Explained! The Delta High Leg (Center Tapped) 4 Wire Transformer (Electrical Power PE Exam)

In this video, we review a four-wire, three-phase, 240 volt delta connected transformer secondary with a 208V "high leg" created by center tapping the B phase winding.
Do you know what the most common mistake is when calculating the high leg voltage magnitude from a four-wire delta transformer with a center-tapped phase winding?
It is making the mistake of NOT using complex numbers (only using magnitudes), or, forgetting to include the angles in your calculations.
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[@electricalpereview]

0:05 - Problem statement 0:17 - Setting up the transformer circuit values 2:21 - Setting up the center tapped winding voltages (VBN and VNC) 5:44 - KVL equation to solve for the high leg voltage (VAN = VAB + VBN) 7:29 - Determining the voltage rating of the high leg delta transformer 7:50 - Reviewing the "trick" to this problem 8:08 - Delta high leg applications and purpose (theory)

[@germanaleyva8128]

Thank you for the tutorial high leg 4 wire delta transformers

[@electricalpereview]

glad it helped!

[@cdiology14]

I'm actually from Jamaica and I must say that, I really apprecilove this video... because many times, even here, a lot of electricians don't know why, or how we have these varied voltages with a delta or a star secondary connected winding transformer. So, i must say again, keep on doing this field a great service by dispersing knowledge like this! Really fulljoyed your video. Blessings and respect!

[@electricalpereview]

Thanks for the kind words, glad the video has helped, didn't realize that open delta was that common in Jamaica!

[@offgridsystems1166]

Electrician here, this was excellent, thank you

[@electricalpereview]

Thanks, glad you enjoyed it. Any other topics you would like to see?

[@ChoompMedia]

This is by far the best explained tutorial of a high leg 4 wire delta transformer I've ever come across. Thanks for spelling it out for us and walking us through how to obtain the three voltages. Thanks Zach - your PE Review course is by far the best I've come across.

[@boxedtheatre]

This is literally one of Zach's best videos, I even shared it with my coworkers. It's easy to get confused on the high leg transformer, and what that means for ratings and wiring.

[@josueayalallanas7142]

Oh god! The explanation i eas looking for months on youtube, i finally found it. This gold for me thabk you. Keep on doing what you do this is great! Ive been learnng a lot thru youtube for the last months, thabk you thank you thank you

[@electricalpereview]

My pleasure.

[@aaroncurry6281]

OMG. I work in the electrical utility industry and not until after watching this video it really made sense. I mean I knew the basics of high leg delta transformers, but this video really breaks down the HOWs and WHYs behind it.

[@danbeesmer8554]

Zach has made the center tapped transformer finally make sense. In general, most books are very confusing. This video makes it very easy to understand.

[@MoisesFeliz]

It was not until I found this video by Zach that I really learned about the topic - he explains it very clear and simple!

[@nasheetarman1908]

Excellent explanation of High leg delta. See this in older substations a lot and your explanation helped.

[@kaels2012]

This is a great explanation on a high leg delta system, and I've used it to help explain the center tap to some of my trainees! Thanks!

[@therealprofessor976]

Amazing explanation!

[@electricalpereview]

Glad you enjoyed it.

[@sunilpsoni5]

zach, this is a very simple explanation useful for a quick lesson

[@AIArchiveLabs]

Very helpful to see it broken down in each part to show the different voltage and magnitudes. Sometimes the terminology can be a little tricky.

[@Sparky-ww5re]

Great video! You see this sometimes in commercial installations, usually older facilities. A very rare high leg delta also exists, most experienced electricians will never have worked with it but only know they exist, and it is a 480/415/240 volt high leg delta, the voltage between A - C phase and neutral is 240 volts, the high leg to neutral is 415 volts. Basically identical to the 240 volt high leg delta, but the voltages doubled. Interestingly enough, you are allowed by the NEC to install a high leg delta system even today, but are kind of pointless.

[@electricalpereview]

Thanks for your comment @Kyle 1500! I still see high leg delta's in older farm installations where it is cheaper to have a single-phase and three-phase power source of different voltages for a small number of loads compared to two separate power supplies.

[@zhumusic-ng9tr]

Man this helped alot.

[@electricalpereview]

Glad to hear it

[@y000tooob]

Awesome video thanks! It definitely makes it look simple compared to other sources out there.

[@ibrahimelsharkawy1617]

great explanation thanks

[@electricalpereview]

Glad you liked it.

[@sohailjanjua123]

Hi, I like your video. Thanks

[@aymanantoun2083]

Very helpful I greatly appreciate your help and time for your great post 👍

[@liger88OZQ8]

This sth new to me, this video gave me a good understanding. I like the videos on this channel and hope for more

[@MotorsportsX]

can you explain neutral and ground connections when using these down stream in a plant? Im curious how a step down transformer (480 to 240) is connected in regards to the neutral connections, and how to prevent ground loops.

[@soushians555]

What software/ hardware you are using for writing?

[@ApplianceMan96]

What is the benefit of having 208? Is it just there as a natural option due to how the transformer is tapped? Is there a reason to use a 208 circuit over 240?

[@electricalpereview]

Some loads are rated for 208 volts, others are rated for 240 volts. The delta high leg gives more flexibility in offering more than voltage from the same transformer.

[@sidneychotalal7807]

Love this

[@Sparky-ww5re]

9:10. Normally you never connect the high leg to neutral to supply a 208 single phase load, the exception being certain fluorescent and high intensity discharge fixtures in commercial buildings. There are a few problems with doing this. First off the breaker must have a straight rating, meaning 240V, not 120/240, and to my knowledge and research, single pole, 240 V breakers do not exist. I guess you could use a straight 240V rated two pole breaker and just use the high leg side, or use both poles with the other side feeding another 120 volt circuit. What makes a 3 phase 4 wire delta a sort of dangerous system, is it's very easy to mistakenly connect a single pole breaker to the high leg and burn up equipment. In addition load balancing tends to be poor. Also many 240v loads will operate satisfactory with 208v, many air conditioners are rated 208/230V for example. For these reasons high leg systems are rarely installed today, instead the most common is 120/208Y, or 277/480Y with a 480 - 120/240 transformer for running smaller equipment while larger equipment can run off 277 or 480.

[@electricalpereview]

Agreed. Typically you only see high leg delta in much older installations or extremely specific installations, however, the PE exam is still expected to test on this form of transformer connection so it is important to be familiar. Thanks for your comment.

[@sethtaylor5938]

The winding with the neutral is used for general convenience outlets. The 240 delta is for large machines and equipment on the premises. Often the "lighting pot is at least 50 KVA and the wing points are minimum of 25 KVA. ST, ex First Energy JCP&L

[@electricalpereview]

Thanks Seth! I agree. The best example that most are familiar with is their own residential home here in the United States. 120 volts for all lights and receptacles, and 240 volts for HVAC, and Washer/Dryer.

[@jolyonwelsh9834]

You can also use basic trigonometry or Pythagoras theorum to calculate the voltages. Although it would only give you the magnitudes.

[@shawnwaterssw]

Great Video.

[@electricalpereview]

Thanks Shawn, glad you enjoyed.

[@Whizkid61438]

So what was the calculation used to actually come up with the 208 volts? When combining the 240V

[@electricalpereview]

That is the calculation, it is the sum of two complex numbers in polar form. Most approved calculators for the PE exam can perform this without interruption. If you're not studying for the PE exam, and not familiar with complex numbers or summing polar numbers, then you'll have to convert the two polar numbers in your comment from polar to rectangular first before you can add them together, then convert the result back to polar to get the final voltage magnitude and angle. Here is our complex number KZbin Playlist: kzbin.info/aero/PL8UEnCgTyXQv4Z9pM4WpsoXvG3YLUUmUu The first video goes over how to manually convert between polar and rectangular and vice versa. The following videos in the play list give examples with numbers. These are some of our older videos so the quality is not as good as our newer content, but they are still great videos to learn from.

[@MINHAJUDDINKhan]

Is there any further explanation how AN is 208...where AC is 240 and CN is 120 or AB is 240 and BN is 120.

[@jessicagriffin6836]

I got lost at how adding 240v with 120v < -120° equals 208. I thought that the extra resistance of going through more wire caused the drop in voltage. But it has to do where the other phase is? So, the other phase is taking the power somehow? Really confused. Is there a better video to explain this.

[@electricalpereview]

The increase in voltage comes from the additional voltage source from B to N in series with the voltage source from A to B.

[@LaminarSound]

I guess I don’t understand “complex math”. How do you derive 208v from adding 240 + 120? That’s hanging me up.

[@electricalpereview]

You have to include the phase angles. Complex numbers in polar form have both a magnitude and an angle, you can't just add the magnitudes together you have to include the angles. This is the same as adding two vectors.

[@LaminarSound]

@@electricalpereview ok. I still don’t see how the math comes out to 208v. Even including the angles. How do you factor the angles in and come to 208? Do you have another video where you explain that part more?

[@electricalpereview]

@@LaminarSound Most approved calculators for the PE exam can perform complex number addition in polar form without interruption. If you're not studying for the PE exam, and not familiar with complex numbers or summing polar numbers, then you'll have to convert the polar numbers to rectangular first before you can add them, then convert the result back to polar to get the final voltage magnitude (208V) and phase angle (-30 degrees). Here is our complex number KZbin Playlist: kzbin.info/aero/PL8UEnCgTyXQv4Z9pM4WpsoXvG3YLUUmUu The first video goes over how to manually convert between polar and rectangular and vice versa. The following videos in the playlist give examples with numbers. These are some of our older videos so the quality is not as good as our newer content, but they are still great videos to learn from.

[@bobbrumley3964]

@@LaminarSound Maybe this helps, mark on the ground your standing on your start location. Walk 120 inches in any direction, turn 120 degrees and walk 120 inches, mark that location. Now measure start location to finish location and it will be 208 inches. Hope that helps!!!

[@fethilakhdari1078]

Unfortunately, you did not show the most challenging and interesting question, namely how do we get the 30 Degree phase angle. This takes away a lot from the quality of the video.

[@electricalpereview]

Hi Fethi, thanks for your comment. If you are referring to the 30 degree angle at 11:48 for VAN, then the answer is that it comes from the sum of the complex voltage VBA and the complex voltage VBN including both magnitudes and phase angles. This channel is to help electrical power engineers with the NCEES® Power PE Exam, complex math is a fundamental component of the majority of power PE exam questions. If you are new to complex math with complex numbers, or if you are not an engineer getting ready to take the PE exam, then there are other videos available on our channel that review the basics of complex numbers in both polar and rectangular form and how to multiply, divide, subtract, or perform addition with complex numbers.

[@Jaybaby153]

Wow I went through a 5 years of a program and it just seems like he's making it more complicated than it needs to be and you always put b phase on the highlight side and it has to be configured with an orange piece of tape

[@electricalpereview]

Hi Jason, are you studying for the Electrical Power PE Exam for engineering? This video reviews how to calculate the different voltages available with a center tapped delta winding due to the complex phasor relationships specifically in ways that you can expect to be tested during the PE exam.

[@drvinson8947]

Thought this was called a wild leg. 🧐

[@electricalpereview]

Goes by lots of names in the industry. Wild leg, ghost leg, high leg, etc

[@drvinson8947]

Electrical PE Review Not arguing, I just thought there was a difference between a high leg, wild leg and stinger leg. Are all these the same or is there a difference? Could you explain? Thanks 🙏🏼

[@electricalpereview]

@@drvinson8947 Same thing, just a different name.