

Hello Everyone!
I need help verifying wire size to a transformer. Below is the application and the variables.
My thirty by fifty workshop is currently single phase. I currently use a couple of phase converters for my machine tools. I have twohundred amps of 480 threephase onehundred sixty feet away. There is plenty to spare on the threephase service. I plan to run the threephase to my workshop into a transformer that will step thethree phase down to around onehundred amps of single phase and maybe fifteen amps of 220 threephase. I have not bought a transformer yet. I have a transformer at the three phase service that is 15 KW. I was told it was equal to about one hundred plus amps of single phase. All it does it run a few small items. The transformer is protected with a thirty amp breaker.
I found a wire size calculator and entered the variables for thirty amps of three phase 480 and came up with a wire size of #14 copper wire. That seems way on the light size to me.
I am adding the transformer to eliminate a meter and have true three phase power in my shop. My one hundred amp service is adequate for what I do. Other that a five horse power air compressor, I never run two tools at once. The shop is air conditioned with electricity and heated with gas.
I am open to any and all suggestion as to transformer type, transformer size, and wire size. In the past, I have worked with three phase or single phase but not both together with the exception of a small transformer in a three phase box.
Thank you for you help.
Grits

"I found a wire size calculator and entered the variables for thirty amps of three phase 480 and came up with a wire size of #14 copper wire."
Wire sizes are as follows:
15 amps  14 AWG
20 amps  12 AWG
30 amps  10 AWG
40 amps  8 AWG
50 amps  6 AWG
70 amps  4 AWG
100 amps  2 AWG
30 amps is 30 amps is 30 amps.
30 amps at 240 volts is 10 AWG and can deliver 7,200 watts.
30 amps at 480 volts is also 10 AWG but can deliver 14,400 watts.
These are singlephase watts. Multiply watts by 1.732 to get threephase watts.

This propisition will not be as expensive as I thought? I reentered the vaules and came up with #10 AGW also. I must have entered something wrong.
Do you have any suggestions on the size trnasformer I need? Is 30 KW enough for what I am trying to do? What are the better brands?
Thanks again
Grits

15,000 watts is 31.25 amps at 480 volts, and 62.5 amps at 240 volts.
In order to get 100 singlephase amps at 240 volts and 15 threephase amps, you're going to need a main transformer rated about 24,000 watts, and two auxiliary transformers rated about 3,600 watts.
I would probably install a 25 KVA main transformer and one or two 5 KVA auxiliary transformers, depending upon whether I wanted an open ∆ or a closed ∆.
Figure on #2 AWG conductors, on account of the required capacity of the singlephase transformer.
#6, if you're sending 480 volts to your shop and transforming it there.
As usual, it is best to transmit at the higher voltage, but 480 volts is in the "600 volt class", and some jurisdictions have concerns about ownerbuilder installations which are in the higher voltage class.

You have not stated what your lower voltage loading is. Everything else is essentially meaningless. Determine that maximum amp loading on any phase, then size the transformer to supply it. The cable on the low voltage side of the transformer, feeding the panelboard that is going to distribute your lower voltage, is sized based on the transformer maximum amperage capacity, rounded up to the next size as necessary, and then the breaker necessary to protect it. It's a bit more complex than most people realize, that's why you pay an electrician!
So for example, if you determined that your maximum load on any phase of the transformer is going to be 30A at 240V, then you will need a 15kVA 3 phase transformer. They only come in certain sizes and a 9kVA is the next size down, but it only provides 21.6A at 240V. The 15kVA transformer is CAPABLE of 36A, but if you don't want to use it all, that's OK.
Per code, you need to size the conductors for 125% of your load so 30A x 1.25 = 37.5A. That means you must use 40A rated wire (#8, the next size up from 30A), and since you can only load a circuit breaker to 80% of it's rating, 40 * .8 = 32A. So now you are within your needs, the transformer's rating, the breaker's rating and the conductor rating.

Grits ,.. just stop for a sec let me ask you a question you say you have exsting 100 amp single phase power supply right now ?
If so where that come from your house or what ??
where is the 3 phase power comming from ??
if that is new supply ? or at other shop or other ??
because this will get little tricky here
some POCO [ powercompany ] will not provide both single and three phase supply at one building at all but they do for exsting area if specal permit approved
but there is the sisuation i dont know if you were aware or not if you have delta system [ there is two verison of this ] or wye system it will make the diffrence there
The delta system can come with or without netural but you have to becarefull with 4 wire verison [ with netural ] because there is a wild leg there if you get the idea
The wye system is very common now days it come 120/208 or 277/480 or 347/600 [ not too often you will see this ]
the lower voltage is refered from line to netural but higher voltage read line to line.
Now let get to this point real quick here if you bring delta system to your shed or shop and you want single phase from this it can be done with transformer [ single phase verison only not the 3 ph versison because unbalnced load can do funny things with transformer on delta system ]
also you have to understand the 480 volts system use more expenivse fuse / breaker compared to 208 or 240 volt system
really my suggest do this way here run 100 amp 480 volts if you have wye or delta [ let me know this first ] to the subfeed box at your shed/shop from there you can use 15 KW single phase transformer to down step to 120/240 volts single phase for your lighting and some single phase 240 volts devices and used the 480 volts to run 3 phase machines
let you know 15 KW single phase is plenty big for your shop 15000 watts = 62.5 amp @240 volts
the 3 phase at 100 amp = 83.1 KW so you get the idea how much so i really only suggest you can run 50 amp from 3 phase panel to your shop/shed that roughly about 40 KW or so @480 volts so you get the idea how much power you have on hand.
to run 50 amp wires from exsting 3 ph box to new location at your shop/shed at 160 feet away you can used # 8 copper wire you have 2.2% voltage drop so you will be fine with it with this setup
if you have more question please do ask me i will try to help ya with this
Merci , Marc

Let me explain my current setup. This in in rural Arkansas. I want to do the job properly but there is no code to contend with.
I am a turf farmer. The 480 three phase runs to a turf washing facility. When we bought the service, we had several options to choose from. The option that best suited us was a 200 amp service due to the the way it was billed, not power requirements. Two hundred amps is way more than needed.
The three phase is run to a meter loop and then to a breaker panel. There are several three phase motors and blowers that are protected by breakers from the three phase panel. The transformer is connected to a thirty amp breaker in the three phase panel. The transformer will provide both low voltage three phase and 110 single phase power. Everything three phase in the washing facility is 480. There is a single phase panel connected to the transformer that is used to run some lights and a few small items. There is probably no more than a thirty amp draw from the single phase panel. I will not use that transformer in my shop described below.
I have a fifteen hundred foot shop, onehundred sixty feet from the three phase panel. That shop contains all my tools and machine, both woodworking and metal working. It is used by me for ninety percent fun and about ten percent work. It is not a residence. There will be no problem with the power company.
I intend to run 480 three phase from the panel at the washing facility to a transformer in my shop. The transformer will power a 240 volt three phase panel and a also run two legs of 110 to the existing single phase panel. After the conversion, the single phase box will be relieved of about fifteen horse power. I will never use over ten horse power from the three phase side at one time. That is assuming I convert my five horse power air compressor to three phase. I will only convert the air compressor if I find a deal on a good three phase motor. It runs about one hundred hours per year. I have an hour meter on it.
Three phase power is much cheaper, in my area, than single phase. Also it will simplify maters by eliminating two phase converters. I have a home for my ten horse phase converter in my farm shop. I will also eliminate the fixed charges that are tacked on the single phase meter. Total payback will be about two and one half years, not to mention a much better setup. My biggest power hog is my central AC. It is hot in Arkansas. In our climate, you can not turn your ac off and expect it to cool down a hot building quickly. I keep it about eighty and turn it down when I am using my shop. The ac also removes a lot of moisture from the air which we have plenty of.
I hope this clarifies the matter.
I will hire an electrician if I get over my head; however, I will run all the wire and purchase (scronge) all the equipment.


I would go with that transformer.
Just remember you will be getting three 120 volt singlephase buses and one 208 volt threephase bus, in the one panel.
This is probably the most flexible system, as you can make maximum use of all buses, whether all singlephase, or all threephase, or anywhere in between.
Your feeder would be at least #3 AWG, and would be protected by a 70 amp threephase breaker at the 480 panel.
As you are 160' from your 480 panel, you should consider upsizing your feeder to lower the voltage drop.

Peter
Would it make more since to use two transformers? One for two legs of 120 and one for my low voltage three phase? This is getting a little over my head.
Thank you,
Grits

The simplest approach is to send 480 to your remote building and convert it to 120Y208 there.
Using several sizes of transformers complicates the protection system, and reduces flexibility.

Peter
Almost all my three phase motors list voltage as 208/220 etc. Will 208 run 220 motors without any problems. Is it wired as low voltage vs. 480. I do not understand the reason and interchangeability of the different voltage on the low side.
Thanks,
Grits

Motors rated 208240 run equally well on 208 as on 240. Current is inversely proportional to voltage, as one might expect.
Motors rated 240, only, are not expected to run on other than 240 +/ 5 percent, which is 228252.
Here, 240, nominal, was 255 for the longest time, perhaps 20 years or so.
Then the socalled energy crisis hit, thanks to Enron, and the regional utility decided to lower the system voltage to lower the energy consumption.
220 was tried, briefly, but there were too many claims for motor burnouts.
The voltage was raised to 230 and the claims stopped.
Not a surprise, as 220 is 8.3 percent low, whereas 230 is only 4.1 percent low.
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