Transformer secondary protection required?

Status
Not open for further replies.

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Since there is a panel installed off the secondary of the xfmr, would not this be the a good way to do the install? clean and simple IMHO.

207ecm24fig2.jpg

where is the 180a rating for the 3/0 coming from?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
The transformer may not need Sec OCPD (450) but the conductors do (240). Because the conductors do (240) the transformer table (450) caps the maximum size and 200A don't make it.

This is simply not correct. Whether or not the transformer secondary conductors require OCP, has NOTHING to do with "Primary and Secondary" protection of the transformer per 450.3(B). 450.3 say "Overcurrent protection of TRANSFORMERS shall comply with 450.3(A), (B), or (C)."

If the OCPD on the primary of the Transformer does not exceed 125% of the primary rated current (or next standard size up,) then the Transformer can be protected by "Primary Only Protection" per 450.3(B). The secondary of the TRANSFORMER will NOT REQUIRE overcurrent protection, per 450.3(B). The secondary conductors will still need overcurrent protection in accordance with 240.21(C).

The OP asked if the install was legit. Not just the transformer.

Actually, he only asked about the transformer...

One more transformer protection question for the day

If he was asking about the complete install, you would think he would have let us know what the primary and secondary conductor sizes were. Without knowing the conductor sizes, there is no way to comment on the complete install.

Had he stated the secondary was 175A or less you'd be okay. Had he asked if transformers in general need secondaries then you'd be okay. But your advice is violating table 450.3(B) Note 2 as a cascade requirement from 240.

My advice is not violating table 450.3(B). Again, table 450.3(B) is for Transformer protection only, not conductors. Note 2 says "where secondary overcurrent protection is required..."

Now look at Table 450.3(B) under protection method. It says "Primary only protection" and "Primary and secondary protection." Looking at the "Primary only protection" line under the "Primary Protection" column, you will see that the Maximum Setting for the primary will be 125% of the primary rated current, or the next standard size up. The 45kVA transformer has a primary rated current of 54A. 125% of that is 67.5A. 70A is the next standard size up from 67.5A. So by using a 70A primary c/b on the 45kVA transformer we do not exceed the maximum setting for "Primary only protection."

Next, continue along the "Primary only protection" row until you get to the "Secondary Protection" column. You will see "Not Required." Secondary protection of the TRANSFORMER is NOT REQUIRED, if the "primary only protection" provisions are met. Since a 70A primary OCPD meets the requirements for "primary only protection," the secondary conductors OCPD can be any size that you desire. The secondary conductors must have an ampacity that is greater to or equal to the secondary conductors OCPD per 240.21(C).

So...

In the OP, the 45kVA transformer with a 70A primary OCPD can have a 200A secondary conductor OCPD, but the conductors cannot be smaller than 3/0. It could have a 250A secondary conductor OCPD, but the conductors cannot be smaller than 250mcm, or it could have a 300A secondary conductor OCPD, but the conductors cannot be smaller than 350mcm, etc.


You did not "make the installation compliant" by only adjusting the primary.

Yes, you would make the installation compliant by only adjusting the primary OCPD, assuming the conductors are already properly protected per 240.4 and 240.21(C).
 
Last edited:

pfalcon

Senior Member
Location
Indiana
... I'm looking at an appliation ... From what I see the secondary 200A breaker does not provide adequate secondary protection of the transformer. ...

... Actually, he only asked about the transformer...

You're splitting hairs David. He asked about his application in the original post. And specifically asked if the secondary breaker was correctly sized. That he expressed it as "protection of the transformer" doesn't make your suggestion compliant. It only means he may not understand the purpose of the secondary OCPD. Whether he does or not isn't relevant to whether it meets code. At 200A it doesn't.
 

pfalcon

Senior Member
Location
Indiana
... In the OP, the 45kVA transformer with a 70A primary OCPD can have a 200A secondary conductor OCPD, but the conductors cannot be smaller than 3/0. It could have a 250A secondary conductor OCPD, but the conductors cannot be smaller than 250mcm, or it could have a 300A secondary conductor OCPD, but the conductors cannot be smaller than 350mcm, etc.
...

No. This is a specific application not a general question. It's a D-Y transformer therefore the conductors REQUIRE the secondary. It's not optional. Since they're required 450.3(B) Note 2 sets a maximum value at 175A.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
No. This is a specific application not a general question. It's a D-Y transformer therefore the conductors REQUIRE the secondary. It's not optional. Since they're required 450.3(B) Note 2 sets a maximum value at 175A.
The transformer does not require secondary protection. The secondary conductors require protection and must be protected at or below their ampacity. You can use any size OCPD for the secondary conductors as long as the rating of the conductor is equal to or greater than that of the OCPD.

450.3 has NOTHING to do with the protection of any conductor. The rules in that section are only for the protection of the transformer itself, If 70 amp primary OCPD is used for this 45kVA transformer, then transformer secondary winding does not require any protection as the primary protection does not exceed 125% of the rated current.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
You're splitting hairs David. He asked about his application in the original post. And specifically asked if the secondary breaker was correctly sized. That he expressed it as "protection of the transformer" doesn't make your suggestion compliant. It only means he may not understand the purpose of the secondary OCPD. Whether he does or not isn't relevant to whether it meets code. At 200A it doesn't.

I am not splitting hairs. You clearly don't understand what the OP was asking. And you are clearly misapplying code provisions.

He specifically asks if the secondary breaker was correctly sized RELATIVE to the size of the primary breaker...

Primary breaker is an 80A breaker and secondary main breaker in panel on secondary is a 200A breaker. From what I see the secondary 200A breaker does not provide adequate secondary protection of the transformer.

Since the 80A primary breaker is larger than 125% of the transformers primary rated current then secondary protection is required for the transformer. The required secondary protection can be a maximum of 125% of the transformers secondary rated current.

Since the primary breaker is larger than 125% of the rated primary current, "Primary only protection" per 450.3(B) cannot be used, therefore, the secondary breaker cannot be larger than 125% of the rated secondary current per "Primary and secondary protection" in 450.3(B). 200A exceeds the 125% of the secondary, therefore the 80A-200A set up is not compliant.

This is a specific application not a general question. It's a D-Y transformer therefore the conductors REQUIRE the secondary. It's not optional. Since they're required 450.3(B) Note 2 sets a maximum value at 175A.

This is where you are misapplying the code. A D-Y transformer requires secondary CONDUCTOR protection, not TRANSFORMER secondary protection. These are two different issues. The provisions of 240.4 and 240.21(C) apply ONLY to conductor protection. The provisions of 450.3 apply ONLY to transformer protection. You are trying to take the secondary conductor protection requirement from 240.21(C) and force it into transformer protection requirements in 450.3. This is wrong.

Note 2 in Table 450.3(B) says that where transformer secondary overcurrent protection is required, the OCPDs shall be not more than 6 breakers or sets of fuses. But the determination of whether secondary overcurrent protection is required comes from Table 450.3(B), not from 240.21(C).

Table 450.3(B) tells us that if the primary OCPD is not more than 125% of the primary rated current (or next size up,) then secondary protection is NOT REQUIRED. If secondary protection is not required, then Note 2 does not come into play.

In the OP, if the primary circuit is reduced to 70A then the transformer protection would fall under "Primary Only Protection" per T450.3(B), and therefore, secondary protection of the transformer would be "Not Required." If there is no transformer secondary protection required, then there is no maximum size (ie, 125%) for the secondary conductor OCPD. The 200A secondary conductor OCPD could remain, as long as the secondary conductors are 3/0 are larger (to comply with 240.21(C)).
 

pfalcon

Senior Member
Location
Indiana
... 450.3 has NOTHING to do with the protection of any conductor. The rules in that section are only for the protection of the transformer itself, ...

Note 2 on the table does not qualify WHY the secondary protection is required. Only that if required it must be less than ...

NFPA70:2011:450.3(B) Note 2 said:
2. Where secondary overcurrent protection is required, the ...

240 requires the OCPD for the conductors. Note 2 in 450 doesn't care WHY you required it. You're adding that qualifier yourself. It doesn't read "Where secondary overcurrent protection is required for the transformer ...". Unless of course you're claiming we get to add our own text to the NEC.
 

pfalcon

Senior Member
Location
Indiana
... The provisions of 450.3 apply ONLY to transformer protection. You are trying to take the secondary conductor protection requirement from 240.21(C) and force it into transformer protection requirements in 450.3. This is wrong.

Note 2 in Table 450.3(B) says that where transformer secondary overcurrent protection is required, the OCPDs shall be not more than 6 breakers or sets of fuses. But the determination of whether secondary overcurrent protection is required comes from Table 450.3(B), not from 240.21(C). ...

And where in 450.3(B) does it say
... apply ONLY to transformer protection. ...

Because Note 2 only says
Note 2 said:
Where secondary overcurrent protection is required, ...
There's no added qualifier to let you ignore the protection required by the conductors. You've added that. Code restrictions cross articles all the time. Why is this one being treated differently?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
And where in 450.3(B) does it say

apply ONLY to transformer protection.
240.3 Equipment shall be protected against overcurrent in accordance with the article in the Code that covers the type of equipment specified in Table 240.3 - "Transformers and transformer vaults - Article 450."

240.4 Protection of Conductors

240.21 Overcurrent protection shall be provided in each ungrounded circuit conductor...

240.21(C) Transformer Secondary Conductors. Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

450.1 Scope. This article covers the installation of all transformers.

450.3 Overcurrent Protection. Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C).
.........FPN No. 1: See 240.4, 240.21, 240.100 and 240.101 for overcurrent protection of conductors.

Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less (as a Percentage of Transformer-Rated Current)


It seems pretty clear that 240.4 and 240.21 are for the protection of conductors, and 450.3 is for the protection of transformers.


There's no added qualifier to let you ignore the protection required by the conductors. You've added that. Code restrictions cross articles all the time. Why is this one being treated differently?

I haven't ignored the protection required by the conductors. I've applied the protection for the transformer required by 450.3, and I've applied the protection for the conductors required by 240.4 and 240.21.

Per 450.3(B), my transformer primary protection is at 125% (70A) from the "Primary Only Protection" row. In this row, transformer secondary protection is "Not Required." I meet the requirements of 450.3 for protection of the transformer.

My primary conductors are #4. Per 240.4, the 70A circuit breaker on the primary side is properly protecting the #4 conductors.

My secondary conductors are #3/0. Per 240.21(C), and 240.4, the 200A circuit breaker on the secondary side is properly protecting the #3/0 conductors.

I have properly protected the transformer in accordance with Article 450, and the conductors in accordance with Article 240. The installation is therefore compliant.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
There's no added qualifier to let you ignore the protection required by the conductors. You've added that. Code restrictions cross articles all the time. Why is this one being treated differently?
Protection of the conductors is outside the scope of Article 450. The fact that the secondary conductors have to have protection per the rules of Article 240 has no effect on the rules in 450. If the transformer has primary protection at 125% or less of the rated primary current there is no protection required for the secondary of that transformer. At that point the rules of Article 450 have been complied with. There is nothing in 450 that can be construed in any manner to restrict the size of the secondary conductors and the overcurrent protection of those conductors.
 

pfalcon

Senior Member
Location
Indiana
Protection of the conductors is outside the scope of Article 450.
Never argued otherwise. Argued that 240 requires the protection for the conductors.
The fact that the secondary conductors have to have protection per the rules of Article 240 has no effect on the rules in 450.
Yes it does. Per Note 2 of the table. The secondary OCPD is there. It is required (though not by 450). Therefore it has a max size per 450.
Never does it say "When secondary overcurrent protection is required by this article ...".
It says only "When secondary overcurrent protection is required ...".
If the transformer has primary protection at 125% or less of the rated primary current there is no protection required for the secondary of that transformer.
Not per 450. But it is required by 240. Again, table 450.3(B) does not question WHY the secondary is required. It's there. It's required. Therefore it must conform to Note 2.
At that point the rules of Article 450 have been complied with.
Except you haven't complied with Note 2. You have a secondary OCPD that is REQUIRED.
There is nothing in 450 that can be construed in any manner to restrict the size of the ... overcurrent protection of those conductors.
It's not construed. It's explicit. It's Note 2.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Not per 450. But it is required by 240. Again, table 450.3(B) does not question WHY the secondary is required. It's there. It's required. Therefore it must conform to Note 2.

Honestly, what part of "NOT REQUIRED" do you not understand.

Note 2 is a note to Table 450.3(B), which is the "Maximum Rating or Setting of Overcurrent Protection for Transformers"
Note 2 applies to Table 450.3(B), not to Article 240.

Table 450.3(B) has two columns, one for Primary Protection, one for Secondary Protection. You will see Note 2 referenced at the top of the Secondary Protection Column. The note applies only to the Secondary Protection column,

The Secondary Protection column gives 4 possible conditions for the Maximum Setting for the Transformer Secondary Overcurrent Protection. On the lower row "Primary and Secondary Protection," where the primary OCPD is not more than 250%, the Max secondary OCPD is 125% where the current is 9A or more(1), and the Max secondary OCPD is 167% where the current is less than 9A (2).

On the upper row "Primary Only Protection," where the primary OCPD is not more than 125% (or 167% or 300%), the secondary OCPD for currents of 9A or more is "NOT REQUIRED" (3), and the secondary OCPD for currents of less than 9A is "NOT REQUIRED" (4).

So, for the 70A primary OCPD in the OP, the Primary OCPD falls under the Max 125% in the primary only, so secondary protection is "NOT REQUIRED" (condition #3 above.) Now look at note 2, as referenced in the Secondary Protection column. It says "Where secondary overcurrent protection is required..." The table has already told us that secondary overcurrent protection is NOT REQUIRED. The note is n/a since secondary overcurrent protection is not required.

It is really no more complicated than that.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...Except you haven't complied with Note 2. You have a secondary OCPD that is REQUIRED.

It's not construed. It's explicit. It's Note 2.
Please read the scope of Article 450. The protection that Note 2 is talking about is the secondary protection of the transformer secondary windings. There is nothing in Article 450 that has anything to do with the protection of the secondary conductors and the fact that those conductors need protection does not have anyting to do with the secondary transformer winding protection.
 

pfalcon

Senior Member
Location
Indiana
Found a shorter way to say this:

450 requires a primary overcurrent protection device for the transformer.
240.21(C)(1) requires a secondary overcurrent protection device for the (OP's) transformer. (Yes I know, it's for the sake of the conductors not the transformer: It's still required and still there)

Therefore: You have a "Primary and secondary protection" transformer installation.

The scope, as Don wanted me to review:
NFPA70:2011:450.1 Scope said:
This article covers the installation of all transformers. ...
emphasis added

Note that it doesn't say it covers the protection of all transformers. This installation has a secondary therefore the second row is used.
Nor does 450.3 say it only applies for the protection (verb) of the transformer but instead specifically points you back to 240.21 that might force a secondary overcurrent protection (noun).
Nor does Table 450.3(B) say it only applies to protecting the transformer. Rather it addresses the type of installation that you have. Which in this case is row two.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...

450 requires a primary overcurrent protection device for the transformer.
240.21(C)(1) requires a secondary overcurrent protection device for the (OP's) transformer. (Yes I know, it's for the sake of the conductors not the transformer: It's still required and still there)

Therefore: You have a "Primary and secondary protection" transformer installation.

The scope, as Don wanted me to review:
emphasis added

Note that it doesn't say it covers the protection of all transformers. This installation has a secondary therefore the second row is used.
Nor does 450.3 say it only applies for the protection (verb) of the transformer but instead specifically points you back to 240.21 that might force a secondary overcurrent protection (noun).
Nor does Table 450.3(B) say it only applies to protecting the transformer. Rather it addresses the type of installation that you have. Which in this case is row two.
You have the answer in the part of your post that I changed to red text. There no required secondary protection of the TRANSFORMER.

As far as the scope, the conductors are not part of the transformer and are not in any way covered by the rules in 450.3. In the same way the required protection of the secondary conductors has nothing to do with Note 2. These are COMPLETELY independent issues.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
450.3 Overcurrent Protection. Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.

Informational Note No. 1: See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.

The code section is very clear that the overcurrent protection required is for the protection of the transformer. This is emphasized by Informational Note #1 that refers you to 240 for the overcurrent protection of the conductors.
 

pfalcon

Senior Member
Location
Indiana
You have the answer in the part of your post that I changed to red text. There no required secondary protection of the TRANSFORMER.

As far as the scope, the conductors are not part of the transformer and are not in any way covered by the rules in 450.3. In the same way the required protection of the secondary conductors has nothing to do with Note 2. These are COMPLETELY independent issues.
Didn't say it was required by 450. Never said it was required by 450. Said the secondary OCPD was there because of 240. And the secondary OCPD is a part of the transformer installation. Therefore it's covered under the scope.

The code section is very clear that the overcurrent protection required is for the protection of the transformer. This is emphasized by Informational Note #1 that refers you to 240 for the overcurrent protection of the conductors.

450.3 Overcurrent Protection. said:
Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.

Informational Note No. 1: See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.

Read more carefully Don. The scope in 450.1 says installation not protection. The requirements from 240 may not be protection for the transformer but they most assuredly are part of the installation. The word "Protection" throughout 450 is being used as a noun not a verb. Put a specific type of OCPD into that paragraph:

450.3 Fuses (modified) said:
Fuses of transformers shall comply ...
Table 450.3(B) Maximum Rating or Setting of Fuses for Transformers (modified) said:
600 Volts and Less ...
This installation (450.1) has secondary OCPD therefore "Primary Only Protection" doesn't apply in Table 450.3(B). No where in 450.3 does it say "only if the transformer needs it", nor does it say anywhere "only if article 450 requires it". It says in the table that when you have secondary OCPD then you size them accordingly.
 
Status
Not open for further replies.
Top