Long service run/transformer

dirtpoor

Hello,

I'm here from electricaltalk.com and thought this might be an appropriate forum for my question/situation. Thank you in advance for your help.

I have a 370' (one way) run from a 25KVA transformer to the meter can mounted at the house (new construction). I need 400amp service at the house (actual load is less than 400 amps, +/- 320). The 25KVA transformer at the pole currently services one other house (existing). I have spoken to the POCO and he said the transformer is sufficient for their service and our 400amp service. POCO has told me we must use a CT 320 can. Power at the house runs from the meter can to (2) 200amp panels in the house; one panel is 50' away from the meter, the other is 100' away. On the 100' run we used 4/0 aluminum. I am new to this and working with a "master" electrician. I have my doubts on the wire size and the POCO's assessment of the 25KVA transformer. As I said, I am new, so please have some mercy. Thank you in advance for your help. To be concise, my questions are as follows:

1)What wire size is required from the transformer to the can, and will it work with a CT 320 can?

2)Is the 25KVA transformer sufficient as described above?

3)Is 4/0 aluminum acceptable on a 100' run from the meter to our panel inside?

Thank you

bobelectric

That far,the pogo should have run 7200 to a separate xfmr.

dirtpoor

Thank you for such a quick response. If getting another transformer is not an option, is the 25KVA adequate? If it is adequate, what wire size is required? Thank you.

Stubbie

25 KVA single phase transformer .. is nowhere near what you need for a 400 amp service. Are you sure you heard correctly?

Use this formula: A = (KVA x 1000) / LLV. For example, if you have a 25 KVA, 120/240 volt, single-phrase transformer with an LLV of 240 volts, you can calculate the amp capacity: A = (25 KVAx1000) / 240 = 25000 / 240 = 104 amps.

dirtpoor

Yes, the POCO rep stated it several times. Before I could question him he said,"You're master is going to say it isn't, but 25KVA is sufficient." Hence my doubts, and that's what's led me to question the wire sizes from the transformer as he said 500mcm should work. Any thoughts on the wire sizes in the original post. I genuinely appreciate your responses. By the way, it's single phase 120/240.

frenchelectrican

Very simple with most POCO they will know and sized the transfomer to closest to the average load as possible so I know few larger home with 320/400 amp service did use 25 KVA transfomer and it is pretty common unless you show them the load demand caluation.

The only time they will bump up if the load demand caluations mention tankless electrique water heaters that useally kill them then they will bump up to either 37.5 or 50 KVA can

370 feet is pretty much over the limit for most POCO will use that for lateral run ( underground run ) most case AFAIK useally 200 feet average unless upsizing the conductor.

With larger conductor check with your POCO for approved meter socket for that size due there are few differnt requirement for it.

And most POCO rather load up the smaller transfomer than use the larger transfomer and run light loaded which they will try to advoid it much as possible.

Merci,
Marc

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dirtpoor

Thanks for your response.

As I'm sure you know, we're responsible for running the wire from the transformer to the meter at the house; the POCO said he didn't care what wire size we ran. Other than a problem with the wire size matching the meter requirements, do you see a problem with us upsizing the conductor to get from the transformer to the house?

AllanJ

What size wire you run and the size of the pole transformer are two separate topics.

If it turns out that the pole transformer gets overloaded then the power company will upgrade it at a later date.

(copied from another forum)

My uncle has a 200 amp service and heat pumps. Some years ago he complained that there were voltage dips of a few seconds, not often but to less than 105 volts. Finally the power company upgraded the pole transformer and the problem stopped. My best explanation was that the power dip happened when air conditioning or heat pump units of two homes kicked on at the same time.

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mpoulton

They really need to run primary and set another transformer closer to your house. That's far too long of a run for 400A secondary. With an undersized transformer and that long of a service feeder, you will have lights dimming and other voltage drop problems at peak load. They just don't want to do the work, so they're trying to make you buy a huge amount of giant wire instead.

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I am a lawyer, but not your lawyer. And who cares anyways? We're here to talk construction. This is DIY advice, not legal advice.