1-Voltage, 2-Wire Secondary

[Jerramundi]

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

Well that's the thing... the argument I'm currently making, while IMHO a valid attempt to clarify 240.4(F), just takes us back to the very beginning of this thread and the initial question posed by my OP... does this transformer, when wired for a two-wire single voltage output, qualify... despite the clear existence of more than two secondary coil wire leads.

But now that I've bounced ideas off you all and received your much appreciated input, I think post #80 clarifies what the NEC is trying to prevent here.

That you may be correct about the transformer as field wired, not as manufactured.

 

[Jerramundi]

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

Although I still think "two-wire, single voltage" may be about "as manufactured."

If you think about it... if for whatever reason, one of the two coils on the secondary side failed, you would be passing the full load through only half of the secondary and this wouldn't trip the primary before the transformer incurred irreparable damage (as also pointed out by other previous posts by others that I don't feel like citing but will give credit to).

 

[LarryFine]

Is there a place where this question can be asked and definitively resolved? Perhaps Mr. Holt himself, or an NEC department?

 

[Jerramundi]

Is there a place where this question can be asked and definitively resolved? Perhaps Mr. Holt himself, or an NEC department?

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I might take a minute to contact both. Good suggestion.

For the purpose of getting this job done, I think I'm just going to error on the side of caution and assume that, even if field wired to produce a single voltage with two points of connection, this particular transformer is NOT a single voltage, 2-wire secondary... and just provide secondary conductor protection.

Always better to error on the side of caution.

 

[jim dungar]

If you think about it... if for whatever reason, one of the two coils on the secondary side failed, you would be passing the full load through only half of the secondary and this wouldn't trip the primary before the transformer incurred irreparable damage (as also pointed out by other previous posts by others that I don't feel like citing but will give credit to).

Transformer protection, per the NEC, is not about preventing 'irreparable' damage to a transformer. I think it could be argued that the NEC simply wants a failed transformer removed prior to damaging the building wiring/conductors.

How do you know how many coils are internal to a transformer, and what difference does it make?
If two coils are connected in series to provide a single voltage of 240V, the failure of a single coil would not result in an overload condition.

 

[Jerramundi]

Transformer protection, per the NEC, is not about preventing 'irreparable' damage to a transformer. I think it could be argued that the NEC simply wants a failed transformer removed prior to damaging the building wiring/conductors.

It's just a side bar regarding the whole "2-wire (single voltage) secondary" debate and whether or not the particular transformer in the OP qualifies as such if (1) field wired as such, as opposed to (2) manufactured as such.

That is, this particular transformer as shown in the OP, wired for 120V only (i.e. two 120V coils wired in parallel) in which each coil produces 120V and the load amps divides between the two coils.

If one were to fail, the remaining coil would still produce 120V, but be tasked with handling the full load current and is not manufactured to do so (i.e. only half of the secondary coil remaining reduces the VA rating on the secondary from 3kVA to 1.5kVA and 120V *20A = 2400 kVA, resulting in failure and potentially damage).

Which to me suggests that erring on the of caution would be a better solution (i.e. concluding that this particular transformer is NOT 2-wire (single voltage) secondary and to... again, err on the side of caution, and just provide secondary conductor protection.

 

[mivey]

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?

Exactly. Those terminal jumpers may be internal or external and the jumper conductors don't necessarily match the supply conductors.

 

[jim dungar]

It's just a side bar regarding the whole "2-wire (single voltage) secondary" debate and whether or not the particular transformer in the OP qualifies as such if (1) field wired as such, as opposed to (2) manufactured as such.

A re-connectable transformer that shows two wire configurations, on its nameplate, is not being 'field modified', it is being used the way it was manufactured.

Personally, it has been probably 30 years since I had an installation with a general purpose transformer without secondary protection.

 

[Jerramundi]

A re-connectable transformer that shows two wire configurations, on its nameplate, is not being 'field modified', it is being used the way it was manufactured.

I never meant to convey modification in saying "field wired." I'm in agreement in that it is being wired the way it was manufactured to be... I'm just not 100% solid on the idea that this "field wiring" (even as intended by the manufacturer) equals "2-wire (single voltage) secondary."

Part of me still feels like... and this is why I brought up the potential failure above... it would be better to have a single coil 120V secondary rated for 3kVA as opposed to a two coil 120V secondary wired in parallel rated for 3kVA.

 

[Jerramundi]

Personally, it has been probably 30 years since I had an installation with a general purpose transformer without secondary protection.

Good to know. I don't typically work with SDS' as a mainly single family home, residential electrician. I'm trying to cover all my bases when quoting low to high and seeing if there was an approved way to avoid installing secondary protection, save on cost, and still be NEC compliant/safe.

 

[mivey]

I never meant to convey modification in saying "field wired." I'm in agreement in that it is being wired the way it was manufactured to be... I'm just not 100% solid on the idea that this "field wiring" (even as intended by the manufacturer) equals "2-wire (single voltage) secondary."

Part of me still feels like... and this is why I brought up the potential failure above... it would be better to have a single coil 120V secondary rated for 3kVA as opposed to a two coil 120V secondary wired in parallel rated for 3kVA.

No big deal. The single coil could also fail. You might even get two internal coils with internal jumpers that make it have one external voltage. Internal or external jumpers don't matter and we see transformers with both configurations every day.

 

[jim dungar]

.... save on cost, ....

Unless you are paying yourself <$15.00/hr, the amount of time you have spent on this exercise vastly exceeds the cost of providing secondary protection for a single 20A 120V circuit.

 

[Jerramundi]

No big deal. The single coil could also fail. You might even get two internal coils with internal jumpers that make it have one external voltage. Internal or external jumpers don't matter and we see transformers with both configurations every day.

Yes, but in the case of a 20A OCPD on the secondary, with two 120V coils wired in parallel, and rated for 3kVA cumulatively, one failing could result in the other pulling up to 2400VA on a 1500VA rated coil without tripping the secondary OCPD. With a single coil rated for 3kVA, it could never surpass the rated kVA without tripping the secondary OCPD.

Unless I misunderstand transformer kVA ratings, which is entirely possible, and both secondary coils are rated for 3kVA in a single transformer rated for 3kVA.

 

[Jerramundi]

Unless you are paying yourself <$15.00/hr, the amount of time you have spent on this exercise vastly exceeds the cost of providing secondary protection for a single 20A 120V circuit.

Eh somewhat agree and disagree. I'm investing personal time in advancing my electrical education. It's worth it in my eyes. This exercise goes beyond any one individual job. I'll be including some office hours in the quote, but not the full amount of time. It's the sacrifice I make to advance my knowledge base. I don't have kids or a wife to support, so yea, lol.

 

[jim dungar]

I'm investing personal time in advancing my electrical education.

So did you learn anything, that you can use in the future? From what I can tell you will be installing the system the way you intended to from the beginning.

 

[Jerramundi]

So did you learn anything, that you can use in the future? From what I can tell you will be installing the system the way you intended to from the beginning.

Absolutely. Every single comment on this thread has been valuable. Even the ones I've ultimately disagreed with. Such is the value of discourse. Bouncing ideas off one another, discussing code references, electrical principles, etc. All worth while. I've yet to find any relationship in the field as valuable as the impersonal ones on this forum. It's just too competitive. IMHO, this is how it should be. This is how we should treat one another... and I plan to return the value by helping others on this forum as I grow.

I only posted the OP with the intention of utilizing a particular product, but was not set on whether I would/should provide secondary protection or not.

 

[winnie]

Yes, but in the case of a 20A OCPD on the secondary, with two 120V coils wired in parallel, and rated for 3kVA cumulatively, one failing could result in the other pulling up to 2400VA on a 1500VA rated coil without tripping the secondary OCPD. With a single coil rated for 3kVA, it could never surpass the rated kVA without tripping the secondary OCPD.

Unless I misunderstand transformer kVA ratings, which is entirely possible, and both secondary coils are rated for 3kVA in a single transformer rated for 3kVA.

I believe that you have hit on a real distinction between a 120/240V secondary wired for 120V versus a 'true' 120V secondary.

However I don't believe that this distinction makes a practical difference; even if you install secondary protection it will be for the entire secondary, not the individual coils of the secondary.

IMHO the 'real' reason you might be forced to use both primary and secondary protection is that if you choose to forgo the secondary protection the required primary protection might be subject to 'nuisance tripping' because of transformer inrush. Compare the maximum allowed primary OCPD if you have separate secondary OCPD vs the required primary OCPD if it must also protect the secondary.

I believe you will be OK, and code compliant, but this is a design issue to consider.

-Jon

 

[LarryFine]

I might take a minute to contact both. Good suggestion.

Please do.

 

[Jerramundi]

I believe that you have hit on a real distinction between a 120/240V secondary wired for 120V versus a 'true' 120V secondary.

However I don't believe that this distinction makes a practical difference; even if you install secondary protection it will be for the entire secondary, not the individual coils of the secondary.

IMHO the 'real' reason you might be forced to use both primary and secondary protection is that if you choose to forgo the secondary protection the required primary protection might be subject to 'nuisance tripping' because of transformer inrush. Compare the maximum allowed primary OCPD if you have separate secondary OCPD vs the required primary OCPD if it must also protect the secondary.

I believe you will be OK, and code compliant, but this is a design issue to consider.

-Jon

I'm digressing into a design issue now, yes... but this design issue is contingent upon a code requirement. If this 240V/120V secondary, wired for 120V, is in fact a "2-wire (single voltage)" secondary... it makes a certain design plausible (i.e. no secondary protection required). If not, said design option is no longer plausible (i.e. secondary protection now required).

 

[Jerramundi]

Please do.

Maybe not immediately, but I will. I'll message when/if I hear from anyone.