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cdru
Go Colts
MVM
join:2003-05-14
Fort Wayne, IN

cdru to Jack_in_VA

MVM

to Jack_in_VA

Re: The renovation continues ... today new 400 amp service!

said by Jack_in_VA:

The transformer is 75 KW and feeds 6 homes. When built, all homes in the neighborhood had 75 or 100 amp service. Most now have 200 amp.

If all the other 5 crank up their usage to 200 amp that's 1000 amp which is 600 above what you say the POCO says they can supply.

Something about this setup doesn't make sense

Even 400 amps doesn't fit a 75 kVA transformer. 240 volts * 400 amps = 96 kVA. It would be 125% overloaded at 400 amps alone, not to mention the other 5 homes. And if Pandora's home is needing all 400, it's likely the other homes are also needing a considerable amount of power. That is, unless Pandora is running the grow lights operation supplying the other 5 homes.

I think that guaranteed 400 amps of continuous service is shared among all the users of the transformers, meaning Pandora's house is likely never to have 400 amps available just to him.

leibold
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join:2002-07-09
Sunnyvale, CA
Netgear CG3000DCR
ZyXEL P-663HN-51

leibold

MVM

said by cdru:

Even 400 amps doesn't fit a 75 kVA transformer. 240 volts * 400 amps = 96 kVA. It would be 125% overloaded at 400 amps alone, not to mention the other 5 homes.

Technically correct, but not relevant. The 75 kVA rating is the load limit in order to maintain the manufacturer specified service life of the transformer.
Utility companies are well aware that they can far exceed that rating at the expense of a shorter lifespan for the transformer. Given how many pole mounted transformers around here show clear signs of overheating (and how often some of them pop) it appears that our utility company prefers the more frequent replacement to proper sizing of the transformers.

tschmidt
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join:2000-11-12
Milford, NH
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tschmidt

MVM

said by leibold:

Utility companies are well aware that they can far exceed that rating at the expense of a shorter lifespan for the transformer.

Yup - I can attest to that from first hand experience.

When we built our house installed a 200A service plus 30A separately metered service for hot water. Except for kitchen stove appliances are electric. Even space heating is electric to backup wood stove.

We are 600 feet off the road so have our own pole pig. Power company installed a 10 KW transformer. Being the good engineer that I am felt obligated to point out that 200A x 240V is 48 KW. The supervisor smiled knowingly and said that was true but transformer has tremendous overload capacity. If we were able to overload it they would be happy to replace it.

Well that was 30+ years, a direct lightning strike, and two kids ago and the transformer is still going strong. It will probably still be there after I'm in the ground.

/tom

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

Jack_in_VA

Premium Member

said by tschmidt:

said by leibold:

Utility companies are well aware that they can far exceed that rating at the expense of a shorter lifespan for the transformer.

Yup - I can attest to that from first hand experience.

When we built our house installed a 200A service plus 30A separately metered service for hot water. Except for kitchen stove appliances are electric. Even space heating is electric to backup wood stove.

We are 600 feet off the road so have our own pole pig. Power company installed a 10 KW transformer. Being the good engineer that I am felt obligated to point out that 200A x 240V is 48 KW. The supervisor smiled knowingly and said that was true but transformer has tremendous overload capacity. If we were able to overload it they would be happy to replace it.

Well that was 30+ years, a direct lightning strike, and two kids ago and the transformer is still going strong. It will probably still be there after I'm in the ground.

/tom

That's interesting since my transformer that feeds 4 homes started out at 15 kva, changed to 25 kva, then 50 kva and now 75 kva all without any problems I'm aware of. I'm sure my 300 amp upgrade didn't trigger it since the original drop wire from when the house was 60 amp was installed.

mattmag

join:2000-04-09
NW Illinois

mattmag to tschmidt

to tschmidt
said by tschmidt:

Being the good engineer that I am felt obligated to point out that 200A x 240V is 48 KW. The supervisor smiled knowingly and said that was true but transformer has tremendous overload capacity.

I'm trying to recall exactly what the POCO guy said about the installation here. We're on a 15kW transformer by ourselves. Service for house is 200A. Seems he told me they figure it a bit differently than you did as 15kW / 120V = 125A, and that is *per leg*, so we would essentially have excess capacity with the current configuration. This of course assumes perfect load balancing.

Now, I may be wrong on that, its been awhile since that conversation came up. Just throwing it out for those who know more than me to cuss and discuss...

DKS
Damn Kidney Stones

join:2001-03-22
Owen Sound, ON

DKS to tschmidt

to tschmidt
said by tschmidt:

Well that was 30+ years, a direct lightning strike, and two kids ago and the transformer is still going strong. It will probably still be there after I'm in the ground.

And full of PCB's...

tschmidt
MVM
join:2000-11-12
Milford, NH
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tschmidt

MVM

said by DKS:

[And full of PCB's...

Unlikely, but not impossible.

The transformer was installed in 1980 when we first moved onto the property. This was around the time of the big PCB scare. I was assured the transformer did not contain PCB. Utilities were going through a big replacement phase at the time so I have no reason to doubt the accuracy of that statement.

In the US manufacture of PCBs was banned in 1980. According the the EPA PCB use in transformers ended in 1977.

»www.epa.gov/reg3wcmd/ts_pcbs.htm

But your post prompted me to double check so I submitted a request to our utility.

/tom

whizkid3
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join:2002-02-21
Queens, NY

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whizkid3 to mattmag

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to mattmag
said by mattmag:

We're on a 15kW transformer by ourselves. Service for house is 200A. Seems he told me they figure it a bit differently than you did as 15kW / 120V = 125A, and that is *per leg*...Now, I may be wrong on that

Yes, its wrong. 15kVA / 240V = 62.5A. Total. Or per leg. Makes no difference, there is no other way to do the math.

This could be exceeded when necessary, the transformer can be overloaded at a reduction of its life. 15kVA works fine for most homes - on average. That is why utilities often put up to 4 on one 25kVA transformer, which can basically only provide 100A total at rated load.

The OP's 400A service? Note the transformer its fed off of. 50kVA at best, and its shared. It puts out 200A - shared - at full rated load. But a 400A service certainly makes for a nice thread, more than most of us could post if we were trying to impress people with our service.