Electrician Talk banner
21 - 29 of 29 Posts

·
Registered
36th year apprentice & Floor Sweeper
Joined
·
1,624 Posts
One thing not mentioned, that I always go by, is that a slash rating will require a neutral. So looking at equipment cuts, if it calls for 120/208 or 120/240, you better pull a neutral. If the equipment cuts say 208 or 240, then it doesn’t need a neutral. It should also specify single phase or three phase. So 120/208 single phase would need two hots and a neutral. If it says 208 single phase, then it just needs two hots. Same with any other voltage and phase setup. / = neutral in my book.
 

·
Bilge Rat
motors and controls.........
Joined
·
8,605 Posts
One thing not mentioned, that I always go by, is that a slash rating will require a neutral. So looking at equipment cuts, if it calls for 120/208 or 120/240, you better pull a neutral. If the equipment cuts say 208 or 240, then it doesn’t need a neutral. It should also specify single phase or three phase. So 120/208 single phase would need two hots and a neutral. If it says 208 single phase, then it just needs two hots. Same with any other voltage and phase setup. / = neutral in my book.
True for most stuff but something like a motor that's rated 230/460 doesn't need a neutral but something like a dryer does.

Also, a lot of stuff that has a slash rating will need to be reconnected for the different voltages where something that has a - (like 208-230) doesn't, it indicated a range of voltages. This is especially true on electronic stuff, if it's 120/240, you'll need to flip a switch or change wiring but if it's 120-240, it'll accept either voltage.
 

·
Chief Flunky
Field Service Engineer
Joined
·
2,545 Posts
The one people are dancing around is 120/240 high leg delta.

So with the “200ish” 3 phase systems there are 4 choices.

There is 208/120 eye which is a 5 wire system with a true neutral. Once in a while a few loads struggle with the low 208 voltage but balancing the neutral is usually easy.

There is 240 delta ungrounded. Popular in the 1950s. Please just don’t do it.

There is 240 corner grounded dekta, easiest way to fix 250 ungrounded. It’s grounded but one phase only and 120 is not available. Not a bad system, just not popular.

The other one is 120/240 high leg delta. There are two versions depending on if it’s an open or closed delta. With either one two of the phases are center tapped so you get 120/240 single phase center tapped. The third leg can be created with two more transformer coils in a true delta or with only one coil which is often just another single phase transformer. It’s a cheap method with say just one pump or HVAC unit but one phase has lousy impedance.

The same thing can exist at 480 plus resistance and reactance grounding but the split voltages aren’t that useful. I do occasionally run into 277 lighting.
 

·
Registered
Joined
·
4 Posts
The one people are dancing around is 120/240 high leg delta.

So with the “200ish” 3 phase systems there are 4 choices.

There is 208/120 eye which is a 5 wire system with a true neutral. Once in a while a few loads struggle with the low 208 voltage but balancing the neutral is usually easy.

There is 240 delta ungrounded. Popular in the 1950s. Please just don’t do it.

There is 240 corner grounded dekta, easiest way to fix 250 ungrounded. It’s grounded but one phase only and 120 is not available. Not a bad system, just not popular.

The other one is 120/240 high leg delta. There are two versions depending on if it’s an open or closed delta. With either one two of the phases are center tapped so you get 120/240 single phase center tapped. The third leg can be created with two more transformer coils in a true delta or with only one coil which is often just another single phase transformer. It’s a cheap method with say just one pump or HVAC unit but one phase has lousy impedance.

The same thing can exist at 480 plus resistance and reactance grounding but the split voltages aren’t that useful. I do occasionally run into 277 lighting.
I was learning about high leg delta from my ugly's reference book yesterday. Very interesting. It can do two legs of 120. One leg of 208. And three legs of 240. Is that correct. Most of your explanation of high leg delta went over my head, could you simplify and explain it once more please?
 

·
Chief Flunky
Field Service Engineer
Joined
·
2,545 Posts
I was learning about high leg delta from my ugly's reference book yesterday. Very interesting. It can do two legs of 120. One leg of 208. And three legs of 240. Is that correct. Most of your explanation of high leg delta went over my head, could you simplify and explain it once more please?
You can get 208ish but I’ve never seen it used that way.

So just draw a triangle on a piece of paper. Do it in pencil. Draw as close to an equilateral triangle as you can (3 equal sides). Mark the two upper sides 240. Now at the bottom draw a ground symbol in the middle of the side. That’s your ground/neutral. Mark the two half lines on either side as 120 and the whole side is 240 again. You can make these voltages with three single phase transformers where each one powers one “side”. The bottom transformer needs to be a center tapped 120/240 transformers. The sides do not have to be. They often are though and we just don’t hook up the center tap because we don’t need it.

The only tricky thing here is the “high leg” (top corner). At the bottom corners we get 120 V to neutral and 240 phase to phase just like a single phase system. But if you drew the triangle accurately it’s obvious that the voltage from the third corner at the top to neutral is a lot more than just 120 V. In fact it’s nearly double that. It’s 208. So you cannot just hook up 120 V loads anywhere. Only three phase loads should be hooked up to the high leg. Current NEC Code says it MUST be orange instead of the usual black red blue or brown orange yellow which is convention, not mandatory. Even then, ALWAYS test voltages to neutral if you are working with three phase/single phase mixed systems because it’s easy to mix them up or assume you are working with 208/120 when it may be 120/240 high leg or 240 corner ground.

Finally while the drawing is out just erase the line on one of the 240 legs. The VOLTAGE and phase angles (120 degrees) stay the same but you need one less transformer. Obviously since the third side draws power through two series transformers the impedance is doubled so it won’t quite act right but unless you have heavy motor loads most if the time this is a cheap way to get three phase. The remaining high leg transformer can be smaller and is just sort of a “helper” for the three phase load where often most of the load in an office is on the single phase 120/240 side.

This system may seem strange if you are used to everything is all symmetrical because it’s anything but.
 

·
Registered
Joined
·
4 Posts
Hand Handwriting Sleeve Gesture Finger

I've attached my drawing as you instructed. Did I represent the three single phase transformers with the bottom one center tapped correctly? What is the purpose of using these transformers in this kind of circuit? Does the impedance double because the transformers are connected in series? Would a wye connection avoid this impedance? What Is impedance?
Hand Handwriting Sleeve Gesture Finger
Hand Handwriting Sleeve Gesture Finger
 

·
Registered
Joined
·
4 Posts
I find it interesting that these drawing are attempting to represent voltage to scale. Both linear distances and angular representations of phase. I'm trying to understand how these drawing relate to real life systems which I assume are seldom shaped as a triangle or delta symbol. What I want to understand is: how does the inputs look, through what type of connections, wires, gaps and coils do they go through before becoming a usable output? Why must I hook up a three phase load to 208 as opposed to a single phase load to 120?
 

·
Registered
industrial E,I&C
Joined
·
3,764 Posts
I find it interesting that these drawing are attempting to represent voltage to scale. Both linear distances and angular representations of phase. I'm trying to understand how these drawing relate to real life systems which I assume are seldom shaped as a triangle or delta symbol. What I want to understand is: how does the inputs look, through what type of connections, wires, gaps and coils do they go through before becoming a usable output? Why must I hook up a three phase load to 208 as opposed to a single phase load to 120?
You need to understand what a ac sine wave is before some of this will make sense.

on a simple split phase 120/240 home circuit 180 degrees out of sync you will see that when one wave peaks in the positive while the other wave is also peaking into the negative. The peak of either wave is 120 from the center line. As we ground the center line we can now say that we have 2 legs of 120v to the grounded conductor (neutral) and leg to leg we have 240v. (remember voltage is pressure so your meter is not reading 2 waves it reading the pressure difference between the 2 waves. E.g its reading the pressure differential which will look like a 3rd sine wave).

A common misconception that was taught in school was that voltage flow was pushed which makes sense if your teaching basic DC to kids but that makes no sense that on AC. You have to think that the power generator is actually pushing and pulling. If one line pushes 120 and one pulls 120 then it makes sense that you have 240 between them.

Now if you look at 3 phase you will see the sine waves are 120 degrees out of sync. Pick a spot on one phase then compare it to sign wave of the other 2 phases and you will see why its 208 rather than 240v.

High leg delta we start with a standard split phase home transformer which gives us phase A , N and phase C. Now we add a extra transformer and connect one end to phase A which gives us pressure between phase A to phase B. As the waves are 120 degrees out of sync we end up with pressure between phase C and B with out having to add the third transformer.
The 208 ish was never really designed to be used on a high leg delta but that does not mean you can not use it. If you google the sine waves for each system you will see how the waves interact with each other.
 
21 - 29 of 29 Posts
Top