I have 3 phase power that has a high leg of 211 volts to ground. The other two legs are 120 volt to ground. Is it fine to run a three phase machine onthis voltage?

 

This arrangement is known as "bitch leg" or "stinger leg" delta. The middle of one leg of the delta is the ground point, rather than the center point of the star, as in Y-secondary transformers. The source will appear as a 240V three phase delta to the load.
--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

I'm guessing this is a 3 phase Y connected 208/120 system. In this system, you get 120 volts between neutral and a leg. If you want 208 volts, you read Line-to-Line. (Neutral is connected where the three lines that form the letter "Y" meet)

Are you sure you're looking at ground?
The only other possibilities are major 3 phase imbalance or a funky/unusual transformer design with taps as you'd listed. I'm not familar with systems other than the most common ones (120/208Y, 480V Delta, 480V/277V Y). I guess it could be a delta system with the neutral tapped halfway between two phases (the two you see 120V on).

If the machine you're connecting is truely three phase (doesn't use a neutral, just Phase A, B, C, and ground), all that should matter is the line-to-line voltages. You need to measure them to be sure it matches nameplate of the machine.

Hope this helps. Good luck and don't blow anything up!!

Jim Zupich
[email protected]

 

I'd hesitate to say yes to "machine" as some may rely on balanced leg voltages. But 3 phase motors and anything without a neutral connection should never know the difference. Here in the US. many factories have this setup.

Regards

cww

 

I have a similar situation and can't say with any degree of certainty that that a "high leg" won't burn up my motors. I feel that the best solution is to install a "Y" transformer in order to achieve balanced power.

D.L.

 

Without a neutral reference, it _is_ balanced power. But there might be an issue with insulation as there is higher voltage to ground. But this has been done in many installations as an economy measure. I don't know if it's common any more as it does unbalance loads from the power
company's perspective. Lots of small shops have it.

Regards

cww

 

Curt,

The wire used in buildings (THHN, etc.) or for power wiring in machines (MTW, TEW) is all rated for 600 volts, so a measly 208V to ground is no problem at all.

Steve

 

Hi Steve

I was more concerned with the motor insulation. Wire insulation has a much higher safety factor.

Regards

cww

 

It's called a "B-High" leg and is not uncommon in rural areas.

 

yes, this is a good source for 3 phase 240 volt loads. you have a 3 phase 240 volt delta tapped winding. be careful with any single phase loads you supply with this as you can get that hi leg onto a 120 volt system and burn up equipment.

 

I am having the same issue in a way. My building has a high leg and we are hooking up a machine that needs 208Y. I am having to buy a converter to get it from 240 down to 208. If it reads 212 with a ground, shouldn't there be some way to use that for 208? It may sound stupid but I thought it made since rather than buying a transformer. The machine installation tech is used to seeing all three legs being 120 and says if they are not then you have to buy a transformer to convert or "step down" to 208Y.

[email protected]
Chad

 

This is normal for Created 3 phase power. The Hi leg is a lighting leg and should be about 277v.
That's were 277 volt lighting come from.
Hook the line between the three phases, not ground or neutral.
All should be well.

 

I'm having trouble following all that. can you say it differently?

 

So the high side is 277v to ground and the other legs are 120v. so from leg to leg it should be? 208v or 220?

 

Excuse my saying so, but some of the information offered in this thread is not only completely (and dangerously) wrong, it also gives a not too rare insight into the incredible level misunderstanding of a subject capable of
killing the person taking the advice as dead as a hammer!

I know this is an old thread, but I can't stay silent any longer.

In an area like this, if you don't know what you are doing, don't ask for help here. Get some education or hire a credentialed professional.

My opinion and my opinion only. Not that of my employer.

David McConnell

 

Responding to Anonymous' query:

(1) The 'high-leg' 3-phase system is more properly referred to as a center-tapped (high-leg or red-leg) 4-wire, delta system. It is important to note that this system finds favor if the 120 V 1-ph, loads are small compared to the 240 V, 3-ph, loads.

(2) Visualize the delta-connected secondary of a 3-phase transformer. The terminals are A, B. & C. Conductors connected to the three terminals are labeled ph-A, ph-B, and ph-C, respectively.

(3) Although a misnomer considering the IEEE definition of the term "neutral," the center-tap (midpoint) of the A to C phase winding is labeled N. It's conductor is also labeled N. (A more correct label is the groundED-conductor.) At this point the N terminal is connected to ground or earth. Phase B is now considered the 'high-leg' or 'red-leg'.

(4) If the rated phase-to-phase voltage of the transformer secondary is 240 Volts, then the corresponding terminal-to-terminal voltages follow:

Phase-to-Phase
A - B, 240 V.
B - C, 240 V.
C - A, 240 V,

Phase-to-Neutral.
A - N, 120 V.
B - N, 208 V. [Sqrt(3)/2]x[Vph-ph]=the 'high-leg')
C - N, 120 V.

Please note that the above system is not to be confused with the corner-grounded delta system. That's another topic!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

You've got a problem in your trig. Try drawing it to scale on paper.

 

Phil,

You can't tell it because the threads aren't indented anymore, but my reply about the trig was for Anonymous' Jan 11 post, not yours of Jan 14. Sorry if there was any confusion.

Steve

 

Steve, an apology is unnecessary.

I thought, based on David McConnell's post, that a more complete response was in order. Hopefully, it will clear up some obvious (and dangerous) interpretations by one or some of the anonymi.

An aside... I'm not sure about the plural of anonymous... is it "mi", "mous'", or "mouses". Same dilemma for caboose, goose, or moose?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

I think this person has a three phase 120/240 with a high leg. it's not 480 which would give you 277 on a single phase. Your question is yes, you can hook up a 240 v three phase device up if it indeed is a 240v not a 480 volt device.