120/240 single phase vrs 3 phase..

[TNGuy81]

Hey im not sure this is the correct place to ask this question but I didnt know where else to post it. Im still new to this trade and learning and just ran across something I cant wrap my head around. I have worked with 120/240 single phase, 120/208 3 phase, 277/480 3 phase but never a 3 phase 120/240v system.. I always thought 240v was single phase and I dont really understand how this works with 3 phase or why for that matter..? Also is this a common method still used or is it a older Delta system. I do appreciate any info on this.

 

[LarryFine]

The 3ph voltages you mentioned are Wye systems, in which one end of the secondaries connect together as the neutral, and the other ends of each secondary are the line conductors.

A Delta system has the three secondaries connected in a triangle, which provides the three lines. If a neutral is desired, it is derived by grounding the center tap of one of the secondaries.

In a 240v 4-wire system, the secondary with the center tap is exactly the same thing as your 1ph 120/240v service. The high-leg Delta began as a modification to existing 1ph services.

The triangle is made by the third line, the "high leg." As with any system, each conductor's voltage to ground depends on the difference between it and the one we choose to ground.

 

[augie47]

Drawing may help.
If you have not already noticed (and many of us learned this the hard way ) one phase will be 208v to neutral.

 

[Rick Christopherson]

Here is a drawing I put together a few years ago. It is not intended to be a transformer configuration, just voltage representations.

 

[G._S._Ohm]

For the last one,
120 at 0 degrees plus 208 at ? degrees = the upper left 240 at how many degrees?
The right hand center 240 must be at 0 degrees.

Now I definitely need a drink.

 

[TNGuy81]

Hey thanks for the relpies and the diagrams. Its a little clearer now but Its still confusing to me.. Would prob make more sense to me if I was in the panel with a meter. Hot to ground, hot to hot, hot to neutral etc. What are the benefits to this and is this obsolete or still used? Why would you need 3 phase 240V i guess is my question?


The diagrams are like looking at hieroglyphics to me..

 

[LarryFine]

What are the benefits to this and is this obsolete or still used? Why would you need 3 phase 240V i guess is my question?

Start with the understanding that electrical systems have evolved over the years. We didn't always have stop signs and stoplights, but our experience has taught us which intersection types require which control without people having to die there first.

Imagine you own a little factory of some sort, and one day, a salesman comes by with the new-fangled Binford 2000 machine, but it requires a 3-phase motor because of the load. All you have is a 120/240v 1ph service.

The simple solution is to add one (open Delta) or two transformer(s) to the one you're on, add one conductor to your drop, replace the meter and base, add a separate 3ph disco for the new machine.

Now, you're being supplied by your original two lines for all your existing loads as before, and those two plus the new one provides the 240v 3ph Binford, and all with minimal disturbance to your business.

The two existing lines and the neutral form the previous 120/240v 1ph supply, and the three lines form a 240v 3ph supply, as well as three 240v 1ph supplies. The two are basically superimposed.

The voltages between either existing two lines and the neutral are still 120v, and the voltage between any two lines is 240v. The voltage between the new line and the existing neutral is 208v.

Today, almost nobody would start from scratch wanting a high-leg, but there are advantages; with a relatively small 3ph load portion, two transformers will work, and require only two primary lines.

 

[jim dungar]

Today, almost nobody would start from scratch wanting a high-leg, but there are advantages; with a relatively small 3ph load portion, two transformers will work, and require only two primary lines.

Actually we also see them when there are a large 3-phase loads and only minor single phase, like a center pivot irrigation system (large motor, small amount of controls).

 

[Mgraw]

Today, almost nobody would start from scratch wanting a high-leg, but there are advantages; with a relatively small 3ph load portion, two transformers will work, and require only two primary lines.

120/240 3ph is still common for new construction in some areas. Mostly in small commercial buildings. It depends on the POCO and the area.

 

[Little Bill]

120/240 3ph is still common for new construction in some areas. Mostly in small commercial buildings. It depends on the POCO and the area.

A lot of 3pf motors (industrial) were designed for 240/120. They run a little better and use less power. You can think of a bicycle and the two peddles. You only have two positions to start peddling. If you add a third peddle you would have a third position to start peddling your bike. Same with a 3 phase motor, you have a third connection which makes starting and running the motor easier. This is a very simple explanation that was taught me and helped me understand 3ph motors.
One thing you have to remember on a 3ph 240/120 system is, as mentioned, the high leg. If you were running a single phase 120v circuit you would need to check all the phases to find the high leg and not use it for that circuit.
Hope this helps.

 

[Mgraw]

I think it is a regional thing. When I was in the New Orleans area I worked for a company for over 20 years that specialized in convenience stores. All but 3 out of about 400 were 120/240 3ph, two were 1ph and one was 120/208 3ph. Where I live now very few electricians know what 120/240 3ph is. There is a small town about 30 miles away that still has 120/240 3ph. But what I find interesting is two POCOs are in both areas yet they do things differently.

 

[ecirplr]

Corner tap

Corner tap

I have also heard of a corner tap on a delta. Its is sometimes called a "deadman's leg". Never seen it. I believe it is strictly for 3 ph.

 

[kwired]

I have also heard of a corner tap on a delta. Its is sometimes called a "deadman's leg". Never seen it. I believe it is strictly for 3 ph.

You are probably talking about a system with a grounded phase. You could still operate single phase loads on this system but the loads would need to be whatever the line to line voltage is as there is no neutral in these systems.

 

[TNGuy81]

Ok I think I got a better grasp on it now thanks guys. Some great analogies in there as well, good stuff. Yea here its not common at all. Its for the most part all 277/480 or 120/208 in or commercial sector. I was looking at you tube videos and came across the 3 phase 240v panel and was like what??? SO I came here for expert advise. Much appreciated, though I still think if I could get my hands on one of these bad boys it would be 100% clear for me. That would suck to land a "120v circuit" on the high leg of the delta.... All the smoke would be gone!! That would be a stressful panel to make up. Hope all the numbers stayed on the wires!!!

 

[jumper]

That would suck to land a "120v circuit" on the high leg of the delta.... All the smoke would be gone!!

Been there, done that. 12 years ago I fried a couple of space heaters.

As mentioned before, you can find a 240/120 high leg delta in a residence. It is not limited to commercial.

 

[jumper]

Also see 240.85 for breakers in a delta panel.

 

[kwired]

Ok I think I got a better grasp on it now thanks guys. Some great analogies in there as well, good stuff. Yea here its not common at all. Its for the most part all 277/480 or 120/208 in or commercial sector. I was looking at you tube videos and came across the 3 phase 240v panel and was like what??? SO I came here for expert advise. Much appreciated, though I still think if I could get my hands on one of these bad boys it would be 100% clear for me. That would suck to land a "120v circuit" on the high leg of the delta.... All the smoke would be gone!! That would be a stressful panel to make up. Hope all the numbers stayed on the wires!!!

Not at all stressful to hook up. All you need to remember is that every third pole in the panel is the high leg, and you can not use MWBC's that use all three phases. There are delta systems all over the place around here - even new systems. Especially on the farms where maybe the majority of the load is three phase motors on grain handling facilities. The advantage over a grounded phase system is that you do have a neutral so you don't need a transformer to supply any 120 volt loads you may have which are usually somewhat minimal.

The open delta is handy for situations where there is only minimal three phase load and a lot of 120/240 loads. In those cases you do not want to use a three phase panelboard because one third of the panel spaces are wasted because you have no need for the high leg, instead install whatever is needed for three phase loads and install a separate single phase panelboard for all of the 120/240 load.

 

[TNGuy81]

Opps thats not good Here the little resi I have done ( my house and family, few friends) its all 120/240v single phase. I was chatting with my old boss and he told me its very rare in my neck of the woods but in some areas mainly commercial/industrial its still around, thats interesting to know about the homes though. What devices are they using 3 phase on in a residential unit?

 

[TNGuy81]

nm guess that was on my end

All you need to remember is that every third pole in the panel is the high leg

could this not change pending on the POCO or at the meterbase?

 

[jumper]

Opps thats not good Here the little resi I have done ( my house and family, few friends) its all 120/240v single phase. I was chatting with my old boss and he told me its very rare in my neck of the woods but in some areas mainly commercial/industrial its still around, thats interesting to know about the homes though. What devices are they using 3 phase on in a residential unit?

Larry would be a better one to answer this, it has something to do with the first A/C units that were used in homes. As I stated, I found out the hard way.