4 (insulated) wires from electric pole for underground 3-phase service

[Elquery]

TL;DR Summary

I notice it is common for an underground 3-phase service (goes underground at distribution voltage) to double tap one of the phase conductors. Why is this?

Hi all.

This is probably a simple question, but I am hoping for some guidance on why a service would be delivered with 4 wires as seen in the image. It appears fairly common around here (northeast U.S.), if not standard, for one of the phase-conductors to be 'double-tapped' and go underground as two separate insulated conductors. I can't wrap my head around the purpose of this.

I've seen it for school campuses, larger businesses, etc. (not residential). In the image below, you can (hopefully) see that the conductor on the far right is tapped twice and sent underground as two separate insulated conductors.

My best guess is that they are sending 3 conductors to a 3-phase transformer and the fourth to a separate single phase transformer, but I'm not sure why they couldn't make the split after the burial, or why they would do that to begin with.

 

[DaveE]

It looks to me like two single phase loads, perhaps to different transformers/panels. There does seem to be a symmetry problem with 3-phase, or 3-phase plus single phase. That current has to come back somehow.

Electrical codes don't like splices. Not that you can't do it, but it costs money to add an interim distribution box to do it. What they do like is standard configurations, standard wires that go to standard devices in standard panels.

 

[Elquery]

Interesting. Is it pretty common to step down off two phase conductors? In the rural parts where I reside, residential service is almost always delivered from a phase conductor to a neutral. Maybe not always, but by far most common. (Cities and apartment complexes perhaps become a different soup?)

I didn't think to check if the neutral was run into the conduit. I can see it in the pic, but I'm not sure if it goes into the pipe or just to ground.

I'm not sure I understand the advantage of the set-up as you mention vs running 2 phase conductors with a neutral and sharing the neutral (leaving you with a 1:1:1 current configuration when balanced, right?). Would it just be the increased voltage available for the primary?
It would seem running four insulated conductors would cost more than just running 2 with a neutral... but there are a lot of components I'm not considering or familiar with.

 

[DaveE]

Interesting. Is it pretty common to step down off two phase conductors? In the rural parts where I reside, residential service is almost always delivered from a phase conductor to a neutral. Maybe not always, but by far most common. (Cities and apartment complexes perhaps become a different soup?)

I didn't think to check if the neutral was run into the conduit. I can see it in the pic, but I'm not sure if it goes into the pipe or just to ground.

I'm not sure I understand the advantage of the set-up as you mention vs running 2 phase conductors with a neutral and sharing the neutral (leaving you with a 1:1:1 current configuration when balanced, right?). Would it just be the increased voltage available for the primary?
It would seem running four insulated conductors would cost more than just running 2 with a neutral... but there are a lot of components I'm not considering or familiar with.

IDK. There are a lot of possible configurations out there. It could be a split phase neutral as you say. Maybe they need to run insulated wires in the conduit for that? If so I would think you'll see some marking near the ends, like a wrap of tape.

One possibility is that they want to distribute 3-phase and for customers that only need 1-phase they run line to line into a transformer. Then they can deliver 3-phase to those who need it and can also balance multiple 1-phase loads in their branches. It's certainly not what we see in suburban distribution, but may be optimized for flexibility. If they want to run higher voltages, then they'll need transformers anyway. You may understand it better if you can figure out where that conduit goes and look at the box it's connected to.

I used to work for a company that sold equipment that required about 20KW or 50KW 3-phase that was rather sensitive to balance issues. Occasionally, I would hear of some pretty strange (very) rural distribution schemes, like 2-phase with 90 degrees, which always sounded just plain stupid, but I'm sure they knew better than me.

 

[wirenut]

That could be a single (split) phase with the 2 phase conductors on the left, and the grounded conductor on the right. If you look closely at one of the "double tapped" conductors it seems to be at a very sharp angle to enter the conduit. I think that is used to bond (ground) the metal conduit to neutral. Using a second coductor is easier than bonding the neutral. In NY state any pipe coming out of the ground , subject to vehicle traffic, must be rigid or protected from damage. Any metal parts must be bonded. That looks like galvanized rigid conduit going up the pole.

 

[Elquery]

If you look closely at one of the "double tapped" conductors it seems to be at a very sharp angle to enter the conduit. I think that is used to bond (ground) the metal conduit to neutral.

I'm not positive I understand what you are describing. It's also possible the picture angle is not great, but having seen this in person, I can tell you that the double tap is coming off one of the phase conductors. There is also a neutral below those conductors (seen in the image) which goes to ground. Are you saying that they are grounding one of the phase conductors, or is it just the confusion of the image?

Here's another angle. Doesn't help see that wire you describe, but you can see what's going on with the neutral a bit better.

I'm not working on this, so not likely to see the interior equipment, but if I find someone local in the know I'll report back.

 

[Averagesupernova]

That looks like galvanized rigid conduit going up the pole.

Looks plastic to me. Look at the bell type connector part way down the pole.

 

[DaveE]

If you look closely at one of the "double tapped" conductors it seems to be at a very sharp angle to enter the conduit. I think that is used to bond (ground) the metal conduit to neutral.

Nope. Not that.
Ground is ground and neutral is neutral, they only connect at specific locations, like a transformer or a service panel. This is to avoid currents flowing through grounds or potential differences between grounds. If that conduit is metallic and buried, it is grounded. If it's metallic and not buried, then it will be connected to a ground system (which isn't neutral). If it's plastic, it's not grounded.

 

[Windadct]

Residential and other small loads will be single phase - single phase transformer, if you have two then you need 4 leads : Trans 1 = Ph A and B, Trans 2 =Ph A and C.

Over the total distribution feed there will be many drops and the load is balanced by managing the connections to the other drops.

Another option is the load could be open delta, kinda rare these days, but feasible for sure.

I have never seen split phase at the MV level.

 

[wirenut]

Looks plastic to me. Look at the bell type connector part way down the pole.

Look at the next section down. It is metal.

 

[Elquery]

For a bit of clarity:

Most of these I'm seeing have plastic at the top, then transition to metal. It doesn't appear as though any neutral is being delivered, but I'm usually driving when I see em, so...

These are common to see around here and not at all an outlier. I am seeing them 'everywhere' now that I've clued into it. It is specifically when the wires go underground without first going to a transformer. As said earlier, it's going to commercial type supplies, or things like high-schools. There also seems to always be two or so other capped conduits at varying heights.

Residential and other small loads will be single phase - single phase transformer, if you have two then you need 4 leads : Trans 1 = Ph A and B, Trans 2 =Ph A and C.

This is something I've been wondering. Is delivering between two phase conductors quite common in parts? As I mentioned earlier, all I've personally observed are single phase residential loads being delivered via a phase conductor and a neutral; not between two phase conductors as you describe. But maybe that's what's going on here, though I can't say I understand why to do that vs just delivering a 3-phase service?

 

[DaveE]

This is something I've been wondering. Is delivering between two phase conductors quite common in parts? As I mentioned earlier, all I've personally observed are single phase residential loads being delivered via a phase conductor and a neutral; not between two phase conductors as you describe. But maybe that's what's going on here, though I can't say I understand why to do that vs just delivering a 3-phase service?

If the utility chooses to distribute 3-phase, but a customer only needs 1-phase, then delivering phase to phase to a transformer makes a lot of sense. The customer may not want to pay for 3-phase infrastructure. It allows the utility to distribute high voltage delta and save on wiring costs in distribution. Single phase loads can be balanced by permutations of which phases are tapped for different customers. If there is a residential section they can put a transformer into create single phase with neutral, like the transformers up on the poles.

 

[sophiecentaur]

but a customer only needs 1-phase, then delivering phase to phase to a transformer makes a lot of sense.

If I saw the setup shown, I'd automatically think it was a 3 Phase supply with a neutral. But I have a UK Electric brain. In UK, if a customer needs one phase then they would get one of the phases and the neutral to their house. That gives them one side of their supply that's near Earth. Pretty well every house I the UK gets that. Houses in a street will get phase1,2,3,1,2,3 etc as you go from house to house. People seldom have a three phase domestic supply but larger buildings will have the phases shared out between floors.
Giving them phase to phase wouldn't achieve a low potential neutral..
PF has certainly opened my eyes about the amazing range of supply philosophies around the World. I have learned never to say never.

 

[DaveE]

If I saw the setup shown, I'd automatically think it was a 3 Phase supply with a neutral. But I have a UK Electric brain. In UK, if a customer needs one phase then they would get one of the phases and the neutral to their house. That gives them one side of their supply that's near Earth. Pretty well every house I the UK gets that. Houses in a street will get phase1,2,3,1,2,3 etc as you go from house to house. People seldom have a three phase domestic supply but larger buildings will have the phases shared out between floors.
Giving them phase to phase wouldn't achieve a low potential neutral..
PF has certainly opened my eyes about the amazing range of supply philosophies around the World. I have learned never to say never.

Yes, there are so many variations on how distribution can be done. There is enough money at stake that utilities will often optimize on their side of the circuits, especially historically, before large grid interconnects.

Of course the big difference between Europe and the US is 230V vs. 120V. You guys get to use less copper. Most residences here actually get 240V signal phase with a neutral center tap. The one thing in common everywhere is that utilities don't want to distribute low voltage/high current.

Ultimately, the single phase customer does need a neutral regardless. Hence the transformer requirement, which is expensive, and best done at a large scale vs. house by house. So the 3-phase delta distribution is really only a great idea if you need to minimize distribution costs with fewer wires and higher voltage. If you distribute high voltage, then you need transformers anyway and making a single phase with neutral is nearly free. I think it would only really be seen in very industrial areas. Residential zones would have a substation and centralize the conversion before local distribution.

I'm no expert, but I think 3-phase wye in utility distribution in the US is uncommon. However, it is very common inside industrial buildings (also 480V 3-phase). I think it mostly boils down to the cost of copper up on poles versus transformers on the ground. If I were to bet on the most common from substation to smallish factory here, it would be 480V delta distributed to a transformer that makes 208/120 wye. Big factories get higher voltage.

 

[Baluncore]

The picture in the OP shows three insulated wires, with one wire below. The wire below appears to be a common earth, bonded to everything, rather than a neutral.

The three insulated wires could be two 120 V, 180° phases on the left, with a neutral on the right.

The four insulated wires entering the conduit then supply neutral and 120V to two separate distribution boards.

 

[sophiecentaur]

Ultimately, the single phase customer does need a neutral regardless.

Whether or not the customer shares a 'floating' phase between other users, there will always be an 'about neutral' conductor, due to some leakage, somewhere in the circuits, your own or someone else's. Fusing of systems is a consideration; the LN system, with a fuse in the L, is the nearest to a fail safe. Fusing in a floating system can suddenly change which is the 'de facto' neutral on the circuit can change to the other leg (removing an unbalanced current). Double pole fusing is not good (in most / many systems) so which side would you fuse?

 

[Elquery]

I think there is still some confusion as to what is in the picture. But I don't blame anyone, rather the picture or my communication of it.

The picture in the OP shows three insulated wires, with one wire below.

No, the picture shows 3 uninsulated phase conductors coming directly off a distribution grid. Then those wires are 'tapped' (proper term?) and transition to insulated wires which enter the conduit. The catch is that one of the phase conductors is 'tapped' twice and sent into the conduit as a separate wire.

I have another (potential) clue or two:

The 4 wires have color coding (tape or whatnot): red, orange, white, and blue. At least on the one I checked, the red and orange were the two coming off the same phase.
Here's another picture, and it's of a different pole going to a school. Unfortunately I couldn't get a good angle, and you'll have to take my word that the insulated wires are coming off the phases as I describe. Note this one also has a more typical uninsulated tap and neutral feed which goes to a clearly visible pole mounted transformer. I.e. a separate delivery. There also appears to be a large ground mounted transformer or something near that other pole.

Another clue, is that I have since also noticed two wire systems where a SINGLE phase conductor is double tapped (at distribution voltage) and sent underground. What the heck? This just seems so strange to me. It is nothing more than parallel conductors at the same potential, unless I'm missing something. I may find a pic of that later and post if this thread is still going.

 

[Baluncore]

No, the picture shows 3 uninsulated phase conductors coming directly off a distribution grid.

Obviously the wires in the conduit will be insulated. I refer to the porcelain insulators that support the aerial wires, otherwise insulated by air.
Surely the Earthed wire without porcelain insulators is not the neutral. So there are only three distribution conductors involved. If one of those is neutral, then there can be only two phases.
Can you not go to the meter box / distribution panel end of a conduit somewhere, to identify the service being provided?

 

[Elquery]

Can you not go to the meter box / distribution panel end of a conduit somewhere, to identify the service being provided?

Perhaps someday. I have one in mind with possible access, but by and large not likely for me. Some may scoff but this is really just an exercise in concept for me, not serving any practical need.
I won't continue to post endless photos, but here's one last one with a clearer shot of the 'taps.' This is yet another service (they really are all over the place). In this case the orange and blue tagged wires are the ones sharing a phase conductor, so that's not consistent.

The two wire system coming off one phase I mentioned in the previous post turned out to go into two separate conduits, whereas this all goes into one. So perhaps not related at all. It almost seems like this could be an open-delta, open-delta config, but wirenut says they are not common and this config seems pretty common.

 

[Elquery]

If one of those is neutral, then there can be only two phases.

This is confusing to me. THis is from the distribution grid. How would one of the phase conductors from a 3-phase dist. grid be used as a primary neutral? I understand you could have a neutral on the secondary. Or are you positing that there's a transformer upstream doing some reconfiguration?

 

[DaveE]

Double pole fusing is not good (in most / many systems) so which side would you fuse?

All of the non-grounded current conductors in distribution. As you said, you don't break the neutral in distribution, you need it to stabilize the voltages.

In single phase equipment (i.e. no distribution, known loads) then it is common to fuse everything so you don't have to worry about reversals. I have no experience with wye connected equipment, except one unusual laser from East Germany with no documentation, it took us an embarrassingly long time to figure out that it wouldn't turn on because we wired it as a delta load.

Maybe you're worried about one phase of a three phase system tripping a breaker? Yes, this is a problem. Yes it really happens. This is why there is often a mechanical link between the phases in a 3-phase circuit breaker. Of course you can't count on that, there are other failure modes, like fuses.

If your lucky, the load current increases enough in the other phases to trip them as well. It is particularly a problem, in my limited experience, with motors. 3-phase motors that lose a phase can continue to run (poorly), if lightly loaded, without drawing enough to trip the other breakers. But, they can get really hot doing that, or not do their job properly. Other more sensitive equipment, like our big Ion lasers will have to deal with big current imbalances. This is a failure mode that equipment/system designers have to address somehow. In our case, we monitored phase balance and would shut down. Our older systems were tested to verify that our internal circuit breakers would trip reliably. This sort of thing is one reason why there is so much (mind boggling) choice in trip/blow characteristics when you specify overcurrent protection.

The breaker/fuse's only job is to prevent fires from too much current, everything else needs a different solution. It's not a problem for the power companies, they are just protecting their stuff; it's a problem for the end user.

 

[Fisherman199]

I've seen this before and it's more common on Underground riser poles than overhead. No reason apart from there just not being enough overhead room for more than 3 transformers. The answer is probably as simple as one phase is feeding a separate single-phase panel close to another panel servicing a 3 phase load. The only way to know for sure is to get a look at the load center(s). It's just costs less to have one pole serving 2 or 3 load points than having a separate pole for every load point.

Of course, I could be seeing the pictures wrong. It looks like there are 2 conductors on one of the phases.