# Residential POCO transformer question



## Mark Twenhafel (Dec 23, 2006)

A general new guy-type question. (BTW, I miss the *real* new guy: newguy07.) 

Does a typical POCO transformer intended for residential use have a secondary winding w/ a center neutral tap on all three phases?

Reason I'm asking is that on a typical schematic drawing of a transformer with 120/240 service winding, the 120/240 winding w/ center tap is always shown on the A phase, with the B and C phases just brought out straight.

There doesn't seem to be any reason not to have all three phases serve 120/240 power in a residential neighborhood, so I just kinda' figured that it was drawn that way to simplify the drawing.

Thanks,
Mark


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## Tiger (Nov 21, 2007)

Typically in this area 3PH isn't in residential neighborhoods.

Dave


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## Mark Twenhafel (Dec 23, 2006)

So the POCO justs distributes out a single phase of a higher voltage along the streets of a new subdivision? 

Our little green transformers seem to service either 2 or 3 houses--makes me wonder how many transformers the POCO can connect to a single distribution line--anyone know? It's probably variable, I guess. Does anyone know what voltage is typically distributed to the primary sides of residential neighborhood transformers? 

Thanks,
Mark

P.S. I bought some books this morning, it may cut down on my newbie questions.

P.P.S. We're working short hours this week--had three days off. :-( Could have done landscaping at the boss's house, but after one day of that, I preferred to stay home and finish Art. 250 of the code.


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## frenchelectrica (Sep 26, 2003)

Typically in my area the 3 Ø system for the resdentail is pretty rare unless you got one of the monster sized house like 8,000 sq feet or larger and the service size is 600 amp or larger we will go with 208Y/120 3 ph system.

but there is some older resdentail area still have delta system but that getting more rarer now days most POCO will just go with Wye system and be done with it.

the other reason why the Delta system is the wild leg they are useally 208 volt line to netrual and you get any 120 volt device hook up to this leg you can kiss the stuff goodbye very fast.

i dont rember which forum but it have pretty good debate here i think it was here i think it was few months back but not for sure.

and with most older resdentail area if you see 4 wire overhead that useally a surefire sign that is delta connected system the only time they used the 3Ø is for the airconditor system that about it. [ it was common in 60's to early 70's era ] and if you do see 4 wire there then check the meter face it will have a marking a triangle marking then it is a delta system.

and by the current NEC code the wild leg on delta system are marked on orange colour but one warning with some older system they will marked in RED as well so just watch out with it.

Merci, Marc


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## thom (Nov 3, 2006)

Around here three phase is not available in residential areas. Only a single phase is run through the neighborhood.

The very largest residential service they will install is 300A. They don't have the triplex for a larger system and will not split a quad-plex.

11 years ago I did a system where we installed [email protected] services on a single residence with approval and plans for a third. We had to provide engineering and get building official and power company approvals. It was a pita, though it paid well. Because this is generally a NEC Violation, we were required to install large red embossed labeling and explanations on everything and the mains and meters had to be exterior and adjacent so everything would be obvious for emergency personnel to deal with.


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## Magnettica (Dec 19, 2006)

Mark Twenhafel said:


> So the POCO justs distributes out a single phase of a higher voltage along the streets of a new subdivision?
> 
> Our little green transformers seem to service either 2 or 3 houses--makes me wonder how many transformers the POCO can connect to a single distribution line--anyone know? It's probably variable, I guess. Does anyone know what voltage is typically distributed to the primary sides of residential neighborhood transformers?
> 
> ...



Most single phase primary distribution through residential neighborhoods is normally between 7200 and 4160 volts. Transformers can be 25, 50, 75 kva depending on the load, obviously. 

Keep reading/ buying them books. It'll eventually all make sense but you have to put the time in to get anything out of it. Going to school certainly helps too.

Also keep in mind that nobody knows everything.


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## Magnettica (Dec 19, 2006)

Mark Twenhafel said:


> A general new guy-type question. (BTW, I miss the *real* new guy: newguy07.)
> 
> Does a typical POCO transformer intended for residential use have a secondary winding w/ a center neutral tap on all three phases?
> 
> ...



The reason the B and C phases are untapped is probably because they're going upstream to service something else. The A phase is tapped to feed the transformer and that same A phase maybe used for a 3-phase set up somewhere else. The 3-phase distribution system ultimately comes from a substation that also has it's own kva rating. How it's all distributed is up to the POCO (power company).


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## gragor (Apr 14, 2006)

Magnettica said:


> The reason the B and C phases are untapped is probably because they're going upstream to service something else. The A phase is tapped to feed the transformer and that same A phase maybe used for a 3-phase set up somewhere else. The 3-phase distribution system ultimately comes from a substation that also has it's own kva rating. How it's all distributed is up to the POCO (power company).




Like he said. They alternately tap the transformers to the different phases, one at a time, as they go down the street to balance the load between the phases. 

Voltages around here (BC) are 120/240v from a 14,400v 1 phase conductor.
120/208v and 347/600v from 14,400v per conductor to ground /25,000v phase to phase. Wire size looks to be about 350 kcm.... wow thats a lot of power.

gragor


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## Speedy Petey (Sep 30, 2003)

Most resi neighborhoods around here only have ONE primary run throughout. 

There are still some places with two primaries so then can provide 120/240v delta three phase using two transformers, but this is pretty rare. 

Neighborhoods near big roads and busier areas do have three primaries where they mix resi and light commercial.


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## macmikeman (Sep 12, 2005)

Gragor is right. The poco usually will alternate the phase taps along stretches of residential housing to balance the loading on the primary wye. Some places it is not as they go down the street, but rather a set number of secondary transformers which might be 2 streets or 5 streets or whatever they decide.


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## Mark Twenhafel (Dec 23, 2006)

Thanks, y'all. Good information. Thanks.


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