Easy one for the Swimp.

[wtdoor67]

Okay Gregg, you're the serviceman. You're working in a Muni. with a 7200/12470 primary system.

The boss calls on the radio. It's an informal place. Gregg go run a temp service over at that new const. site. It's just a saw pole etc. Just get the power off that abandoned 120/208 bank. They gotta have 120/240 though.

No problem, you say. In about 15 minutes you and your helper arrive. Looks like about 50 ft of triplex to the meter base pole and pull her up and make it up. Set the meter and go.

Your helper grins. Looks like you gonna have to climb up, take the lid off that one pot and put the coils back in series on this one bud.

You laugh. Kid, let the old Swimpus show you a trick I learned in Saudi.

What did your steel trap mind come up with?

[wtdoor67]

Just get the power off that abandoned 120/208 bank. They gotta have 120/240 though.

[topgroove]

swamp will never get this one without help. there's only one way to get 120/240 without opening the lid and center-tapping one of the transformers. its kinda funky and you better know the electrician at the construction site real well so he wires the panel correctly. you won't find this one in a book of standards.

[wtdoor67]

You didn't tell me anything before. This is not against any code and it doesn't require any electricians approval or any thing. At least not in the 4 or 5 states I've worked in.

Abandoned BANK! A bank with the pots left hanging on the pole, the service gone, the cutouts open. I have seen plenty like this. The customer went belly up, moved to a different location or whatever. The crew couldn't get to the removal or maybe they were shorthanded.

Don't BS me that you would have climbed up and wrote split or whatever. Most co's put a 208 sticker on them, tape the blank lugs or something.

For some reason people try to complicate things. This is simple. You recognize the bank, hell in a small area you probably ran the original service. It's not rocket science.

Just think about it. You're only gonna need 2 pots. These pots each are capable of 120 volts in their present state. Do you need a picture?

Also someone said you must have the system neutral for 277/480. That is true for a Wye/Wye but there are plenty of Delta/Wye 277/480's.

[wtdoor67]

I never called the Orlando or wherever electric dept. and talked about floating the "Wye/Wye" as you asserted. It wasn't me but I remember who it was.

Alzimer's is a ***** isn't it?

[Fiberglass Cowboy]

I'm waiting to read your answer as well. Educate me. Always interested in learning more about transformers and connections. Was told once there was at least 27 different applications a single transformer could be hooked up in/for; including single phase, 2 phase and 3 phase applications.

As for the floating wye? Well we did come across some interesting "floating" wye secondary connections in Houston, on Centerpoint Energy property during the hurricanes a few years back. Feeding sewer lift stations and other applications of that nature. Wye/ Floating Wye is what I suppose it would be considered. 277/480v wye transformers, 2 bushing. Primary - 19.9/34.5 kv ground-wye sytem. Highside of transformer bank hooked up in a very standard ground-wye connection. H1 's all connected separated and used as primary lead, H2's all bonded together (grounded), tied to pole ground and tied to main line system neutral through the pole ground. Secondary side typical 277/480 wye transformer, x1 and x2 secondary bushings. X1's all used as hotlegs, connected separately to the open wire secondary. X2's all tied together, but not grounded. They were floated. The pole ground was bonded to the secondary neutral (main line neutral was in the static position) just as it was bonded to main line neut as I said earlier. I imagine the secondary neut was used more for a "case" ground/"equipment" ground reference for the motor and controls; versus a neutral. 3 phase motors are delta anyways, and don't require a neutral, as most know. So the voltage at the motor ends up being 480 volts phase to phase, with no true (good) single phase to ground voltage reading from the bank, same as a primary delta system. We found this interesting; to be able to use wye transformers for a straight 480v power bank that would otherwise require a delta connection. But it was neat to be able to see these other things different electric companies do, and quite the learning experience.

P.S. Is this what he meant by "floating wye" ???

Thanks DB
- Eric Elder :cool:

[MI-Lineman]

Well I'll give her a shot seein how Swamp's ran out of "Life Lines!":D

I think it's simple like Door says......take 2 pots and look at'em as 1! Sister the high side on a phase! I think but I'm still drivin my self crazy trying to figure if ya take the 2 sec legs on one of the cans and switch'em??? That way when ya look back at it you'll have from left to right a 120 leg, your neutral, and then your other leg! I believe this way you'll get your 240 instead of 208 again when ya go phase to phase on the sec side???

One big single phase can? I think?:o

AW SH!T! I WAS WRONG! KEEP THE LEGS AS THEY WERE!! OH WELL I WAS CLOSE!

[wtdoor67]

How's the Kansas flash? We had some oil field applications that "floated" the secondary Wye. I remember once in Wyoming the supt. in Casper talking to some hands that were hooking some of those up. I remember him saying over the radio. Yes, that's right you "float" the wye on the secondary.

I told this before but it was funny. A crew was winching a pole across the tracks. We heard it on the radio. Old Ed Spitz. He happened to drive up when the pole was partially across the tracks. First thing he said. "There's a train coming." Second thing said with some concern in his voice. "There's a train coming." Last thing he said and almost hysterical. "There's a train coming. There's a train coming." They made it before the train arrived but old Ed got a little nervous.

[topgroove]

Well I'll give her a shot seein how Swamp's ran out of "Life Lines!":D

I think it's simple like Door says......take 2 pots and look at'em as 1! Sister the high side on a phase! I think but I'm still drivin my self crazy trying to figure if ya take the 2 sec legs on one of the cans and switch'em??? That way when ya look back at it you'll have from left to right a 120 leg, your neutral, and then your other leg! I believe this way you'll get your 240 instead of 208 again when ya go phase to phase on the sec side???

One big single phase can? I think?:o

AW SH!T! I WAS WRONG! KEEP THE LEGS AS THEY WERE!! OH WELL I WAS CLOSE!exactly... sister the high side of two tubs and series the secondary side of them that'll give you 240 volts accross the entire coil, only thing is ya got to ground one side of the 240 coil . now you have a 240 volt leg phase to ground. and the untouched tub gives you 120 volts phase to ground. the thing that worries me is when the electrician wires the two hotlegs in to the main breaker instead of wiring the 240 leg into a seperate double pole breaker( one pole to neutral the other to hotleg):eek:OOPS... my bad. no need to put the coils in series, just leave em the way they are and don't even use the third untoched tub.

[wtdoor67]

Have taken 2 house transformers 120/240, hooked them to the same primary phase , bussed the X1 from one pot to the X3 of the other, made this the neutral connection. Used the remaining X3 and the remaining X1 as the hot legs. Got 480 single phase. No reason it wouldn't work on 2 pots with the insides cut AC BD.

The adjacent ends of the windings would be bussed together and make your neutral. The far ends should be your hot legs. Have to, of course make sure the ground strap is curled back on one side as it'll become hot. Shouldn't be any 240 to ground winding. Just a normal 120/240 volt secondary.

Someone try it and report back.

[topgroove]

your right I was over thinking it. your right if you had oppisite polarity on each tub by connecting x1 on one tub to x3 on the other ( assuming x2 is empty of cource cause they're cutover) and made this your nuetral connection than you would read 240 accross the two hotlegs

[SwampRat JR.]

So tell me, what the **** is a delta/y 277 480 bank used for, what is the ****in point, what does does a motor want to see, I guess Danny we have not all worked in these Delta systems, but I would like for you to go into detail. But I know cheap utilities, why bring in neut, for that matter, why have a system neut at all , **** the return I guess, just hook everything up delta on the high side, just by all sorts of cans, seen alot of three wire primary, but have never seen a delta/ Wye, configuration, companies or utilities only offer certain voltages, not gonna have a infinate # of cans or voltages on hand, maybe a few excemptions, but your goofy ****in exception to the rules is goona **** alot of the next guys in the field.

You can do alot of off beat ****, but in that last example, 240v across, but how do ya get your 120? Please explain. I get the deltas, only work on a 480 bank, thanks, but that goes back to the phase to phase voltage, so again, how in the **** are you getting 277 to dirt? Where is your tie point.

I will say I have not cracked a book in about 13 years, probably shows, but I have never blew up, or wired a bank wrong, never delivered the wrong voltage to a customer, never used the wrong name plate, but I will say, if I roled up into a area, with alot of strange lookin ****, I would have to say, Danny must have been experimenting again, check and double check, he can build some good traps.

[wtdoor67]

If you will go and look at a 167 KVA or bigger it's plain to see. All the secondary lugs are external and visualization is easy. Try it.

[MI-Lineman]

"Close"...counts in horse shoes. and gernade throwin.
Don't worry Youngen...
It will happen again too.:D
Ya just move on, and Learn.:cool:

Dude you don't even have a clue what we're talkin about so don't try you sh!t with me! Next time you brag about havin "40 years experience" I'll be takin that with a grain of salt!:rolleyes:

If you're gonna stick your neck out you can't be afraid of gettin your head chopped off! I'd rather be wrong on here then out in the field any day!!!

HEY SWAMP! IF I EVER RUN INTO THIS SITUATION I'LL GIVE YOU A CALL SO YOU CAN COME CLOSE IT IN THOUGH!! :o

[wtdoor67]

Don't give me to much credit. I do much of this stuff just by rote and memorization like a lot of people.

It's sometimes hard to explain simple things that if you were face to face would take about 30 seconds.

On the hooking two AC BD pots together and getting 120/240, just forget about what the outside of a pot looks like. Just imagine 2 coils, separated in the middle, each having 120 turns. Just like in a regular pot. Then join them together in the middle and it becomes just a regular pot, except that each coil is in a separate container in this case.

On the 277/480 hookup I have never found anything bragging about the advantages of the Wye/Wye, except some vague references to it being safer. All the literature I've read seems to say that other hookups are more efficient. Of course as long as it's straight 3 phase being served things are pretty good as 3 phase is naturally balanced and essentially no imbalance return. However I know some areas use the 277 also. Then pot sizing becomes part of the equation, with sizing taking into account the expected imbalance. It would be a good question to pose to a good engineer. Perhaps someone can get CPope to help us out here.

I must admit some of the literature I have is quite old. Some of the Pac. Pwr. stuff was put out in 77. Maybe there have been some advances made these last few years.

The old engineering book I have was first published in 1954. It has about a paragraph warning not to float the high side of Capacitors and yet Barehander and Loner I believe say it's quite common where they work. Probably after 50 years some advancement has come along.

Anyway the more we discuss things the better informed it makes us and hopefully you guys still working will become even better.

Also if you're a person who works alone or even with a crew, boom up at some transformer bank you're building or passing by and do a little volt meter messing around. I used to do that some. Keep a diary of strange things that happen. I neve kept a diary but sometimes I wish I had. Memory is a poor thing to rely on.

[MI-Lineman]

Don't give me to much credit. I do much of this stuff just by rote and memorization like a lot of people.

It's sometimes hard to explain simple things that if you were face to face would take about 30 seconds.

On the hooking two AC BD pots together and getting 120/240, just forget about what the outside of a pot looks like. Just imagine 2 coils, separated in the middle, each having 120 turns. Just like in a regular pot. Then join them together in the middle and it becomes just a regular pot, except that each coil is in a separate container in this case.

On the 277/480 hookup I have never found anything bragging about the advantages of the Wye/Wye, except some vague references to it being safer. All the literature I've read seems to say that other hookups are more efficient. Of course as long as it's straight 3 phase being served things are pretty good as 3 phase is naturally balanced and essentially no imbalance return. However I know some areas use the 277 also. Then pot sizing becomes part of the equation, with sizing taking into account the expected imbalance. It would be a good question to pose to a good engineer. Perhaps someone can get CPope to help us out here.

I must admit some of the literature I have is quite old. Some of the Pac. Pwr. stuff was put out in 77. Maybe there have been some advances made these last few years.

The old engineering book I have was first published in 1954. It has about a paragraph warning not to float the high side of Capacitors and yet Barehander and Loner I believe say it's quite common where they work. Probably after 50 years some advancement has come along.

Anyway the more we discuss things the better informed it makes us and hopefully you guys still working will become even better.

Also if you're a person who works alone or even with a crew, boom up at some transformer bank you're building or passing by and do a little volt meter messing around. I used to do that some. Keep a diary of strange things that happen. I neve kept a diary but sometimes I wish I had. Memory is a poor thing to rely on.

Actually I just got back from a 3 phase bank refresher yesterday! Yeah I know kind of good timin AND I NEVER TURN DOWN ANY TRAININ OFFERED BY THE CO. ESPECIALLY WHEN THEY'RE PAYIN FOR IT!!!:o

ANYWAYS! We went out and created different trouble scenarios and took the reads! Course the **** so-called instructors couldn't explain it good enough!:( I've got'em all written down so I can reference them in the future!

[RDawgs]

Okay Gregg, you're the serviceman. You're working in a Muni. with a 7200/12470 primary system.

The boss calls on the radio. It's an informal place. Gregg go run a temp service over at that new const. site. It's just a saw pole etc. Just get the power off that abandoned 120/208 bank. They gotta have 120/240 though.

No problem, you say. In about 15 minutes you and your helper arrive. Looks like about 50 ft of triplex to the meter base pole and pull her up and make it up. Set the meter and go.

Your helper grins. Looks like you gonna have to climb up, take the lid off that one pot and put the coils back in series on this one bud.

You laugh. Kid, let the old Swimpus show you a trick I learned in Saudi.

What did your steel trap mind come up with?

I don't know where you come from don't care... If your called to do something do it right. You said the bank is no longer in use. Retire it. Hang a 120/240 tub. done. We don't leave chit hangin' . And if communication is as bad as it is everywhere don't leave traps for fellow lineman. SIMPLE do it retire the bank hang a knew one why set traps right!

[wtdoor67]

I don't know where you come from don't care... If your called to do something do it right. You said the bank is no longer in use. Retire it. Hang a 120/240 tub. done. We don't leave chit hangin' . And if communication is as bad as it is everywhere don't leave traps for fellow lineman. SIMPLE do it retire the bank hang a knew one why set traps right!

If you think 2 pots being fed by 1 cutout and having 120/240 secondary is a trap, then you better stay on the right of way crew where you probably work.

You wanta see some traps, try working where things are paralleled together and split banks are common. You gotta keep cut in,in some parts of the country not like that Muni or REC you probably work for.

I had a foreman once with an IOU. I mentioned using transformers on Wye systems in a phase to phase mode as is sometimes done. He didn't believe it. Get out of the boonies dip. I've worked with guys who did not believe you could build a 2 pot bank and use the H1 on one pot and the H2 on the other. Know what they said? I don't care if it will work, it ain't the way we do it.

I can only imagine what kind of hand you are.

[rcdallas]

I like diagrams... this should be it.

Though really, I don't see why if you WANTED to keep the ground strap attached on the left pot I mean it's already grounded through the tie bus, or is it just the golden rule you do it once?

[rcdallas]

I had a foreman once with an IOU. I mentioned using transformers on Wye systems in a phase to phase mode as is sometimes done. He didn't believe it. Get out of the boonies dip. I've worked with guys who did not believe you could build a 2 pot bank and use the H1 on one pot and the H2 on the other. Know what they said? I don't care if it will work, it ain't the way we do it.

I can only imagine what kind of hand you are.

I agree you need to know this ****, it was about 7 months ago we were out in the oil field to change out a failed pot in a open wye-delta bank and the fed in from H1 on one and H2 on another, if we didn't catch that we'd get some jacked up voltage on the secondary... could of been a case where we'd have to go another 50 miles back to town to get another transformer because we thought it was bad.

It might be a standard with the guys you work with now to do it one way NOT to build a trap, but that doesn't say **** when you go out on storms or different areas or even your own area from the guys that built it originally.

Hell maybe one day you'd get a call you need to do this and that's all thats available...??

[rcdallas]

You know...I had a foreman once bring this up when I first started with the company I'm at, about things you could do if you had a customer on the end of the line and the company you worked for if they didn't want to spend a lot of money to install regulators or boosters, whatever to get the voltage up for one customer at the end of the line...

If it was three phase or open-wye (b phase, v phase whatever you want to call it on your system)

Say you take the primary lead and try all different phases and the voltage is still to low, try hooking up delta, see what that does. :)

[Lineman North Florida]

If you think 2 pots being fed by 1 cutout and having 120/240 secondary is a trap, then you better stay on the right of way crew where you probably work.

You wanta see some traps, try working where things are paralleled together and split banks are common. You gotta keep cut in,in some parts of the country not like that Muni or REC you probably work for.

I had a foreman once with an IOU. I mentioned using transformers on Wye systems in a phase to phase mode as is sometimes done. He didn't believe it. Get out of the boonies dip. I've worked with guys who did not believe you could build a 2 pot bank and use the H1 on one pot and the H2 on the other. Know what they said? I don't care if it will work, it ain't the way we do it.

I can only imagine what kind of hand you are. Why would you want to use H1 on 1 pot and H2 on the other unless 1 was additive and the other subtractive? or were you just stating that it would work.

[wtdoor67]

it very common. I've seen it everywhere. Usually just to get more clearance with jumpers.

[Lineman North Florida]

it very common. I've seen it everywhere. Usually just to get more clearance with jumpers. I've only seen it where additive and subtractive were banked together in an open delta, but I've never worked outside of the southeastern US, thank's for the info.:D

[wtdoor67]

You know...I had a foreman once bring this up when I first started with the company I'm at, about things you could do if you had a customer on the end of the line and the company you worked for if they didn't want to spend a lot of money to install regulators or boosters, whatever to get the voltage up for one customer at the end of the line...

If it was three phase or open-wye (b phase, v phase whatever you want to call it on your system)

Say you take the primary lead and try all different phases and the voltage is still to low, try hooking up delta, see what that does. :)

I think that would be a little dicey. If later the voltage responded to a boost on some regulator it might get a little high. Never saw it done, but it might work. Worked one place where a lot of the pots had taps. Used to tap em up a little if it was low.

Also capacitors help with low voltage some. I spec other folks on here got a better take on this subject than I.

On a pot with 7200 turns on the primary I wouldn't want to try it on a nominal 12470 phase to phase. Probably blow. I can't remember but there's a basic insulation level (BIL) given for transformers. If I find it I'll put it on here. Your co. might have that info.

[topgroove]

I've seen 4160 hooked up delta on duel voltage csp's but there's no way it would work on 13.2kv. I have moved taps around to improve voltage problems mostly on 4800 delta. it seams most lineman prefer to use the two outside phases and the middle phase is always allittle higher. Cap banks do help it corrects the power factor and ya can squeeze out a couple more volts on the customer side, nothing beats a regulator bank for voltage problems .

[SwampRat JR.]

Couple things here, first, Dallas you are useing single bushing pots in you diagram, second, what Danny said, about clearance on your jumpers, correct again, much cleaner, but I see most places I have worked other than where I started, all seem to build subtractive 120/208 or 277/480 banks, which is cool. Most engineers like Y systems, but hate Y/delta banks, alot of unbalanced loads. I like to check loads at the subs on different feeders, then check my Kyles for info, always try and check them when I got a feeder locked out or any larger outages, so I kinda know what I am lookin for before patrolling, how high or low the fault current is, tells me where to start looking for my cause.

One of my instructers had an electrical back ground in the Navy, then a lineman, and also had a meterman ticket, real good for a instructer, my other one was a engineer, real smart, but could not relate to the field side.

Anyway, the JL/JM always wanted guys to refresh our knowledge as we get used to the Y systems, the same hook ups. But we never took the time to do it.

It is hard to retain, or at least it is for me, it has been a long time, but I do give ya credit Danny, even though you are retired, you worked alot of different systems, and you have seen a big change in how things happen, or certain companies change, or it depends on the engineer at the time at some of the smaller joints, this board has helped me see how ****ed up different parts of the country are, but most only offer certain sec. voltages, but that old **** is still out there. I do all I can to get rid of all that 3 wire Y ****, and delta is all but gone or is gettin phased out around here, but as long as the customer does not up grades his equipment, we got to give them what is there, I am still scrathin my head about that delta/wye ****, but thats why I love this trade, ya learn something new every day, always hated books, reading or even typing, but this site has changed all that.

But I still hate books, ****in hate them, I would take the time to read one if it was written by a JL and it was written from a JL perspective, how he related it to the field work, mistakes he has made or others he has worked with, a new JL's Bible, alot has ****in changed, you know how ****in dry those books are, just confuseing as **** to a guy like me, but if the field experience was there, along with the diagrams, where ya might find some of these oddball ****ers, it would help, but I don't like going away without the answers. I would like to have a refresher course on banks, but 18 years, ya get through your ape****, and into the field ya go, and if you are not exposed to the systems or hook ups, even if I go through it again, might forget it the next week.:confused:

[SwampRat JR.]

Danny, so you hook two 15kva pots into the same cut out, making one 30kva transformer? or what would it be? I know what you are saying about idle facilities, but what if that customer decided he wanted his bank back? What then or the single phase customer?

One more, why do places like the 120/208 voltages? I never got that. Is that customer drivin or what?

I guess I get the padmounts, load issue, need the KVA, why don't they need the 240v? **** Dude I am tired, my head is goin to explode, thanks.

[lewy]

Banking 2 single phase transformers to double your kva was taught to us in second year & it was nice to know, but it is something we never did in the field, if we needed more capacity we would just hang a bigger TX easier & cleaner. Up here our customers are limited on there voltage choices, 120/240 or 120/208 & 347/600. The new guys coming along up here might never work on a delta bank. I have worked on very few. As far as tap changers we only have them on our lower voltage transformers & when you want to raise the voltage you lower the tap, I am sure WT knows this , but maybe some of the young guys might not.

[wtdoor67]

Danny, so you hook two 15kva pots into the same cut out, making one 30kva transformer? or what would it be? I know what you are saying about idle facilities, but what if that customer decided he wanted his bank back? What then or the single phase customer?

One more, why do places like the 120/208 voltages? I never got that. Is that customer drivin or what?

I guess I get the padmounts, load issue, need the KVA, why don't they need the 240v? **** Dude I am tired, my head is goin to explode, thanks.

If you hook two 15 KVA pots to the same phase and parallel the secondaries, yes you would have a 30 KVA pot. The secondary voltage would remain the same. However if you place the secondary coils in series (end to end), then you have a different secondary voltage. I've seen a similar thing done with car batteries.

On the necessity of 120/208 I expect someone else may have an upto date take on it. If I find a pertinent statement about it I will post it.

On the padmount thing. A 480 single phase voltage was wanted. PSO did not stock pad mts. in single phase 480. Therefore 2 standard 120/240 volt pad mts. were set and their secondaries were placed in seriesl. Make a drawing of a conventional overhead pot. It's the easiest to understand. You will readily see that the X1 of the left pot, connected to the X3 of the second pot becomes your neutral buss. From the X3 of the left pot to the X1 of the right pot you have 480. You have 120 volts available and also 240 volts. As far as I know those are not used. You have 120 volts from X3 to X2 on the left pot ungrounded. From X2 to X1 120 volts with a grounded end and vice versa on the other pot. From X2 to X2 you have 240 volts. Just the fact that the two X2's are available makes it goofy looking. If they were not exposed it would look simplier.

To be truthful I did not help in making this setup. I only told management how to do it and another crew did the job. They got a little confused at first and tried to hook them up as if they were a 2 pot bank. Couldn't make it hold a fuse. The next morning they went over it and once again I said you have to put them in series etc. The second try they got it working and were amazed. It is for lights in a ball park.

[wtdoor67]

Can't help much there. Used to work on a few and am somewhat familiar with them but never had anything to do with the regulation. Some of the little setups I was around didn't even have regulators. Best ask Top Groove, MI and probably Edge and if there's ever any LAWP people again they know those systems.

[rcdallas]

I think that would be a little dicey. If later the voltage responded to a boost on some regulator it might get a little high. Never saw it done, but it might work. Worked one place where a lot of the pots had taps. Used to tap em up a little if it was low.

Also capacitors help with low voltage some. I spec other folks on here got a better take on this subject than I.

On a pot with 7200 turns on the primary I wouldn't want to try it on a nominal 12470 phase to phase. Probably blow. I can't remember but there's a basic insulation level (BIL) given for transformers. If I find it I'll put it on here. Your co. might have that info.

All I was getting at is instead of spending the money to install a line device to boost the voltage or even home brewing a booster a 2 bushing transformer that might be collecting dust at the yard might be a cheaper route for one customer.

Out here we have 7200/12470 and 12470/21600, we do have 12470/21600 transformers wired delta on the high side on our 7200.

I've been told don't put a pot labeled 7200/12470 wired delta on 7200 for the same reason you mentioned.

[wtdoor67]

When dinking with pots, basically just keep in mind the number of turns on the primary winding. You mention that you keep 12470/21600 pots. Now those would have 12470 turns on the primary winding. They could be hooked up phase to phase or delta in a 12470 wye circuit. Now the number of turns doesn't have to be exact. They just have to be close. I've never heard a rule on how close but I expect there's a range where you're too far one way or the other.

Also become familiar with the term nominal voltage, which just means about average, or the one named. Nowaday nominal house voltage is 120/240, but we know it's not always that. The 5% rule is common. If it's too high or low by 5% then you need to raise it or lower it. Nominal voltage also is used when referring to primary voltages. 7200 or 12470 may not read exactly that when read on a good digital voltmeter.

Back to the turns thing. I have worked where 12000/20800 pots were hooked phase to phase in the same type of system you're working in sometimes. I expect 14400/24900 pots might work also. Have to set the taps all the way down. You can get a little clue from the name tag. Have also used 14400/24900 pots phase to phase in a 13200 system. Had to move the taps one position I think. Used 19900/34500 pots phase to phase in a 20800 system. Etc. etc.

Something to always keep in mind if you have a bad pot or something you can sometimes go phase to phase and be just fine.

Sometimes figuring out the ratio of the primary and secondary windings will tell you approx. what voltage. It's all different though. Sometimes they'll have the regulators boosting the voltage up where you're at and things won't work like you think. Sometimes just have to hang it, heat it up, take a reading and kill it and adjust the taps accordingly. Having taps on pots actually is very handy I think.

On your diagram I think I'd ask someone else to close it.

[topgroove]

you could close one door but the second would blow instantanoius:D

[rcdallas]

When dinking with pots, basically just keep in mind the number of turns on the primary winding. You mention that you keep 12470/21600 pots. Now those would have 12470 turns on the primary winding. They could be hooked up phase to phase or delta in a 12470 wye circuit. Now the number of turns doesn't have to be exact. They just have to be close. I've never heard a rule on how close but I expect there's a range where you're too far one way or the other.

Also become familiar with the term nominal voltage, which just means about average, or the one named. Nowaday nominal house voltage is 120/240, but we know it's not always that. The 5% rule is common. If it's too high or low by 5% then you need to raise it or lower it. Nominal voltage also is used when referring to primary voltages. 7200 or 12470 may not read exactly that when read on a good digital voltmeter.

Back to the turns thing. I have worked where 12000/20800 pots were hooked phase to phase in the same type of system you're working in sometimes. I expect 14400/24900 pots might work also. Have to set the taps all the way down. You can get a little clue from the name tag. Have also used 14400/24900 pots phase to phase in a 13200 system. Had to move the taps one position I think. Used 19900/34500 pots phase to phase in a 20800 system. Etc. etc.

Something to always keep in mind if you have a bad pot or something you can sometimes go phase to phase and be just fine.

Sometimes figuring out the ratio of the primary and secondary windings will tell you approx. what voltage. It's all different though. Sometimes they'll have the regulators boosting the voltage up where you're at and things won't work like you think. Sometimes just have to hang it, heat it up, take a reading and kill it and adjust the taps accordingly. Having taps on pots actually is very handy I think.

On your diagram I think I'd ask someone else to close it.

Yes sir, understood. 1.25 volts per turn I do remember is what I was taught to keep in mind.