3-phase panel used for single phase service

[sw_ross]

Worked at a business that is going to have a mini-split installed.
My task was to run a new circuit to the outside with disco.


When I pulled the panel cover off the first thing that caught my eye was the 100 amp breaker with a jumper. At first I didn't know what to think. Then when I looked at the conductors feeding the panel I realized it was a 3-phase panel.

I already knew this building was supplied with only single phase. It's an older strip mall type of structure and I've done work in some of the other units. All the other units I've been in had single phase panels.

It's strange that this unit had this panel.

Regardless, there was no markings (or warnings!) about this setup and the fact that phase B and C are the same phase....
The way I see it, all the breakers that are in the "C" location are being fed by the jumper wire on the 100-amp breaker. And if you put a 2-pole breaker in a spot that is a B phase and C phase you're feeding that load with the same phase....

Obviously a variety of violations,
How serious would you consider each item?
My brain keeps going back to the jumper wire on the 100 amp breaker. I'm not sure what size it is. Maybe #4 at most.

 

[texie]

But then why is there what appears to be 3 phases coming in to the main breaker?

 

[sw_ross]

Yeah, I wonder about that also...
Where does the C phase (blue) wire come from...

It looks like a typical 120/208 3-phase service.
Voltage between A and B is 240v. The jumper gives 240v between A and C also. B and C are the "same" phase" so potential between B and C is 0v.

Maybe originally there was a need for 3-phase at this unit- I can't imagine what. The main level is retail with only lights receps and a PTAC unit. Above that is an apartment.

Regardless, is it wrong (code-wise) to use this 3-phase panel for this purpose?

 

[Hv&Lv]

Looks to me like the Jumper is used as you say..
but the wire up to top, (Blue) Is backed to what??
weatherhead? Old XF location on the pole outside?

what I’m trying to figure out is if it is jumpered and is back feeding back up through the breaker, where does the back feed terminate??

 

[goldstar]

Read Mike Holt's recent newsletter - https://www.mikeholt.com/newsletters.php?action=display&letterID=2181

 

[Hv&Lv]

Read Mike Holt's recent newsletter - https://www.mikeholt.com/newsletters.php?action=display&letterID=2181

Interesting read.
although the part about the meter isn’t quite right...
The 120V has little to do with it. It’s the CTs

 

[sw_ross]

Looks to me like the Jumper is used as you say..
but the wire up to top, (Blue) Is backed to what??
weatherhead? Old XF location on the pole outside?

what I’m trying to figure out is if it is jumpered and is back feeding back up through the breaker, where does the back feed terminate??

I agree. I know there’s a transformer out behind the building and everything is underground.

I’m not sure if the blue phase conductor is just taped off in the xfmr tub or what.

 

[Hv&Lv]

I agree. I know there’s a transformer out behind the building and everything is underground.

I’m not sure if the blue phase conductor is just taped off in the xfmr tub or what.

It would be nice to know for someone’s safety on the outside....
I would hope no one ever goes to connect it on the other end.
it really needs to be removed from the breaker IMO

 

[Beaches EE]

Yes, and it would be good to know exactly to what, if anything, the other end of that blue conductor is connected (or not).

 

[texie]

If this originally really 3 phase and they used MWBCs then you now have a bunch of overloaded neutrals.

 

[Coppersmith]

I see black, red and blue feeders so it looks like the panel is being fed with 3 phases.

To simplify the case, if split-phase feeders were connected to just the N, and A and B phases of a 3 phase panel, the panel would work exactly like a split-phase panel with the note that C phase slots don't have power. Jumpering the B and C phases together just powers up C phase for additional 120v slots and A-C for additional 240v slots. Obviously B-C would not work. A plaque to that effect would be appropriate. It would be useful to place lightly loaded breakers on the C phase since it is limited to 100 amps by the backfed 100 amp breaker. None of this would be a code violation AFAIK.

 

[GoldDigger]

Is the jumper sized properly?

 

[sw_ross]

Is the jumper sized properly?

I'm not sure. Your talking about 3 inches of conductor and I can only tell you what it appears to be. I'm guessing it would be a #4 thhn? Which doesn't get you to 100 amps.

 

[sw_ross]

If this originally really 3 phase and they used MWBCs then you now have a bunch of overloaded neutrals.

Good point about MWBC's not actually being MWBC's!

 

[sw_ross]

Jumpering the B and C phases together just powers up C phase for additional 120v slots and A-C for additional 240v slots. Obviously B-C would not work. A plaque to that effect would be appropriate. It would be useful to place lightly loaded breakers on the C phase since it is limited to 100 amps by the backfed 100 amp breaker. None of this would be a code violation AFAIK.

A plaque, or at least a sharpie pen description would be nice! It would be good to know that B and C are the same phase and that C is limited due to the 100 amp breaker.
Also knowledge of where the blue supply conductor goes would be nice. Disconnecting that conductor from the main breaker might provide some comfort.

I find it hard to believe that this is a code compliant install, but I can't cite any specific violations unless there is a UL listing saying the panelboard can't be set up this way.

 

[kwired]

If this originally really 3 phase and they used MWBCs then you now have a bunch of overloaded neutrals.

If it was originally high leg delta supply maybe not. Depends on what changes were made since "the conversion". Originally any MWBC's would not have involved the high leg and if unchanged would still be on opposite legs.

And don't assume red tape means there was no high leg. Seen many old installs with red marking the high leg.

 

[goldstar]

Interesting read.
although the part about the meter isn’t quite right...
The 120V has little to do with it. It’s the CTs

I guess it depends on the POCO. There are 3 POCO's in the areas I work in and all 3 are the same. High leg on the C phase of the meter (if that is what you were referring to).

 

[jap]

I'm still trying to figure out how you all know the 2nd energized phase is coming in on red and back feeding the blue?

It could be that the 2nd energize phase is coming in on blue and back feeding the red through the jumper.

Regardless, one or the other needs to be disconnected from the Main Breaker.

JAP>

 

[Beaches EE]

see post 9...

 

[jap]

see post 9...

Yes, and post 9 is phrased as though the blue is being backfed, not incoming, seeing as how if the blue "wasn't connected to anything" and it was the 2nd needed phase, the panel would only be 120 volts in it from the black phase seeing as how the panel is fed single phase.

Jap>