High Leg Voltage

[Seven-Delta-FortyOne]

I’ve heard that one of the reasons the high leg is generally not used for line-neutral loads, like say running a 208v motor, is because the voltage is very often unstable.

I was on a service call the other day and I was checking voltages in a panel, and I noticed that the readings were normal, except when I would check B phase to ground, and then it would be all over the place for a few seconds and then settle down.

2 questions:

Is unstable high leg voltage the result of open delta transformer configurations, and would it be normal if 3 transformers were used?

And, what is it that causes the unstable voltage?

 

[CoolWill]

Think about what is happening to get that high leg voltage. Any other potential difference in that system is across a whole winding or half a winding. So the voltage may vary according to the loading on that winding due to voltage drop in the winding itself, but it will be consistent across that coil. But the high leg voltage is across 1.5 windings, and the voltage drop across each winding will vary according to the load on each. Since each winding can be loaded differently at any given time, the voltage drop can be inconsistent compared to the drop across a single phase.

I hope that makes sense.

 

[ActionDave]

Was this a high leg service or something like a 480V to 240V step down transformer. If it is a service I'd be interested in the service neutral. If it is a customer step down transformer I'd be interested in the bonding of the X4.

 

[Birken Vogt]

It is stable enough. Your readings are wonky for some other reason.

The real reason it is not used is that breakers are not readily available for it, and it is an inefficient use of the transformer copper.

Think about what is happening, you are making a connection across the high leg transformer and then bucking the voltage down for no reason with half of the low tx, all the while increasing load on the low tx for no reason. Why would you not just connect line to line across the high tx and get a full 240?

The only reason I can think is that the panel is out of breaker spaces, and 208 would be good enough, and that is not a very good reason. OTOH, most high leg tx banks I see are very underloaded, and use of the 208 leg would not harm anything either.

 

[jim dungar]

And, what is it that causes the unstable voltage?

A transformer bank that was not sized to handle a 208V L-N.
This connection puts additional load on 1/2 of the center tapped winding at weird angle versus and is almost never planned on when the bank was designed. The unstable voltage would probably be tolerable if the was little to no120V loading on that part of the winding.

 

[hillbilly1]

Open delta’s are generally used where the three phase load is small, and the majority of the load is single phase. The high leg is seldom used for single phase loads. This works well with utilities because the single phase load transformer is larger than the second transformer the high leg comes from. With a full delta, it overloads the one winding that the neutral taps from, that’s why the manufacturer limits the neutral to phase loads.

 

[CoolWill]

Plus you would need a non slash rated single pole breaker.

 

[electrofelon]

I would make a guess that its really not an issue in most cases. Your POCO probably says not to do it FWIW. Seattle City light says to connect any L-L loads only to the lighter pot, but its unenforceable and I almost always connect L-L NOT across the lighter to balance things out on my side of things

 

[ggunn]

I would make a guess that its really not an issue in most cases. Your POCO probably says not to do it FWIW. Seattle City light says to connect any L-L loads only to the lighter pot, but its unenforceable and I almost always connect L-L NOT across the lighter to balance things out on my side of things

Some AHJs will not allow a PV system interconnection to the B (high leg) phase.

 

[electrofelon]

Some AHJs will not allow a PV system interconnection to the B (high leg) phase.

Wouldn't that be more of a POCO thing?

Helped a guy with a PV system here a few months ago, and it was open Delta and the POCO closed it. Single phase inverters were connected ab, bc, ca, etc.

 

[Birken Vogt]

I know one closed delta bank (rare, usually open) here that has 2 large solar arrays, one on the lighter and one on one of the wings.

It makes no difference in the grand scheme because every single phase array setup is only connected to 1 phase anyway, randomly distributed across the system. The Poco has bigger fish to fry.

 

[ggunn]

Wouldn't that be more of a POCO thing?

A POCO is an AHJ as defined in Chapter 1 of the NEC.

 

[Hv&Lv]

I would make a guess that its really not an issue in most cases. Your POCO probably says not to do it FWIW. Seattle City light says to connect any L-L loads only to the lighter pot, but its unenforceable and I almost always connect L-L NOT across the lighter to balance things out on my side of things

We say not to connect 208 loads to neutral through that bank also

 

[electrofelon]

A POCO is an AHJ as defined in Chapter 1 of the NEC.

Hmmm, ok. I guess fair enough a POCO does have to approve an "installation or procedure". I think most of us on this forum would treat POCO and AHJ as being separate and distinct.

 

[Seven-Delta-FortyOne]

Was this a high leg service or something like a 480V to 240V step down transformer. If it is a service I'd be interested in the service neutral. If it is a customer step down transformer I'd be interested in the bonding of the X4.

Straight 240D/120 service. No transformer. I still run into these around here in older commercial buildings.

Might have just been a bad initial connection on the ground. Subsequent readings were normal. It just kind of got me thinking…

 

[tortuga]

I would make a guess that its really not an issue in most cases. Your POCO probably says not to do it FWIW. Seattle City light says to connect any L-L loads only to the lighter pot, but its unenforceable and I almost always connect L-L NOT across the lighter to balance things out on my side of things

The reason they require that is the NEC does not require identifying the open phase set, same theory as 208 line to neutral a 240 L-L load across the open phase instead of the stinger pot then you reduce the total kVA rating of a open delta bank, as your load is now on the stinger and the lighter.

 

[hillbilly1]

I know one closed delta bank (rare, usually open) here that has 2 large solar arrays, one on the lighter and one on one of the wings.

It makes no difference in the grand scheme because every single phase array setup is only connected to 1 phase anyway, randomly distributed across the system. The Poco has bigger fish to fry.

Closed banks are not so rare, they are used for industrial applications. Open banks are used mainly for light three phase loads such as convenience stores, or rural areas where only two of the three phase lines are run. Saves the poco a transformer, and a line.

 

[electrofelon]

Closed banks are not so rare.

Yeah it's one of those regional things. Out in seattle, they are all open. I don't think City light even provides a closed Delta. Here in central New york, I've never seen an open Delta, they're all closed.

 

[Birken Vogt]

Closed banks are not so rare, they are used for industrial applications. Open banks are used mainly for light three phase loads such as convenience stores, or rural areas where only two of the three phase lines are run. Saves the poco a transformer, and a line.

I meant they are rare in this area.

Out here we have delta primaries so it does not save the poco a line either way.

Anyplace urban that gets a padmount they seem to make it 208Y.

 

[jim dungar]

Open banks are used mainly for light three phase loads such as convenience stores, or rural areas where only two of the three phase lines are run. Saves the poco a transformer, and a line.

And an open delta can easily be designed to have a very large amount of 120V while it is more difficult with closed deltas, especially in padmount construction.