using 3 phase a single phase transformer

[dluck]

Hello,
I hope these questions are not out of bounds here.
I manage a sm/dayall 5000 sqft warehouse that until recently use electricity only a couple times a month. There is a 30kva 3 phase 480/277v transformer in the warehouse supplying power.
First question. With all circuits disabled I am still seeing 250 watts used. Is this normal for this size transformer when idling?
2nd. We only use a couple 120v circuits. There is no 208 or 3 phase equipment we are using. Could I disconnect one or two of the phases to the transformer leaving only one active to provide the stepped down 120? And hopefully reducing the idle watts to 2/3 or even 1/3 of the 250 watts being wasted now?
3rd, would adding a second single phase 480-120 3kva transformer in parallel to the existing 30kva transformer to power typical requirements and install a manual 3 phase disconnect switch be legit.
The issue is we are trying to save on electrical costs and just keeping that transformer hooked up all the time seems a waste. thanks for any help
dale

 

[roger]

Let's keep this to a discussion of the power / transformer issues verses "how to" information.

Thanks

Roger

 

[augie47]

My first thought would be that if this space is separately metered, the "Minimum charge" by POCO would probably be in effect no matter what you did.
My second thought, if you have no 24 loads, why not just turn off the power to the transformer when not in use ?

 

[dluck]

My first thought would be that if this space is separately metered, the "Minimum charge" by POCO would probably be in effect no matter what you did.
My second thought, if you have no 24 loads, why not just turn off the power to the transformer when not in use ?

Thanks for the quick reply.
Answer to second question first:
Yes, you are right, and we have been turning the power off at the main breaker in the PG&E shed for several years. There is no breaker in the warehouse as far as I could tell. However because of rain/humidity issues we find we need to run a dehumidifier nearly 24/7 now. The dehumidifer is a hi-edry 195 and takes 120v 12amp.
Minimum charge from pg&e is about $14 (figured that out because we had a month with 0 kwh billed with main breaker turned off). However the bill went to $30/month with the breaker on and everything else turned off for the month, summer rates.
With the transformer taking 250 watts and my 24/7 equipment only taking 1000 watts. Thats more than 20% waste for 90% of the time. There are periods where we do use an additional 2-3 kw for a few hours at a time on weekends. This is mainly the overhead flourescent lighting. and testing out arcade games.
again I appreciate any advise.
dale

 

[augie47]

I don't have the transformer knowledge to answer your first two questions, but as to #3, in the simple form you stated it wold not work because the 3kva would backfeed the 30 kva (unless you also switched the secondary) and the 30 kva exciting load would still be present.
In addition you would have to assure proper protection for the 3 kva.
You might pursue the 3 kva install with a 2nd secondary panel for the selected loads, but at what would probably be a $10 month or so savings it might take a while to see "payback". (of course, if you are doing it for "green" cost would not matter)

 

[dluck]

Thanks to SmartMeter I can show you a graph of the energy usage. I've installed a 7 day 15 amp timer to control the dehumidifier so that it cuts out during 6 hours a day when we go to a Time of Use rate later. You can see it working pretty clearly. You can also see the 250 watts being wasted as everything else is turned off. The spike you see prior to the 6 hour period is me in the warehouse with some lights on that morning PG&E has some problems with their website for commercial accounts as the chart is mislabeled. It is actually showing 15 minute usage, not hourly usage so all the kwh need to be multiplied by 4 to get kw.

 

[dluck]

I was going to run the 3kva to just a 20 amp powerstrip that would be dedicated to dehumidifier duty, and some small lights.

 

[templdl]

Just to energize a 30kva transformer is 250 watts which is simply done by applying power to it with no load.

 

[mivey]

I don't see where you are going to save enough to pay for installing the second transformer.

 

[dluck]

Unless my calculations are wrong, I estimate the 30 kva is using 250 watts, a 3 kva would use about 25 watts. This is a savings of 225 watts almost 24/7 365 days a year. I estimate savings at about $90c/day or about $330/year.

A used/surplus 3kva transformer is about $100 maybe less. I've just about everything else, breaker panels, heavy duty switches. There are many surplus electrical places to get stuff.
Its not that heavy so getting it to my location myself won't be a problem. Being a computer engineer I of course think I can wire up anything myself by doing enough research and taking the time to do it right, but I've also a couple electrician friends that I could tap. Now I'm not asking for directions on how to do it. I'm just asking if any of the ideas for reducing the energy usage make sense. If my savings calculations don't make sense, or my estimate of the time to accomplish this (not more than an afternoon) then I'd appreciate some additional education.

What about my other question about just disconnecting the unused legs going into the 30kva transformer since I'm only using 120v and never using more than 5kv for short times, and typically only use 1.5kv? that would be the cheapest option and could save as much as 180 watts of the 250 watts presently being burned up.
Is that possible? is it dangerous? would it cause some sort of instability? Thats actually the original subject of this thread: Can I use a 3 phase transformer as a single phase transformer by only connecting one (or maybe 2) of the 480v/277v lines.
thanks again.

 

[mivey]

Being a computer engineer I of course think I can wire up anything myself by doing enough research and taking the time to do it right, but I've also a couple electrician friends that I could tap.

That may be so, but the forum rules here prevent us from helping you do that.

Now I'm not asking for directions on how to do it. I'm just asking if any of the ideas for reducing the energy usage make sense.

Well, anytime the savings outweigh the costs, it makes economic sense. Don't know why you need input to reach that conclusion. If a 60 watt light bulb can serve the purpose in place of a 100 watt light bulb, then you can wait until the 100 watt bulb blows and replace it with a 60 watt, or calculate the energy saved to see if the savings will pay for the 60-watt bulb within its life.

If my savings calculations don't make sense, or my estimate of the time to accomplish this (not more than an afternoon) then I'd appreciate some additional education.

I think providing that info would be stretching the forum rules.

What about my other question about just disconnecting the unused legs going into the 30kva transformer since I'm only using 120v and never using more than 5kv for short times, and typically only use 1.5kv? that would be the cheapest option and could save as much as 180 watts of the 250 watts presently being burned up.
Is that possible? is it dangerous? would it cause some sort of instability? Thats actually the original subject of this thread: Can I use a 3 phase transformer as a single phase transformer by only connecting one (or maybe 2) of the 480v/277v lines.
thanks again.

I think the original question was more along the lines of you doing the disconnecting. However, there will still be flux through the "unused" coils. Beyond that, I'll offer no more input, including time to complete, as you obviously intend to do this yourself, whichever route you choose.

 

[augie47]

I will have to agree with mivey. I had hoped, as Roger cautioned, that you would get the "technical" answers you desired before the thread turned more to "how to". From your latest post, it appears any advice you gain here will lead to a DIY situation.
If anyone wishes to advise you on the transformer specifics, they may PM you.
It seems best to close this thread in accordance with to Forum rules.