# Can someone explain the secondary of a 120/208v transformer to me?



## speedfreakian (Aug 29, 2010)

A 120/240v single phase transformer makes perfect sence to me. the neutral comes from the center tap of the 240v winding. but obviously the neutral on a 120/208v system doesn't come from the center of a 208v winding, you'd have 104 volts. someone please explain!


----------



## 480sparky (Sep 20, 2007)

It's all in what's called 'electrical math'.

Single-phase is 120/240 because it's really just a 240v circuit with a center tap.... both 'legs' are 180 ° (electrical degrees) apart.

But with three phase, each leg is 120° apart, so the two voltages are not a simple addition problem.

Since you're dealing with sine waves, you use the SINE function. So the Sin(120) = 0.866025. Multiply that by 240, and you get 208.

However, with only one phase of the system, you don't have any ° relation to anything else, so you end up with 120 volts.

You may find a better explanation here.


----------



## wildleg (Apr 12, 2009)

208/Y120 is a three phase transformer, not a center tapped single phase transformer like 120/240. look up transformer on the web, you will find a wealth of reading material.


----------



## Bob Badger (Apr 19, 2009)

Does this help?










The neutral is the one on the right connected to all the windings.


----------



## 480sparky (Sep 20, 2007)

Another way to look at it is this:











You will notice that only one phase peaks at a time, while none of the others are directly opposite of it.

Basically, the peaks & valleys are, in order, -Red, +Black, -Blue, +Red, -Black, +Blue. A + and - are never directly in line with each other.


----------



## Jlarson (Jun 28, 2009)

Try drawing a phasor diagram, it really helps some people understand. You need a pencil, ruler, protractor, and paper. To start place a dot at the center of the paper then draw a line straight up from that 1.2 inches long. 

Then from the center point use your protractor and make a mark 120 degrees to the left of the center and 120 degrees on the right of the center. Draw a 1.2 inch line from center out to each of these points. 

Now label your points A, B, C going around clockwise and label the Center point N. Now measure from A-N, B-N, and C-N and multiply you measurements by 100. The measure A-B, B-C, C-A and multiply your measurements by 100.

This may be hard without a good ruler.


----------



## speedfreakian (Aug 29, 2010)

480sparky said:


> It's all in what's called 'electrical math'.
> 
> 
> 
> But with three phase, each leg is 120° apart, so the two voltages are not a simple addition problem.


Thanks! that makes perfect sense.


----------

