Regarding code of the day

[Jpflex]

Where a transformer supplied a panel, the overcurrent protection shall be on the secondary side of a transformer but DO THEY MEAN on the LINE SIDE OR LOAD SIDE OF SECONDARY WIRE OR EVEN BOTH?

If the OCPD is on the load side of the secondary of transformer then the secondary wires will not have protection upstream from the breaker panel

Also I thought code said transformers can have either primary or primary and secondary protection but not secondary only as code of the day says

 

[augie47]

1st thing you have to do is mentally separate the rules for overcurrent protection of transformers, Art 450 and protection of transformer conductors {secondary = 240.21(C)}
Transformer secondary conductors rules are similar to "tap rules" in that you are allowed specific distances from the connection to the overcurrent device {See 240.21(C)} which can be separate from the panel or in the panel taking the allowed length into account.

In addition 408.36 requires over-current protection either internal or ahead of a panelboard so you have to have OCP on the secondary side of the transformer to meet that requirement. Todays;' quiz takes that rule into account.

 

[infinity]

If the OCPD is on the load side of the secondary of transformer then the secondary wires will not have protection upstream from the breaker panel

Which is one of the reasons why the length of the secondary conductors is limited.

 

[Jpflex]

In addition 408.36 requires over-current protection either internal or ahead of a panelboard so you have to have OCP on the secondary side of the transformer to meet that requirement. Todays;' quiz takes that rule into account.

So this makes it sound like there must be OCPD on line side of secondary and the normal main breaker to the panel on the load side of the secondary wire which would make two breakers

The OCPD ahead or on top internal to panel board is upstream to the panel board before branch circuits

 

[augie47]

In most cases, transformer secondary conductors require an over-current device (there are exceptions but I won't muddy the water) and per 408.36, panels need a main OCP device. It is possible for one device to serve both functions if all the criteria for each is in place.

 

[PaulMmn]

Now you're confusing me-- this talk of line side / load side of the secondary wires... This is what I think you're all saying:
.

 

[don_resqcapt19]

Now you're confusing me-- this talk of line side / load side of the secondary wires... This is what I think you're all saying:
.
View attachment 2568043

The conductors in what you call "no man's land" are protected by the OCPD closest to the transformer and no addition protection is needed assuming both the conductors and the panel bus have an ampacity equal to or greater than the rating of that first OCPD.

 

[Jpflex]

Now you're confusing me-- this talk of line side / load side of the secondary wires... This is what I think you're all saying:
.
View attachment 2568043

Yes the line side of a wire is the beginning where it normally starts with a breaker OCPD and the load side of the wire is where it ends NORMALLY at the load such as a light bulb

In your picture you have 2 breakers as it seems some are asking for but seems excessive

 

[ActionDave]

So this makes it sound like there must be OCPD on line side of secondary and the normal main breaker to the panel on the load side of the secondary wire which would make two breakers

The OCPD ahead or on top internal to panel board is upstream to the panel board before branch circuits

Do you put your line side secondary breakers on the lugs? Or do you put them on the first turn of the secondary winding to protect as much of the line side of the secondary as possible?

 

[augie47]

IF you can satisfy all the requirements of a specific transformer secondary rule {240.21(C)*} and meet the requirements of 408.36 with just one breaker, that is allowed.
For example, IF you have a 75kva transformer (208Y120 secondary) and you connect 3/0 ti the secondary lugs, run 10 ft to a pane; with a 200 amp M/B, then the panel breaker is the only one needed,. (See Drawing below)

Note: That only applies when the conductors meet the requirements of 240.21 as to size, length, termination, etc.

 

[Jpflex]

IF you can satisfy all the requirements of a specific transformer secondary rule {240.21(C)*} and meet the requirements of 408.36 with just one breaker, that is allowed.
For example, IF you have a 75kva transformer (208Y120 secondary) and you connect 3/0 ti the secondary lugs, run 10 ft to a pane; with a 200 amp M/B, then the panel breaker is the only one needed,. (See Drawing below)

Note: That only applies when the conductors meet the requirements of 240.21 as to size, length, termination, etc.

View attachment 2568052

So there are possibilities depending on length of secondary where two breakers will be needed? I never seen it done this way but only seen a breaker on the line side of a distribution panel

I’ll look into those codes you listed if I can because my 2020 NEC book has a torn up page in transformer section

 

[Jpflex]

Do you put your line side secondary breakers on the lugs? Or do you put them on the first turn of the secondary winding to protect as much of the line side of the secondary as possible?

The transformers have spread apart lugs to connect secondary wires to the transformer secondary windings so you couldn’t connect a breaker typically directly to secondary windings but some wire would be needed to do this.

I just read before that either primary only OCPD or primary and secondary OCPD for transformers were required but apparently secondary only OCPD is allowed?

 

[ActionDave]

The transformers have spread apart lugs to connect secondary wires to the transformer secondary windings so you couldn’t connect a breaker typically directly to secondary windings but some wire would be needed to do this.

Exactly. And that wire would be called a transformer secondary conductors, and there are special rules for it because you can't protect the line side of them.

I just read before that either primary only OCPD or primary and secondary OCPD for transformers were required but apparently secondary only OCPD is allowed?

No

 

[don_resqcapt19]

So there are possibilities depending on length of secondary where two breakers will be needed? I never seen it done this way but only seen a breaker on the line side of a distribution panel

I’ll look into those codes you listed if I can because my 2020 NEC book has a torn up page in transformer section

If designed correctly, the code would never require two breakers for a single set of transformer secondary conductors and the panelboard.
Where the length of the conductors between the transformer secondary terminals and the panelboard exceeds the lengths specified in 240.21(C), you simple install one OCPD in a location that meets the 240.21(C) requirements. That OCPD is sized so it does not exceed the ampacity of the load side conductors and the panelboard bus rating. When you do that the panelboard can be main lugs only.

 

[infinity]

To add to what Don correctly stated you'll need a single OCPD to protect the panel and you're limited as the secondary conductor length between the transformer and that OCPD. You may be able to use a single main CB in the panel to comply with both the OCPD rule and the conductor length rule.

 

[cadpoint]

This is a great thread

 

[Jpflex]

That OCPD is sized so it does not exceed the ampacity of the load side conductors and the panelboard bus rating. When you do that the panelboard can be main lugs only.

But what about the NEC rule that says (without looking it up) something similar to secondary OCPD ampacity not being sized greater than the ratio of primary voltage / secondary voltage x primary OCPD ampacity? In other words VA of primary and secondary being equal while voltage and current on primary and secondary are inverted to each other

 

[don_resqcapt19]

But what about the NEC rule that says (without looking it up) something similar to secondary OCPD ampacity not being sized greater than the ratio of primary voltage / secondary voltage x primary OCPD ampacity? In other words VA of primary and secondary being equal while voltage and current on primary and secondary are inverted to each other

That only applies to secondary conductors and not the OCPD for the secondary conductors. It also never comes into play where there is only one set of secondary conductors unless the transformer was drastically over sized for the connected load.