explain 120/208v - Page 2 - Electrician Talk - Professional Electrical Contractors Forum

nick · 06-23-2009, 06:25 PM

Well If a lamp was connected first to a dc voltage and then to a ac voltage the lamp will light up more brilliantly when connected to dc .This is do to dc voltage its constant it remains at lets say 100 voltages continuously whereas the ac voltage reaches 100 volts peak only at two points during the cycle .
In order for the lamp to light with equal brilliance on ac as well as dc we must raise the ac voltage to 141 peak volts .
Effectively then 141 peak volts of ac will light up a lamp as does 100 volts of dc . Thats were they came up with the formula E peak = 1.41 x Eeff

EFFECTIVE VALUE of an ac wave can be calculated from the peak value by the following formula Eeff= 0.707 x E peak

For all sine waves whether voltage or current the value given to an ac wave will always be the effective value unless stated otherwise .
AC voltmeters and ammeters will always read the effective value of the ac wave unless it is indicated otherwise .

Take care

Buddha In Babylon · 06-23-2009, 06:36 PM

To expound on what nick is explaining so brilliantly...
Effective voltage is also known as RMS which is an acronym for RootMeanSquare (which nick also mentions)....which is a laborious calculation of voltage readings at every angle of rotation in a sine wive...all 360 instantaneous values....The Root squared of the averages will give you effective voltage.... or is it the average root squared??? or the squared average root??? i dont know...its a complicated calculation but it works out, that 1.414 times rms will give you peak voltage...and peak voltage times .707 will give you rms... that is WHERE those values come from. RMS

nick · 06-23-2009, 06:51 PM

Well yes and yes RMS is what is read and its the effective value .

The meter takes many readings it averages these many readings to calculate the true R M S value . Take care

guitarboyled · 06-23-2009, 09:26 PM

According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipatingthe same power (heat) by an equivalent resistance. Your example explains it better.

But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).

nick · 06-23-2009, 09:50 PM

Quote:

Originally Posted by guitarboyled

According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipatingthe same power (heat) by an equivalent resistance. Your example explains it better.

But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).

Well you dont need to know unless you just want to know your meter does that for you .
The same value is current or voltage they are both treated as the same in the same circuit .
power companys voltage changes hundreds of different values during the normal day up or down .

Heres how i know some times we install a voltage current monitor kit day/week logger on a breaker or switchboard to monitor its resent problem or history like lots of tripping.

Our service dept will monitor a service and look at the results of the week to check out stuff the voltage changes on a paper graph of record voltages can vary from 480 volts down to say 450 then back up to 495 this is the power company loading the lines or switching the lines or just plain old generator generating .

Its lots of different voltage changes up or down my personal home service is coming in at 126 volts and 245 volts single phase its my poco supply .

Take care

John · 06-23-2009, 09:51 PM

Quote:

Originally Posted by John

Try this.....

Electricity is actually just simple geometry.

Here is how to explain a 277/480 VAC, 3 phase electrical system visually using geometry.

Using the scale of .1” equals 1 volt

On a floor draw a circle with a radius of 27.7” (27 ¾”) that is equal to 277 volts and mark the center of the circle with an “X”

Attachment 1473

For this next step a magnetic compass or a drawing protractor and a straight edge is required.

Attachment 1476
Attachment 1477

Draw a line from the center ”X” of the circle to the outside line of the circle. Then draw another line 120 degree from the first line from the center “X” of the circle to the outside line of the circle. Then draw another line 240 degrees from the first line from the center “X” of the circle to the outside line of circle. Label the lines A, B & C where they intersect the outside circle. Now it should look like this.

Attachment 1474

Now measure the distance from “A” to “C”

Attachment 1475

The measurement should be about 48” (converted to scale would equal 480 volts) depending on how accurate you draw and measure. The measurements from “A” to “B” and “B” to “C” should be about the same.

There are more complex mathematical equations but this is the simplest and a neat thing to do for someone new to the trade.

Carry On!

For those of you that haven't been around too long this is a rerun of the best of John.

And this will work for most of all 3 phase voltages.

Buddha In Babylon · 06-23-2009, 10:16 PM

hahaha the best of john...
yeah but i did get a kick out of that. never seen it before.

John · 06-23-2009, 10:20 PM

Quote:

Originally Posted by Buddha In Babylon

hahaha the best of john...
yeah but i did get a kick out of that. never seen it before.

Glad you liked it...but can you do it?

I did on lunch break on a large jobsite one time and most of the electricians on the job didn't understand it....

guitarboyled · 06-23-2009, 10:36 PM

Great explanations guys thanks, I’m getting a pretty good idea of the relation between the line current and the phase current.

120 / 208
277 / 480
347 / 600

But to get back to what Raider1 said, is the peak voltage really 170v in a 120v nominal supply line? If my service provider makes sure the voltage is never higher than 127v how can it reach 170v?

Wouldn’t be the other way around, the peak value supplied is 120v therefore :

Efficient Value (RMS) is only 85v = 120v/1.414 or 120v x 0.707

alimaj · 06-23-2009, 10:58 PM

Hi guitarboyled I would suggest you re read nick's posts they have the info you want. But just to rspond to your last post which was answered by nick in a previous post, you have NO CONTROL what the peak voltage is , the peak voltage is RMS *1.414 SO WHAT EVER IS THE RMS the peak voltage would be 1.414 times higher. If you rectify an AC voltage and connect a capacitor the capacitor voltage would rise to approx. the peak voltage.

guitarboyled · 06-23-2009, 11:14 PM

I understand that I have no control over the peak voltage but are you saying that when the supply company guarantees the voltage will always be between 106v and 127v, they are talking about Effective Value (RMS) and not Peak value?

That would mean that the peak can go up to 179v (127v x 1.41).

If this is the case, when do we use the peak value?

Therefore I also presume that all motor voltage are in RMS

alimaj · 06-24-2009, 11:26 AM

As stated in other posts AC voltages and current are RMS values unless stated otherwise, your multitester measures RMS values. Voltages going to motors are RMS values.

raider1 · 06-24-2009, 11:42 AM

Quote:

If this is the case, when do we use the peak value?

Peak value is not used in most applications, it is more a theroretical principal than a usable measurement.

The utility will supply you with a nominal 120/240 volt system for most residential systems. This voltage is expressed as the effective (RMS) voltage and not the peak.

Chris

guitarboyled · 06-24-2009, 12:48 PM

Thanks a million guys, sorry for being slow at grasping these concepts. We almost did no electrical at school, reason why I’m here acquiring as much knowledge as I can.

If there is any good book to learn basic electrical please feel free to recommend it.

I’m still not sure about the web site claiming that in a 3 phase unit the power delivered to the load is the same at any instant. Is it really? Doesn’t the RMS voltage oscillate constantly between 208v (position C) and 240v (position B) as illustrated in the previous graph? Or is that not how it works.

drsparky · 06-24-2009, 12:52 PM

You can read peak value with an oscilloscope, and then convert it to RMS. I don't know many who need to do it, but I have found problems with the AC power buy looking at the sign wave.

guitarboyled · 06-24-2009, 01:08 PM

For instance, when I need to dimension or size conductor (wiring), breakers, power consumption, etc. do I simply use the 208v value?

raider1 · 06-24-2009, 01:21 PM

Quote:

Originally Posted by guitarboyled

For instance, when I need to dimension or size conductor (wiring), breakers, power consumption, etc. do I simply use the 208v value?

Yes, when doing calculations for loads you would use the nominal voltage.

Here are a couple of NEC definitions and sections dealing with voltages.

Quote:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

Quote:

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.

Quote:

Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.

Quote:

110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Quote:

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
(B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.

Chris

guitarboyled · 06-24-2009, 07:30 PM

My next question concerns the Delta 3 phase configuration

In the Y (Wye or star) configuration:

Line Current = Phase Current
Line Voltage = Phase Voltage x √3

120 / 208
277 / 480
347 / 600

But in the Delta configuation:

Line Current = Phase Current x √3
Line Voltage = Phase Voltage

So why do we use the 120/240 nomenclature? Since there is no neutral in a delta configuration how do you go from 120 to 240?

nick · 06-24-2009, 07:58 PM

Quote:

Originally Posted by guitarboyled

My next question concerns the Delta 3 phase configuration

In the Y (Wye or star) configuration:

Line Current = Phase Current
Line Voltage = Phase Voltage x √3

120 / 208
277 / 480
347 / 600

But in the Delta configuation:

Line Current = Phase Current x √3
Line Voltage = Phase Voltage

So why do we use the 120/240 nomenclature? Since there is no neutral in a delta configuration how do you go from 120 to 240?

Well check this there is a neutral http://en.wikipedia.org/wiki/File:Ce...ransformer.svg

guitarboyled · 06-24-2009, 08:43 PM

Thanks Nick

So the center tap acts as a neutral…

I’m confused, you can get 120v, 208v and 240v all from the same configuration. I presume in the 3 phase mode voltage is 240v. When would get 208v? Is there such a thing as two phase?