Utility Service Conductors

[electrofelon]

You would be surprised at amp draw on a substation. I have seen an entire circuit of residential as low as 25 amps on the entire circuit feeding several HUNDRED people.
In the summer that same circuit can be up around 200-250 amps, MAYBE a little higher on really hot days

Yeah I see a 480 or even 208 feeder drawing close to 200 amps and am like "wow that's a lot of juice", but that same current at MV, now THAT'S cranking.

 

[don_resqcapt19]

Is that why the conductors provided from the POCO from the pole to the house always seem way undersized compared to what we must run to meet their lines?

Their wire sizing is based on their knowledge of the actual loads, along with using free air ampacities for overhead lines. The load calculations in Article 220 result in a service size is oversized by a factor of 1.5 to 2.5.

 

[NewtonLaw]

Hi All,
This question may not be appropriate in this forum, just wanted to ask how are the utility service conductors are protected. Say a set of overhead service conductors feed a customer. I suppose the overload protection can be provided at the mains breaker of the panel/meter supplied by these conductors.
How about protecting against the short circuit anywhere along the length of the service conductors before they enter the customers disconnect device. I haven't seen protective devices installed immediately downstream of the OH transformer, is the typical assumption by utility here is that we're dealing with a short run to the customer's disconnecting means, and in case of a fault, the fuses on the primary side of the utility OH transformer would clear the fault?

Thanks,
EE

My Utility protects the transformer and the secondary cables/service cables with a primary fuse located on the high side of the transformer. A typical 25 kVA transformer we use is fused with a 6k fuse. and the nameplate impedance is typically 1.2%. A fault at the transformer terminals would give a primary fault current of 289.4 amps (8681 amps on the secondary) which would clear in 1.5 cycles. Our typical secondary service drop is limited to 100 feet. At this length the secondary maximum fault using #1/0 Al. triplex the fault would be about 5821 amps or about 200 amps on the primary side. The fuse would clear the fault in approximately 2 cycles.

It shows that there is protection on these conductors sufficient for their use and application. Including Florida which uses a similar protection scheme.

Hope this helps.

 

[NewtonLaw]

As the son of retired lineman i always questioned this as well. His response to me was the OH or UG service conductors are the sacrificial portion of their feed. There is no protection for them. The fuse above the xfmr protects only their mainline going back to the substation. The main on our side protects our stuff. The part in between is not protected and replaceable. This is FPL’s view and approach.

Not sure this is correct. Do you have any of the FPL specs on htis?

 

[NewtonLaw]

Is that why the conductors provided from the POCO from the pole to the house always seem way undersized compared to what we must run to meet their lines?

The Utilities follow NESC guidelines for construction and clearances. Cable sizing is based on thermal limitations and load factor. The NEC sizing is based on maximum allowable currents as per NEC guidelines which have to take into consideration that the conductors are not in free air, later adjustments will not be made for load factor changes and appropriate updates would not be made. They look at Safe 1st. Load 2nd. Load factor 3rd for initial installation.

 

[Open Neutral]

Here, PEPCO had two fuses on some xfmrs. I asked a lineman why.
The lower current, cheaper, fuse was for overload.
The higher price & amp fuse was faster. It was for lightning hits.

 

[Hv&Lv]

Here, PEPCO had two fuses on some xfmrs. I asked a lineman why.
The lower current, cheaper, fuse was for overload.
The higher price & amp fuse was faster. It was for lightning hits.

Yes, it’s a current limiting fuse right at the hot line clamp in series with an expulsion fuse above the transformer.

 

[tom baker]

Some one once told me the POCO only has two sizes of service drop cable on their trucks, 2 and 6 AWG.

 

[Hv&Lv]

Some one once told me the POCO only has two sizes of service drop cable on their trucks, 2 and 6 AWG.

#2 for all houses and #6 for the lights!

 

[MD Automation]

Just chiming in with a datapoint of my own, the 25KVA transformer on the pole in front of my house failed in May of 2020 – and since I was around I watched and learned a lot.

The fuse that my POCO used on the little 25 KVA pot (powers only my house) is rated 10A and has a “DI” rating. So that’s a bit higher than the 6K posted above that Newton Law indicates they use up in PA, or the 5D that Hv&Lv says he uses. Not saying my POCO got it right or wrong or better or worse, I just know that this is what they put in the holder earlier this year to magnetize the new unit.



I found data for this fuse here..

https://indelbauru.com.br/wp-content/uploads/2019/03/curve_english_ef_di.pdf


Typical data for K rated fuses is here…

https://www.sandc.com/globalassets/...ts/tcc-number-165-6.pdf?dt=637267440321552315

If I am reading these charts correctly, the 10DI appears to work similarly to a 10K rated fuse on the “slow-blow” end (both seem to open ~20A at 300 sec), but on the other end of the curve this DI fuse allows greater current at the .01 sec trip. Something like twice the amount, ~450A for the 10K and ~900A for the 10DI. That’s obviously on the primary side – which is 7680V in my rural area.

At any rate, it would seem that each POCO has figured out how they want to protect the pot and secondary service conductors from obvious (bolted) faults. And the system will ride thru smaller and quicker overloads w/o opening the cutout. I assume trying to take the “nuisance” out of nuisance tripping.

Lastly, with regards to the OPs original question about protecting service conductors – I read all the time that it’s best to consider them unfused. Tom Baker made that point in this thread, MBrooke made the same point in my thread about my transformer replacement. It’s possible for a ton of amps to flow on the primary side w/o clearing a fault quickly, if at all.

 

[NewtonLaw]

MD Automation's information is spot on for the 10D "DI" fuse. And if you compare the S&C K-Link, the 6K is more sensitive in two respects. The minimum has to exceed about 13 amps for the 6K and about 20 amps for 10 fuse. DI from INDEL Bauru, Eaton's TCC for the 10D (curve R240-91-16) is similar. The k-link is faster acting for a fault current of 70 primary amps the MM is 0.1 seconds, the 10DI is about 3.5 seconds. This maybe why that POCO considers the service cables unprotected. 3.5 seconds is a long time electrically.