO published rules, and every AHJ in the area requires that they be followed.

From the Arizona Public Service manual:
"Arc Flash hazard calculations should be performed by a Registered Professional Engineer. The dynamic electric utility system grid makes it impractical to provide any further information other than the fault current values noted in Table 800.2. APS will provide only maximum fault current values at the service entrance section. Impedance values, X/R Ratios, primary fault current data and coordination information will not be provided, except primary services see section 1000.2-1."

From the Salt River Project manual:
"The minimum interrupting rating of service equipment shall be the Breaker sizing value in the table."

Arc flash hazard calculations are more complex then just finding available fault current. Plus 110.24(A) requires us to mark service equipment with available fault current for everything except dwellings. This is not the same thing as determining arc flash hazard levels, it is just a start at doing that.

Are you sure you fully understand what is required by your laws?

If they want an engineer to provide that calculation then I guess so be it, find someone qualified and mark your service equipment with the results they give you so you can comply with 110.24(A). Arc flash hazards calculations are not part of NEC requirements.

 

[petersonra]

Arc flash hazard calculations are more complex then just finding available fault current. Plus 110.24(A) requires us to mark service equipment with available fault current for everything except dwellings. This is not the same thing as determining arc flash hazard levels, it is just a start at doing that.

Are you sure you fully understand what is required by your laws?

If they want an engineer to provide that calculation then I guess so be it, find someone qualified and mark your service equipment with the results they give you so you can comply with 110.24(A). Arc flash hazards calculations are not part of NEC requirements.

note it used the word "should".

I think there is wide spread confusion regarding the role of the available short circuit current and what the incident energy might be in the event of a fault.