POCO transformer 4 feet from service

[Greg1707]

I have a project to replace a 200 amp panel at a house with an underground service. The main electric panel is four feet from the POCO transformer.
How do I determine what type of main breaker that will be needed?

 

[augie47]

It is very unlikely that you will need anything greater than a 10k (AIC) rated breaker but your POCO should be able to confirm.

 

[wbdvt]

It is very unlikely that you will need anything greater than a 10k (AIC) rated breaker but your POCO should be able to confirm.

possibly but there is not enough detail to know for sure. For example: is this in a residential development with a decent size padmount feeding multiple houses?

Not likely since the panel is so close to the transformer which sounds like maybe a pedestal setup?

 

[Greg1707]

possibly but there is not enough detail to know for sure. For example: is this in a residential development with a decent size padmount feeding multiple houses?

Not likely since the panel is so close to the transformer which sounds like maybe a pedestal setup?

This is in a residential development. There are houses up and down the street. The next house is 20 feet away. I assume they all share the same transformer.

So, is the POCO where I find the answer to this question?

 

[roger]

If you can not get actual numbers the POCO should have some numbers posted somewhere like Duke does starting on page 48 of the link below.

https://www.duke-energy.com/_/media/pdfs/partner-with-us/construction-toolbox/white-book-3.pdf?la=en

Roger

 

[beanland]

10kA or more

10kA or more

Be careful assuming 10kA. As suggested by others, if this transformer serves multiple services, it can be large and thus, have a high fault current. For a single home, a 25kVA is fine but these are typically 2-3% impedance and fault current is below 10kA. But, if the transformer is 100kVA, then it may be over 20kA.

The other concern is that "what of the utility replaces the transformer in the future?" Best bet is to ask the utility for the AFC.

 

[electrofelon]

Note that unless you use a regular 10k backfed as a main, or one of those small circuit breaker enclosures, pretty much everything else will be 22k rated. Even with a 100kva tranny, with under 1.9% impedance, you would still be fine.

 

[Greg1707]

POCO Book

POCO Book

This is what I found in the Dominion Energy Blue Book.

On initial services and reconnection after alterations to existing services, the Company will limit
the available fault current on single family detached homes (including mobile homes not in mobile home
parks) to 10,000 symmetrical amperes. This is provided the service size does not exceed 200 amperes.s is what I found in the Dominion Energy Blue Book.


So this means the POCO will make adjustments to limit the fault current?

 

[LarryFine]

I imagine the "adjustment" is made with the sizing of the service conductors.

 

[electrofelon]

That's interesting they commit to that. Usually POCO's are like, "fault current is almost always like 100k, at least"

 

[kwired]

I have a project to replace a 200 amp panel at a house with an underground service. The main electric panel is four feet from the POCO transformer.
How do I determine what type of main breaker that will be needed?

4 feet between enclosures or 4 feet of conductor? Length of conductor is what is most important here, at such a short distance every foot can make a difference.

If you can somehow make it so there is at least 10 feet of conductor it may get you below 22kA in most instances, maybe not for a 75-kVA or larger transformer.

Aluminum instead of copper could make enough of a difference in some cases.

 

[oldsparky52]

I would request the fault current for that service at that address in writing (email) from the PoCo.

Aren't most manufacturer's series rating their stuff so you could just change out the main to a 22kaic unit and series rated protect the 10kaic branch breakers? It's not really that much money, is it (or is it)?

 

[electrofelon]

Aren't most manufacturer's series rating their stuff so you could just change out the main to a 22kaic unit and series rated protect the 10kaic branch breakers? It's not really that much money, is it (or is it)?

Pretty much anything with a main will be rated 22k, and series rate with 10k branches.

(As I said previously, there are a few exceptions, mostly when using "regular" 10k as a main, say when using a MLO 2-6 setup, or a circuit breaker enclosure. Meter packs with disconnects might have 10k too if you didn't request 22's)

 

[kwired]

I would request the fault current for that service at that address in writing (email) from the PoCo.

Aren't most manufacturer's series rating their stuff so you could just change out the main to a 22kaic unit and series rated protect the 10kaic branch breakers? It's not really that much money, is it (or is it)?

Yes most "loadcenters" will have 22kA rated mains, but depending on size of this transformer and with the very short run, this could exceed 22kA.

Plugged in some numbers on the fault current calculator found on Mike Holt's main site, and it depends on transformer impedance, but looks like you need to have 100 or even 167 kVA transformer (single phase) before this short of distance may be too much for 22kA mains.

And at that short distance the fault current is higher for line to neutral faults than for line to line faults.

 

[AtTheKeyboard]

I agree that the Utility's connection standards (aka Blue Book) should be followed. In the past, I used Fairbanks GVEA's commercial standards as my tool for standard-less utilities, as they referenced the ANSI standards for minimum impedance in MV transformers. I no longer do that, as I found one - new - transformer in the field that was 0.2% below said minimum. Real data wins over assumptions. I'm lucky that my predecessor had specified a high enough SCCR at the breakers to handle this.

I disagree with what I saw earlier in this thread, where someone indicated that at 22kAIC breakers could be safe with a 100MVA transformer. - The fault current can be much higher than that.

100kVA implies a single phase transformer, and I usually do rough calculations for 120/240V transformers as:


100000VA / 120V / 0.019 = 44kAIC (More like 35kAIC really... ...but I'll do a more formal calc if the first one goes over.)


This is because of how a split-phase transformer is wired, unlike three-phase transformers. While faulting, the transformer can effectively become more like a X-120V, instead of a X-120/240 transformer. Literature indicates reality isn't as conservative as my rough calculation, but it's still not divided by "just" 240V.


Busmann's paper, below, would take it at 240V, and then multiply the resulting available fault current by 1.5 for this effect. Yet, there's even more details, like tolerance, etc. I don't like how the 1.5X is tied to a range, thus the simplified 120V calc...


http://www.cooperindustries.com/cont...l_Formulas.pdf


I will occasionally use the following app to make my life easier, but I generally prefer to hand-calc (spreadsheet) so I know where the number came from.


http://www.cooperindustries.com/content/public/en/bussmann/electrical/resources/fc2.html


Beyond the above, some utilities, such as Colorado's Xcel, are requiring the use of a current-limiting fuse before feed-through meter sockets to drop the current to 10kAIC or less. I'm not sure why they need that at the meter socket, and I would assume there's many, many meter sockets not compliant with this criteria. It's also nice to leave some headroom, in case the utility increases the "firmness" of their system - with a bigger transformer outside.