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#68600 - 08/11/06 04:22 PM Pole Pig Fault Current  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
I ordered a hard-to-find 200A main breaker to upgrade an old Murray panel to its full rating. But when it arrived, it was only 10kAIC, vice the 22kAIC breaker I wanted.

The Pole Pig (240V 1-phase secondary) is 25kVA- at 100% load, that's only 104A, yet this pole pig is feeding two homes with 200A services, is that normal? I know they can operated overloaded, but to actually install one like that?

But my big question is, what's the fault current for a 25kVA pole pig? I'd think the core would saturate LONG before it gets close to 10kA, but I want to make sure.

[This message has been edited by SteveFehr (edited 08-11-2006).]

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#68601 - 08/11/06 05:28 PM Re: Pole Pig Fault Current  
LK  Offline
Joined: Jan 2003
Posts: 1,429
New Jersey
"feeding two homes with 200A services, is that normal?"

No that is not normal, usually more, in my neighborhood they have 12 drops from one transformer.

#68602 - 08/11/06 05:34 PM Re: Pole Pig Fault Current  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
My home is on one transformer with four or five other homes.

Bob Badger
Construction & Maintenance Electrician

#68603 - 08/11/06 08:02 PM Re: Pole Pig Fault Current  
e57  Offline
Joined: May 2003
Posts: 2,876
Lets not talk about Bobs transformer.... [Linked Image]

Mark Heller
"Well - I oughta....." -Jackie Gleason

#68604 - 08/11/06 08:06 PM Re: Pole Pig Fault Current  
RODALCO  Offline
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
If you know the impedance of the transformer you can do a rough calculation as follows.

Most transformers have an impedance of 4 to 5 %.

Continuous output current of transformer is 104 Amps.

if impedance is 4% then 100/4 = 25 * 104 gives 2600 Amps.
allowe for a factor of two so a 6 kA braker would be fine.

The 10 kA you have should have no problem at all braking a fault.

Then other factors which will reduce the available fault current are the length of the cables from the Transformer to the braker, the actual loading at the time a fault occurs conditions of the terminations, overhead underground etc.

edited for typo's (ROD)

[This message has been edited by RODALCO (edited 08-11-2006).]

The product of rotation, excitation and flux produces electricty.

#68605 - 08/11/06 10:36 PM Re: Pole Pig Fault Current  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
LOL guys! I was looking at the other houses in my neighborhood, and they seem to have 5-10 of them per 25kVA transformer, too. I'm on the end of a row, which is the only reason why we just have 2, I think How can that work? Do they rely on freezing temperatures to keep the transformers from bursing into flames on cold winter days when everyone's heat pump goes to emergency resistive heater and there's suddenly a 150kVA demand? Is it even worse on down the distribution line? Guy I ran into on the street today while I was looking at the pole pigs told me a story of how MIS Utility mis-marked the underground feeders on a jobsite and he drove a metal stake into one- and watched the nearest pole pig explode into flames, and a few minutes later, the big neithbordhood transformer 2 blocks down exploded into flames as well.

Seems kinda pointless to upgrade the 150A breaker to 200A when the transformer is only rated for 104A.

Also, don't the cores saturate? I thought they were actually designed to saturate about 100% load in order to protect the conductors. Are they typically capable of 300% the rated kVA? That seems to be what I gather from what I could find on google. 2600A is about 25x the rating... How can they have a fault current so much in excess of the saturation point of the core? This one happens to be right in front of my house- maybe 100' of cable. Looks to be 4/0 aluminum, but it's hard to tell.

[This message has been edited by SteveFehr (edited 08-11-2006).]

#68606 - 08/12/06 12:02 AM Re: Pole Pig Fault Current  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Hello Steve.

In regards to your question about PoCo's Pole Mounted Transformers / Pots, and even PMTs (Pad Mounted Transformers);
They are normally the lower KVA ratings in Residential areas - mainly due to the fact that Residential Loads are rarely both high and continuous (even with heat pumps running).
They (Heat Pumps) will eventually cycle off before the 180 minute "Limit Line" is reached.

As to the SCA (Short Circuit Amperes) of a 25 KVA 1Ø 3 Wire Transformer with a Secondary Voltage rating of 120/240VAC, here's a "Worst-Case Scenario Figure":

25 KVA 120/240V 1Ø 3 Wire, Z = 1.58%:
SCA at the Transformer's Terminals would be 11,574 Amps.

PoCo Service Feeders & Secondary Distribution Feeders on Overhead Service Connections, are typically Aluminum (typical of areas using Triplex Cable to the Service Riser) - or at least Copper Clad Aluminum (in older areas, where individual Conductors are strung between poles and to the Service Riser).

Also, the Conductor sizes are small - as compared to what we who use the NEC would consider to be "Proper Size".
The PoCos use Ampacity Tables from the NESC to size Service Feeders - and the Load Calcs generated for a Project is what the Service Design Engineers base the maximum Load of a Customer's Service on.

Computed Load figures are what they use to find minimum feeder conductor sizes - so even if the Service Equipment has a 400 Amp Maximum rating (320 Amp continuous), if the Load Calcs show a Load of, say - 230 Amps, the PoCo will install something like 2/0 XHHW Aluminum for the Ungrounded Conductors, and maybe a #2 XHHW Aluminum for the Grounded Conductor.

Conductors strung between poles are something like #2 Aluminum I believe (need to verify this).

Using these small sized Aluminum Conductors greatly affects the total Fault Current between the Transformer and Customers' side of the kWh Meter.

Adding to that, the distances of which the Service Conductors + Secondary Distribution Conductors are run, has an additional affect on the overall SCA.

Now apply this to the 11.5 KAIC rating at the 25 KVA Transformer, and the SCA becomes a 2.0 KAIC fault rating at the Service, for a Customer located 4 Houses away from the Transformer.

The Residential Distribution Feeders will help limit the SCA at the Customers' end, due to the Line Impedance.

If a Bolted Fault condition occurs, this will load up the Distribution Conductors, and will result in a considerable drop in System Voltage.
As the Voltage decreases, so will the sustained Fault Current, so this will also affect the total let-through value for a fault condition.

In the Neighborhood where we live, there are 11 Houses (one of which is my wife and me) on a 37.5 KVA Pole Mounted Transformer.
Secondary Distribution Conductors are individual #4 RHW Copper-Clad Aluminum strung between Poles.
Service Drops are Three individual #6 RHW Copper-Clad Aluminum Conductors.

Average Square Footage of each House is like 1,500 Sq. Ft.

We are the last Customer on this Secondary Circuit - a good 400 Feet from the Transformer.

If that Transformer's Secondary is 1.56% Z (Impedance), the SCA at the Transformer Terminals would be no more than 17,400 Amps.

I would imagine an L-L Bolted Fault at our Service's Load side would result in maybe 4,000 Amps at the very most.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#68607 - 08/12/06 12:14 AM Re: Pole Pig Fault Current  
LK  Offline
Joined: Jan 2003
Posts: 1,429
New Jersey
Nice post Scott' good job!

Thank you

I think this will help, everyone understand a little more, about utility distribution.

[This message has been edited by LK (edited 08-12-2006).]

#68608 - 08/12/06 12:20 AM Re: Pole Pig Fault Current  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,122
On my street we have 3 transformers spaced about 5 houses apart feeding a common bus the drops are tapped from. I guess they do that for load balancing.

Greg Fretwell

#68609 - 08/12/06 01:59 AM Re: Pole Pig Fault Current  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
Gfret, I be that, if you look closely, you'll find "egg" insulators in the lines that separate the transformer secondaries from one another, meaning that any given house is only supplied by one transformer.

That's how they do it around here, anyway. I guess it's possible that they really do parallel the transformer secondaries, but that could lead to a larger outage or too much current to safely be inetrrupted.

Larry Fine
Fine Electric Co.

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