1-Voltage, 2-Wire Secondary

[Carultch]

Do you consider a single winding with a center tap, which is not wired, as meeting 240.4(F) and 240.21(C)1?

If the center tap remains unconnected to any external circuit: yes

If you are using the center tap to generate a split phase system on the secondary: no

 

[jim dungar]

If the center tap remains unconnected to any external circuit: yes

And I would say, only if the transformer nameplate shows that as an acceptable connection.

 

[Jerramundi]

If you have a 3KVA transformer constructed with 2 secondary coils which are 'field wired' in parallel, the secondary protection would be for the _parallel_ coils, not for the individual coils.

This is what I meant by a clear distinction that does not make a practical difference. Yes, if you have two coils in parallel versus a single coil, you have a potential failure mode where one of the parallel coils gets disconnected and the other coil gets overloaded. However secondary protection _if required_ would not protect from this failure mode. In cases where secondary protection is required, it is for the _entire_ secondary, not portions of the secondary.

Now if you had a 20A secondary consisting of two parallel 10A coils, with separate 10A OCPD in each coil that would be a different story, but NEC doesn't require this.

-Jon

I'm clear on the ideas that secondary protection is not intended to protect "portions of the secondary" and never meant to imply that it was intended for that purpose.

I was just giving a hypothetical that I thought put a little more value behind the idea of the secondary being a SINGLE 3 kVA coil as opposed to TWO 1.5 kVA coils wired in parallel because if my secondary is a SINGLE coil rated for 3kVA, there is NEVER an instance in which that coil could see more than it's rated kVA without first tripping the secondary 20A OCPD.

Perhaps this is not mandated by the NEC, but I think it's still somewhat relevant and even practical.

 

[Jerramundi]

And I would say, only if the transformer nameplate shows that as an acceptable connection.

I get what you all are getting about the nameplate configurations in regards to "2-wire (single voltage) secondary" and honestly, you may be correct.

The only thing that still bothers me about this is (1) the potential failure I've identified and (2) my Handbook notes, which I concede are not technically enforceable, that state that this "2-wire (single voltage) secondary," which subsequently allows for primary protection only, is supposed to be, and I quote, a "special case."

Based on my perusing of various products available, finding such a single coil secondary would in fact be a "special case." I know I'm rehashing an old argument and we're just going in circles, but I'm just stating my personal feelings about this.

If all multiple coil secondaries capable of being wired for a "2-wire (single voltage) secondary" are permissible, this would no longer be a "special case." That, coupled with the potential failure identified, has me leaning towards choosing to see this as NOT permissible for primary protection only...

...but I concede that you all may be correct and my arguments are more hypothetical and personal as opposed to explicitly NEC based.

 

[jim dungar]

My position is how on Earth can two secondary windings ....

Also, let me add that the word winding has several different uses when it comes to describing transformers. The uses can be somewhat confusing, so their context needs to be explored.

For example, two of the most common uses are:
Referring to the primary and secondary windings of transformers as singular items regardless what the internal components really are. I would say a 3-phase delta-wye transformer is a 2 winding device, and a 3-phase wye-delta-wye is a three winding one.

Saying that a winding is the input or output coils of a transformer with a specific voltage across its ends of the winding. It would be correct to say, in the single-phase transformers output windings may be connected in series, parallel, singular, or center-tapped.

 

[jim dungar]

If all multiple coil secondaries capable of being wired for a "2-wire (single voltage) secondary" are permissible, this would no longer be a "special case." That, coupled with the potential failure identified, has me leaning towards choosing to see this as NOT permissible for primary protection only...

What makes it a special case is the 2-wire output, not the transformer connection.
The NEC is not really concerned with the transformer failing internally. It is concerned with the external circuit conductors.
In a properly protected system with primary only protection, the transformer secondary conductors would not be under-protected even if one of the paralleled windings were to fail.

 

[Jerramundi]

What makes it a special case is the 2-wire output, not the transformer connection.
The NEC is not really concerned with the transformer failing internally. It is concerned with the external circuit conductors.
In a properly protected system with primary only protection, the transformer secondary conductors would not be under-protected even if one of the paralleled windings were to fail.

I get what you're saying... but I feel like Article 450 and the requirements surrounding overcurrent protection for the actual transformer primaries and secondaries (not the primary or secondary conductors) lends credence to the argument that the NEC IS IN FACT concerned with protecting the transformer itself and not just the conductors attached to it... no?

There is a clear distinction between Articles 450 and 240 in regards to protecting the transformer itself and the conductors attached to it. It's just that they can overlap and serve each other's requirements.

 

[Jerramundi]

I just wrote up a 1,000 word + analysis of this discussion for my own educational purposes and you all actually have a point. I'd be happy to share it with anyone whose interested, but will just summarize it below...

If you disclude my hypothetical failure...

This particular transformer w/ primary protection only (per 450.3) could allow up to 1.6 x the xfmr kVA rating across the secondary before tripping the maximum allowable primary OCPD.

This particular transformer w/ primary and secondary protection (per 450.3) could allow up to 1.4 x the xfmr kVA rating across the secondary before tripping the maximum allowable secondary OCPD.

Even if you switch to my proposed "indisputable '2-wire (single voltage) secondary" ... that is single coil secondary...

That particular transformer with primary protection only (per 450.3) would still allow 1.6 x the xfmr kVA rating across the secondary before tripping the primary OCPD

That particular transformer with primary and secondary protection (per 450.3) would still allow 1.4 x the xfmr kVA rating across the secondary before tripping the secondary OCPD.

So essentially, without my hypothetical failure in the picture, the results are the same regardless.

It's only when you include my hypothetical failure that those rates get skewed and one could argue that the single coil secondary would clearly be a better option in terms of protecting the transformer itself.

 

[Jerramundi]

I don't really have much more to add and would like to thank everyone for their input.

I will still be contacting Mike Holt and the NFPA to get some more official clarification, but again, appreciate the input.

It's been a ride. Stay well brothers.

 

[kwired]

Perhaps a poor choice of wording, but I think you know that I meant "primary protection only" when I said "if I want to NOT have secondary conductor protection." Or at least I feel like you should in the context of this whole thread. But yes, begrudgingly I admit that is a valid clarification. *grumble* *grumble*

Read what it says carefully. It isn't that there is no secondary protection, it is permitted via the primary to secondary ratio to protect the secondary in those specific situations by a device on the primary side. Think about it - if you have only a single voltage in and single voltage out you have a fixed ratio, in our case we have been discussing 2:1. You put a 10 amp OCPD on the primary you can not draw more than 20 amp on secondary or you will blow the primary device - it is providing 20 amp secondary protection. It is when you are utilizing multiwire output (120/240) you can have unbalance output and overload part of secondary but still remain under the primary rating and in that case you generally must have separate secondary protection.

My position is how on Earth can two secondary windings qualify as a "single voltage, two wire secondary" ?

How slightly different but yet similar - dual voltage motor where you connect in series or parallel to change applied voltage. You still get same VA rating either way. If one section of winding fails when in the high voltage configuration you have open circuit and it won't run. If same section fails when in low voltage configuration you only have open circuit in that one segment and other segment has to draw more to make up for it, try to run at full VA rating and you eventually burn it out.

Another thing that is similar about the motor as the transformer - we don't put overload protection in each individual segment of winding, we put it in the supply conductor and size according to which voltage we are connected for.

This is very common on most all general purpose motors from fractional HP to about 50 or 60 HP. Above that you start to see them wound for single voltage only, but may still see a few dual voltage ones here or there.

 

[LarryFine]

How slightly different but yet similar - dual voltage motor where you connect in series or parallel to change applied voltage.

Using your example, if a "single voltage, two wire motor" was specified, would you counter that a dual-voltage motor qualifies, because it will be connected as a single-voltage motor when it's installed?

 

[Jerramundi]

Read what it says carefully. It isn't that there is no secondary protection, it is permitted via the primary to secondary ratio to protect the secondary in those specific situations by a device on the primary side. Think about it - if you have only a single voltage in and single voltage out you have a fixed ratio, in our case we have been discussing 2:1. You put a 10 amp OCPD on the primary you can not draw more than 20 amp on secondary or you will blow the primary device - it is providing 20 amp secondary protection. It is when you are utilizing multiwire output (120/240) you can have unbalance output and overload part of secondary but still remain under the primary rating and in that case you generally must have separate secondary protection.


How slightly different but yet similar - dual voltage motor where you connect in series or parallel to change applied voltage. You still get same VA rating either way. If one section of winding fails when in the high voltage configuration you have open circuit and it won't run. If same section fails when in low voltage configuration you only have open circuit in that one segment and other segment has to draw more to make up for it, try to run at full VA rating and you eventually burn it out.

Another thing that is similar about the motor as the transformer - we don't put overload protection in each individual segment of winding, we put it in the supply conductor and size according to which voltage we are connected for.

This is very common on most all general purpose motors from fractional HP to about 50 or 60 HP. Above that you start to see them wound for single voltage only, but may still see a few dual voltage ones here or there.

I appreciate you taking the time to expand on this, especially the motors part because that is another area I need to expand my knowledge in, but you need to go back and read what I wrote.

Everything you've just taken the time to describe (aside from the motor stuff) implies that I was unaware that it's not "no secondary protection, but secondary protection via the primary" and that is unequivocally untrue. Every time I've used the wording "no secondary protection," I've been referring to "primary only." So once again, a poor choice of words on my part.

I even explicitly admitted that my choice of wording was poor. (see *grumbling*)

 

[jim dungar]

It's only when you include my hypothetical failure that those rates get skewed and one could argue that the single coil secondary would clearly be a better option in terms of protecting the transformer itself.

If the NEC was really concerned with protecting the transformer it would likely contain provisions for sizing the transformer, like it does for services, feeders and branch circuit conductors.

As far as your hypothetical situation. The transformer has already failed, otherwise one winding would not have opened or been lost. How does the NEC prevent that from occurring?

 

[kwired]

I appreciate you taking the time to expand on this, especially the motors part because that is another area I need to expand my knowledge in, but you need to go back and read what I wrote.

Everything you've just taken the time to describe (aside from the motor stuff) implies that I was unaware that it's not "no secondary protection, but secondary protection via the primary" and that is unequivocally untrue. Every time I've used the wording "no secondary protection," I've been referring to "primary only." So once again, a poor choice of words on my part.

I even explicitly admitted that my choice of wording was poor. (see *grumbling*)

That is fine, just making sure you understand the concept of why it is allowed to protect the secondary with the primary device.

There are other items that are dual volt out there as well, heating elements that get connected series or parallel to change volts yet maintain same output rating is another example.

Generators also can have dual volt ability by connecting windings in series vs parallel - and would be similar to the transformer situation since they are both sources instead of loads. Packaged portables or standby units are wired for whatever intentions are for it to supply, but there are more "generic" units out there that you can configure for your needs and have to provide your own prime mover for them as well.

 

[kwired]

Using your example, if a "single voltage, two wire motor" was specified, would you counter that a dual-voltage motor qualifies, because it will be connected as a single-voltage motor when it's installed?

Absolutely.

Besides, unless it is a definite purpose or OEM motor, most general purpose motors up to about 60 HP are going to be dual voltage motors if they are three phase, single phase usually up to 2 HP is dual voltage, maybe can find an occasional 3 HP.

 

[LarryFine]

Absolutely.

Then we're at an impasse.

 

[Jerramundi]

That is fine, just making sure you understand the concept of why it is allowed to protect the secondary with the primary device.

I just feel like all of this is creating FAR TOO MANY scenarios where primary protection only is justifiable as opposed to being the rare exception I think it should be... but then again that opinion, and it is an opinion, results from me being hung up on a handbook note which isn't enforceable.

 

[david luchini]

as opposed to being the rare exception I think it should be...

I'll point out the obvious...neither the Code nor the handbook commentary that you refer to mentions "Rare Exception."

 

[kwired]

I just feel like all of this is creating FAR TOO MANY scenarios where primary protection only is justifiable as opposed to being the rare exception I think it should be... but then again that opinion, and it is an opinion, results from me being hung up on a handbook note which isn't enforceable.

If you have a 2:1 ratio transformer, why wouldn't 10 amp OCPD protect the secondary at 20 amps? If it has dual voltage ability but you connect it for a single voltage only, you still have same thing. Yes you could have failure of half the windings - the thing is toast anyway if that happens.

 

[Jerramundi]

I'll point out the obvious...neither the Code nor the handbook commentary that you refer to mentions "Rare Exception."

Not true. The 2014 Handbook has explicit author's commentary following 240.4(F) that explicitly refers to these circumstances as "special cases."

Am I taking a little bit of creative freedom in going from "special case" to "rare exception?" Yes, but that is hardly a stretch.
Both phrases imply a degree of rarity.. and I maintain that the ability to wire any dual winding, multiple voltage output xfmr as a "2-wire (single voltage) secondary" (even if listed on the nameplate) is a direct contradiction to said rarity.