three phase panel - 240Y/139 instead of 240 delta

Given a circuit breaker panel rated for 240 volt delta, center tapped (which means 120 volts to ground for A and C phases, and 208 volts to
ground for B phase), what is the chance this panel could work OK with 240Y/139 (which means 139 volts to ground for all three phases)? The concern would be that the manufacturer skimped on the design of the A and C phase busbars such that they can handle 120 volts to ground but not 139 volts.
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240v Highleg delta is A phase 120v to ground, B phase is 240v to ground and C phase is 120v to ground. All phase to phase voltage is 240v. I have never seen or heard of 240v Wye system here is the U.S. But to answer your question the buss bars will handle voltage above 120v.
Given a circuit breaker panel rated for 240 volt delta, center tapped (which means 120 volts to ground for A and C phases, and 208 volts to ground for B phase), what is the chance this panel could work OK with 240Y/139 (which means 139 volts to ground for all three phases)? The concern would be that the manufacturer skimped on the design of the A and C phase busbars such that they can handle 120 volts to ground but not 139 volts.
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| 240v Highleg delta is A phase 120v to ground, B phase is 240v to ground and C phase is | 120v to ground. All phase to phase voltage is 240v. I have never seen or heard of 240v | Wye system here is the U.S. But to answer your question the buss bars will handle | voltage above 120v.
I disagree with you on the B phase. Draw the triangle with 240 volt sides, horizontal on the bottom. Now put a ground dot at the center on the bottom (not at a corner). The B leg voltage is from the apex to the ground. Since we know that the side of a right triangle is the square root of the sum of the square of the two right sides, we can verify that my voltages are right.
120 volts * sqrt(3) = (approx) 207.8460969 volts
120 ^ 2 = 14400 207.8460969 ^ 2 = 43199.99999656418961 = (approx) 43200 14400 + 43200 = 57600 sqrt(57600) = 240
240 / sqrt(3) = (approx) 138.5640646 = (roughly) 139
Assuming A and C phases can handle 139 volts, then 240Y/139 provides a safer configuration of power that should be able to run all 240 volt three phase motors. This voltage can be produced by many three phase generators, derived via transformers (such as with a 277/139 split phase secondary), and I've even found it offered by at least one power company:
http://www.cityofseattle.net/light/contractors/resc/resc.pdf http://www.cityofseattle.net/Light/engstd/Docs/ConStd/u977.pdf
The issue is whether it can be used with circuit panels rated for 240 delta, or whether one has to upgrade the panel where the next available is for 480Y/277 (double that of 240Y/139).
A panel rated for 240 delta certainly must have insulation rated for a minimum of:
240 volts between bus bars 208 volts between bus bar B and ground/chassis 120 volts between bus bars A or C and ground/chassis
But 240Y/139 raises the voltage slightly for A and C, and a panel that is designed to the bare minimum for 240 delta could actually be exceeded with 240Y/139 by applying 139 volts on the phases, and exceed the possible 120 volt rating to ground of A and C.
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The basic listing for panelboards (QEUY) says 600v or less. Beyond that you only have the manufacturer's labelling and installation instructions.
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| The basic listing for panelboards (QEUY) says 600v or less. Beyond that you | only have the manufacturer's labelling and installation instructions.
Square D QO and NQOD say they are rated for 208Y/120 and 240 delta. Same for Cutler-Hammer CH, BR, and PRL1 series. Next up is Square D NF series and Cutler-Hammer PRL2a which are rated for 480Y/277 but not 480 delta. Above that go with Square D I-Line and Cutler-Hammer PRL3-5.
I'm wanting to know if Square D QO and NQOD and Cutler-Hammer CH can be used with 240Y/139.
This also makes me realize an interesting issue. If one is running 240 delta with center tapped ground in a 3-phase 3-bus panel such as QO, then 1/3 of the phase slots are going to give 208 volts, not 120, and so, installing 120 volt circuits will require care to be sure they are not attached to the B phase. Of course with 240Y/139, you can't do any 120 volt circuits at all.
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Why "wye"?
I know this might be a good EE test question but it doesn't make much sense in the real world. If you don't have any 139v loads why not delta?
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| Why "wye"? | | I know this might be a good EE test question but it doesn't make much sense in | the real world. If you don't have any 139v loads why not delta?
There are pluses and minuses for either wye or delta. The big plus for wye is that the line to ground voltage is never higher than 139 volts. Utilities might prefer wye because it discourages center tapped delta imbalances where the lighting load on the 240/120 winding is made too high.
Wye does have a disadvantage under harmonic loads.
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All the more reason why that should just be 120v (208 wye) Any designer who really wants wye would design for 208. The computer systems I worked with could be tapped either way with no change in expected reliability.

They also like as few "standard" services as possible.

Not if there is no neutral load
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|>There are pluses and minuses for either wye or delta. The big plus for wye |>is that the line to ground voltage is never higher than 139 volts. | | All the more reason why that should just be 120v (208 wye) | Any designer who really wants wye would design for 208. The computer systems I | worked with could be tapped either way with no change in expected reliability.
Existing 240 volt motors don't work well with 208.
|> Utilities |>might prefer wye because it discourages center tapped delta imbalances where |>the lighting load on the 240/120 winding is made too high. |> | | They also like as few "standard" services as possible.
240/120 CT provides the needed 240 volts, and 5% for lighting. This can be abused by users who overload the center tapped winding. Only those who have 95% of their loads as 240 volt systems that can handle 208 volts above ground can use this properly. Single phase 240 volt loads often cannot handle this on other than the lighting winding.
240Y/139 provides the needed 240 volts, and discourages imbalanced lighting loads by requiring it to be obtained or derived by other means. It also works well with single phase 240 volt loads. And it is safer.
If I had to offer just one of these, it would be 240Y/139. Then users can either get 208Y/120 simultaneously, or use their own transformers to get 240/120 out of each 240 volt side.
|>Wye does have a disadvantage under harmonic loads. | | Not if there is no neutral load
Except for the neutral at the WYE transformer bank. Wanna see what happens when you burn out the neutral on your transformers?
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One advantage I havn't heard mentioned yet of doing 240Y instead of 240 Delta is that it is common for closed Delta banks to blow one of the line fuses when the primary line looses a phase. This is true for both D-D banks and Y-D banks. (Y-D banks with no primary neutral connection are common in irrigation, at least around here) This is a major pain in areas with lots of irrigation pumps becuase every time one line opens, all of the three phase banks must be re-fused. Even worse is when the Delta banks back feed the fault and get an ususpecting lineman.
When a Y-Y transformer is installed, the motor will loose a phase and trip on overload (if properly protected) when a primary phase is lost. When the phase is restored, the motor may be restarted.
208V on irrigation pumps is a non-starter because most motors have a service factor of 1.15 on 240V and 1.0 on 208V. The service factor is needed to get the irrigation lines filled before the pressure comes up. (Cent. pumps have MORE load when the output pressure drops.)
Matthew
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On Sat, 12 Jun 2004 02:45:07 GMT Matthew Beasley
| One advantage I havn't heard mentioned yet of doing 240Y instead of 240 | Delta is that it is common for closed Delta banks to blow one of the | line fuses when the primary line looses a phase. This is true for both | D-D banks and Y-D banks. (Y-D banks with no primary neutral connection | are common in irrigation, at least around here) This is a major pain in | areas with lots of irrigation pumps becuase every time one line opens, | all of the three phase banks must be re-fused. Even worse is when the | Delta banks back feed the fault and get an ususpecting lineman.
If you lose a phase on Y-D, the winding no longer powered by the primary will be backfed from the secondary. It's like an open delta but with a winding on the open ends taking power and transforming it back to the primary on the missing line. There might be some single phase loads on that phase somewhere, which these Y-D banks will now be powering. That is in addition to the increased current on the remaining two phases due to it operating like an open delta. Blowing fuses does not surprise me.
So why are they Y-D?
| When a Y-Y transformer is installed, the motor will loose a phase and | trip on overload (if properly protected) when a primary phase is lost. | When the phase is restored, the motor may be restarted.
Some motors can still function on 2 phases. Some can even function on 1 phase. It depends on the ratings, I presume. But it is best to keep them protected at the proper starting and running current levels.
A D-Y transformer is another option. But when you lose a primary phase line, the windings it was connected to are now singled phased in series with the remaining phase lines. You end up with half voltage on two of the secondaries, and in the same phase as the remaining. D-D would do something like that and end up mimicking an Edison center tapped single phase circuit while powering loads that expect three phase.
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wrote:

240
both
connection
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opens,
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me.
To produce a delta secondary, the options are a Y-D or D-D. If the area is already Y primaries for single phase 240V, then the same transformers can be used for Y-D 240V. Less types of transformer to stock.

and
lost.
keep
phase
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single
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On Sun, 13 Jun 2004 20:35:40 GMT Matthew Beasley
|> So why are they Y-D? | To produce a delta secondary, the options are a Y-D or D-D. If the area | is already Y primaries for single phase 240V, then the same transformers | can be used for Y-D 240V. Less types of transformer to stock.
So why are they *-D? E.g. why is having a delta secondary the goal?
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wrote:

area
transformers
Well, I have an answer, but I am not sure it's a good one.... I believe it is so that standard 240V transformers can be used instead of 139V transformers. It also allows 120V single phase loads on the same system, but in many of the pump setups I have seen, the 120V is only used for the contactor coil. In smaller dairies, the 240V red leg delta is used to feed conventional 120V/240V edison three wire single phase plus 240V three phase for motors. Larger diaries use 480V with stepdowns for 120V.

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On Mon, 14 Jun 2004 13:43:28 GMT Matthew Beasley
| wrote: |> |> |> So why are they Y-D? |> | To produce a delta secondary, the options are a Y-D or D-D. If the | area |> | is already Y primaries for single phase 240V, then the same | transformers |> | can be used for Y-D 240V. Less types of transformer to stock. |> |> So why are they *-D? E.g. why is having a delta secondary the goal? | | Well, I have an answer, but I am not sure it's a good one.... | I believe it is so that standard 240V transformers can be used instead | of 139V transformers. It also allows 120V single phase loads on the | same system, but in many of the pump setups I have seen, the 120V is | only used for the contactor coil. In smaller dairies, the 240V red leg | delta is used to feed conventional 120V/240V edison three wire single | phase plus 240V three phase for motors. Larger diaries use 480V with | stepdowns for 120V.
I can understand the answer. They do make 208 volt motors (but mostly for three phase since availability of 208 on single phase is low), so you could have used 208Y/120. Since 277 volt transformers are common, another option would be 480Y/277 (but the utility could charge more).
If the connection wiring for the motors allows wiring each winding totally separately, one option is to wire the motor with each winding branching off a neutral (e.g. requires WYE for power), and wire the transformer bank as 416Y/240. Same motor, same transformers, but no more backfeeding except what the motors themselves do.
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|>There are pluses and minuses for either wye or delta. The big plus for wye |>is that the line to ground voltage is never higher than 139 volts. | | All the more reason why that should just be 120v (208 wye) | Any designer who really wants wye would design for 208. The computer systems I | worked with could be tapped either way with no change in expected reliability.
Adding to my previous followup ...
The load might really be lots of 240 volt single phase loads which would normally be evenly spread over the phases. Not many single phase devices are designed for 208 volts.
I'm sure a computer switched to 240 volt would work fine on 208 volts. But I've definitely seen motors designed for 240 volts burn out when doing a lot of work with only 208 volts.
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This was when "computers" had 3 phase motors. They were one of the things that didn't get tapped when we went from 208 to 240.
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snipped-for-privacy@ipal.net writes:

Having the maximum voltage being 139V vs 208V doesn't seem to be much of an advantage. Presumably all the 240V motors are all insulated sufficiently for 240V (or at least 208V). Also load imbalance for lighting off a center tapped phase shouldn't be an issue if the power co. knows of it and installs proper transformers. I've seen a few installations which appear to be 240V open delta with a large transformer with a grounded center tap and a small one for the B phase. (Including several in a residential neighborhood in Baton Rouge LA. Whole house air conditioners using 3 phase?)
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I was thinking the same thing. I had a lot of small data centers with 3 phase mainframes and significant office space plus the usual 120v load, 240v copiers and such. They ran off of a open vee redleg delts. It was like you said. They had a big center tap transformer and a smaller one for the wild leg. We never had power related issues unless someone assumed 208 3p when they installed something and didn't tap it properly. The IBM default was 208 but everything would run on 240 if you set the taps.
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On Fri, 11 Jun 2004 19:54:09 +0000 (UTC) Michael Moroney
| snipped-for-privacy@ipal.net writes: |
|>| Why "wye"? |>| |>| I know this might be a good EE test question but it doesn't make much sense in |>| the real world. If you don't have any 139v loads why not delta? | |>There are pluses and minuses for either wye or delta. The big plus for wye |>is that the line to ground voltage is never higher than 139 volts. Utilities |>might prefer wye because it discourages center tapped delta imbalances where |>the lighting load on the 240/120 winding is made too high. | | Having the maximum voltage being 139V vs 208V doesn't seem to be much | of an advantage. Presumably all the 240V motors are all insulated | sufficiently for 240V (or at least 208V). Also load imbalance for | lighting off a center tapped phase shouldn't be an issue if the power | co. knows of it and installs proper transformers. I've seen a few | installations which appear to be 240V open delta with a large transformer | with a grounded center tap and a small one for the B phase. (Including | several in a residential neighborhood in Baton Rouge LA. Whole house | air conditioners using 3 phase?)
If going from 139V to 208V (line to ground) is "not much", then going up to 277V (line to ground) would be "not much more". So why not run 480Y/277 everywhere that three phase is needed. Europeans already use 400Y/231 which is in between these.
Sure, proper transformers can allow more lighting power. But then it's not a simple delta anymore. But the power company doesn't like this extra cost even if it's a small teaser for the B phase.
In cases where the only load is 240V delta, though, 240Y/139 is still a lower line to ground voltage than a straight delta (corner grounded) or a center tapped delta.
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