Transformer Tap rule

[wwhitney]

The physics of the transformer hasn't changed. The two halves of the secondary are in parallel.

No, for a 120/240V secondary configuration, the two halves of the secondary are in series. A 200A 120V load and a 100A 240V load will cause the same current in the primary.

Cheers, Wayne

 

[jim dungar]

No, for a 120/240V secondary configuration, the two halves of the secondary are in series. A 200A 120V load and a 100A 240V load will cause the same current in the primary.

Cheers, Wayne

Oops.
They are in series. I corrected my mistake.
But my analysis is still correct. 100a on the secondary of a 1:1 transformer will be seen by the primary as 100A. It makes no difference if the load is L-L or L-N.

 

[wwhitney]

But my analysis is still correct. 100a on the secondary of a 1:1 transformer will be seen by the primary as 100A. It makes no difference if the load is L-L or L-N.

No, that is not how a 240V : 120/240V transformer behaves. If on the secondary I just connect one load L-L, then it behaves as a 1:1 transformer. But if instead I just connect one load L-N, it behaves as a 2:1 transformer.

Edit: this difference is the whole reason there is an issue in this thread.

Cheers, Wayne

 

[Strathead]

Oops.
They are in series. I corrected my mistake.
But my analysis is still correct. 100a on the secondary of a 1:1 transformer will be seen by the primary as 100A. It makes no difference if the load is L-L or L-N.

This has been an enlightening discussion. I have to say that the passage makes more sense to me now, and I just had an epiphany. To remind everyone, the code section says, "The secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied"

We have been debating something that is actually clear when we step back. It doesn't say, transformer voltage ratio, so the ratio that would need to be used would be the voltage of the secondary wire we are sizing to the voltage of the primary breaker. Unless the entire conversation has gone over my head, this seems pretty cut and dry. Now we can move the discussion over to having unbalanced secondary loads. LOL

 

[jim dungar]

No, that is not how a 240V : 120/240V transformer behaves. If on the secondary I just connect one load L-L, then it behaves as a 1:1 transformer. But if instead I just connect one load L-N, it behaves as a 2:1 transformer.

Edit: this difference is the whole reason there is an issue in this thread.

Cheers, Wayne

No.

The physics of the transformer do not change, based solely on it's loading. 100A of secondary current will 'produce' 100A of primary current if the transformer ratio is 1:1. The only way this can not be true is if you have physically changed the transformer windings so it is no longer a 1:1 ratio.

 

[wwhitney]

The physics of the transformer do not change, based solely on it's loading. 100A of secondary current will 'produce' 100A of primary current if the transformer ratio is 1:1. The only way this can not be true is if you have physically changed the transformer windings so it is no longer a 1:1 ratio.

In the above formulation, are you calling a 240V : 120/240V a "1:1" transformer?

If so, then your statement is incorrect. For a center-tapped secondary like 120/240V, you need to use the average current on the entire secondary coil, which would be 50A when you just have 100A flowing L-N, with no current on the other L. If you have the same amps on half the turns, you have half the amp-turns.

Or if you don't consider a 240V : 120/240V transformer to be "1:1", then my concern is not about transformers with a single fixed ratio, it's about transformers with multi-voltage secondaries, where the current transformation ratio depends on the circuit configuration.

Cheers, Wayne

 

[wwhitney]

the ratio that would need to be used would be the voltage of the secondary wire we are sizing to the voltage of the primary breaker.

This statement is a bit imprecise--voltage is measured between two points. So what exactly do you mean by "voltage of the primary breaker" and by "voltage of the secondary wire"?

Cheers, Wayne

 

[jim dungar]

In the above formulation, are you calling a 240V : 120/240V a "1:1" transformer?

If so, then your statement is incorrect. For a center-tapped secondary like 120/240V, you need to use the average current on the entire secondary coil, which would be 50A when you just have 100A flowing L-N, with no current on the other L. If you have the same amps on half the turns, you have half the amp-turns.

Or if you don't consider a 240V : 120/240V transformer to be "1:1", then my concern is not about 1:1 transformers, it's about transformers with multi-voltage secondaries, where the current transformation ratio depends on the circuit configuration.

Cheers, Wayne

Your reasoning is flawed.
You do not average the current through both halves of a series circuit. The proper method is to take the sum of the currents through the two windings halves.

You are correct in that twice the current through a single half winding is required, but your starting point is wrong.
Your initial premise was a 1:1 transformer 240V to 240V. 100A on the secondary yields 100A seen by the primary protective device.
Now, if you split the secondary winding into a series of multiple pieces, it doesn't change the relationship between the primary and secondary currents, 100A on one side is 100A on the other.
For a 120/240 secondary with two winding halves each half only needs to have 50A through it in order to produce the 100A on the primary. But one half of the winding could have 40A and the other 60A and the total on the secondary is still only 100A.
Now we get to the extreme unbalance of 100A on one half and 0A on the other. The total secondary current is still 100A even though one half of the winding now sees 2X the amount of current as the original condition.
This takes us back where we started: 100A on the secondary and 100A on the primary.

 

[LarryFine]

Sorry, Jim, but Wayne is correct. If you load only half of the secondary, the unloaded half is irrelevant.

A 240-to-120/240 transformer with only half of the secondary used is effectively a 2:1 transformer.

Use VA numbers instead of current. It takes twice the current at half the voltage to be the same power.

If dual-voltage secondaries wired in parallel share a 200a load, the primary current is 100a, not 200a.

 

[wwhitney]

You do not average the current through both halves of a series circuit. The proper method is to take the sum of the currents through the two windings halves.

If you are treating a 240V : 120/240V transformer as 1:1, then you would take the sum of the two half-winding currents, and divide by 2, which is the average.

Your initial premise was a 1:1 transformer 240V to 240V.

Nothing I've said in this thread is in regard to a 240V 2-wire : 240V 2-wire transformer, so no, that was not an initial premise.

For a 120/240 secondary with two winding halves each half only needs to have 50A through it in order to produce the 100A on the primary. But one half of the winding could have 40A and the other 60A and the total on the secondary is still only 100A.

This is your mistake. Each secondary half-winding has to have 100A through it in order to to produce 100A on the primary.

Take the simple case that on the secondary, the currents are L1 = 50A, N = 0A, and L2 = 50A, and the loads are linear. Then as the neutral current is 0, we can just unhook the neutral at the transformer without changing the load behavior or currents. Now we unequivocally have a 1:1 240V 2-wire : 240V wire transformer. As L1 = L2 = 50A, the primary current is 50A.

Cheers, Wayne

 

[jaggedben]

If the primary protective device is 100A how is it going to allow twice as much current to flow through it simply because you are now using only 1/2 of a secondary winding?

The primarily protective device allows twice as much current to flow through the secondary conductors, not the primary ones.

 

[jim dungar]

Nothing I've said in this thread is in regard to a 240V 2-wire : 240V 2-wire transformer, so no, that was not an initial premise.

This is basic transformer theory, so it would apply equally to a 2-wire secondary as to a multi-tapped one.

In post #43, you said a 240V to 120/240V transformer would act as a 1:1 ratio transformer if the secondary was connected L-L, so that is what I used in my example.

 

[wwhitney]

In post #43, you said a 240V to 120/240V transformer would act as a 1:1 ratio transformer if the secondary was connected L-L, so that is what I used in my example.

OK, we can agree on that.

But you are still mistaken on what happens with such a transformer when there is just a load L1-N, and L2 is unloaded. As multiple people have pointed out.

Once you overcome that, the issue raised in the OP and under discussion here should be clear.

Now, if you split the secondary winding into a series of multiple pieces, it doesn't change the relationship between the primary and secondary currents, 100A on one side is 100A on the other.

The above is true as long as you understand that 100A through the secondary is 100A through all of the series pieces.

Cheers, Wayne

 

[Strathead]

This statement is a bit imprecise--voltage is measured between two points. So what exactly do you mean by "voltage of the primary breaker" and by "voltage of the secondary wire"?

Cheers, Wayne

Why are you being obtuse? Do you truly not understand what I meant? I would assume anyone who is a journey man would understand. I just didn't feel like typing "nominal voltage rating or the circuit as designated by the plans and provided by the local utility company" twice.

 

[Strathead]

Your reasoning is flawed.
You do not average the current through both halves of a series circuit. The proper method is to take the sum of the currents through the two windings halves.

You are correct in that twice the current through a single half winding is required, but your starting point is wrong.
Your initial premise was a 1:1 transformer 240V to 240V. 100A on the secondary yields 100A seen by the primary protective device.
Now, if you split the secondary winding into a series of multiple pieces, it doesn't change the relationship between the primary and secondary currents, 100A on one side is 100A on the other.
For a 120/240 secondary with two winding halves each half only needs to have 50A through it in order to produce the 100A on the primary. But one half of the winding could have 40A and the other 60A and the total on the secondary is still only 100A.
Now we get to the extreme unbalance of 100A on one half and 0A on the other. The total secondary current is still 100A even though one half of the winding now sees 2X the amount of current as the original condition.
This takes us back where we started: 100A on the secondary and 100A on the primary.

Everything you said here seems correct to me, but tie that back to my original question.

 

[wwhitney]

Why are you being obtuse? Do you truly not understand what I meant?

Geez, no need to be hostile. My question was sincere, as there's a difference between L-N voltage and L-L voltage, which is the point of your OP. And for, say, a 240V 3P4W high leg delta secondary, L-N varies. [For weird systems like 2P4W and 2P5W, L-L voltage also varies, but let's ignore 2-phase.]

I believe the ratio that 240.21(C)(6) references should be, or work out to be equivalent to, the "largest possible primary to secondary conductor current ratio." So far I haven't come up with a succinct way to express that in terms of voltages.

I think the following rule works for "primary-to-secondary voltage ratio" for many typical transformer topologies: when one side of the transformer has multiple voltages present, choose the lowest non-zero voltage for that side.

Not sure about wye : wye transformers, though: is supplying a wye : wye with only 3 primary wires (no neutral) an option, and if so should that change the ratio used in 240.21(C)(6) versus supplying it with 4 wires?

Cheers, Wayne

 

[jim dungar]

The above is true as long as you understand that 100A through the secondary is 100A through all of the series pieces.

Cheers, Wayne

Of course it is, and 100A through only one piece with 0A through the others is alao possible.

 

[wwhitney]

Of course it is, and 100A through only one piece with 0A through the others is alao possible.

Yes, but the latter case won't give you 100A in the primary. You only get 100A through the primary if you have 100A through all of the secondary pieces.

The amp-turns on the primary and the secondary are equal. So if, say, the full primary and secondary are 10 turns, and we have 100 amps through, say, 3 turns on the secondary, and 0 amps through the other 7 turns on the secondary, then we'd have 300 amp-turns. The primary current would be 300 amps-turns / 10 turns = 30 amps. [This for the case the primary has only two connections at the ends, and the secondary has a 30% tap.]

Cheers, Wayne

 

[Tulsa Electrician]

I give this three popcorns and one coke.
Very enjoyable read.
Obtuse , shawshank redemption. Love that part.
I still vote for line to line 480/208 since Mr. Holt said 1/3rd (33%) and nothing about the rest. Maybe some one should ask him. Can a mod do that. I would love to read his take on it. Or a nice video. Love those videos.

 

[wwhitney]

I still vote for line to line 480/208 since Mr. Holt said 1/3rd (33%) and nothing about the rest.

Mr. Holt is one commentator among many. In your earlier reference, he may just have read the language and gone with the obvious meaning without considering the underlying physics.

Cheers, Wayne