120/240 and 120/208 Volt Transformer Secondaries

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Explanation of how we derive 120/240 V and 120/208 V from various Transformer Secondaries.

PLEASE NOTE: The arrows above the left schematic show that this is one winding that has simply been tapped in the center. The waveforms below it in the lower left can give the wrong impression that the "halves" of the winding are out-of-phase with each other. This is because I'm using the neutral as a reference and as I mention in the video, from the perspective of the neutral you look "up-arrow" to one end and "down-arrow" to the other end of the winding. BUT, from either end (i.e: black wire to red wire, or vice-versa) you will be looking "up-arrow" or "down-arrow" for the entire length of the winding (i.e: in-phase).
You can dig through the comments for further discussion on this. I have made a video addressing the phase relationship within 120/240 V systems, and I hope to create other videos focusing on current flow in each of the scenarios that will further our understanding of these transformer configurations.
Thank you,
Dave

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This man is a master at teaching... so clear. his body movements, his eye movements, hands all work in coordination with each other. his voice, easy on the ears... His brain the conductor of the whole operation. A pleasure to learn from...Thank you Dave.

ronmurphy

I am electromechanical engineer, your explanation is very very clear. THANKS a lot teacher.

eduardoconde

Truly a master in explaining, makes it easier to understand the concepts

nassimzouaoui

As a EE, I have to commend you on a well diagrammed and spoken lesson. Good job!

rty

The 120/240V thing was actually a late 19th century thing - all derived from the Great Fire of Chicago. Edison lamps were made out of carbon, and used 110V, however, other companies figured out how to make lamps using tungsten, a much more resilient metal and could work at 240V (carbon lamps burn up at 240V). Since adoption of electricity was much slower in Europe, the European utilities went with 240V as it saved money on copper and is more efficient. Our 110V system was derived as a split-phase system, because Edison noticed that some people's lights were brighter than others. So he designed the system as two 110V generators (this was DC, mind you) and connected them together to form a third neutral line. He realized by monitoring the current flowing on the neutral line, he could determine how bad the imbalance was and rebalance the grid so the same number of lamps were on each half so everyone's lights were the same brightness. In addition, if you needed more power, you had 240V (DC) available.

Now, electrification was huge in the US - especially after the Great Fire of Chicago which meant everyone started from scratch, and skyscrapers were a thing. Gas lamps couldn't be lit more than a few stories up, and everyone wanted electric lighting. So much so that demand for light builbs was so big even though carbon lamps were obsolete, Edison was still making millions of them as the tungsten lamp makers couldn't keep up (in the late 19th century, they made around 10 million tungsten lamps, and Edison made nearly 60M carbon lamps. This changed about 20 years later in the early 20th century when carbon lamps were finally obsolete and replaced with tungsten lamps. However, by now the die has been cast - 110V had been around for so long, there was no appetite to change over to 240V..

In Europe, electric lamps were seen as a novelty, and very few people had it. The rich folks refused to install it in their castles, so when it came time, it was trivial to switch over.

Oh, and the first war of the currents was between Edison and Westinghouse, not Tesla. The second war of the currents was between GE and Westinghouse which dictated if we'd use two-phase (Westinghouse, via Tesla's patents) or three-phase (GE, Edison had been kicked out) AC. Tesla is confused in these because Westinghouse bought Tesla's AC motor patent (and later all of Tesla's patents). That patent showed how a motor would work at 60Hz two-phase. You have to remember though, that just because of this, power was still not quite standardized as you could get DC, 60Hz, or 133Hz AC power (133Hz because Westinghouse managed to find a design for an electric meter but that required 133Hz to run properly).

tlhIngan

I was so lucky to be in his classes. Dave, you rock, man.

abrahamjaime

Nice work Dave. This is one of the most thorough and understandable descriptions on 120/240 and 120/208....excellent.

_rich

Thank you, as a mechanical engineer I’ve always struggled with electrical concepts and this helped me understand much more about how the different windings affect voltage.

lxbanos

i feel lucky that people like dave are on youtube and shares their knowledge, great info!

BobbaFett

Dave your videos are so immensely understated. You're an absolute hidden gem and I can't thank you enough for making this material available. I'm an IBEW electrician apprentice in my 3rd year and I constantly supplement my school work with your videos.

zacharybob

You have simplified months of frustration in trying to understand these concepts. I cannot wait to check out the rest of your videos. I so appreciate the clarity you deliver. THANK YOU!

litocroy

Dave, I was looking for the definition of a great teacher, and now I got it on this YouTube video. You are beyond excellent and keep up doing this amazing teaching style.

tmengistu

This man is a truly gifted teacher!!!!

georgeswindoll

Other than you kept referring to the peak voltages as 120V which would be confusing to a beginner when actually Vp ≈ 170Vac or Vp-p ≈ 340Vac hence the need for capacitors with 370V or 440V ratings.

Root Mean Square is just the DC equivalent hence there would be a straight line drawn through the sine wave to represent that average. Vrms = Vpeak/√(2). RMS is the only way to average a sine wave in case your wondering. It's also used to take an average in statistics to minimum the effects of outliers 😉

- 1st drawing - Single Phase
- 2nd drawing - Three Phase (Wye)
- 3rd drawing - Three Phase (Delta)

I suggest adding "Ugly's Electrical References" book to your toolbox because it covers all this and much more in case you forget.

Great video 😊

weavercattlecompany

This is one of the best videos I have watched out of a lot out there that explains with this much clarity and expertise. Thanks for passing your knowledge how it should be with all.

hamskidoo