1-Voltage, 2-Wire Secondary

[Jerramundi]

So, you're saying that "single voltage, two wire secondary" also applies to a dual-voltage, four wire secondary, as long as you wire it for a single voltage.

Again, I disagree. IMO, they could and would have said so if they meant it that way.

I hear you Larry. I think you make a sound argument as well and can find several statements that, IMHO, support what you're saying.. or at least seem to. I'm just stating what the majority consensus here seems to be. Then again, while I value the input of everyone, this isn't exactly a proper sample size, lol.

If I were to error on the side of caution, I would agree with you.

 

[Jerramundi]

So, you're saying that "single voltage, two wire secondary" also applies to a dual-voltage, four wire secondary, as long as you wire it for a single voltage.

Again, I disagree. IMO, they could and would have said so if they meant it that way.

The problem is, as I'm learning... there are two (2) types of "transformer secondary" protection.

(1) That which refers to protection of the transformer itself (governed by 450.3) and (2) that which refers to protection of the secondary conductors (governed by 240.4, 240.21, 240.100, and 240.101).

If I were to conclude that this transformer was NOT a "single voltage, two wire secondary," and thus that secondary protection of the transformer itself were required... I would have to utilize a Breaker Integrated Transformer (BIT).

As far as I'm aware, this is the ONLY way to provide this type of protection.

I found some BIT's on Eaton's website, but NONE of them go below 300V, which implies to me that there must be a way, for lower voltage circumstances, to not utilize a BIT.

 

[Jerramundi]

I can't find anything that allows for "transformer secondary protection" (of the transformer itself, not the secondary conductors) in which the OCPD is NOT an integral part of the transformer.

The only thing that comes close, is when the code talks about grounding/bonding at the first disconnect of an SDS, but that is the "disconnect," NOT the OCPD.

 

[LarryFine]

I'm really not into the discussion of the OCP, only the definition using "single voltage, two wire secondary."

If that description isn't intended to differentiate between a single-winding secondary and a dual-winding secondary that can be wired for a single voltage (the only exception being the center-tapped configuration), then why did they use such a specific description? They could have simply specified any secondary wired for a single output voltage.

To me, this is a case of "it says what it says" and not "what you want it to mean" or "what you wish it said."

 

[Jerramundi]

To me, this is a case of "it says what it says" and not "what you want it to mean" or "what you wish it said."

Well, I'm not one to ignore a minority opinion just because it's a minority. I think you make a somewhat valid argument.

 

[LarryFine]

Well, I'm not one to ignore a minority opinion just because it's a minority. I think you make a somewhat valid argument.

That happened when we debated the phase-vs-polarity topic a while back. I think I won that one (too).

 

[wwhitney]

So, here's the thing, 240.21(C)(1) actually says "two-wire (single voltage)," not "single voltage, two-wire". That puts a different complexion on the language, the term "single voltage" is not a further limitation of "two-wire" but a simple elaboration of the meaning.

That, coupled with the fact that 240.21 is about the location in the _circuit_ of OCPD, makes it clear that the language in 240.21(C)(1) is about the secondary circuit, not the internal design of the transformer.

Cheers, Wayne

 

[LarryFine]

That, coupled with the fact that 240.21 is about the location in the _circuit_ of OCPD, makes it clear that the language in 240.21(C)(1) is about the secondary circuit, not the internal design of the transformer.

And my contention is that "single voltage, two wire secondary" does indeed refer to "the internal design of the transformer."

As I said:

If that description isn't intended to differentiate between a single-winding secondary and a dual-winding secondary that can be wired for a single voltage (the only exception being the center-tapped configuration), then why did they use such a specific description? They could have simply specified any secondary wired for a single output voltage.

I I'm wrong then exactly what does "single voltage, two wire secondary" refer to?

 

[wwhitney]

I I'm wrong then exactly what does "single voltage, two wire secondary" refer to?

It refers to a secondary circuit which is two wires, which is perforce single voltage. That's why it's in parentheses as (single voltage), it's a redundant description.

The actual text at the start of 240.21(C)(1) (2014):

Conductors supplied by the secondary side of a single phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to . . .

Clear to me it does not refer to the internal design of the transformer. The proper boundary for the determination is the case of the transformer, and it's about what is going in and out of the case, not what the wiring is inside the case. Not worth it to debate further.

If it bugs you that much, when you're using 240.21(C)(1), you can irreversibly connect the 4 wires on a dual voltage secondary to the desired configuration. Now it's definitely a 2 wire secondary.

Cheers, Wayne

 

[winnie]

I would contend that since 240.21(C)(1) is about protection of _conductors_ connected to transformers, then "single voltage, two wire secondary" refers to the system connection rather than the particular way the transformer was manufactured.

IMHO it doesn't matter the the transformer _could_ be connected with a 120V secondary, or a 240V secondary or a 120/240V secondary. What matters is the actual connection, and "single voltage, two wire secondary" describes a particular connection.

Consider the other device permitted under this rule: a delta:delta transformer with a 3 wire secondary. If one of the secondary coils has a center tap that is unused, then I believe that transformer install would fit the requirements of 240.21(C)(1) ; but as soon as you use the center tap you violate the '3 wire' requirement'.

-Jon

 

[Carultch]

The problem is, as I'm learning... there are two (2) types of "transformer secondary" protection.

(1) That which refers to protection of the transformer itself (governed by 450.3) and (2) that which refers to protection of the secondary conductors (governed by 240.4, 240.21, 240.100, and 240.101).

If I were to conclude that this transformer was NOT a "single voltage, two wire secondary," and thus that secondary protection of the transformer itself were required... I would have to utilize a Breaker Integrated Transformer (BIT).

As far as I'm aware, this is the ONLY way to provide this type of protection.

I found some BIT's on Eaton's website, but NONE of them go below 300V, which implies to me that there must be a way, for lower voltage circumstances, to not utilize a BIT.

What "single voltage, two wire secondary" means, is that you are not actively using the center tap. Don't count the EGC as one of the two wires, but DO COUNT the neutral. The alternative is 120/240V as the secondary voltage, which is a three wire secondary. Consider 240V straight, or 120V straight. You abandon the center tap, if there is one at all. You configure the taps of the transformer, so you are only using one voltage setup on the secondary.

The rules of 240.21 that allow you to not have secondary side OCPD, require that you have a topology that qualifies. To qualify, the transformer topology must be set up so overcurrents line-up winding-to-winding, so that faults don't redistribute onto the primary windings and go "unnoticed" by the primary OCPD. Delta:delta three phase transformers also qualify, as long as there is no centertap either. Anything with a wye or centertap doesn't qualify.

I use the term "protect by proxy" when describing this situation. Given a 480V primary to 240V secondary transformer, a 15A OCPD on the primary effectively acts as if it were a 30A OCPD on the secondary. It can do this, as long as there is no possibility to redistribute the current on the primary of the transformer.

If you had a centertap, you could draw a 50A overload to line 1 to neutral, while applying 0A line 2 to neutral. This becomes 12.5A on the 480V primary. A 15A OCPD on the primary would not measure this as an overload, and therefore not trip. If you are counting on only 30A worth of conductors on the transformer secondary, this 50A overload would exceed their capacity.

 

[LarryFine]

Okay, for now, I'll concede that they mean the set-up.

"Captain Kirk! I shall consider it!"

 

[Jerramundi]

So, here's the thing, 240.21(C)(1) actually says "two-wire (single voltage)," not "single voltage, two-wire". That puts a different complexion on the language, the term "single voltage" is not a further limitation of "two-wire" but a simple elaboration of the meaning.

That, coupled with the fact that 240.21 is about the location in the _circuit_ of OCPD, makes it clear that the language in 240.21(C)(1) is about the secondary circuit, not the internal design of the transformer.

Cheers, Wayne

The debate was never about whether or not 240.21(C)(1) is about the "internal design of the transformer," but whether or not the phrasing "two wire (single voltage) referred to (1) the transformer as manufactured or (2) the transformer as field-wired... all as a PREMISE to what is allowed by article 240.

I was never arguing that 240 was about the transformer, but about the interpretation of a phrase that serves as a premise/conditional statement for what is allowed by 240.

 

[Jerramundi]

I guess it depends on what section of code you're looking at...

For example 240.4(F) Transformer Secondary Conductors says...

"Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device.

If "2-wire" and "3-wire" here were talking about the transformer itself, the word "transformer" would/should be plural.

The two items in red are clearly adjectives referring to the collective transformer secondary conductors. And it makes sense because the section is about transformer secondary conductors.

The problem is, if you keep reading, the latter part of 240.4(F) says...

"Conductors supplied by the secondary side of a single phase transformer having a 2-wire (single voltage) secondary,
or a three-phase, delta-delta connected transformer having a 3-wire (single voltage) secondary, shall be permitted to...

"...2-wire (single voltage)..." and "...3-wire (single voltage)..." HERE are clearly referring to the transformer secondary itself.

The only thing that makes sense, aside from (1) the code being redundant or (2) an outright error of some kind, is that these two different parts of 240.4(F) are talking about two distinct things: (1) The transformer secondary conductors, and (2) The transformer secondary (itself) as a conditional statement.

The question that remains for the latter part of 240.4(F) is... does, for example, "2-wire (single voltage)" mean (1) as manufactured or (2) as field wired??

 

[Jerramundi]

240.4(F) PART 1 of 2

• Single Phase (2-wire) transformer secondary conductors
  CAN be protected by the primary OCPD.
  
  
• Single Phase (other than 2-wire) transformer secondary conductors
  CANNOT be protected by the primary OCPD
  
  
• Multiphase (delta-delta, 3-wire) xfmr secondary conductors
  CAN be protected by the primary OCPD
  
  
• Multiphase (other than delta-delta, 3-wire) xfmr secondary conductors
  CANNOT be protected by the primary OCPD.

240.4(F) PART 2 of 2

• Conductors supplied by the secondary side of a single phase xfmr having a 2-wire (single voltage) secondary
  SHALL be permitted to be protected by the primary OCPD.
• Conductors supplied by the secondary side of a three-phase, delta-delta connected transformer having a 3-wire (single voltage) secondary SHALL be permitted to be protected by the primary OCPD.

 

[Jerramundi]

I believe if I'm looking at this correctly... what this section is doing is giving you two (2) conditional statements for allowing (or prohibiting) secondary conductors to be protected by the primary OCPD...

The first conditional statement is about the condition of the secondary conductors.
The second conditional statement is about the condition of the transformer secondary ITSELF.

Both are conditions that allow (or prohibit) the secondary conductors to be protected by the primary OCPD.

I think this helps clarify what they are referring to in regards to whether it's (1) the secondary conductors or (2) the transformer secondary...

95% resolved... but the question, I feel, is still up in the air, as to whether "2-wire (single voltage) secondary" refers to
(1) as manufactured or (2) as field wired.

 

[wwhitney]

On 240.4(F), the two sentences are just (almost) redundant. If we added the word "only" to the beginning of the first sentence, we could delete the first half of the second sentence (we need to keep the part starting with "provided"). And as in 240.21(C)(1), this is about the connections that pass through the case of the transformer, not about anything inside the case of the transformer.

You're definitely overthinking this one. Let the physics be your guide; if the wording is ambiguous, use the reasonable interpretation.

Cheers, Wayne

 

[winnie]

Remember though: the code says what it says, not what you want it to say.

I stated previously: if there are two reasonable interpretations of what the code says, then use the physics of the situation to pick between them.

Jon

 

[mivey]

There is the legalistic code and there is the physics of the situation. They should align but sometimes this doesn't work out. IMHO if there are two plausible interpretations of the legalistic code, then the one that matches the physics should prevail.

As long as only 2 wires go in for the primary and come out for the secondary, the current on the secondary conductors should be proportional to the current on the primary (disregarding magnetizing current) and primary OCPD can plausibly protect the secondary, presuming correct sizing per the turns ratio of the transformer and conductor sizes.

I can see the legalistic argument that what matters is the number of conductors coming off the secondary _as connected_ and the legalistic argument that what matters is the number of conductors that could possibly come off the secondary. I believe that the _as connected_ conductor count makes more sense.

-Jon

Well said. I agree.

 

[kwired]

'Control power transformers' as typically found in industrial control panels often only have a single voltage output. Door bell and thermostat transformers are also usually only have two terminals.

However, the NEC does not apply to the factory supplied wiring 'inside' of a transformer enclosure. Those conductors, whether they are wires or bus pads and jumpers are part of the device called a transformer. These items must be properly interconnected, only then do you have the 'terminals' to which you connect your secondary conductors, which are subject to the NEC.

240.4(F) refers you to 450.3 which says a transformer may be a group of two or more devices acting as a single unit. So your 'single transformer' is actually contains two sets of windings, each of which is an individual transformer, which you interconnect into a single voltage output. The resultant single voltage transformer which also complies with the 450.2 requirement for a single nameplate.

Two wire in, two wire out - even control power transformers with two wire out often have dual voltage input abilities.