Grounding one leg of 120/240V transformer

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jim dungar

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So why then are we saying that L2-G is 120V if L2 is the grounded conductor? Wouldn't they be at the same potential and therefore be 0V?

Effectively I am renaming the terminals as L1, L2 and Common/Ground. I no longer consider the 'center-tap' as a neutral point because I do not intend to have a 1PH 3W circuit, I have (2) 1PH 2W ones instead.

If you continue to call it a neutral point then it must be grounded, 250.26(2) and this discussion becomes moot.

Would I really do this with 120/240V - probably not, but the logic applies to other voltage voltages as well.
 

LarryFine

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So when we remove the ground from the center tap and move it to what was the L2 (far right) leg then we essentially have moved the neutral and the center tap now becomes L2 and the far right leg the neutral. Any down side to doing this?
Not correct. The voltage between any two points remain the same, and the neutral remains the center tap.

What does change with a change in bonding is the voltage from each conductor to ground (earth.)
 

iwire

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Effectively I am renaming the terminals as L1, L2 and Common/Ground. I no longer consider the 'center-tap' as a neutral point because I do not intend to have a 1PH 3W circuit, I have (2) 1PH 2W ones instead.

The NEC definition says the mid point is the neutral point without dealing with how it may or may not be used.
 

jim dungar

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The NEC definition says the mid point is the neutral point without dealing with how it may or may not be used.
No, the definition says: "... mid point on a single-phase, 3-wire system...", it relies on how the point is used in the system.

If the system is not 3-wire, the mid-point does not need to be called a neutral. The FPN tries to clarify this by using the phrase "that utilize the neutral".

My point is that (2) 1PH 2W systems sharing a common connection do not automatically become (1) 1PH 3W system.
 

pfalcon

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Indiana
Neutral: Where the sum of the phases cancel each other - not necessarily 0 volts.
Ground potential: An unknowable potential we reference with the arbitrary value of 0 volts.

Three phase 208/120Y has the neutral at the center no matter where it is grounded.
Two phase 240/120 has the neutral at the midpoint no matter where it is grounded.
Single phase 120 does not have a neutral.
Three phase delta does not have a neutral.

On the described system:
L1 to N :: +/- 120VAC
L2 to N :: +/- 120VAC
L2 to L1 :: +/- 240VAC

In order of L1, N, L2:
L1 grounded - 0, 120, 240 (same polarity at N, L2)
L2 grounded - 240, 120, 0 (same polarity at L1,N)
N grounded - 120, 0, 120 (Opposite polarity at L1,L2)
Floating :: Relationship of voltage in first section remains the same. Actual potential is unknown.
 

jim dungar

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Neutral: Where the sum of the phases cancel each other - not necessarily 0 volts.
Ground potential: An unknowable potential we reference with the arbitrary value of 0 volts.

The NEC definition of 'neutral point' never uses the word voltage. But, its FPN (which are used for clarifications) says neutral is the point where the vectorial sum of the phases voltages is 0.

To measure volts we must have (2) points. We can not say ground = 0V. As far as the NEC goes, ground is simply dirt (well actually earth). The intent of article 250, is to bond things, including dirt, so the potential difference to ground or to each other is 0V.

250.20(B)(1) requires you to create a connection that establishes a maximum ungrounded voltage to ground of 150V. 250.26(2) requires this connection to be the neutral point of a 3W system.
 

jghrist

Senior Member
250.20(B)(1) requires you to create a connection that establishes a maximum ungrounded voltage to ground of 150V.
Which means that your connection does not meet code. If it is possible to ground the system so that the voltage to ground does not exceed 150 V, then it is required.
 

jim dungar

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Which means that your connection does not meet code. If it is possible to ground the system so that the voltage to ground does not exceed 150 V, then it is required.
My connection creates one L-G of <150V and one L-G of>150V. My midpoint tap is not part of a 3-wire system and is not intended to serve L-L loads. So, I believe there is a methodology that would consider my connection to meet code. As I have stated before, my logic would apply to a 12/24V or 16/32V lighting transformer or a 24/120V control transformer, but should probably be avoided on a 120/240V one.

However, the OP was about 'what happens', not if it is a code compliant installation. As most of the contributors have pointed out the system to ground bond simply creates a reference point, the 'laws of physics' do not care which conductor is bonded, but the NEC does.
 

pfalcon

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Location
Indiana
Neutral: Where the sum of the phases cancel each other - not necessarily 0 volts.
Ground potential: An unknowable potential we reference with the arbitrary value of 0 volts.

The NEC definition of 'neutral point' never uses the word voltage. But, its FPN (which are used for clarifications) says neutral is the point where the vectorial sum of the phases voltages is 0.

I had hoped to say what I meant in the way I said it. :grin:

Yes. I agree.

If we arbitrarily assume the neutral is at 0V potential then the sum of the phase-neutral potentials also equals 0V. The actual potential at the neutral is unknowable. In practice we assign (also arbitrarily) the nearest grounded surface as 0V. We then measure from the neutral (or any other point in the system) to the assigned ground and claim that to be the potential of that point. And now several people with glazed eyes need medical attention. :grin:
 
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