10KAIC breakers in a 200A service

Location
South Carolina
Occupation
Electrician
I was looking at an existing house that has 5 of the 2-pole GE THQL disconnects at the 200A single phase service. I have one 60A breaker, one 50A breaker, and three 30A breakers. All of the service disconnects are rated at 10kaic. I am wanting to remove an inside fuse panel and move the circuits outside. I was thinking about adding a panel. The old fuse panel is on the 60A 2-pole breaker. I would probably feed the new panel off the same breaker. But, I am wondering if 10kaic is enough. I am use to installing 200A services with one disconnect and over 20 kaic protection. So, I am not sure if 10 kaic is enough. If I add the panel, I could add a panel with a main breaker that has a greater aic. But, because I am being asked to look over the electrical for a potential owner, I feel that I need to at least be concerned about the other breakers that will not be changed out. If it is too low a rating, then I need to recommend changing out the other breakers for a correct aic. Or, if that is not possible, I should recommend changing out the panel. Should I be concerned about the 10 kaic?
 

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hillbilly1

Senior Member
Location
Atlanta,Ga
I misread your post, and thought you had a 200 amp main. Probably need to get with the utility company to find out the available fault current at the transformer, and do a calculation taking in wire size and length. There are several online calculators you can use.
 

Carultch

Senior Member
Location
Massachusetts
You would need to know the kVA and impedance of the transformer, to get the starting point for the fault current (short circuit current) at its secondary. Then you would implement the length and size of the service conductors, to determine how it diminishes with distance. More impedance means less fault current. More KVA of the transformer means less fault current.

See "Calculator: Point-to-Point Method by John Sokolik" on this page of resources.

You might be able to see the KVA rating on transformer housing with a pair of binoculars. Usually there is a big number stamped on the housing like 10 or 25, that indicates this. The largest standard size for single phase is 167 kVA, and units this large are uncommon. The impedance (%Z) cannot easily be known, without seeing a detailed nameplate. A typical impedance of a single phase pole-mount transformer is between 2% and 2.5%. The utility can also provide fault current at the service for you, if you give them the account and meter number.
 
Location
South Carolina
Occupation
Electrician
You would need to know the kVA and impedance of the transformer, to get the starting point for the fault current (short circuit current) at its secondary. Then you would implement the length and size of the service conductors, to determine how it diminishes with distance. More impedance means less fault current. More KVA of the transformer means less fault current.

See "Calculator: Point-to-Point Method by John Sokolik" on this page of resources.

You might be able to see the KVA rating on transformer housing with a pair of binoculars. Usually there is a big number stamped on the housing like 10 or 25, that indicates this. The largest standard size for single phase is 167 kVA, and units this large are uncommon. The impedance (%Z) cannot easily be known, without seeing a detailed nameplate. A typical impedance of a single phase pole-mount transformer is between 2% and 2.5%. The utility can also provide fault current at the service for you, if you give them the account and meter number.
Thank you.
 

NewtonLaw

Senior Member
In my company our single phase pole mounted transformers range from 10 kVA to 167 kVA as previously reported. The suggestion to go look at the transformer to determine the size is a good starting point. The transformer size is determined by the Utility based on the total number of customers served and the expected maximum demand and load factor.

For your purposes I will consider the nameplates on my lowest impedance transformers, so a 25 kVA transformer with 1.2%Z will have a maximum fault duty of (25/.24)/0.012= 8,681 amps assuming an infinite primary bus. Most new pole mounted transformers will vary impedance values based on cost and voltage regulation parameters set by the Utility. If the transformer is a 50 kVA unit the maximum my transformer would give you is 17,361 amps with a %Z of 1.2 and an infinite primary bus. You indicated that the service drop is overhead. My Utility would extend #1/0 AL. , XLP triplex cable up to 100 feet away. If yours is the same, the fault duty at 100' of #1/0 I suggested, the fault duty drops to about 5,800 amps. If the service cable length is as short as 37' the fault level would be 10,000 amps. If the service drop cable was #4/0 AL XLP, the service length would have to be at least 50 feet in length to be at 10,000 amps. I assumed your secondary service voltage is 240/120 volts. (PS: my 100 kVA units have a 1.5%z and my 167 kVA units have a 2.2%Z)

I hope this helps.
 
A couple other comments. 408.36 requires a panelboard bus to be protected at it's rating, however there is an exception for MLO service panelboards, which is what you have.

Regarding the 10kaic rating, if you find that to be inadequate, or just want to be sure you are good, you could install a main disconnect ahead that series rates with those 5 service disconnects. Those breakers are common so I am sure they rate with lots of fuses and other GE breakers
 
Location
South Carolina
Occupation
Electrician
In my company our single phase pole mounted transformers range from 10 kVA to 167 kVA as previously reported. The suggestion to go look at the transformer to determine the size is a good starting point. The transformer size is determined by the Utility based on the total number of customers served and the expected maximum demand and load factor.

For your purposes I will consider the nameplates on my lowest impedance transformers, so a 25 kVA transformer with 1.2%Z will have a maximum fault duty of (25/.24)/0.012= 8,681 amps assuming an infinite primary bus. Most new pole mounted transformers will vary impedance values based on cost and voltage regulation parameters set by the Utility. If the transformer is a 50 kVA unit the maximum my transformer would give you is 17,361 amps with a %Z of 1.2 and an infinite primary bus. You indicated that the service drop is overhead. My Utility would extend #1/0 AL. , XLP triplex cable up to 100 feet away. If yours is the same, the fault duty at 100' of #1/0 I suggested, the fault duty drops to about 5,800 amps. If the service cable length is as short as 37' the fault level would be 10,000 amps. If the service drop cable was #4/0 AL XLP, the service length would have to be at least 50 feet in length to be at 10,000 amps. I assumed your secondary service voltage is 240/120 volts. (PS: my 100 kVA units have a 1.5%z and my 167 kVA units have a 2.2%Z)

I hope this helps.
Thank you
 
Location
South Carolina
Occupation
Electrician
A couple other comments. 408.36 requires a panelboard bus to be protected at it's rating, however there is an exception for MLO service panelboards, which is what you have.

Regarding the 10kaic rating, if you find that to be inadequate, or just want to be sure you are good, you could install a main disconnect ahead that series rates with those 5 service disconnects. Those breakers are common so I am sure they rate with lots of fuses and other GE breakers
Thank you.
 

kwired

Electron manager
Location
NE Nebraska
In my company our single phase pole mounted transformers range from 10 kVA to 167 kVA as previously reported. The suggestion to go look at the transformer to determine the size is a good starting point. The transformer size is determined by the Utility based on the total number of customers served and the expected maximum demand and load factor.

For your purposes I will consider the nameplates on my lowest impedance transformers, so a 25 kVA transformer with 1.2%Z will have a maximum fault duty of (25/.24)/0.012= 8,681 amps assuming an infinite primary bus. Most new pole mounted transformers will vary impedance values based on cost and voltage regulation parameters set by the Utility. If the transformer is a 50 kVA unit the maximum my transformer would give you is 17,361 amps with a %Z of 1.2 and an infinite primary bus. You indicated that the service drop is overhead. My Utility would extend #1/0 AL. , XLP triplex cable up to 100 feet away. If yours is the same, the fault duty at 100' of #1/0 I suggested, the fault duty drops to about 5,800 amps. If the service cable length is as short as 37' the fault level would be 10,000 amps. If the service drop cable was #4/0 AL XLP, the service length would have to be at least 50 feet in length to be at 10,000 amps. I assumed your secondary service voltage is 240/120 volts. (PS: my 100 kVA units have a 1.5%z and my 167 kVA units have a 2.2%Z)

I hope this helps.
Hit on much of what I was prepared to say.

Bottom line is if there is at least 50 feet of conductor between the transformer and point of concern, chances are the available fault current at that point is less than 10kA.

If the conductor is small, say 60 amp or less you probably can have as little as 25 feet of conductor and many times the impedance of the conductor is enough that it will never be over 10kA at the load end even with a pretty large transformer as the source.

This I assume is at least part of why we aren't even required to mark available fault current on single family dwellings - nearly never is high enough to be a concern when it comes to available fault current vs service equipment rating. 22kA mains that come standard in many loadcenters is overkill in majority of single family dwellings, but that is what they put in them and might cost you even more if you wanted to special order one that wasn't 22k so just use what is on the market.
 

Mevlevi

Member
Location
Massachusetts
Occupation
Engineer
Typical kAIC for single phase transformers
25kVA & below: 10,500kAIC
50kVA: 13,900kAIC
75kVA: 20,900kAIC
100kVA: 27,800kAIC
167kVA: 46,400kAIC

You need to find out the size of the transformer and use the online calculator mentioned in the posts above. The safe method is to call the utility company for the available fault current. Once you have that, the type of cable and length you should be able to calculate the kAIC at the panel.
 

Carultch

Senior Member
Location
Massachusetts
I realize I neglected to edit my copy and paste. I wrote "More KVA of the transformer means less fault current".

What I meant was:
More KVA of the transformer means more fault current.
 

NewtonLaw

Senior Member
Hit on much of what I was prepared to say.

Bottom line is if there is at least 50 feet of conductor between the transformer and point of concern, chances are the available fault current at that point is less than 10kA.

If the conductor is small, say 60 amp or less you probably can have as little as 25 feet of conductor and many times the impedance of the conductor is enough that it will never be over 10kA at the load end even with a pretty large transformer as the source.

This I assume is at least part of why we aren't even required to mark available fault current on single family dwellings - nearly never is high enough to be a concern when it comes to available fault current vs service equipment rating. 22kA mains that come standard in many loadcenters is overkill in majority of single family dwellings, but that is what they put in them and might cost you even more if you wanted to special order one that wasn't 22k so just use what is on the market.
Is this based on the transformer size of 50 kVA or less?
 

NewtonLaw

Senior Member
Typical kAIC for single phase transformers
25kVA & below: 10,500kAIC
50kVA: 13,900kAIC
75kVA: 20,900kAIC
100kVA: 27,800kAIC
167kVA: 46,400kAIC

You need to find out the size of the transformer and use the online calculator mentioned in the posts above. The safe method is to call the utility company for the available fault current. Once you have that, the type of cable and length you should be able to calculate the kAIC at the panel.
The AIC you give for the 25 kVA single phase transformer would have to be 0.99%Z or 1%Z. Do you have transformers this size with that low of an impedance value? Just curious. Kind of the same question on the 167 kVA with a 1.5%Z which also seems kind of low.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I've heard also that the input voltage plays a part too. I remember years ago in EC&M magazine they had an article about poco's upgrading line voltage (usually doubling the voltage) which affected the available fault current, and services that previously had sufficient aic ratings, no longer would be sufficient.
 
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