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Old 07-30-2008, 02:58 PM   #1
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Pole Transfomer size question


I'm upgrading my electrical service from 60 to 200 amp. My home and outbuildings (its a farm) are the only load on the pole transformer. My PoCo told me that the current pole transformer is 25kVA, and that it would be adequate for my 200 amp service.

Isnt that too small a transformer ? I thought that I would need at least a 50kVA ( 200amps x 240 volts = 48kVA)

What am i missing ?
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Old 07-30-2008, 05:17 PM   #2
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Pole Transfomer size question


It is normal for the POCO to undersize their transformers [when they serve noncommercial], for the reason that you will probably never pull close to 200 amps ever, probably not even 100. They can tell how much power you're using by analyzing the KWH's used for each month with a computer that tells them about how much load is on the transformer at a given time. Those transformers can also take considerable overload, up to 50% when it is cold outside.

In my neighborhood, 6 houses share a 25 single Kva transformer. (2 with 100 A, 4 with 200 A service)
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Old 07-30-2008, 06:10 PM   #3
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Pole Transfomer size question


Consider that POCO's follow their own rules, and will typically do what they want. Just because you are upgrading your service does NOT mean they will change the transformer, or even the wire from the pole.
They are an independent lot.
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Old 07-30-2008, 07:02 PM   #4
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Pole Transfomer size question


they generally wait until the transformer burns up or there is a problem before they replace it.
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Old 07-30-2008, 07:50 PM   #5
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Originally Posted by junkcollector View Post
In my neighborhood, 6 houses share a 25 single Kva transformer. (2 with 100 A, 4 with 200 A service)
So the maximum average continuous current draw for each house must be less than 25/6 = ~4 kva. Last May my house pulled 0.5 kva, average, so 4 kva sounds generous.

240 kva on a 25 kva xformer. . .it sounds like Demand Factor in the NEC.

Poco seems to be gambling that even if the demand ever goes to 240 kva, that all six houses will be drawing peak load at the exactly the same time, it will be for so few seconds that the xformer will stand it.

But, the xformer might actually see this peak load right after a power failure with all those refrigerators starting up at exactly the same time.


Last edited by Yoyizit; 07-30-2008 at 08:04 PM.
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Old 07-30-2008, 08:43 PM   #6
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Pole Transfomer size question


There's a sort of unwritten rule among the older POCO engineers; 'if you don't burn up a transformer once in a while, you're oversizing them'.

An oil-filled transformer, pole-mount or pad-mount, can take about a 200% load for about 2 hours, or about a 400% load for about 1/2 hour. If it's cold out, it can take even more for an even longer time. POCOs know this, and they also know that the typical peak load on a residential transformer occurs at around 6PM, when everyone is cooking dinner. It usually lasts for less than an hour.

They don't have codes of any kind to follow, they size equipment solely on years of experience.

The full-load current of a 25KVA transformer operating at 240 volts is 104 amps. The chances of any residential 200 amp service drawing more than 104 amps for more than a few minutes is very small.

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Old 07-30-2008, 08:44 PM   #7
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Pole Transfomer size question


I see red lights lit on transformers all the time. I've even called the POCO a couple of times to let them know. They don't seem to care.
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Old 07-30-2008, 08:56 PM   #8
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Pole Transfomer size question


Wish I had a 25 kva on my house, only a 15 and I have a 200 amp service. My only complaint is a very noticeable flicker when my ac (5 ton) starts up.
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Old 07-30-2008, 09:14 PM   #9
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An oil-filled transformer, pole-mount or pad-mount, can take about a 200% load for about 2 hours, or about a 400% load for about 1/2 hour. If it's cold out, it can take even more for an even longer time.
That's one long thermal time constant, but it is also one big transformer.
And, just like a fuse, it has an (I^2) x T curve.

Last edited by Yoyizit; 07-30-2008 at 10:33 PM.
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Old 07-30-2008, 11:15 PM   #10
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Pole Transfomer size question


The reason, obviously, is it has a lot of oil to heat up. The oil also pretty much eliminates hot spots in the windings. A dry-type transformer would never take that kind of abuse.

Rob
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Old 07-31-2008, 09:09 AM   #11
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Pole Transfomer size question


Today I have learned a lot about PoCo and their stuff.
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