1-Voltage, 2-Wire Secondary

[kwired]

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??

But you are ignoring "conductors supplied by" part of that statement.

 

[kwired]

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.

There is no neutral on a two wire system. You can ground one of the conductors though.

 

[jim dungar]

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 transformer secondary does not exist until the proper 'internal' transformer winding and tap connections have been made. Once the windings are properly interconnected, you then can determine if your transformer has a single voltage secondary.

Why else would a nameplate show both a 240V 2-wire output connection and a 120/240V 3-wire connection as listed wiring options?

 

[synchro]

The transformer secondary does not exist until the proper 'internal' transformer winding and tap connections have been made. Once the windings are properly interconnected, you then can determine if your transformer has a single voltage secondary.

I agree with this and especially the emphasis on "proper" connections of the windings inside the transformer. For example, if only one of the two 120V secondary windings was brought out it would be a 2-wire output but it could only support 1/2 of the transformer's rated kVA. In this case having an OCPD on the primary that is sized for the transformer's kVA rating would not adequately protect the transformer's secondary winding.

 

[Jerramundi]

But you are ignoring "conductors supplied by" part of that statement.

No, I'm not. The "conductors supplied by" part of that statement is letting you know that this is ultimately referring to overcurrent protection of secondary conductors. The entire section is about that.

But the "supplied by the secondary side of a single phase transformer having a 2-wire (single voltage) secondary"
is a CONDITIONAL STATEMENT that allows or prohibits the protection of these secondary conductors by the primary OCPD.

 

[Jerramundi]

But you are ignoring "conductors supplied by" part of that statement.

In regards to single phase ONLY, what they are telling you is that IF you want to protect the secondary conductors via the primary OCPD (2) conditions have to be met.

1) The secondary conductors themselves MUST be single phase (2-wire)
2) The secondary conductors MUST be SUPPLIED BY the secondary side of a single phase xfmr that has a 2-wire (single voltage) secondary.

It's giving you two conditional statements about (1) the condition of the secondary conductors themselves and (2) the condition of the secondary side of the xfmr. These conditions ultimately allow or prohibit the protection of the secondary conductors via the primary OCPD.

 

[Jerramundi]

There is no neutral on a two wire system. You can ground one of the conductors though.

Well this is about the definition of a two wire system. I'm trying to understand the verbiage of 240.4(F) first... and I think I've broken it down correctly.

Once we agree on that... then we can decide what (1) single phase (2-wire) (w/ respect to the secondary conductors) and (2) 2-wire (single voltage) secondary (w/ respect to the transformer secondary) means.

 

[Jerramundi]

On 240.4(F), the two sentences are just (almost) redundant.

I thought the same at first. Then I picked out that the first sentence explicitly says "transformer secondary conductors" and the second the sentence says "the secondary side of 2-wire (single voltage) secondary."

Clearly these are talking about two different things: (1) The condition of the secondary conductors and (2) the condition of transformer secondary.

Both serving as conditional statements that ultimately either allow or prohibit the protection of the secondary conductors via the primary OCPD.

 

[david luchini]

I thought the same at first. Then I picked out that the first sentence explicitly says "transformer secondary conductors" and the second the sentence says "the secondary side of 2-wire (single voltage) secondary."

Clearly these are talking about two different things: (1) The condition of the secondary conductors and (2) the condition of transformer secondary.

Both serving as conditional statements that ultimately either allow or prohibit the protection of the secondary conductors via the primary OCPD.

"Transformer Secondary Conductors" are conductors "supplied by the secondary side" of a transformer. The two sentences are talking about the same thing.

 

[Jerramundi]

"Transformer Secondary Conductors" are conductors "supplied by the secondary side" of a transformer. The two sentences are talking about the same thing.

I partially disagree. Being so redundant would be pointless.
Ultimately, yes, ALL of this is about transformer secondary conductor protection by the primary OCPD...

...but the two sentences utilize two very distinct phrases: (1) secondary conductors and (2) transformer secondary.

I believe both (1) and (2) are conditional statements, (1) addressing the condition of the secondary conductors, and (2) addressing the condition of the transformer secondary itself, that ultimately allow or prohibit the secondary conductors to be protected by the primary OCPD.

It's basically telling you that IF you want to protect the secondary conductors via the primary OCDP, 2 conditions have to be met (1) the condition of the secondary conductors and (2) the condition of the transformer secondary itself.

 

[Jerramundi]

"Transformer Secondary Conductors" are conductors "supplied by the secondary side" of a transformer. The two sentences are talking about the same thing.

Yes but "Conductors supplied the secondary side of a transformer..." is not the complete sentence. It goes on to say "Conductors supplied the secondary side of a transformer WITH a 2-wire (single voltage) secondary." This is giving you a condition of the transformer secondary itself as a conditional statement for protecting the secondary conductors.

I think this section is talking about two different conditions required for ultimately protecting the secondary conductors. (1) The condition of the secondary conductors themselves and (2) the condition of the transformer secondary itself.

 

[david luchini]

I partially disagree. Being so redundant would be pointless.
Ultimately, yes, ALL of this is about transformer secondary conductor protection by the primary OCPD...

...but the two sentences utilize two very distinct phrases: (1) secondary conductors and (2) transformer secondary.

I believe both (1) and (2) are conditional statements, (1) addressing the condition of the secondary conductors, and (2) addressing the condition of the transformer secondary itself, that ultimately allow for secondary conductors to be protected by the primary OCPD.

The first sentence tells you that secondary conductors shall not be considered protected by the primary overcurrent device (other than single phase-2 wire and three phase 3-wire.)

The second sentence tells you that the single phase 2-wire and three phase 3-wire secondary conductors can be protected by the primary overcurrent device PROVIDED that the protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

In other words, the first sentence prohibits protecting the secondary conductors with the primary ocpd, except as permitted in the second sentence.
You must meet the conditions in the second sentence (450.3 and ampacity times ration) in order to provide secondary conductor protection with the primary ocpd.

It's not really that complicated.

 

[wwhitney]

I partially disagree. Being so redundant would be pointless.

Welcome to the NEC.

Cheers, Wayne

 

[Jerramundi]

Welcome to the NEC.

Cheers, Wayne

You said it bud, lol. One would think relying on a professional third party resource to dictate what is the "right" way to do things would be easily definitive. Nope, lol.

 

[Jerramundi]

The first sentence tells you that secondary conductors shall not be considered protected by the primary overcurrent device (other than single phase-2 wire and three phase 3-wire.)

The second sentence tells you that the single phase 2-wire and three phase 3-wire secondary conductors can be protected by the primary overcurrent device PROVIDED that the protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

In other words, the first sentence prohibits protecting the secondary conductors with the primary ocpd, except as permitted in the second sentence.
You must meet the conditions in the second sentence (450.3 and ampacity times ration) in order to provide secondary conductor protection with the primary ocpd.

It's not really that complicated.

You're completely ignoring the part of sentence two which clearly references the transformer secondary itself.

 

[Jerramundi]

In other words, the first sentence prohibits protecting the secondary conductors with the primary ocpd, except as permitted in the second sentence.
You must meet the conditions in the second sentence (450.3 and ampacity times ration) in order to provide secondary conductor protection with the primary ocpd.

I think you may actually be right about how your describing this.... Sentence one is prohibitive. Sentence two is permissible.

BUT you have to acknowledge that sentence two gives an explicit CONDITION of the transformer secondary itself as a condition for protecting the secondary conductors.

 

[david luchini]

You're completely ignoring the part of sentence two which clearly references the transformer secondary itself.

No I'm not. If the secondary of the transformer is 120v, then its single phase, 2 wire.

 

[wwhitney]

I think you may actually be right about how your describing this.... Sentence one is prohibitive. Sentence two is permissible.

BUT you have to acknowledge that sentence two gives an explicit CONDITION of the transformer secondary itself as a condition for protecting the secondary conductors.

Sure, but "transformer secondary" is not a defined term in the NEC. They just switched up the language a little to combat the mind-numbing redundancy. In both cases what matters is the circuit configuration.

Cheers, Wayne

 

[Jerramundi]

No I'm not. If the secondary of the transformer is 120v, then its single phase, 2 wire.

All I'm saying is that the actual condition of the transformer secondary itself matters.

That these two sentences are addressing (1) the secondary conductors and (2) the secondary of the transformer... BOTH as conditions for protecting the secondary conductors.

It is definitely a bit redundant, but quintessential NEC to cover their ass by making both (1) the condition of the secondary conductors and (2) the condition of the transformer secondary supplying them.... relevant to whether or not the secondary conductors can be protected by the primary OCPD.

 

[Jerramundi]

Basically this is telling me that I can't wire this particular transformer for 120V/240V output and supply only 120V conductors from that. That the transformer secondary has to be wired for a 120V 2-wire output and supply 120V two-wire secondary conductors... which you would think would be obvious, but alas.