120v/208v 3 phase residential service

[ActionDave]

Dave, us Coloradoan's didn't have A/C back in the early days. I grew up in a house in the deep south, built in '49 or '50, with such a service for A/C. Also I should have mentioned in my previous post that all these services were CT metered as self contained metering above about 100 amps of calculated load were not yet made. All 3 phase was CT in those days. I'm dating myself. PS:you getting any smoke over there from the fires? The wind has been in our favor and we have none.

Your right. I don't have A/C now and did not growing up here.

Not getting much smoke in the air here, sure can see it though. That West Fork fire looked like a mushroom cloud from my house yesterday afternoon.

 

[Fliz]

Here is one, even 30 years of experience can't explain. Oh yeah, and it cost me my job. 208v 3 phase, the one with the wild-leg.


Question #1: The utility wanted the wild-leg to be on "B" phase. The city inspector wanted it to be on "C" phase. I made the switch between the meter socket and main breaker, was this wrong and if so how should it be done?


Question #2 Voltage readings from phase to phase was 238v. (A-B, B-C, A-C) The customer complained that his 240v range top was getting too hot. If the voltage is the same, and range elements have not changed their resistance, how can this be possible? After all there's Ohm's Law right?

There is a very serious safety issue with putting it on the C phase.
- It was required to be on the B phase because before it was part of the code, electricians would electrocute themselves depending on which way it is facing, not knowing if they were dealing with "a" or "c".
- Since the code requires it to be on "B" phase, wiremen will make mistakes when they install breakers and equipment if they are not paying close attention or it is phase-colored incorrectly, harming personnel and equipment.

480 delta services are actually quite common in high-end residential homes that have elevators.

 

[kwired]

There is a very serious safety issue with putting it on the C phase.
- It was required to be on the B phase because before it was part of the code, electricians would electrocute themselves depending on which way it is facing, not knowing if they were dealing with "a" or "c".
- Since the code requires it to be on "B" phase, wiremen will make mistakes when they install breakers and equipment if they are not paying close attention or it is phase-colored incorrectly, harming personnel and equipment.

480 delta services are actually quite common in high-end residential homes that have elevators.

Why were electricians electrocuted? seems there would just be a higher number of incidents of burning up 120 volt equipment when 208 was applied. Risk of electrocution probably remains near the same, unless you were one of those that used the wet finger method of voltage testing

 

[ggunn]

A possibly stupid question here...

If 240 three phase is taken off a transformer wired this way (center tapped between two phases with a high leg to neutral), is there an absolute phase?

 

[GoldDigger]

A possibly stupid question here...

If 240 three phase is taken off a transformer wired this way (center tapped between two phases with a high leg to neutral), is there an absolute phase?

Just what do you consider an "absolute phase" to mean? There are three delta phases in the normal phase relationship, but there is no "neutral" in the strict sense relative to the three phase lines. If you ignore the (120/240)-only neutral, you have a perfectly normal three phase delta relative to the primary phase voltages on the POCO side.
And is a perverse sort of way you have a two phase system with unequal phase voltages and no neutral if you take the fourth wire into account. (That is, the high leg to neutral is rotated 90 degrees from the 120/240 phasing.)
You could also look at it as a Scott-T with some pieces missing.

 

[masterinbama]

Most stove eyes operate at 120 volts with only the oven and broiler element operating at 240 volts.

That in and of itself explains the elements overheating.

 

[ggunn]

Just what do you consider an "absolute phase" to mean? There are three delta phases in the normal phase relationship, but there is no "neutral" in the strict sense relative to the three phase lines. If you ignore the (120/240)-only neutral, you have a perfectly normal three phase delta relative to the primary phase voltages on the POCO side.
And is a perverse sort of way you have a two phase system with unequal phase voltages and no neutral if you take the fourth wire into account. (That is, the high leg to neutral is rotated 90 degrees from the 120/240 phasing.)
You could also look at it as a Scott-T with some pieces missing.

What I mean is, when you have a three phase transformer with wires coming out of it, is there a wire that is unambiguously the A phase, or can you just pick one to designate as "A" which then determines what the other two are?

 

[GoldDigger]

What I mean is, when you have a three phase transformer with wires coming out of it, is there a wire that is unambiguously the A phase, or can you just pick one to designate as "A" which then determines what the other two are?

There is nothing that you can unambiguously call the A phase. What you will have, and may or may not care about are:

1. A relative phase rotation compared to the phase rotation of the primary. ABC on the primary should produce ABC, BCA or CAB on the secondary, all of which will produce the same motor rotation direction. Make sure your labeling agrees with that.
2. A specific phase offset between the primary phase voltage and the secondary voltage on the corresponding phase. This may be zero, or some multiple of 30 degrees depending on how the transformer is wound and wired. Your labeling of the phases should match the phase offset spec on the transformer nameplate or you will confuse people. This only matters when you will be paralleling the output of two transformers or switching hot between them.
3. If you have a center tap on one secondary winding, that is by convention and by code the A-C (or C-A) winding. The other line must be B (if you use or ground the center tap. If you leave it unconnected it does not matter.)

 

[ggunn]

There is nothing that you can unambiguously call the A phase. What you will have, and may or may not care about are:

1. A relative phase rotation compared to the phase rotation of the primary. ABC on the primary should produce ABC, BCA or CAB on the secondary, all of which will produce the same motor rotation direction. Make sure your labeling agrees with that.
2. A specific phase offset between the primary phase voltage and the secondary voltage on the corresponding phase. This may be zero, or some multiple of 30 degrees depending on how the transformer is wound and wired. Your labeling of the phases should match the phase offset spec on the transformer nameplate or you will confuse people. This only matters when you will be paralleling the output of two transformers or switching hot between them.
3. If you have a center tap on one secondary winding, that is by convention and by code the A-C (or C-A) winding. The other line must be B (if you use or ground the center tap. If you leave it unconnected it does not matter.)

Thanks. Now, earlier in this thread there was a discussion of which phase should be the high leg on a 240/120 three phase with a stinger. Taking what you have said it seems to me that the question is really what you should call the phase that is the high leg, since (if I understand you correctly) no phase is explicitly A, B, or C until you define one of them.

 

[suemarkp]

On a high leg delta you can define B. Which poles are exactly A and C is more problematic, and you can flip A and C if rotation is wrong. But you know where B is (it touches the two winding that don't have the center tap).

 

[GoldDigger]

Thanks. Now, earlier in this thread there was a discussion of which phase should be the high leg on a 240/120 three phase with a stinger. Taking what you have said it seems to me that the question is really what you should call the phase that is the high leg, since (if I understand you correctly) no phase is explicitly A, B, or C until you define one of them.

As noted, the high leg is uniquely identified, and is by NEC definition B. As for rotation, there are actually sometimes differences in the rotation of POCO labelled connections as either ABC or CBA. But we do not really care how POCO labels them as long as all of the equipment on a site gets the right rotation.
As a totally separate issue, the meter base for three phase will usually identify A, B and C and for the meter to read correctly POCO may require the high leg to be attached to C in the meter base.
The standard solution to that common situation is to attach the high leg to C at the meter and B on panels and subpanels and not worry about the apparent contradiction between the two.
If the high leg delta is created as an SDS using a customer's own transformers, then the NEC identification of B as the high leg still applies, and metering is irrelevant.

 

[Fliz]

Why were electricians electrocuted? seems there would just be a higher number of incidents of burning up 120 volt equipment when 208 was applied. Risk of electrocution probably remains near the same, unless you were one of those that used the wet finger method of voltage testing

It's just what I heard when I was was taking code class, explaining the intent of the code. Something about panel boards being accessible from both sides causing confusion. Does sound like more of an equipment issue though, it's been awhile.

 

[kwired]

On a high leg delta you can define B. Which poles are exactly A and C is more problematic, and you can flip A and C if rotation is wrong. But you know where B is (it touches the two winding that don't have the center tap).

You can also move the grounding from one mid point to another and change nothing else (on a bank made of three single phase transformers anyway) and touch nothing else. Rotation never changed phases are still in same relation to the primary, high leg is in a different position though. IMO there is no true A B or C until you identify them. POCO may have what they call A B and
C on the primary, but in reality it means nothing to most customers, all that matters is rotation and proper position of a wild leg if there is one.

It's just what I heard when I was was taking code class, explaining the intent of the code. Something about panel boards being accessible from both sides causing confusion. Does sound like more of an equipment issue though, it's been awhile.

But why should it matter from a touch perspective. If high leg is not in the position where it is expected you still shouldn't touch it, that is why I don't understand why there would be an increase in electrocutions. An increase in burning up equipment because it was connected to the high leg is pretty understandable.

 

[GoldDigger]

You can also move the grounding from one mid point to another and change nothing else (on a bank made of three single phase transformers anyway) and touch nothing else.

True if POCO or SDS uses three center-tapped transformers. The customer would only be aware of that if all three center points were present at the service.
If only one transformer secondary is center tapped, then there is no choice.

If POCO did something to change which primary phase the center tapped secondary was connected to, then absolutely nothing would change. If they changed which transformer secondary they supplied the center tap from, they would also have to change which one they grounded.

I have seen in another forum a lineman describing the primary phases as "road", "yard" and "top" based on the wire position on the pole. Unless they rotate the wire positions at some point, or the line crosses from one side of the street to the other, that does the identification job they need. It would be a pleasant coincidence if the top phase fed the high leg.
Underground wiring, well.....

 

[kwired]

True if POCO or SDS uses three center-tapped transformers. The customer would only be aware of that if all three center points were present at the service.
If only one transformer secondary is center tapped, then there is no choice.

If POCO did something to change which primary phase the center tapped secondary was connected to, then absolutely nothing would change. If they changed which transformer secondary they supplied the center tap from, they would also have to change which one they grounded.

I have seen in another forum a lineman describing the primary phases as "road", "yard" and "top" based on the wire position on the pole. Unless they rotate the wire positions at some point, or the line crosses from one side of the street to the other, that does the identification job they need. It would be a pleasant coincidence if the top phase fed the high leg.
Underground wiring, well.....

Overhead lines cross streets all the time and they do not change orientation of conductors. Then you get cases where all the conductors are either in a vertical or horizontal plane, even if only for a span or two. Then there are times where they are not adjacent to a road.


I did have a case where POCO had an open delta bank on a pole but this was supplied by underground conductors because of frequent large machinery traffic in the area and the hazard that would have been present with overhead lines. The underground was three med volt cables but they were only using two to supply the open delta bank. Cable had been there for a number of years and was getting to the point where there was occasional failures. POCO would simply switch the faulted line with the spare for a quick fix and then repair the fault when convenient. One time this happened the guys checked rotation at the service but didn't check voltage, and ended up with correct rotation but the wild leg was in the wrong position. Lets just say POCO and/or their insurance paid for repairs to all the equipment that was damaged on that one.

 

[ggunn]

In a related story...

A colleague of mine installed a microinverter solar array on an apartment/condo building which was new construction. When he went to do the interconnections, he found that whoever wired the split phase service panels for the units wired them as if the main feed to the building were 208/120 when actually it was 240 with a high leg, so 2/3 of the panels had a 120 leg and a 208 leg. Oops. I'm glad I didn't have to approve that change order.