Transformer to panelboard & transformer secondary tap rules

[don_resqcapt19]

I get what you’re saying for the most part and I didn’t intend to imply that other educational books were to be followed as code but only a source for further detail, theory, instructions and explanations.

The protection of the secondary conductor only at its load end but not its line side would not protect the conductor to its ampacity if a short were upstream of the load side terminating breaker, and if it were not for the primary side breaker / counter electromagnetic force resistance between primary and secondary.

Service conductors, tap conductors and transformer secondary conductors are only protected from overload at their load end. That is why the length of the tap conductors and transformer secondary conductors is limited.

 

[Jpflex]

Service conductors, tap conductors and transformer secondary conductors are only protected from overload at their load end. That is why the length of the tap conductors and transformer secondary conductors is limited.

Based on how NEC defines a tap, a transformer secondary conductor may be considered a tap, regardless if this was not what NEC intended.

Is there a code in NEC that specifically states that secondary conductors terminating without overcurrent protection on their line side are not taps? Just asking in case there is

 

[augie47]

Is there a code in NEC that specifically states that secondary conductors terminating without overcurrent protection on their line side are not taps? Just asking in case there is

Yes. 240.2 in defining a tap includes the words "has overcurrent protection at the point of supply".
That is one reason there is a 240.21(B)for "taps" and 240.21(C) for transformer secondary condutors even though they are similar.

 

[don_resqcapt19]

Based on how NEC defines a tap, a transformer secondary conductor may be considered a tap, regardless if this was not what NEC intended.

Is there a code in NEC that specifically states that secondary conductors terminating without overcurrent protection on their line side are not taps? Just asking in case there is

Transformer secondary conductors meet the definition of a tap conductor, but they are a special type of tap conductor and have their own overcurrent protection rules in the first level subdivision titled "transformer secondary conductors". There are not many differences between the rules in 240.21(B) for tap conductors, and the rules in 240.21(C) for transformer secondary conductors, but there are some.

There was a Public Input submitted for the 2026 code that would add a definition of "Transformer Secondary Conductor". The First Draft Report will be published in July of this year, and that will tells if this Public Input was accepted by CMP10.

 

[augie47]

don ??


"Transformer secondary conductors meet the definition of a tap conductor,"
vs

Tap Conductor. A conductor, other than a service conductor,
that has overcurrent protection ahead of its point of supply
that exceeds the value permitted for similar conductors that
are protected as described elsewhere in 240.4.

 

[don_resqcapt19]

don ??


"Transformer secondary conductors meet the definition of a tap conductor,"
vs

Tap Conductor. A conductor, other than a service conductor,
that has overcurrent protection ahead of its point of supply
that exceeds the value permitted for similar conductors that
are protected as described elsewhere in 240.4.

it has overcurrent on the primary side of the transformer and in some rare cases, that device is even permitted to protect the secondary conductors.

 

[Jpflex]

It just doesn’t seem clear with NEC when to consider a transformer output secondary leads or transformer secondary tap leads. Look at the first picture. The above text explains that these are secondary conductors but on the corresponding picture its labeled secondary tap connectors?

Also take note of the second picture, where the system bonding jumper is placed at the transformer instead of the first panel disconnect which is especially required if the separately derived transformer is located outdoors.

I just feel that most people would not understand if you did not install the system bonding jumper at the first panel. Such as bonding the first breaker panel neutral buss, grounding electrode conductor, ground buss, equipment ground conductors and the first disconnect cabinet all together.

 

[don_resqcapt19]

It just doesn’t seem clear with NEC when to consider a transformer output secondary leads or transformer secondary tap leads. Look at the first picture. The above text explains that these are secondary conductors but on the corresponding picture its labeled secondary tap connectors?

Also take note of the second picture, where the system bonding jumper is placed at the transformer instead of the first panel disconnect which is especially required if the separately derived transformer is located outdoors.

I just feel that most people would not understand if you did not install the system bonding jumper at the first panel. Such as bonding the first breaker panel neutral buss, grounding electrode conductor, ground buss, equipment ground conductors and the first disconnect cabinet all together.

Once again that is using something other than the code to define code rule. There is no such thing as a secondary tap conductor, because you are not permitted to tap transformer secondary conductors.

The conductors that originate at the secondary terminals of a transformer are very simply transformer secondary conductors and the overcurrent protection rules for those conductors are found in a first level subdivision with the title of "Transformer Secondary Conductors" That first level subdivision being 240.21(C).




As far as the location of the system bonding jumper, the code permits it at the transformer or at the first means of disconnect. For me, the location of the GEC connection for a transformer is simply based on the location that results in the shortest length of GEC that has to be installed.

 

[Jpflex]

Once again that is using something other than the code to define code rule. There is no such thing as a secondary tap conductor, because you are not permitted to tap transformer secondary conductors.

So are you saying there is no such thing as a transformer 10’ foot secondary tap rule? Meaning a feeder at the same ampacity as the secondary windings, terminates to the transformer x connectors then each phase to a power distribution block where 10’ taps following the tap ampacity rules?

 

[jim dungar]

So are you saying there is no such thing as a transformer 10’ foot secondary tap rule? Meaning a feeder at the same ampacity as the secondary windings, terminates to the transformer x connectors then each phase to a power distribution block where 10’ taps following the tap ampacity rules?

All transformer secondary conductors must originate at the transformer terminals and end at OCPD terminals per 240.21(C).

No taps, as defined in 240, may be made to/on transformer secondary conductors.

 

[augie47]

You can check 240.21(C) . I believe all the transformer secondary rules require the conductors to terminate at an overcurrent device

opps jim beat me to it

 

[ptonsparky]

Yes, the conductors are not taps..
The only 'thing' that is a tap is the connection at the transformer secondary.

 

[don_resqcapt19]

So are you saying there is no such thing as a transformer 10’ foot secondary tap rule? Meaning a feeder at the same ampacity as the secondary windings, terminates to the transformer x connectors then each phase to a power distribution block where 10’ taps following the tap ampacity rules?

Yes, I am saying there is no "transformer 10' secondary tap" rule. The conductors are not "transformer secondary taps", they are simply "transformer secondary conductors". There is a 10' tap conductor rule in 240.21(B)(1) but you do not use that rule for transformer secondary conductors. You use the 10' secondary conductor rule found in 240.21(C)(2).

The rule in the parent text in 240.21 clearly prohibits what you are suggesting when you say a "feeder at the same ampacity...with taps using smaller conductors" That text says that transformer secondary conductors "shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4". That means that the full ampacity conductors cannot be "tapped".

My issue is that there are both tap rules and transformer secondary conductor rules, and using the word "tap" with transformer secondary conductors leads to confusion as to what rule needs to be applied.