Neutral conductor: grounded or ungrounded conductor?

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drcampbell

drcampbell

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Strictly speaking, a "neutral" wire is one with no current flowing in it. But that only happens in a perfectly-balanced circuit, which never happens in real life.

The answer depends on what kind of circuit it is. In an old corner-grounded 3-phase delta, the grounded wire is one of the three conductors, carrying the same current as the other two and anything but "neutral". But because it's grounded, it's required to be color-coded white or grey, and the words "white" and "neutral" are often used interchangeably by people who are less persnickety than I am.

The same would be true of a single-ended single-phase American/Canadian 120-volt circuit. One of the two current-carrying conductors is grounded and white, but not neutral.

In a three-phase Y that only supplies three-phase loads, there isn't any white wire or neutral conductor. But the transformer centertap is expected to be grounded. (most of the time -- exceptions exist)

In a three-phase Y that supplies both single- and three-phase loads, the white wire isn't neutral because the single-phase loads assure that the three phases are unbalanced, and consequently, the unbalanced return current is flowing in the white wire. But the transformer centertap (white) is required to be grounded.
 
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GoldDigger

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drcampbell said:
Strictly speaking, a "neutral" wire is one with no current flowing in it. But that only happens in a perfectly-balanced circuit, which never happens in real life.

The answer depends on what kind of circuit it is. In an old corner-grounded 3-phase delta, the grounded wire is one of the three conductors, carrying the same current as the other two and anything but "neutral". But because it's grounded, it's required to be color-coded white or grey, and the words "white" and "neutral" are often used interchangeably by people who are less persnickety than I am.

The same would be true of a single-ended single-phase American/Canadian 120-volt circuit. One of the two current-carrying conductors is grounded and white, but not neutral.

In a three-phase Y that only supplies three-phase loads, there isn't any white wire or neutral conductor. But the transformer centertap is expected to be grounded. (most of the time -- exceptions exist)

In a three-phase Y that supplies both single- and three-phase loads, the white wire isn't neutral because the single-phase loads assure that the three phases are unbalanced, and consequently, the unbalanced return current is flowing in the white wire. But the transformer centertap (white) is required to be grounded.
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Even more strictly speaking, a neutral is a conductor which is at an equal magnitude voltage offset to each of the other conductors, regardless of what the current balance may be and the neutral is at the center of a complete balanced voltage phasor set. The NEC specifically overrides that base definition when it labels the center tap on a high leg delta as a neutral, but they have the power to do that. :)
The grounded conductor of a corner grounded delta is not a neutral even though it is at an identical voltage offset to the other two corners.
The NEC also states that, for certain specified voltage ranges, if there is a neutral conductor in the source circuit it must be grounded. A delta can be operated ungrounded with appropriate restrictions, since it has no neutral.
 
electrofelon

electrofelon

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jaggedben said:
The NEC requires it to be grounded.
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m sleem said:
I think it has to be grounded
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GoldDigger said:
The NEC also states that, for certain specified voltage ranges, if there is a neutral conductor in the source circuit it must be grounded.
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Goldie has it. A neutral doesnt always have to be grounded, although it would be very rare. A system must be grounded if:

1. The system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150V

2. The system is three-phase, 4-wire and is wye-connected and the neutral conductor is used as a circuit conductor

3. The system is three-phase, 4-wire and is delta-connected in which the midpoint of one phase winding is used as a circuit conductor


So you dont have to ground a 480/277 system, but you do if you use the neutral as a circuit conductor, so its mostly academic.

Edit: just wanted to add that, unless you are just going for that satisfying feeling that you got one up on the code man, you would also have to have a REASON to go ungrounded.
 
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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
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GoldDigger said:
It depends. If you consider "than" to be a preposition, then "me" is correct.
If you consider the statement to be an ellipsis of "..than I am", then "I" would be correct.
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Of course; context matters. I use these for examples:

"John likes you more than me." = John likes you more than he likes me.

"John likes you more than I." = John likes you more than I like you.
 
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