120/208/230 240/220V ......

[PPC800]

I'm a mech engineer so this stuff is really confusing me.

How is 115V the same as 120V. I know about the 10% rule, but why is the nominal voltage not always used in discussions, if 120V is nominal why are some people calling it 115V?

Can someone list the types of voltages that are possible in a one phase and 3 phase-5-wire, 4 wire configuration?

Thanks for any assistance. I apologize in advance if my question is unclear... its due to my confusion.

 

[waross]

115V/120V There are two reasons for the difference, both are valid and true. Take ypour pick.
1> 120 volts and multiples of 120 volts are the standard voltages in the US and Canada. 120, 240, 480, and 600 in Canada.
2> 115 volts and multiples of 115 volts are the standard motor voltages in the US and Canada. 115, 230, 460, and 575 in Canada.
But there is also a historical reason.
Voltages used to be lower. In the US and Canada standard voltages were at one time multiples of 110 volts. (This is probably the origin of calling 240 volts 220 Volts.)
The industrial voltage was 440 volts.
The voltages were raised at the rate of abut 1/2 volt a year. 112 Volts became common, then 115 volts, followed by 117 volts and finally 120 volts.
For three phase voltages use the root of three, 1.73
120V x 1.73 = 208V
480V / 1.73 = 277V
600V / 1.73 = 347V


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PPC800]

OK that helps, so is 208V same as 230V?

 

[PPC800]

120V/208-230V 3 phase ...what is meant by this?

 

[waross]

120/208V is the standard 3 phase voltage. 230 volt motors were run on 208 volts for years but it is becoming more common to use 200 volt rated motors on 208 volt systems.
120/240V is the standard single phase system voltage.
230 Volts, where to start. 230 volts is a valid motor voltage.
In the days when delta power systems were in favor 230 volt three phase delta systems were common, (earlier these would have been 220 volt systems).
Now, if a three phase delta system is used it will use standard transformers and will be a 240 volt delta system.
But, be aware that there is a 4 wire delta system that is used regionally. In some areas I understand that it is common, in other areas it is rarely if ever used.
This is one of the delta systems that I mentioned, but one transformer has a center tap brought out and used. This allows 120/240V single phase loads to be used on the same transformer bank as three phase loads. These systems have what is called a "Wild" leg. One phase has a higher voltage to ground than the other two. It is not suitable for single phase loads, only three phase loads are connected to it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PPC800]

Got it! Thanks Bill!

 

[PPC800]

Just though Id add this

Single-phase voltage | Three-phase voltage (rounded)
100 173
110 190
115 200
120 208
127 220
220 380
230 400
240 415
277 480
347 600

 

[benta]

For 95% of Europe, you'll find that the standard line voltage is 230 V phase-to-neutral, 400 V phase-to-phase.

Benta.

 

[jraef]

One phase has a higher voltage to ground than the other two. It is not suitable for single phase loads, only three phase loads are connected to it.

Bill is, I believe, in Canada where I don't think they use this configuration much (if at all). But here in the US, it's still quite common. The purpose of it is for small commercial / light industrial installations where there is a small amount of 1 phase loads compared to 3 phase loads. For example, a small machine shop that has 3 phase motors on the machines, but also needs a few circuits for lights and computers etc. If the 1 phase loading becomes too much of the total power used however, it creates a significant phase current imbalance which then starts to cause other troubles. There is a general rule-of-thumb on the maximum size service where the utilities will consider 120/240 3 phase delta, but I can't remember off the top of my head. I think it may be 20% maximum 1 phase loads?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Compositepro]

I'm trying to understand why load imbalance would be an issue with wild leg(also called red leg) delta. The feed is from three single phase transformers connected in delta with the center tap of one transformer being the grounded neutral.

What would be the consequences if single phase loads reach 80% of one transformer and three-phase loads are 20%?

Are single phase 240 volt loads permitted to be connected to the non-center-tapped transformers? Obviously the phase-to-ground voltages would be strange.

 

[busbar]

Formally, [ANSI/NEMA C84.1-2006, American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz)] 120, 240, 480… is defined as the “service” voltage and 115, 230, 460… is the “utilization” voltage.

 

[davidbeach]

The purpose of it is for small commercial / light industrial installations where there is a small amount of 1 phase loads compared to 3 phase loads.

Gee, I would have described it just the other way. But in reality it can be both ways. If derived from a three transformer bank with three equally sized transformers it is probably intended to supply mostly three phase loads. If it is derived from a two or three transformer bank where one transformer is larger than the other (two) transformer(s) it is probably intended to serve a mainly single phase load with just a bit of three phase. Rather versatile source in many regards, but I'd still like to see them all go away and everybody take a wye service.

 

[waross]

Hi folks, poor wording on my part. I meant to say that the wild leg was used for three phase only, Some installations will use the wild leg for single phase, line to line loads, but in most installations the wild leg is only used for three phase.

Hi David; it is my understanding that the single phase loading of a four wire delta transformer bank, with three transformers in delta, is limited by the capacity of the smallest transformer. (subject to any adjustments for unequal impedances).
This statement is supported by the load sharing of the double delta connection used to convert a three phase generator to single phase service.
I have found the easiest way to describe this is to first consider an open delta connection.
Assume equal transformers.
If you have an open delta bank supplying "A" to "B", and "B" to "C", they can be considered as a virtual transformer at "C" to "A".
If you draw vector sketches of the voltage, voltage drops, reactive voltage drops and anything else that you can think of, the resultant in every case is equal to the same parameter of a single transformer from "C" to "A".
So, an open delta on "A" to "B" and "B" to "C" can be considered as a single phase transformer from "C" to "A".
The current of a single phase load will split evenly between the in phase transformer and the pair of out of phase transformers.
The single phase capacity of a three phase bank is twice the capacity of a single transformer and 2/3 the three phase capacity.
If any one of the transformers is smaller, that will determine the current limit of the bank.

Open delta does let you mix transformer sizes if need be.
But remember that part of the current in the "open delta" transformers is in phase with the third phase. That means that it is leading by 120 deg in one transformer and lagging by 120 deg in the other transformer. Now try to calculate the currents and transformer loading with a mixed single and three phase load with a bad power factor on the three phase load. The same phase relationships apply to 1/2 the current of a single phase load on a three transformer bank.
I have seen quite a few of these installations. They are usually well oversized. They have to be, because the practice there was to also connect the neutral on a wye primary. (Yes, I know, this is contrary to REA standards.)
As a result, most of these banks spent a good part of the time with one primary fuse blown running open delta with one transformer along for the ride.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I owe you an apology, David. You are correct. The smaller transformers will have a greater impedance and will share the current in the inverse ratio of their sizes.
I agree, I like wye a lot better.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[busbar]

 
PCC800, I suppose this is information overload, but for the US ANSI/NEMA standard C84.1 is the “holy grail” of acceptable voltages; generally agreed upon by electric utilities and electrical-equipment manufacturers.

http://6L6.net/elm/etip.ansi-nemaC84.1.Table1xrpt.xls,

and

http://6L6.net/elm/etip.ansi-nemaC84.1.NotesTbl1xrpt.xls