Odd question for neutral connection

Location
Texas
Occupation
Electrician
Ive got a 120vac sources stepping down via autotransformer to 24VAC, then stepping back up via autotransformer to 120VAC. Can I bond my neutral from 2nd 120v back to source neutral. Also I wont be using the neutral from 24v transformer. Should that also tie back in with 120 source neutral?
 

retirede

Senior Member
Location
Illinois
If I am envisioning what you are saying correctly, I don’t see how you could have a complete circuit without having the neutrals all connected.

And why would you want to do this anyway?

 
Location
Texas
Occupation
Electrician
OK, so there is no problem connecting a neutral from 24v source to neutral from 120v source, not a problem but actually required to complete the circuit.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
If you are using an auto-transformer for step down (120 to 24V is an awfully big range), the high side neutral conductor is automatically commoned with one of the LV conductors. (see retirede's sketch)

What do you really have?
 
Location
Texas
Occupation
Electrician
0-130VAC transformer, im using 24v as a guide to the question, the second transformer is my own design that I will adjust until I get 120v from it. Home experiment to help me understand reactance a little better however the neutral connection is the only thing I am having trouble with. And I think yall have already answered my question. That not only can you connect neutrals from the different voltage sources bit in this instance it is required to complete the circuit. Am I right or did I misinterpret?
 
Last edited:

LarryFine

Master Electrician Electric Contractor Richmond VA
For a permanent installation, and if you're using a standard (isolation) transformer, the secondary would be a separately-derived system, and the "neutral" should connect to an electrode system, not the supply grounded conductor.

If you're just experimenting, I wouldn't worry about it. If you're using an auto-transformer, the grounded conductor extends until you end it, and shouldn't be used as a means to bond to earth ground an isolated secondary circuit.
 
Location
Texas
Occupation
Electrician
If I'm hearing you correctly. You are saying that the neutral coming from the variable transformer I will have set to 24 volts, should connect to Earth ground. While my second transformer with 120 volt secondary should then be tied back to The Source neutral.
 
Location
Texas
Occupation
Electrician
I don't know what I'm saying. That doesn't make any sense. The 120 volts from the second Transformer connecting the neutral back to the neutral from source Doesn't seem like it would complete the circuit. At that point. The thing is that my second transformer won't have a neutral. I was thinking that maybe I could bond the current-carrying conductor to an isolated Ground Bar And bond my neutral from my load to the Ground Bar. Would that just create a direct short? Or could the circuit be completed that way?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
If you are using something like a Variac, there will be a common conductor to the incoming and outgoing side of the transformer. Unless you change the 'polarity' of the step down unit versus that step up unit, the common conductor will pass all the way through to your load.

You do not have a separately derived system when you are employing autotransformers. Your circuit is really only two wire, so it may be beneficial to stop calling it a conductor a neutral.
 

oldsparky52

Senior Member
I don't know what I'm saying. That doesn't make any sense. The 120 volts from the second Transformer connecting the neutral back to the neutral from source Doesn't seem like it would complete the circuit. At that point. The thing is that my second transformer won't have a neutral. I was thinking that maybe I could bond the current-carrying conductor to an isolated Ground Bar And bond my neutral from my load to the Ground Bar. Would that just create a direct short? Or could the circuit be completed that way?
Travis, my apologies if this is insulting, but ... do you understand the difference between an autotransformer and the transformers that are used for stepping power down (and up) in the distribution system? The wiring for the two are completely different.
 

gar

Senior Member
200210-1602 EST

Travis:

You need a clear understanding and definitions of the words you use. You also need a better understanding of electrical circuit theory.

Forget the words neutral, and ground for the experiment you want to perform.

What you want to do or were doing was not clear in the first post.

What I now conclude is:

1. You have a voltage source that is a Variac (a variable autotransformer). Unless you have need for a variable voltage, then this could be just as well any fixed isolation or autotransformer with your desired 24 V output.

2. Since you have a Variac or equivalent other brand name like Powerstat it is likely an autotransformer.

3. I want to use the word common or possibly grounded for what you are calling neutral. And grounded does not mean ground (EGC). For your experiment I think it does not matter whether anything is referenced to earth (ground).

4. Your experiment is about a homemade transformer with a 24 V primary to be supplied from your Variac 24 V output.

5. You indicate your experimental transformer is an autotransformer, with possibly 3 leads. One lead connects to common of your source voltage. The other input lead connects to the 24 V lead of the power source. This makes common on everything have the same potential.

6. Use common as your reference for all measurements.

7. Run whatever tests you want on the experimental transformer.

.
 
Location
Texas
Occupation
Electrician
No, I really appreciate your comments, and I'm not offended. I understand the wiring is completely different. I am fairly new to Auto Transformers, though. And my apologies with terminology. My understanding now is that a neutral carries the unbalanced load in a separately derived system? However, since this is an auto Transformer There is simply a common and not a neutral. I guess that Commons only purpose is to complete the circuit. And varying voltages isn't an issue for me to connect my load to that common.
 
Location
Texas
Occupation
Electrician
There are only two leads with the Transformer. The lead that will connect to my 24 volt primary. The other will be my hot on the secondary side.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
If I'm hearing you correctly. You are saying that the neutral coming from the variable transformer I will have set to 24 volts, should connect to Earth ground. While my second transformer with 120 volt secondary should then be tied back to The Source neutral.
No. I was saying that an auto-transformer has a terminal common to both input and output, and that terminal should be connected to the source neutral, which automatically means that one output conductor should already be a grounded conductor.

This has nothing to do with grounding or bonding.

Now, if the second transformer is also an auto-transformer, the same thing applies.


However, you just mentioned that you're using a variable auto-transformer (brand name Variac) which possibly changes things. Most Variacs I've seen are enclosed and equipped with a cord and plug and a receptacle, but there are also open-chassis units.

Do we know for sure which output conductor is common with which input conductor?


So, please give us more information on both transformers, and a wiring diagram.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
My understanding now is that a neutral carries the unbalanced load in a separately derived system? However, since this is an auto Transformer There is simply a common and not a neutral.
Now you're confusing the neutral of a 120/240v 3-wire supply with the neutral of a 120v 2-wire supply. They are the same conductor, to be sure, but we're discussing them for different reasons.

Look at the drawing above. That's how two fixed auto-transformers would be wired. The bottom wire runs unbroken from source to load. Code specifies that auto-transformers be wired this way so the common, fed-through conductor is connected to the source's grounded conductor.

Since there are two, and they (hopefully) are wired thusly, the wire we're talking about already, or rather still is the neutral, or grounded conductor. By "grounded" we're referring to the neutral, white wire, as opposed to the hot, black (or red) conductor.


Now, none of this post has anything to do with grounding, which is the green (or bare) conductor.


If either of your transformers has isolated windings, like a conventional step-up or -down transformer, you have a separately-derived power source, defined by having no conductors connected to the primary or earth. Such a circuit should have one conductor grounded by being connected to earth by a grounding electrode conductor.

Note that this conductor has nothing to do with the source's grounded conductor, which is what we mean when we say neutral. The neutral is indeed grounded by being bonded to the system grounding electrode system via what we call the main bonding jumper, which is normally located in the enclosure housing the main service disconnect.

After the main, the neutral (grounded) conductor and the grounding conductor are not interchangeable, should never be interconnected again, and neither should be used in place of the other.


Is this a hard-wired installation, or a plugged-in bench-top experiment?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
There are only two leads with the Transformer. The lead that will connect to my 24 volt primary. The other will be my hot on the secondary side.
So, you're using the output of a Variac to supply an isolation transformer?

You're using the auto-transformer as an adjustable 24 volt supply?

And the second transformer is being used to step 24 volts "back up" to 120 volts?

Why did you ask the original question? What are you looking for, or to do with the output?

Are you aware what your output could be if you ramp the Variac all the way up?
 
Location
Texas
Occupation
Electrician
Yes to the fire extinguisher, look I appreciate yalls input, I understand grounding vs grounded, I don't work with Commons very much but I understand the concept, I don't need to be taken to school I need some advice. Let me a draw a wiring diagram real quick. And if its not up to snuff inform me of my errors.
 
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