1? / 3? Neutral Amp Question

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George Stolz

George Stolz

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I was looking at (as opposed to understanding) this thread, which prompted an old question of my own:

Imagine a panel of a 3? system.
There is a only a 2-pole breaker in this panel feeding one appliance, say it's 50 amps.
It is a line-to-line load, using phases A and B.
There is a neutral run to the appliance, for fun.
The calculated load of the branch circuit neutral is zero.
The calculated load for the panel feeder neutral is 50 amps.

How does that work? I must be wrong. :)

[ September 19, 2005, 11:47 PM: Message edited by: georgestolz ]
 
Re: 1? / 3? Neutral Amp Question

If the neutral is run for fun, then there would be no load on the feeder neutral. How could there be?
I'm assuming your "fun neutral" is just wirenutted off in the disconnect or outlet.
 
Re: 1? / 3? Neutral Amp Question

What I mean is, if you just know you have a panel with 50 on A, 50 on B, and 0 on C, then according to calculations you'd have 50 amps on the neutral.

But this isn't so. I know I'm missing something, what is it? :)
 
Re: 1? / 3? Neutral Amp Question

George, it is the phase angles you are missing. The current flow is from node A, through the load, through node B, through the neutral node, and back out through node A.

That is to say that the sum of Ia and Ib at the neutral node is zero, therefore In is zero. This is true for every instant in time.
 
Re: 1? / 3? Neutral Amp Question

I think your calculation for the 50A load on the neutral is wrong. I know I have seen a formula posted for this, but I don't have it handy. It should give a 0 neutral load for a single line-line load.

Steve
 
Re: 1? / 3? Neutral Amp Question

The formula would be:

In = Ia + Ib + Ic added vectorially.

But in this case, Ib = -Ia and Ic = 0,

Then,

In = Ia -Ia + 0 = 0

QED, whatever that means.
 
Re: 1? / 3? Neutral Amp Question

Rattus,

You nailed it. Permit me to rephrase a little bit.

George,

Given the thread that you got this thread going from, it is important to remember the configuration of the transformer bank that this 240 Volt 1 ? appliance of yours is connected to.

The supply (transformer bank secondary) is a delta.

With the 50 A load connected only between Line A and Line B, then, at the other end of the Lines, the end connected to the transformer bank secondary, the 50 A load current goes straight through the one secondary winding between A & B. (Now, this could be B & C or C & A, and the 240 Volt 1 ? appliance would work just fine.)

What is distracting is the neutral run, just for fun.

The connection of a neutral at the center tap of the supply secondary still doesn't have a connection in the appliance. So, without the appliance end of the neutral connected there can be no neutral current in the neutral that goes to the appliance location.

Moral: This is electrical and it is serious.

;)
 
Re: 1? / 3? Neutral Amp Question

Q.E.D. (sometimes written "QED") is an abbreviation for the Latin phrase "quod erat demonstrandum" ("that which was to be demonstrated").

Or.

As I colloquialize it:

"Proven as stated."
 
Re: 1? / 3? Neutral Amp Question

You only have two current carrying conductors (i.e. single phase), there can be no phase angle between them. The neutral conductor does not come into any part of the calculations or discussions. Any analysis should be done using line-line voltages. The use of any reference to the line-neutral voltages is like counting cattle in a field by adding the number of legs and then dividing by four, same result just more room for error.

Like Rattus described, the neutral of the transformer feeding this panel is simply one point (node) in a circuit and what goes in must come out.
 
Re: 1? / 3? Neutral Amp Question

Al, I knew what QED means, just being cute.

George owes us a diagram.

I visualize this thing as a 120/208V wye secondary. My formula fits the wye.

If it is a 120/240V delta secondary, then:

The single phase load current would be shared unequally by the three transformers. The neutral, whether connected or not, would not carry any current.

Furthermore, with any combination of line to line loads on a wye, the neutral current is zero.

[ September 20, 2005, 10:44 AM: Message edited by: rattus ]
 
Re: 1? / 3? Neutral Amp Question

what goes in must come out
Click to expand...
I've come to know that as Testlittles Deviation to Fudd's Law of Opposition.

A tip o' the hat to Fire Sign Theater.

:D
 
Re: 1? / 3? Neutral Amp Question

Rattus: QED, whatever that means.
Al: "Proven as stated."
Rattus: Al, I knew what QED means, just being cute.
Click to expand...
Well, I didn't! :D

So under what circumstances would you use the following formula:
?((AA+BB+CC)-(AB+BC+CA) ?

How is this formula accurate if single-phase loads don't comply?
 
Re: 1? / 3? Neutral Amp Question

George, that looks like Boolean algebra to me. I would never use it because I don't understand it. Tell us more.

For a wye, the neutral current is simply,

In = Ia + Ib + Ic (Vector addition)

If the neutral is floating,

In =Ia + Ib + Ic = 0 (Again, vector addition)
 
Re: 1? / 3? Neutral Amp Question

George, what do A, B, and C represent? Current, voltage, whatever? Looks like it could be some formula from solid geometry.
 
Re: 1? / 3? Neutral Amp Question

George,
That is the correct formula for finding the grounded conductor current in a three phase wye system, where there is line to grounded conductor loads. Your original question asked about a line to line load. There is no grounded conductor current when the loads are only line to line.
Don

Additional information...A, B and C represent the line to grounded conductor currents in the phase conductors.

[ September 21, 2005, 07:17 AM: Message edited by: don_resqcapt19 ]
 
Re: 1? / 3? Neutral Amp Question

George,

It hurts my head when A, B, & C are used instead of I to represent current. Makes life tedious.

Natheless, these symbols represent phase currents in the wye. Doesn't matter how the loads are connected.

Now open the C leg, then A = B, and the evaluation of the formula yields zero. You must be using reverse Polish notation on an algebraic calculator.
 
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