# Trying to learn more, ? on 4 wire commercial service



## dcorlfla (Oct 21, 2018)

Hi all, my first post to this forum. I work as a marine tech and have extensive low voltage experience. Enjoy learning and the other day I was looking at the electric service at the shop trying to figure out what we have there. I was hoping someone could help me ID the setup. Outside there are four wires going into the service mast, 3 hot and one neutral. On the poll there are two transformers, the larger one has three taps and I assume it is providing 120/240 split phase. The smaller transformer also has three taps but only two are hooked up. I assume it is providing the 208 volt high leg. The owner of the business next door saw me looking at the transformers and we got to talking. He says it is three phase and runs 3 p motors with it. I'm looking at the poll scratching my head - how could it be 3 phase as there are only 2 high voltage lines feeding the two transformers. I thought 3 phase required 3 high voltage feeder lines and three transformers. The building has 120/240 and there is a AC unit in the back that says 208 volt 1 phase on it. I'm not aware of any three phase in the building. So how is the neighbor running 3 p motors?

Thanks, Dale


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## MechanicalDVR (Dec 29, 2007)

A standard single phase (120/240) service actually has 2 hot legs and a neutral. 

A standard 3 phase service (208 or 480) is 3 hot legs and a neutral.


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## radio208 (Aug 27, 2014)

Your probably seeing an open wye primary / open delta secondary.
The high side uses only 2 hots and the primary neutral


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## dcorlfla (Oct 21, 2018)

MechanicalDVR said:


> A standard single phase (120/240) service actually has 2 hot legs and a neutral.
> 
> A standard 3 phase service (208 or 480) is 3 hot legs and a neutral.


Yes I understand 3 phase has three hot legs but what I do not understand is where or how are the three hot legs getting 120 degree phase separation with only two high voltage lines to the two transformers. One of the transformers does have a center tap but that would provide the split phase at 180 degrees apart. 

The building does have 3 hot legs but I'm not seeing how there could be the 120 phase separation that 3 phase AC would have.

???


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## 460 Delta (May 9, 2018)

It's a open delta, and it may or may not be in open star on the primary. Open deltas are kinda on the way out in part to the acceptance of 208 stars. A quick internet search, with pictures, will yield more info on open and closed deltas than I could ever type.


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## paulengr (Oct 8, 2017)

Start with 3 phase. Let’s say it’s 4160 V phase to phase. Now if we put a 4160:208 V transformer between say A and B we have 208. Now do the same between B and C so you get 208 again. What’s the voltage from A to C? Since we started balanced, it’s still 208 V. Now going a little further we can ground B (and it typically is) which makes it corner grounded delta. The advantage is the transformer is super cheap since you only need 2 sets of coils instead of 3. This is commonly used in substation metering and some very light duty commercial installations. If the high side is also corner grounded delta, the corner is a ground/neutral. There are big advantages to 4 wire wye industrially so that’s what’s in the text books but there are about a dozen different variations. 

Another common commercial system which you might have is 120/240 high leg delta and could also be what you see. In this system start with a single phase tapped transformer so you get 120/240 with the center tap as a grounded neutral. Now add another single phase transformer between either of the phases feeding the 240/120 transformer and the third leg with a 240 V output. This gives you 240/120 and 240 delta outputs again with an open (2 coil) delta. Optionally if the 240 service is higher demand a third transformer can be added or a true three phase transformer with both sets of outputs.

None of this looks like nice neat text book work and it’s not super efficient but when you just have say an HVAC load, cost trumps efficiency.


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## Forge Boyz (Nov 7, 2014)

I think this is what you are looking at. It gets you 3phase with only two primary lines and the nuetral.








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## Signal1 (Feb 10, 2016)

^^Bingo^^


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## radio208 (Aug 27, 2014)

Forge Boyz's diagram is a good one however the OP wanted to know why are only 2 primary phases are used to get 3 phase secondary. Here in northern and central CA converting from 12kv delta to 21kv wye means 2 hots and the primary neutral to existing open delta banks. Trns. would still need 12 kv to grounded neutral-not line to line.


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## Unionpride277 (Oct 31, 2018)

radio208 said:


> Your probably seeing an open wye primary / open delta secondary.
> The high side uses only 2 hots and the primary neutral





Forge Boyz said:


> I think this is what you are looking at. It gets you 3phase with only two primary lines and the nuetral.
> View attachment 129680
> 
> 
> Sent from my Moto Z (2) using Tapatalk



Is this corner grounded delta


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## telsa (May 22, 2015)

No.

The center-tap is grounded.

That's no corner.


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## frenchelectrican (Mar 15, 2007)

Unionpride277 said:


> Is this corner grounded delta


No As ForgeBoyz posted look very carefully it is centre grounded not corner grounded so that photo what ForgeBoyz posted is *open* delta 4 wire system and yes it kinda common in some area where the three phase load is small. common used in old resdentail area way back 70's era (that time they used three phase cental air conding unit but now single phase is more common for that size ) and still used in small to med commercal locations.

Yes some farm location used open delta too. 

The only golden rules you have be aware of open and close delta 4 wire is line to netural voltage wild leg connection ( typically about 190 to 208 volt range )


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