Women attack pole transformer and kill it

[tangerine3]

I dont believe this is a Memorial Day tradition, but a small rural town had a big dinner cooking for the whole town. For a reasonable price, you could get a complete meal with all the fixings an dessert too.

One problem. Many women in town converged on the town's community center building, all equipped with an electric roaster filled with food.

They plugged in roaster after roaster, and had to move several of them after tripping some breakers. They finally got them all working. There were around 25 roasters total, but not all were plugged in at once. However they did manage to plug in about 15 of them. Then they plugged in a half dozen huge industrial sized coffee pots, cranked up the walk in freezer for the ice cream, turned on all the lights in the large dining room, and since it was a hot day, they turned on the air conditioner.

For awhile, everything worked fine. Right when the people were coming in to eat, the place went dark. No lights, all the roasters went off, the air conditioning went off, the water stopped flowing from the sinks (well pump), and everyone was eating in the dark. Minutes later the fire department arrived with a large generator. The generator failed after 3 minutes of use and although the gas engine still ran, it would not put out even one volt of power.

About that same time, the electrician arrived, several neighbors whose homes are nearby came over and said they had no power in their homes. OOPS!!!! The electrician confirmed that there was no power coming from the pole to that building, nor to all the homes for a couple block area.

It was a hot day, as well, so lots of people were running Air conditioners.

Yep, all those roasters and coffee pots and ACs and other stuff, roasted the pole transformer.

It took the power company about an hour to arrive, and another 20 minutes to locate the drivers of cars that were in the way of power company's trucks. They finally got to the top of the pole, when one of the workers began to remove wires, touched the transformer with his bare hands and burned his fingers. (by this time the power had been off for over one and a half hours, so it had cooled down some).

They removed the 25 KVA transformer and replaced it with a 37.5 KVA model.

When I got home, I did some figuring. Those roasters (most are old), consume about 14 amps each. Those huge coffee pots are not much less. So, lets just use round figures and say they averaged 12 amps per unit.

15 roasters, and 6 coffee makers is 21 devices. That works out to a draw of 252 amps at 120V AC. Add to that the freezer, the AC, the lights, and then add all the stuff that was running in homes along that block.

It's no surprise that the transformer failed.

Now comes a question. How many amps can a 25 kva (kila volt amps) produce without overloading? (at 120 volts AC. single phase). I did some googling and found some complex mathematic formulas. I could not find a simple chart. Math is not really my best subject....

Therefore how many amps can a 25 kva transformer safely handle. What about the replacement 37.5 kva. And how about a 15 kva (which is what supplies my house).

Thanks in advance!

[Dean Hoffman]

Snipped rest due to AIOE quotation limit. Try alt.engineering.electrical Someone there would know. Some of the someones have EE after their names.

[NotMe]

Assuming (a lot) with a balance load on two 120 VAC legs that's a bit over

100 Amps per leg. or 200 amps total.

Motors (AC/cooler) complicate things so I've ignored those in my calculations.

I expect (a SWAG -- super wild a** guess) that the original transformer was bigger (but clearly not big enough.) as the one for our church is considerably bigger.

[Bill Gill]

That reminds me of the story my sister used to tell. Back in the 60s for several years the power company had to come out and replace the pole transformer with a bigger one every year. That was when window air conditioners became popular and everybody started buying them. So the load kept going up every year and overloading the transformer.

Bill

[HeyBub]

Company I used to work for sold a transformer management software solution, mainly to rural electric co-ops.

Simply stated, it took the customer's billing information (kwhs) and applied it backwards to the installed transformers. The utility could then monitor when the attached load approached the capacity level of the transformer. Presumably, the utility could get ahead of the failure curve by proactively swapping out the transformer.

Our salesman reported the common objection: "Why manage the transformers? When one blows up, we replace it. Problem solved."

Oh well...

[Metspitzer]

That is a pretty accurate way to know if you have the right size. If one blows, use the next size up. If that one blows, go one size larger. :)

[HeyBub]

When we DID get a contract, the first job was to inventory the system, that is, go house to house, record the meter number, size, serial, and hook up of the service transformer, etc.

Several times, our crews reported finding a customer-owned transformer sitting in the backyard with dodgy wires looped over the primaries!

Now, to your solution: In these cases, the homeowners owned their (illegal) transformers, so it behooved them to have the right size.