208V Wild Leg Question

[SKalkbrenner]

Hello everyone. I have 3 phase service 120/240/208. 2 legs @ 120V and a wild leg @ 208V. Any combination of 2 = 240V.

My question is this: if my PTAC unit specs allow for a range of 193V - 253V, could I just use the 208V leg and N for the PTAC? I realize my current draw would go up. Any other issues I may be overlooking?

Thanks
Steve

 

[Little Bill]

For one, you would have to have a single pole breaker rated for 208V or more. It can't be a slash rated (240/120).
Hardly worth the effort being a 2-pole breaker is both cheap and easy to find.

 

[winnie]

In theory this could work, in practice it isn't done.

1) The loading on the transformers is such that this is frowned upon; the 208V current to the load must pass through (and load) both 240V and 120V transformer coils.

2) Very often single pole breakers for a 120/240V panel are rated for 120V to ground, so you might have to go to the expense of a double pole breaker anyway, just to get the proper voltage rating. Once you've done this, you might as well just run the 240V to the load.

-Jon

 

[LarryFine]

My question is this: if my PTAC unit specs allow for a range of 193V - 253V, could I just use the 208V leg and N for the PTAC? I realize my current draw would go up. Any other issues I may be overlooking?

No, never. You should use two lines.

 

[Jraef]

Not to mention that 2, 5 or 10 years from now, someone will look at that circuit that is a Hot + Neutral and ASSume it is 120V, wire something else to it, fry it, then wonder what the heck you were thinking... (to put it politely).

 

[LarryFine]

Not to mention that 2, 5 or 10 years from now, someone will look at that circuit that is a Hot + Neutral and ASSume it is 120V, wire something else to it, fry it, then wonder what the heck you were thinking... (to put it politely).

Which is why the rule that the high leg be orange when a neutral is present.

 

[Hv&Lv]

Can you find a single phase breaker rated for 208??.

 

[ptonsparky]

Depending on details of panel type he may not find a properly rated two pole.

 

[oldsparky52]

Can you find a single phase breaker rated for 208??.

Would a 120/

In theory this could work, in practice it isn't done.

1) The loading on the transformers is such that this is frowned upon; the 208V current to the load must pass through (and load) both 240V and 120V transformer coils.

2) Very often single pole breakers for a 120/240V panel are rated for 120V to ground, so you might have to go to the expense of a double pole breaker anyway, just to get the proper voltage rating. Once you've done this, you might as well just run the 240V to the load.

-Jon

... and connect the load to the 120V phases, not a 120V phase and a 208 phase.

Typically I think these questions arise when there are no 120V spaces left in the panel.

 

[winnie]

... and connect the load to the 120V phases, not a 120V phase and a 208 phase.

I think that choice would depend on the nature of the supply. If you had a balanced 240V delta supply with a center tap on one side for 120V loads, then I'd use any pair of phases for 240V loads, adjusting for best balance. If you had an open delta supply with a big 120/240V transformer and a smaller 'stinger' to get the third phase, then I'd put a 240V single phase load on the 'main' 120/240V legs.

-Jon

 

[oldsparky52]

I've never seen a closed delta service. Plenty of open delta's, just never a closed delta. I know they exist, but it seems the PoCo's want to hook up 3 transformers in a wye instead of a delta. So, ... I admit, ... I was "assuming" an open delta.

 

[LarryFine]

... and connect the load to the 120V phases, not a 120V phase and a 208 phase.

There's nothing wrong with using the high leg for 240v loads that do not require the neutral.

 

[LarryFine]

I've never seen a closed delta service. Plenty of open delta's, just never a closed delta. I know they exist, but it seems the PoCo's want to hook up 3 transformers in a wye instead of a delta. So, ... I admit, ... I was "assuming" an open delta.

There are plenty of both here; Richmond is an old city.

 

[hillbilly1]

I've never seen a closed delta service. Plenty of open delta's, just never a closed delta. I know they exist, but it seems the PoCo's want to hook up 3 transformers in a wye instead of a delta. So, ... I admit, ... I was "assuming" an open delta.

Pretty common in small Industrial, the poco likes them because they can use smaller transformers, industrial customers like them because of the higher phase to phase voltage. The open deltas are mostly used on convenience stores where the majority of loads are single phase, but some three phase loads are present.

 

[winnie]

I've never seen a closed delta service. Plenty of open delta's, just never a closed delta. I know they exist, but it seems the PoCo's want to hook up 3 transformers in a wye instead of a delta. So, ... I admit, ... I was "assuming" an open delta.

That's because you are an electrician in the field and actually see these installations, whereas I am a pretend engineer doing research and remembering things I've found in old books

 

[LarryFine]

The 3ph open delta started out as a modification to existing 1ph services to supply then-new 3ph equipment like refrigeration. The "delta breaker" was invented for this situation; it was a3p breaker that was four bodies wide, but only had two bus clips. It picked up the two main phases as usual, but received the third phase via a terminal on the fourth body.

Also, a friend grew up in a house that had a small red wire strung with the normal three insulated wires (before triplex) to the house, and the small wire fed only a fused 3p disconnect that supplied only the AC compressor unit. If I remember correctly, they didn't even need to pass the high leg through the meter when they added a high leg.

Found some pics:

Source on right, panel on left:

 

[hillbilly1]

Never seen one of those, all of our open deltas were using regular three phase panels or loadcenters. Looks like they were using an existing single phase panel as you said, and using regular wire for the third “buss”. Interesting!

 

[Jraef]

Which is why the rule that the high leg be orange when a neutral is present.

Yes, assuming of course that the color coding on the single hot conductor makes it all the way out of the panel, and that the person in the future understands what that means when they see an orange taped wire and a white wire in the load connection point. My point was that weird stuff can be fine when installed and you are around to know what you did, it's problematic when you are gone.

Another aspect I forgot is that you must know HOW your 4 wire delta is sourced, because if it is coming from two transformers, not 3, the High leg to Neutral load cannot exceed 5% of the transformer rating in most cases.

 

[LarryFine]

You have to keep in mind the progression of electrical installations. Think of it like stoplights: we now know what kind of traffic control is necessary for given traffic intersection types and sizes, but we didn't always have, or need stoplights for roads with 5 lanes in each direction. We learned by trial and error as traffic evolved.

Nobody today would ask for a high-leg open-delta service (nor would you get one if you asked), but they came into use as the ever-growing need and availability of larger machinery mesh as they grew. Many buildings with electrical installations already existed as new and improved utilization equipment became available.

Which came first: 3ph power or 3ph equipment? Hmmm?

 

[LarryFine]

Another aspect I forgot is that you must know HOW your 4 wire delta is sourced, because if it is coming from two transformers, not 3, the High leg to Neutral load cannot exceed 5% of the transformer rating in most cases.

True. I sometimes show customers how to tell by looking at their transformers, making sure they're following their service drops.