NEC 240.92 D transformer secondary taps

[Jpflex]

I know this area can become ambiguous because code requires not only protection of transformers but the conductors.

In 242.1 D (s) outside

Conductors tapped to a transformer secondary may be considered protected without neccesarily having an OCPD at the conductors line side:

If the sum of the overcurrent devices at the conductor termination limits the load to the conductor ampacity.

The overcutrent devices shall consist if not mote tyam 6 circuit brrakers or set of fuses MOUNTED IN A SINGLE ENCLOSURE, group of separate enclosures or in or on a SWITCHBOARD OR SWITCHGEAR.

THERE SHALL BE NO MORE THAN 6 OVERCURRENT DEVICES GROUPED IN ONE LOCATION!!!!



Therefore, if i were to use this transformer secondary tap to terminate into a SINGLE "subpannel" with a single main breaker (series with phase conductors as typical) and more than 6 breakers from the pannel buss to feed branch circuits, then since there are more than 6 breakers within this SINGLE enclosure, how can many projects be compliant as the code is written?

Obviouly 6 loads or OCPD limit wont meet many needs.


I do understand that a power distribution block may be used to feed 6 breakers with an accumulative ampere sum less than or equal to the feeder tap ampacity but how does this code affect situations mentioned above? Thanks

 

[Jpflex]

And then there is NEC table 450.3 B which opposes the secondary conductor tap rule by forrcing a specific fuse rating on the primary when NEC 242.1 D does not

 

[Jpflex]

And third there is another contradicting code in NEC 240.4 F that says that a transformer secondary conductors shall not be considered protected by the primary OCPD unless the secondary is a single phase 2 wire or 3 phase 3 wire delta (ungrounded systems) and primsry OCPD ampacity does not exceed secondary conductor ampacity x secondary to primary voltage ratio.

This is not mentioned as a requirement in the other sections governing secondary conductors and OCPD

 

[Jpflex]

Does this mean that a 3 wire single phase transformer (with neutral) cannot have its secondary considered protected by the primary OCPD and must therefore redundantly have a breaker on the line side of the secondary and load side of secondary termination. This seems overkill and never or hardly done in the field?

 

[wwhitney]

Therefore, if i were to use this transformer secondary tap to terminate into a SINGLE "subpannel" with a single main breaker (series with phase conductors as typical) and more than 6 breakers from the pannel buss to feed branch circuits, then since there are more than 6 breakers within this SINGLE enclosure, how can many projects be compliant as the code is written?

Your posts are a bit disjointed, so I'm only responding to selected portions which track for me. As to this question, the relevant text in 240.92(D) (on outside feeder taps, not transformer secondaries as per the thread subject) refers to "overcurrent devices AT the conductor termination." In your example, there is only one such overcurrent device, the single main breaker. The branch breakers in the panel are not connected directly to the tap conductors.

Does this mean that a 3 wire single phase transformer (with neutral) cannot have its secondary considered protected by the primary OCPD and must therefore redundantly have a breaker on the line side of the secondary and load side of secondary termination. This seems overkill and never or hardly done in the field?

Yes, any transformer with a wye secondary will require OCPD on both the primary side and the secondary side. Three checks are required for sizing those OCPD: the primary and secondary OCPD need to protect the transformer per 450.3(B) (although some of the options there look at only the primary OCPD size); the primary OCPD needs to protect the transformer primary conductors (although if the primary and secondary conductors total at most 25', 240.21(B)(3) is an option); and the secondary OCPD(s) will need to protect the secondary conductors per 240.21(C) (possibly modified by 240.92, have not reviewed that full text).

Cheers, Wayne

 

[Jpflex]

I will simplify NECs contradictions below:

1) NEC 240.4 F says conductors "tapped" from a transformer secondary CANNOT be considered protected from transformer primary OCPD unless the conductors are 2 wire single phase or 3 wire delta delta....

Repeated 240.21 (1)


2) 240.21 C(2) (3) (4) (5) contradicts above rule by allowing secondary conductors [of any system] without OCPD based primarily on secondary conductor length.

This makes it difficult to justify whether to install 2 breakers, one at line side of transformer secondary and another at load side of transformer secondary for a 3 wire single phase with neutral or 4 wire delta delta with a neutral

 

[wwhitney]

I will simplify NECs contradictions below:

No contradiction, see below.

1) NEC 240.4 F says conductors "tapped" from a transformer secondary CANNOT be considered protected from transformer primary OCPD unless the conductors are 2 wire single phase or 3 wire delta delta....

Transformer secondary conductors are not taps. They are like taps in not having properly sized (complying with 240.4) OCPD at their point of supply. They are unlike taps in having no OCPD (conductively) at their point of supply, while feeder taps have oversized OCPD at their point of supply.

But yes, for a transformer that is 2-wire to 2-wire, or 3-wire to 3-wire three phase, you can have an install where the primary OCPD protects the primary conductors, the transformer itself, and the secondary conductors, with no secondary OCPD required.

2) 240.21 C(2) (3) (4) (5) contradicts above rule by allowing secondary conductors [of any system] without OCPD based primarily on secondary conductor length.

No contradictions; for 240.21(C), you have your choice of which subsection to use. 240.21(C)(1) corresponds to my previous paragraph, so you can choose that.

But if you don't choose that, or can't because of the transformer configuration, then you will need to use one of the other sections, all of which require secondary OCPD to protect the secondary conductors.

This makes it difficult to justify whether to install 2 breakers, one at line side of transformer secondary and another at load side of transformer secondary for a 3 wire single phase with neutral or 4 wire delta delta with a neutral

Transformer secondary conductors as covered by 240.21(C) end at the first OCPD. The only reason you'd need both an OCPD at the transformer on the secondary side, and a further main OCPD at a panel supplied by that first OCPD is if the conductors between them are not protected by the first OCPD, so you are using a 240.21(B) feeder tap rule for those conductors.

Cheers, Wayne

 

[augie47]

I see no contradiction.
Transformer secondary conductors must meet the requirements of 240.21(C)(1)-(6).
(c)(2)-(c)(6) require secondary OCP
(C)(1), for 2 wire or delta-deal does not.

 

[Jpflex]

Yes, any transformer with a wye secondary will require OCPD on both the primary side and the secondary side. Three checks are required for sizing those OCPD: the primary and secondary OCPD need to protect the transformer per 450.3(B) (although some of the options there look at only the primary OCPD size); the primary OCPD needs to protect the transformer primary conductors (although if the primary and secondary conductors total at most 25', 240.21(B)(3) is an option); and the secondary OCPD(s) will need to protect the secondary conductors per 240.21(C) (possibly modified by 240.92, have not reviewed that full text).

Cheers, Wayne

Here are two contradicting codes

NEC 242.1 E:

conductors supplied by the secondary side of a transformer shall be permitted to be protected by the overcutrent protection provided on the primary side of the transformer if the primary device current protection x secondary to primary voltage ratio protects sscondary


Nec 240.4 F

Secondary conductors SHALL NOT be considered to be protected by primary OCPD unless 2 wire single phase or 3 wire 3 phase delta delta...


In your case 242.1 E WOULD NOT require your Wye transformer secondary example to have secondary protection

However 240.4 F would require protection on secondary

 

[wwhitney]

Here are two contradicting codes

First, you mean 240.92(E), not 242.1(E).

In one sense you are technically correct, as there is nothing in 240.4 or earlier that says "except as provided in Part VIII of this article."

But if you look at the start of 240.90, the start of Part VIII of Article 240, it says "Overcurrent protection in areas of supervised industrial installations shall comply with all of the other applicable provisions of this article, except as provided in Part VIII" So clearly Part VIII is meant to be able to amend the earlier parts of the article.

The scope of Article 240 says in 240.1 "Parts I through VII of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal. Part VIII covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 1000 volts, nominal."

Really that first sentence should say "Parts I through VII of this article provide the general requirements, excluding supervised industrial installations, for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal."

Anyway, that is implicit, so 240.92(E) amends 240.4(F).

Cheers, Wayne

 

[wwhitney]

BTW, looking at 240.92(E) and 240.4(F), the first refers to "the maximum effective primary-to-secondary transformer voltage ratio," while the second refers to "the secondary-to-primary transformer voltage ratio."

In the case of say, a 480V Delta: 208Y/120V Wye transformer, the issue with using the primary OCPD to protect the secondary conductors is that a 10A 120V L-N load on the secondary causes a 2.5A L-L load on the primary. While the (nominal) voltage ratio is arguably 480:208, which is less than the 4:1 current ratio. So 240.4(F) with its simpler language is restricted to cases where this does not happen.

However, I would say that the "maximum effective" voltage ratio for such a transformer is in fact 4:1, as the example demonstrates, and that is language used in 240.92(E). As far as I am aware, 240.4(F) could be amended to use the same language (along with a fine print note clarifying the meaning as above), and then it would not be necessary to have the restrictions on transformer type that it does. Similar considerations apply to some of 240.21(B) and (C).

Cheers, Wayne

 

[Jpflex]

First, you mean 240.92(E), not 242.1(E).

In one sense you are technically correct, as there is nothing in 240.4 or earlier that says "except as provided in Part VIII of this article."

But if you look at the start of 240.90, the start of Part VIII of Article 240, it says "Overcurrent protection in areas of supervised industrial installations shall comply with all of the other applicable provisions of this article, except as provided in Part VIII" So clearly Part VIII is meant to be able to amend the earlier parts of the article.

The scope of Article 240 says in 240.1 "Parts I through VII of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal. Part VIII covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 1000 volts, nominal."

Really that first sentence should say "Parts I through VII of this article provide the general requirements, excluding supervised industrial installations, for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal."

Anyway, that is implicit, so 240.92(E) amends 240.4(F).

Cheers, Wayne

So only in a supervised industrial setting for manufacturing control environments are you allowed to ammend 240.4 F.

in these environments you are allowed to Consider the secondary of a transformer to be protected by the OCPD of the primary under criteria.

However i dont think most installs are following code as written because many out door installations may be following 240.92 E regardless if tge envirinment served is industrial?

 

[wwhitney]

However i dont think most installs are following code as written because many out door installations may be following 240.92 E regardless if tge envirinment served is industrial?

For outdoor installations you can use 240.21(C)(4), which allows unlimited secondary conductor length with no minimum ampacity. The only significant limitation is that the secondary conductors terminate on a single OCPD either outside or inside nearest the point of entry. Are you really seeing installations that don't fit within the limits of 240.21(C)(4)?

Cheers, Wayne