1-Voltage, 2-Wire Secondary

[Jerramundi]

Then what does "single voltage, two wire secondary" mean, and why bother mentioning it?

My 2014 Handbook has an interesting "author commentary" that's swaying me in your direction Larry.

Albeit not technically "code" and only "author's commentary," it says....

1. A transformer with a 2-wire primary and a 2-wire secondary...
2. A 3-phase, delta-delta...

Except for those two special cases, transformer secondary conductors must be protected by the use of OCPD's...

"Special case" to me implies a rare circumstance, which is swaying me in the direction of Larry here that "single voltage, two wire secondary" means a transformer quite literally with ONLY two wire leads coming off the secondary coil and the capacity to be wired for only a single voltage output.

It would in fact be a "special case," because I haven't been able to find any transformers with ONLY two wires coming off the secondary coil and ONLY the capacity to be wired for a single voltage output.

It seems most products come with the capacity for at least two secondary voltages...

 

[Jerramundi]

I recall this topic coming up in the past. I am in the camp of the "what is used" people. I certainly see how the wording says/could be interpreted as "what the transformer has" but Im going to let the laws of physics tip the scales. Perhaps a code change/clarification is in order.

Looks like I've stumbled upon something that is actually a point of contention. I feel validated haha

 

[electrofelon]

The description of "single voltage, two wire secondary" refers to how the unit is manufactured, not how it's wired or used.

Does it? I think it is vague. Also, Look at the last paragraph of 240.21(C)(1) and say the words I put in red:

single phase (other than 2-wire ) and multiphase (other than delta-delta) transformer secondary conductors are not considered protected by the primary OCPD

 

[Jerramundi]

Does it? I think it is vague. Also, Look at the last paragraph of 240.21(C)(1) and say the words I put in red:

The only way this works, and it can work, so I agree with the description of it being vague, is if you DON'T count the four wire leads coming off the secondary coil as "secondary conductors" and instead as "terminals" with the "secondary conductors" beginning at the spliced connection.

Otherwise, I'm beginning to think that it's quite clear that you do NOT have two wires.

The NEC lacks a definition of "Terminal" to my knowledge, but Wikipedia *gasp* defines it as "the point at which a conductor from a component, device or network comes to an end." In which case these four wire leads would be "terminals" as opposed to "secondary conductors."

 

[Jerramundi]

If we go with the argument that these four wire leads coming off the secondary coil are "terminals" and not "secondary conductors," we are not in violation of the "secondary conductors" being required to be "single voltage, two wire" because we can connect such hereto.

However, the latter part of 240.4(F) Transformer Secondary Conductors... seems to suggest that this "single voltage, two wire" requirement may be referring to the transformer itself (i.e. including the "terminals") when it reads...

"Conductors supplied by the secondary side of a single phase transformer having a 2-wire (single voltage secondary)... shall be permitted to be protected by overcurrent protection provided on the primary..."

 

[jim dungar]

It would in fact be a "special case," because I haven't been able to find any transformers with ONLY two wires coming off the secondary coil and ONLY the capacity to be wired for a single voltage output.

'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.

 

[kwired]

Two wire means two at the xfmr.

You could take/tap eight conductors out of the transformer. Does that mean it is no longer a two wire?

If utilizing only two of them (per primary/secondary sides), or if series or paralleling different sections (primary or secondary) to come up with a two wire input or output then primary and secondary current are always going to be proportional to one another and in that situation primary overcurrent device can effectively protect secondary conductors.

 

[kwired]

My 2014 Handbook has an interesting "author commentary" that's swaying me in your direction Larry.

Albeit not technically "code" and only "author's commentary," it says....

1. A transformer with a 2-wire primary and a 2-wire secondary...
2. A 3-phase, delta-delta...

Except for those two special cases, transformer secondary conductors must be protected by the use of OCPD's...

"Special case" to me implies a rare circumstance, which is swaying me in the direction of Larry here that "single voltage, two wire secondary" means a transformer quite literally with ONLY two wire leads coming off the secondary coil and the capacity to be wired for only a single voltage output.

It would in fact be a "special case," because I haven't been able to find any transformers with ONLY two wires coming off the secondary coil and ONLY the capacity to be wired for a single voltage output.

It seems most products come with the capacity for at least two secondary voltages...

The transformer secondary conductors that you must protect are the two field installed conductors you run, not the conductors that are factory assembled part of the transformer. Often those factory conductors are higher temp insulation and smaller size than what you will connect to them as well.

Dual volt in most cases is nothing more than putting two coils in series vs parallel to attain desired volts. VA rating remains the same, volts across each coil remains same and amps through each coil remains same.

Other thing you run into is primary taps to adjust for input volts variances, but you only connect to two of the taps and use it as a two wire input.

 

[GeorgeB]

I actually found a 10A 2-Pole Breaker, which I didn't even know existed. Not sure if I can use it as it would be considered a "nonstandard ampere rating," but again... I digress.

Automation Direct now sells Eaton's UL489 DIN mount breakers in a wide current range, both 480/277 (0.5-32 Amp) and 240 (0.5-63 Amp). One local panel shop says they have no problems with them.

 

[Jerramundi]

'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.

True. When I said "rare" / "special case" I was referring to the Hubble/Acme and Square D catalogs I've been looking at for larger transformers, in which I was hard pressed to find ANY instances of literally only two wire leads coming off the secondary coil with only the capacity for a single voltage output... except for one autotransformer in all of two catalogs... which to me seemed to justify the "special case" description and reinforce that "two wire, single voltage secondary" was inclusive of how the transformer itself was manufactured...

...because if we include transformers that are manufactured with more than two wire leads and more than the capacity for a single voltage output, "two wire, single voltage secondary" is no longer a "special case," but extremely common... as a wide variety of transformers could now be wired to produce such a scenario.

 

[Jerramundi]

Automation Direct now sells Eaton's UL489 DIN mount breakers in a wide current range, both 480/277 (0.5-32 Amp) and 240 (0.5-63 Amp). One local panel shop says they have no problems with them.

Yea, but those are inverse time circuit breakers, are they not? Which 240.6 explicitly allows nonstandard ampere ratings for. What I'm talking about would be an instantaneous trip and a nonstandard ampere rating, which as far as I can tell, the NEC does not explicitly prohibit, but does not explicitly approve of either.

 

[jim dungar]

...because if we include transformers that are manufactured with more than two wire leads and more than the capacity for a single voltage output, "two wire, single voltage secondary" is no longer a "special case," but extremely common... as a wide variety of transformers could now be wired to produce such a scenario.

Special does not mean rare or uncommon, the NEC devotes an entire article, 725, to circuits that are typically are two wire and single voltage.

You are hung up on the internal construction of a device, even though that construction is not subject to the NEC.
I am sure the nameplate of your devices says where to connect your 2 secondary conductors to get a single voltage.

 

[ptonsparky]

Yea, but those are inverse time circuit breakers, are they not? Which 240.6 explicitly allows nonstandard ampere ratings for. What I'm talking about would be an instantaneous trip and a nonstandard ampere rating, which as far as I can tell, the NEC does not explicitly prohibit, but does not explicitly approve of either.

I don't think you want an instantaneous trip on transformer protection. Time delay.

 

[Jerramundi]

I don't think you want an instantaneous trip on transformer protection. Time delay.

No motors involved, just LED lights, so I'm not really concerned about an inrush current.

Just happens to be 300+ ft away from the panel so I'm utilizing the 240V at the panel and stepping down at the sign. I've also read that isolation helps protect sensitive equipment and filter out harmonics, so taking that extra step because the sign is expensive, but I'm still learning about harmonics.

 

[ptonsparky]

No motors involved, just LED lights, so I'm not really concerned about an inrush current.

Just happens to be 300+ ft away from the panel so I'm utilizing the 240V at the panel and stepping down at the sign. I've also read that isolation helps protect sensitive equipment and filter out harmonics, so taking that extra step because the sign is expensive, but I'm still learning about harmonics.

LED has a high current at start up.

 

[Jerramundi]

LED has a high current at start up.

Fair enough. I guess I don't typically think about inrush current when thinking about LEDs, but you are correct.

 

[electrofelon]

No motors involved, just LED lights, so I'm not really concerned about an inrush current.

Just happens to be 300+ ft away from the panel so I'm utilizing the 240V at the panel and stepping down at the sign. I've also read that isolation helps protect sensitive equipment and filter out harmonics, so taking that extra step because the sign is expensive, but I'm still learning about harmonics.

I dont think any of that is an issue. I have yet to come across these "sensitive electronics" people always mention.

 

[curt swartz]

The transformer has inrush which may trip an undersized breaker.

 

[Jerramundi]

Appreciate the additional input, but ultimately digression. I will take the comments into consideration on my own time, but prefer to stay on topic.

Seems like the majority opinion here is that the transformer pictured above, when wired for a single voltage output and connected to a set of two wires (beyond transformer's manufactured coil wire leads themselves), would qualify as a "two wire, single voltage secondary."

 

[LarryFine]

Seems like the majority opinion here is that the transformer pictured above, when wired for a single voltage output and connected to a set of two wires (beyond transformer's manufactured coil wire leads themselves), would qualify as a "two wire, single voltage secondary."

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.