Neutral for 600amp open delta system

[augie47]

Ok now, let's talk about using a delta breaker as a main.

That would take me back to days when you dipped connections in solder

 

[kwired]

Im VERY familiar with open delta applications and configurations.. just not with the high leg, precisely for the reasons you've stated. Typically we have the inverse problem: heavy three phase load and small single phase load. Of course, for every application there probably is its inverse.

Really the only significant difference is one has a center tap on one winding, and the other one doesn't. You still have to pay attention to load distribution on either one as you can not load it the same as you could a full delta system. If you for some reason don't have third primary phase and no choice but open delta, you may need larger transformers then you would for a full delta.

Ok now, let's talk about using a delta breaker as a main.

Delta breakers were not used as mains, they were a method of connecting a three pole breaker to a single phase bus panel.

 

[chris kennedy]

Code requires you to (can't remember the section offhand).

408.3(E).

 

[jrannis]

Delta breakers were not used as mains, they were a method of connecting a three pole breaker to a single phase bus panel.

The way I remember it being used was three phase coming into the panel it the breaker with the high leg passing through it and going on to the load.
But, maybe it could have been a MLO panel and the high leg going straight through to the equipment OCP that contained the delta breaker.
Would that be correct?
It seems to work better that way I guess.

 

[don_resqcapt19]

Of course but, since it is physically outside of A and B, how did our industry decide this was a good idea? I can imagine the fist fights at that code panel discussion.
Maybe they proposed it only to trade it for something crazy like ground rods.

The high leg is almost always "B" phase in terms of the phase rotation sequence. Older utility metering equipment needed to have the high leg on the "C" phase position so that their cash registers would work correctly. The utility just landed the "B" phase conductor in the "C" phase position at the metering equipment and we just restore it to its correct position at the service equipment.

 

[don_resqcapt19]

...
Delta breakers were not used as mains, they were a method of connecting a three pole breaker to a single phase bus panel.

Where there was only one three phase load in the panel the delta breaker was the "main" for the high leg. The high leg from the utility was landed directly on the delta breaker. The other two phases were obtained from the panel bus.
If you just turned on the panel main, you still had the high leg energized at the 3 phase equipment, unless you also turned off the delta breaker. Of course turning off the delta breaker did kill all of the conductors to the 3 phase equipment.

 

[kwired]

The way I remember it being used was three phase coming into the panel it the breaker with the high leg passing through it and going on to the load.
But, maybe it could have been a MLO panel and the high leg going straight through to the equipment OCP that contained the delta breaker.
Would that be correct?
It seems to work better that way I guess.

Don described it a little better then I did. The delta breaker plugged on to the two phases of a regular single phase panel. The incoming third phase supplied only one pole of the delta breaker. Part of reason they are no longer acceptable is mentioned in Don's post, turning off the main breaker but not the delta breaker could result in back feed voltage through the load connected to the delta breaker and back onto the panel bus as the third phase from the supply was never interrupted.

Where there was only one three phase load in the panel the delta breaker was the "main" for the high leg. The high leg from the utility was landed directly on the delta breaker. The other two phases were obtained from the panel bus.
If you just turned on the panel main, you still had the high leg energized at the 3 phase equipment, unless you also turned off the delta breaker. Of course turning off the delta breaker did kill all of the conductors to the 3 phase equipment.

 

[mivey]

Not odd in the connection sense, but odd in that youre going to have to de-rate the crap out of that connection.

Not as much as you might think.

In my nerd glasses world im cringing at the voltage regulation on a system like that.

The voltage regulation is unbalanced and that can be a concern if the bank is heavily loaded, even for balanced loads.

The phase to phase 240 is rock solid in this setup

I wouldn't say that. The voltages are unbalanced and can cause issues if the bank is heavily loaded and serving three-phase motors that are near full load.

 

[mivey]

Im VERY familiar with open delta applications and configurations.. just not with the high leg, precisely for the reasons you've stated. Typically we have the inverse problem: heavy three phase load and small single phase load. Of course, for every application there probably is its inverse.

The inverse problem will still have a high leg. Amazing that you have not run across a center-tap delta bank before. I guess it just depends on local practice but I thought they were about as common as dirt.

 

[waedwards87]

The inverse problem will still have a high leg. Amazing that you have not run across a center-tap delta bank before. I guess it just depends on local practice but I thought they were about as common as dirt.

Sorry, been out of the office for the past few days. As far as the derate goes, youre at 57% (line current limited to phase current to prevent overheating) of the single phase KVA from the open delta alone. Then you have to consider the reduction in equivalent KVA when looking at the split phase. When looking at I^2R = W, even cutting the resistance in half is bad when you are doubling the current via halving the voltage. I wonder how much larger that center tapped transformer is?!

I can understand if the utility only provides single phases in light commercial/residential. Most of my experience has been industrial, which would explain why I haven't seen it too often.

Is there any more insight you guys can provide? Am I off base in anything im assuming (on this only.. im sure you guys could grill me on a ton of other stuff :lol?

 

[mivey]

Sorry, been out of the office for the past few days. As far as the derate goes, youre at 57% (line current limited to phase current to prevent overheating) of the single phase KVA from the open delta alone. Then you have to consider the reduction in equivalent KVA when looking at the split phase.

Don't forget the de-rating that may be necessary to help compensate for poor voltage regulation.

When looking at I^2R = W, even cutting the resistance in half is bad when you are doubling the current via halving the voltage.

You would only double the transformer output current if you paralleled both half windings

I wonder how much larger that center tapped transformer is?!

It may very well be the same size for small banks. For others you might even have something on the order of a 37.5 or 50 kVA combined with a 15 kVA.