SCCR Residential - How large transformers can power homes with only 10kVA breakers

[Cody Brown]

This area in electrical code appears to be unknown by a lot of electricians. I have seen that distribution transformers may be as large as 500 kVA. The the case that there is a 500 kVA distribution transformer feeding over 100 homes, with a short impedance of 6%, is there any concern that the available fault current from the 500 kVA transformer exceeds the branch circuit breakers SCCR rating in each of the 100 homes?

Available Fault Current = 500,000 VA / ( 3^(1/3) * 240V * 0.06) = 20.05 kA
Typical house circuit breaker (15A to 30A) SCCR is only 10 kA

In this case does there have to be a smaller distribution transformer 1:1 ratio, or have current limiting fuses just for SCCR protection, expressly for the homes closest to the 500 kVA transformer?

From what I see, the largest distribution transformer you can have power a home that only has 10 kA circuit breakers is 249 kVA (with 6% impendence).

 

[tom baker]

The NEC (since about 2015) now addresses available fault current requiring a label and date of calculation. This is effective if the AHJ requires it. But I agree this is an area that is less known

 

[Jraef]

But also, there are a lot of panels where although the individual branch breakers are labeled as 10kAIC, in series combination with the specific Main breaker, the overall panel can bear a 22kAC label. That would be Code compliant in your scenario.

 

[augie47]

In my experience, it does not necessarily go unnoticed and certainly needs to be addressed on multifamily installations. Thankfully it is rare on the majority of neighborhood residential structures that a large transformer is involved and even in those events the service entrance conductors are long enough and sized small enough that the SCA at the service panels is low. The policy of departments with which I have been associated was not to be concerned on services sized 400 amps of less and this was based on a history of evaluations.

 

[drcampbell]

... the service entrance conductors are long enough and sized small enough that the SCA at the service panels is low. ...

Being the geek that I am, I double-checked that for my own house.
There's about 50 feet of 4 AWG aluminum triplex from the pole to the weatherhead. (older neighborhood, small-ish lots) That alone provides 40 mΩ of DC resistance, which will permit less than 6 kA to flow. (even neglecting the AC impedance of the cable, the transformer impedance, the service-entrance cable, and not being the first house on the transformer)

 

[electrofelon]

But also, there are a lot of panels where although the individual branch breakers are labeled as 10kAIC, in series combination with the specific Main breaker, the overall panel can bear a 22kAC label. That would be Code compliant in your scenario.

And I would even change your "a lot" to a stronger word like "almost all" or the "vast majority". I remember looking into this a while back and I think siemens and eaton BR/zinsco2 had some main breaker loadcenters in their catelog where the main was only 10k, But practically everything I run into now has a 22K MB which 10k branches series rate to.

 

[tom baker]

When I started some 40 years ago, the standard breaker was 10K. Today 22 KAIC is the norm. I had a poco engineer tell me many years ago that the largest transformer they use for secondary distribution was 10 kVA that would limit the AIC at dwelling units to 5K.

 

[David Castor]

It's definitely a concern, but at 240 V, the impedance of the cable between transformer and the panelboard has a significant impact. Even a few feet of cable will reduce the available fault current significantly. You can't ignore the cable at these lower voltages. Also, as jraef notes, series rated panels should be available to deal with it. This is a good application for series-rated equipment.

 

[electrofelon]

This area in electrical code appears to be unknown by a lot of electricians. I have seen that distribution transformers may be as large as 500 kVA. The the case that there is a 500 kVA distribution transformer feeding over 100 homes, with a short impedance of 6%, is there any concern that the available fault current from the 500 kVA transformer exceeds the branch circuit breakers SCCR rating in each of the 100 homes?

Available Fault Current = 500,000 VA / ( 3^(1/3) * 240V * 0.06) = 20.05 kA
Typical house circuit breaker (15A to 30A) SCCR is only 10 kA

In this case does there have to be a smaller distribution transformer 1:1 ratio, or have current limiting fuses just for SCCR protection, expressly for the homes closest to the 500 kVA transformer?

From what I see, the largest distribution transformer you can have power a home that only has 10 kA circuit breakers is 249 kVA (with 6% impendence).

What is the cube root of 3 in your eqation?

Some other comments: I think the case of a 500KVA transformer serving residences is rare. Sure there are network distribution systems and apartment buildings, and AFC should be looked at with these.

One other thing many people skip over is that the L-N FC can be higher with single phase transformers so you should provide a factor for this. Probably a factor of 1.5 over the "simple" AFC should cover it. See the following:

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-electrical-formulas.pdf

 

[electrofelon]

This area in electrical code appears to be unknown by a lot of electricians. I have seen that distribution transformers may be as large as 500 kVA. The the case that there is a 500 kVA distribution transformer feeding over 100 homes, with a short impedance of 6%, is there any concern that the available fault current from the 500 kVA transformer exceeds the branch circuit breakers SCCR rating in each of the 100 homes?

Available Fault Current = 500,000 VA / ( 3^(1/3) * 240V * 0.06) = 20.05 kA
Typical house circuit breaker (15A to 30A) SCCR is only 10 kA

In this case does there have to be a smaller distribution transformer 1:1 ratio, or have current limiting fuses just for SCCR protection, expressly for the homes closest to the 500 kVA transformer?

From what I see, the largest distribution transformer you can have power a home that only has 10 kA circuit breakers is 249 kVA (with 6% impendence).

Note SCCR is the wrong term here. AIC is the correct term for the fault current rating of an OCPD.

 

[Canton]

This area in electrical code appears to be unknown by a lot of electricians. I have seen that distribution transformers may be as large as 500 kVA. The the case that there is a 500 kVA distribution transformer feeding over 100 homes, with a short impedance of 6%, is there any concern that the available fault current from the 500 kVA transformer exceeds the branch circuit breakers SCCR rating in each of the 100 homes?

Available Fault Current = 500,000 VA / ( 3^(1/3) * 240V * 0.06) = 20.05 kA
Typical house circuit breaker (15A to 30A) SCCR is only 10 kA

In this case does there have to be a smaller distribution transformer 1:1 ratio, or have current limiting fuses just for SCCR protection, expressly for the homes closest to the 500 kVA transformer?

From what I see, the largest distribution transformer you can have power a home that only has 10 kA circuit breakers is 249 kVA (with 6% impendence).

In my jurisdiction we only ask the Utility to provide the Available Fault Current on commercial projects, and then check the AIC rating.

I was told, in the past, the Utility only supplies Xformers for residential services sized with impedance/cable length/etc. that the historical data always resulted in fault currents below 10k. So they stopped providing the calcs, and we stopped looking at the AIC for residential services.

Multi-family building service and individual unit feeders is looked at.

 

[garbo]

Side note: In my area the utility company never supplies a fuse or circuit breaker on the secondary of any distribution transformers and sure they install higher ampere rating that the NEC allows on the 4 to 7,000 volts primary. Yes they do not have to follow the NEC but not only is the higher fault current then the 10KAC but the long time a dead short would ever clear the ultility companies primary fuses. Years ago a ultility company linesmen said they rather have your house catching on fire then blow one of their primary fuses. Another thing that I always was dangerous was when lazy cheap ultility company has three phase power in pole they only supply the distribution transformers with one phase and use the neutral to backfeed other side of this autotransformer connection. They connect one side of primary, transformer case & center tap of 120/240 secondary to a ground wire at bottom of pole. One problem in some areas they steal the 6 or 8' of aluminum wire going to pole ground rod.

 

[tortuga]

Check with your public utility commission (PUC) or whom ever regulates the POCO there might be a rule limits the available fault current at the service point for a single family home.

 

[kwired]

It's definitely a concern, but at 240 V, the impedance of the cable between transformer and the panelboard has a significant impact. Even a few feet of cable will reduce the available fault current significantly. You can't ignore the cable at these lower voltages. Also, as jraef notes, series rated panels should be available to deal with it. This is a good application for series-rated equipment.

Correct, dwellings with only 200 amp or less service have limiting enough effects on just 50 feet of service conductor that the available current at the service disconnect is significantly less than at the transformer terminals. You generally don't have to have higher than usual rated equipment for 200 amp and less and often don't for 400 amp either, but starting to get into a situation where you should check a little more closely at least. Also most main breaker loadcenters usually have 22 or 25 kA main breaker that is series rated for the 10kA branch breakers that are listed for that panel.

 

[drcampbell]

... in the past, the Utility only supplies Xformers for residential services sized with impedance/cable length/etc. that the historical data always resulted in fault currents below 10k. So they stopped providing the calcs, and we stopped looking at the AIC for residential services. ...

That seems entirely reasonable, and precisely how catastrophes begin.

In my own house, (erected in 1927, rewired in maybe 1957-ish) there's a main fusebox. It predates AIC labeling, but it's probably no higher than 5kA.
What happens when the PoCo upgrades the neighborhood distribution and just assumes that 10kA is a safe & reasonable criterion, without notifying anybody or checking anything? What happens 50 years from now when "everybody" has a 10kA panel and the PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

If we required a simple available-fault-current form that needed to be checked & signed with every upgrade, 99% of them would be irrelevant, (and almost silly) but we might just identify and correct a few hazards along the way. If we continue to rely on what the historical data has always said, those hazards are only discovered after they escalate to catastrophes.

I've installed current-limiting main fuses, but how many homeowners, landlords & renters just don't have a clue?

 

[electrofelon]

That seems entirely reasonable, and precisely how catastrophes begin.

In my own house, (erected in 1927, rewired in maybe 1957-ish) there's a main fusebox. It predates AIC labeling, but it's probably no higher than 5kA.
What happens when the PoCo upgrades the neighborhood distribution and just assumes that 10kA is a safe & reasonable criterion, without notifying anybody or checking anything? What happens 50 years from now when "everybody" has a 10kA panel and the PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

If we required a simple available-fault-current form that needed to be checked & signed with every upgrade, 99% of them would be irrelevant, (and almost silly) but we might just identify and correct a few hazards along the way. If we continue to rely on what the historical data has always said, those hazards are only discovered after they escalate to catastrophes.

I've installed current-limiting main fuses, but how many homeowners, landlords & renters just don't have a clue?

I dont think distribution upgrades will make a lot of difference in AFC. We all typically assume infinite primary anyway, and the transformer does the lions share of the AFC reduction. Granted the POCO could change serving transformers to lower Z units. But really, how likely is a bolted fault immediately after the OCPD? How many incidents are there of underdutied equipment exploding?

 

[augie47]

That seems entirely reasonable, and precisely how catastrophes begin.

In my own house, (erected in 1927, rewired in maybe 1957-ish) there's a main fusebox. It predates AIC labeling, but it's probably no higher than 5kA.
What happens when the PoCo upgrades the neighborhood distribution and just assumes that 10kA is a safe & reasonable criterion, without notifying anybody or checking anything? What happens 50 years from now when "everybody" has a 10kA panel and the PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

If we required a simple available-fault-current form that needed to be checked & signed with every upgrade, 99% of them would be irrelevant, (and almost silly) but we might just identify and correct a few hazards along the way. If we continue to rely on what the historical data has always said, those hazards are only discovered after they escalate to catastrophes.

I've installed current-limiting main fuses, but how many homeowners, landlords & renters just don't have a clue?

POCO making changes that might effect the available current opens up a whole can of worms predominately "upgrades at whose expense". zIf it's a POCO change the homeowner would likely say it should be a OPCO expense.
My experiecen with POCOs is that they are not goling to mention anything that might effect the bottom $$. Safety is #1 UNLESS there is money involved

 

[drcampbell]

... Safety is #1 UNLESS there is money involved

Ain't that the truth!?

How did we ever let a safety-critical inherent monopoly be taken over by for-profit corporations? If ever there were anything that needs to be publicly owned & operated, it's electric power distribution.

Our local utility refuses to install five-dollar squirrel guards on transformers because that would be a routine expense that would come out of their profits, but removing the remains of a squirrel and replacing a primary fuse can be billed as "emergency" service, and they dispatch (and no doubt, bill for) six people and three trucks when one guy, a station wagon and a hookstick would suffice for 90% of such calls.

(yes, we have frequent outages and closed-casket squirrel funerals here)

 

[GeorgeB]

... PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

(I'm an engineer, not electrician)

I agree with your discussion, but let's look a little deeper. Looking only at possibilities here, my underground service line is in the 100 ft range, ALMOST guaranteeing under 10kA at the meter, certainly after the 60 additional feet to the MBP.

My home was built in 2004 with a 22kA main and a remote (primary) MLB panel with CH 10kA series breakers rated to 22kA with a CH 22kA main. The previous owner had an electrician add a panel in the basement for his home shop fed from a 50A breaker with 6-3/g aluminum. The additional cable certainly reduces SCA to below 10kA. BUT that electrician used a SqD Homeline panel, so no series rating.

No question everything is adequate and likely code compliant even TODAY, but if the in-house wiring was substantially oversized and the transformer run much shorter, the basement panel POSSIBLY could have seen over 10kA SCA. I'm sure the electrician, if he knew how to calculate SCA, would have found even 10kA was sufficient, but would all just not worry? SqD won't be series RATED with the CH main. I've no records of inspections either in 2004 at construction, or 2006 (based on mfg date in the panel) when he did the addition, but would that have been looked at in any event?

 

[Canton]

That seems entirely reasonable, and precisely how catastrophes begin.

In my own house, (erected in 1927, rewired in maybe 1957-ish) there's a main fusebox. It predates AIC labeling, but it's probably no higher than 5kA.
What happens when the PoCo upgrades the neighborhood distribution and just assumes that 10kA is a safe & reasonable criterion, without notifying anybody or checking anything? What happens 50 years from now when "everybody" has a 10kA panel and the PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

If we required a simple available-fault-current form that needed to be checked & signed with every upgrade, 99% of them would be irrelevant, (and almost silly) but we might just identify and correct a few hazards along the way. If we continue to rely on what the historical data has always said, those hazards are only discovered after they escalate to catastrophes.

I've installed current-limiting main fuses, but how many homeowners, landlords & renters just don't have a clue?

That seems entirely reasonable, and precisely how catastrophes begin.

In my own house, (erected in 1927, rewired in maybe 1957-ish) there's a main fusebox. It predates AIC labeling, but it's probably no higher than 5kA.
What happens when the PoCo upgrades the neighborhood distribution and just assumes that 10kA is a safe & reasonable criterion, without notifying anybody or checking anything? What happens 50 years from now when "everybody" has a 10kA panel and the PoCo decides that 22kA is a safe & reasonable criterion, based on the (then-) usual & contemporary practice?

If we required a simple available-fault-current form that needed to be checked & signed with every upgrade, 99% of them would be irrelevant, (and almost silly) but we might just identify and correct a few hazards along the way. If we continue to rely on what the historical data has always said, those hazards are only discovered after they escalate to catastrophes.

I've installed current-limiting main fuses, but how many homeowners, landlords & renters just don't have a clue?

Just not gonna happen in this world. The Utilities do what they want. The AHJs do what they want. There has never been enough documented incidents at the residential level to justify a change.