Was told by power co that there is open-bank, wild-leg, 3-phase available at a property. Is this the same thing as open-delta? There are only three separate conductors on the pole, each with ceramic insulators, so I had assumed it is. Then got to wondering - can anyone shed any additional light on this? Is this satisfactory for running a shop on?
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Open-bank, wild-leg, 3-phase
- Thread starter VSMI
- Start date
- Replies 4
- Views 4,085
Open Delta 120/240V 3Φ 4W Services
If there is only 3 conductors on the service drop, it would be ungrounded 3Φ or corner grounded 3Φ. But my guess is your not counting the bare support conductor, which also acts as the neutral or the fourth conductor.
The wild leg is 208V with respect to neutral, the other two legs are 120V with respect to the neutral. The system was common in older districts but has been generally phased out in preference to Y systems 120/208 or 277/480V.
The disadvantage of it is two things.
First it lends itself to unbalanced loads on the utility system. One transformer pot carries all of the single phase loads and 2/3rds of the three phase loads, and is usually larger to accommodate this. The second transformer pot only carries 1/3rd of the three phase load and is normally smaller.
Second is the user needs to be careful not to connect any 1Φ 120V circuit loads to the 208V wild leg third phase. To prevent this from happening it's best to have two main panels, one 1Φ and one 3Φ, then it's less likely to connect a 120V load to the wild leg. If there is only one 3Φ panel every third connection on the bus is 208V and cannot be used for 120V loads. Meaning you will loose some circuit slots for 120V circuits.
The advantage to it was that both 1Φ and 3Φ loads could be supplied from only 2 transformers, one of them being smaller than the other. The utility took care of the unbalance problem by alternating the primary connections of each set of distribution transformers on the local network.
Other than the things mentioned you will have no problems running your equipment on it. In my area it's still being used widely, in older districts.
SAF Ω
If there is only 3 conductors on the service drop, it would be ungrounded 3Φ or corner grounded 3Φ. But my guess is your not counting the bare support conductor, which also acts as the neutral or the fourth conductor.
The wild leg is 208V with respect to neutral, the other two legs are 120V with respect to the neutral. The system was common in older districts but has been generally phased out in preference to Y systems 120/208 or 277/480V.
The disadvantage of it is two things.
First it lends itself to unbalanced loads on the utility system. One transformer pot carries all of the single phase loads and 2/3rds of the three phase loads, and is usually larger to accommodate this. The second transformer pot only carries 1/3rd of the three phase load and is normally smaller.
Second is the user needs to be careful not to connect any 1Φ 120V circuit loads to the 208V wild leg third phase. To prevent this from happening it's best to have two main panels, one 1Φ and one 3Φ, then it's less likely to connect a 120V load to the wild leg. If there is only one 3Φ panel every third connection on the bus is 208V and cannot be used for 120V loads. Meaning you will loose some circuit slots for 120V circuits.
The advantage to it was that both 1Φ and 3Φ loads could be supplied from only 2 transformers, one of them being smaller than the other. The utility took care of the unbalance problem by alternating the primary connections of each set of distribution transformers on the local network.
Other than the things mentioned you will have no problems running your equipment on it. In my area it's still being used widely, in older districts.
SAF Ω
Thanks, SAF. Definitely only three conductors and no bare support wire or any other kind of fourth wire. I should have taken a picture - the ceramic insulators don't look like anything modern.
After looking at your schematic again - this would be a 240V service, correct?
After looking at your schematic again - this would be a 240V service, correct?
Yes the diagram is for a 240V 3Φ 4W.
My second guess that you only have a 1Φ 3W 240V service existing, but the 3Φ 4W system is available on the local utility network with a building service upgrade and an additional drop conductor added. I would suggest that you check the panel in the building, is it 1Φ or 3Φ? The utility could tell you what is installed currently. When they told you, "open-bank, wild-leg, 3-phase available at a property" that may indicate that it's only 1Φ currently, but could be upgraded?
The 1Φ service would be the same as in the diagram but missing the top transformer and it's connections.
SAF Ω
My second guess that you only have a 1Φ 3W 240V service existing, but the 3Φ 4W system is available on the local utility network with a building service upgrade and an additional drop conductor added. I would suggest that you check the panel in the building, is it 1Φ or 3Φ? The utility could tell you what is installed currently. When they told you, "open-bank, wild-leg, 3-phase available at a property" that may indicate that it's only 1Φ currently, but could be upgraded?
The 1Φ service would be the same as in the diagram but missing the top transformer and it's connections.
SAF Ω
If it is "available", the pole should be carrying 3 phase on it. That does not guarantee availability, across the street is 3 phase, but it is never connected to a residence by the local powerco.
Different for farms, businesses, etc. Powerco will hook them up if asked.
Check your wires. The 3rd should be bare. As far as I know the powerco will NEVER EVER connect 3 phase with no ground, even if it is not a neutral.
Check the service entry... if it goes into conduit, you can't tell, but if it goes to service entry cable, a neutral will be connected to the "shield" of teh cable and not to an actual insulated wire.
Then also, of you had 3 phase, your breaker panel would have 3 buses, and likely some 3 section breakers. If you had 3 phase corner grounded (no neutral), you would have to have a transformer to get 120V.
I also suspect you have regular 240 center tapped, and you "can get" 3 phase. You may NOT want it, because it often comes with a whole other set of billing issues.
Different for farms, businesses, etc. Powerco will hook them up if asked.
Check your wires. The 3rd should be bare. As far as I know the powerco will NEVER EVER connect 3 phase with no ground, even if it is not a neutral.
Check the service entry... if it goes into conduit, you can't tell, but if it goes to service entry cable, a neutral will be connected to the "shield" of teh cable and not to an actual insulated wire.
Then also, of you had 3 phase, your breaker panel would have 3 buses, and likely some 3 section breakers. If you had 3 phase corner grounded (no neutral), you would have to have a transformer to get 120V.
I also suspect you have regular 240 center tapped, and you "can get" 3 phase. You may NOT want it, because it often comes with a whole other set of billing issues.
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