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Old 07-17-2012, 11:47 AM   #1
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Panel Question concerning 3 pole breaker


Less of a question and more of a discussion because I'm trying to figure out more about a 3 pole breaker, and all of its uses:

This is my friends electrical panel. Does any of this seem unusual to y'all?

I haven't seen any three pole breakers before. I think I recall him telling me it connects to a subpanel with three busbars, where he runs A/C from it - it doesn't sound right to me.

What are some possible uses of a three pole?

Also, is it bad to have gaps in between breakers? Should breakers be added and labeled spares?

Is there anything else different about this panel?
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Old 07-17-2012, 11:49 AM   #2
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Panel Question concerning 3 pole breaker


It appears he might have a three phase high leg service. This seems to have been common in some parts of the country for resi HVAC.

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Old 07-17-2012, 01:29 PM   #3
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Panel Question concerning 3 pole breaker


The OP do have three phase delta system in there.

The other issue you will run into with three phase panels that genrally they don't use the tandem breakers at all.

And the other thing I will add if you have straight 240 volts load that is not a issue but if anything do required 120 volts along the 240 volts you have to advoid the wild leg aka B phase otherwise the wild leg voltage is 208 Line to netural so just be aware with that.

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Old 07-18-2012, 09:47 AM   #4
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Panel Question concerning 3 pole breaker


Thank y'all for the help.

Is this true:
1. So the three cables at the top running into the main breaker: 2 are 120V the other is the neutral.
2. Then you have a seperate cable coming in straight to a third busbar which also carries 120V, which is different than a typical residential dwelling.
3. The sine waves of all 3 phases are staggard to allow maximum voltage of what? 480V?




Question: A typical residential panel has two busbars each carrying 120volts that are staggard to allow a max of 240V?
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Old 07-18-2012, 03:46 PM   #5
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by Shinerbalk View Post
Thank y'all for the help.

Is this true:
1. So the three cables at the top running into the main breaker: 2 are 120V the other is the neutral.
2. Then you have a seperate cable coming in straight to a third busbar which also carries 120V, which is different than a typical residential dwelling.
3. The sine waves of all 3 phases are staggard to allow maximum voltage of what? 480V?

Question: A typical residential panel has two busbars each carrying 120volts that are staggard to allow a max of 240V?
No, that is mostly either incorrect or dangerously oversimplified if you need a thorough understanding to work on the system. Unless you know otherwise, connect 120 and 240V single phase loads to the two phases that read 120V to neutral. Only three phase loads should be connected to the phase that is 208 to neutral.

You see 240V L-L between any of the three phases and this can be done in some cases. Those cases are (1) if you know that you do not have an open delta transformer supply from the power company and (2) if the 240V loads are balanced. The transformer design is not for unbalanced loads across the high leg.

From the user end, I prefer to look at my open delta system as two power supplies. One is a like a typical residential single (split) phase system. You use only three three of the feeds for that, two legs and neutral. The other is used only for three phase.
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Old 07-18-2012, 10:13 PM   #6
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by Shinerbalk View Post
Thank y'all for the help.

Is this true:
1. So the three cables at the top running into the main breaker: 2 are 120V the other is the neutral.
2. Then you have a seperate cable coming in straight to a third busbar which also carries 120V, which is different than a typical residential dwelling.
3. The sine waves of all 3 phases are staggard to allow maximum voltage of what? 480V?

Question: A typical residential panel has two busbars each carrying 120volts that are staggard to allow a max of 240V?
Your last question there, about the function of a typical panel, is correct. Everything else is not. Working on 3-phase systems requires a very solid understanding of 3-phase power. It's not something you can bumble through and expect to get a safe or functional result. Example: moving a breaker into one of the empty spaces in the panel will put 208V on a 120V circuit! It would be unwise to work on this panel until you really understand how it functions - which is more complex than we can reasonably teach you on this forum.
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Old 07-18-2012, 10:40 PM   #7
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Panel Question concerning 3 pole breaker


mpoulton. The three phase systems I have worked on always had 3 legs of the same voltage. Is that the characteristic of a WYE system, while a DELTA has two legs of the same voltage and one of higher voltage? And is the high leg always the "B" phase?
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Old 07-18-2012, 11:49 PM   #8
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Panel Question concerning 3 pole breaker


High leg delta is better for running loads like motors, plus you have the added convenience of available 120. Mostly what you've worked on is a Wye configuration. Another advantage is that if you lose a winding in a Wye system you get a voltage increase whereas in a delta you get more current in the windings. Personally I the the high leg delta is sort of useless but that's just me. The high leg is always b phase.

The high leg in that tub should orange though no?
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Old 07-18-2012, 11:54 PM   #9
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by andrew79
High leg delta is better for running loads like motors, plus you have the added convenience of available 120
Why is delta better for motors?

120 is not conveniently available in a wye?
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Old 07-19-2012, 12:02 AM   #10
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by Missouri Bound View Post
mpoulton. The three phase systems I have worked on always had 3 legs of the same voltage. Is that the characteristic of a WYE system, while a DELTA has two legs of the same voltage and one of higher voltage? And is the high leg always the "B" phase?
High leg systems are always delta, but not all delta systems are high leg. Wye systems are inherently symmetrical because there are three secondary windings, one per phase, with one end of each connected to the center point. So the voltage of each phase to the center point (ground/neutral) is the same.

Delta systems can be configured several different ways. The three secondary windings are connected end to end to form a triangle. The ground/neutral point (if there is one - some systems are allowed to be ungrounded) can be placed in a few different locations. If one corner of the triangle is grounded, then you get two hot phases and a grounded phase, with the same voltage between each. This gives you a full three phase system that only has two hot wires and a neutral! If the midpoint of one winding is grounded, then you get a high-leg system: two phases with half the phase-to-phase voltage to ground, and a third with (sqrt3)/2 times the phase-to-phase voltage. In that circumstance, the middle phase is the high leg. This allows the use of a single large transformer with the midpoint-grounded secondary for single phase loads, and two smaller transformers to complete the system and allow the use of small 3-phase loads. You can also eliminate one of the three transformers in a delta system and it still works the same way. That's an "open delta". Saves a transformer on services that have only a small 3-phase component to the total load.
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Old 07-19-2012, 12:17 AM   #11
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by jlmran

Why is delta better for motors?

120 is not conveniently available in a wye?
120 is available in a Wye but in my line of work we usually have to step it down to get it. As for delta being better for motors I. long since forgotten the reason but Im guessing it may have something to do with the fact that i(line) equals i(phase) and the voltage varies between the two in a Wye and e(line) equals e(phase) in a delta and the current gets higher across two windings.
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Old 07-19-2012, 07:10 AM   #12
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Panel Question concerning 3 pole breaker


On a three phase system:

1. If you have 120 volts from each phase to neutral then you will have 208 volts from phase to phase. (a 120/208 volt Y system)

2. If you have 240 volts from any phase to any other phase and 120 volts from phase to neutral for two phases then phase to neutral for the third phase will be 208 volts (a delta system)

These facts are irrefutable.

Other voltages may also be provided in 3 phase systems (for example 277 volts phase to neutral and 480 volts phase to phase Y system) in which case the more common 120 and 240 volts will not both be available without using transformers.

I believe that the advantage of 240 volts over 208 volts for motors is small and is because the higher voltage leads to less voltage drop in the feed conductors. Meanwhile motors can be designed for best performance at 208 volts or 240 volts or whatever voltage is desired.

If you have a 120/240 volt delta (high leg) service it is better to balance the load by hooking up 240 volt only loads using the high leg (usually the B phase) but otherwise the other two legs and the neutral are used as if you had single phase 120/240.

I do not know if there is a standard (but there should be a standard) for 120/240 volt receptacles and 120/240 volt appliances whereby one of the hot slots is standardized as 120 volts to neutral and is interchangeable with 120/240 volt delta systems with the high leg permitted to be connected to the other hot slot.

So long as the electrician knows that he is working with a high leg service, he should not have any problem wiring up the panel and the building, using just the two legs with 120 volts to neutral for the 120 volt and 120/240 volt loads.
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Last edited by AllanJ; 07-19-2012 at 07:27 AM.
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Old 07-19-2012, 07:46 AM   #13
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by andrew79

120 is available in a Wye but in my line of work we usually have to step it down to get it.
Andrew what are you talking about? Everybody has to have it stepped down to get it...unless you think the rest of the world lives on portable generators. I don't ever recall seeing a 120v transmission or distribution lines running alongside the highway.
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Old 07-19-2012, 07:53 AM   #14
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by AllanJ
On a three phase system:

1. If you have 120 volts from each phase to neutral then you will have 208 volts from phase to phase. (a 120/208 volt Y system)

2. If you have 240 volts from any phase to any other phase and 120 volts from phase to neutral for two phases then phase to neutral for the third phase will be 208 volts (a delta system)

These facts are irrefutable.
Where is the neutral point in a delta system?
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Old 07-19-2012, 08:29 AM   #15
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Panel Question concerning 3 pole breaker


Quote:
Originally Posted by AllanJ
On a three phase system:

1. If you have 120 volts from each phase to neutral then you will have 208 volts from phase to phase. (a 120/208 volt Y system)

2. If you have 240 volts from any phase to any other phase and 120 volts from phase to neutral for two phases then phase to neutral for the third phase will be 208 volts (a delta system)

These facts are irrefutable.

Other voltages may also be provided in 3 phase systems (for example 277 volts phase to neutral and 480 volts phase to phase Y system) in which case the more common 120 and 240 volts will not both be available without using transformers.

I believe that the advantage of 240 volts over 208 volts for motors is small and is because the higher voltage leads to less voltage drop in the feed conductors. Meanwhile motors can be designed for best performance at 208 volts or 240 volts or whatever voltage is desired.

If you have a 120/240 volt delta (high leg) service it is better to balance the load by hooking up 240 volt only loads using the high leg (usually the B phase) but otherwise the other two legs and the neutral are used as if you had single phase 120/240.

I do not know if there is a standard (but there should be a standard) for 120/240 volt receptacles and 120/240 volt appliances whereby one of the hot slots is standardized as 120 volts to neutral and is interchangeable with 120/240 volt delta systems with the high leg permitted to be connected to the other hot slot.

So long as the electrician knows that he is working with a high leg service, he should not have any problem wiring up the panel and the building, using just the two legs with 120 volts to neutral for the 120 volt and 120/240 volt loads.
Yes but the high leg is 208 not 240 due to the phase shift of the phases. So your motor is actually drawing less amps and this allows for smaller wire size and cheaper installation.

The neutral is center tapped off the c phase.

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