Transformer secondary protection required?

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mull982

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One more transformer protection quesiton for the day :)

I'm looking at an appliation where there is a 45kVA transformer 480V-pri, 208V-sec. 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. 125% of this transformers secondary rated current is 156A (125A * 125%). The 200A secondary breaker exceeds this 156A value and therefore does not provide adequate required secondary protection.

My recommendation would be to change the 80A primary breaker to a 75A breaker which will be within 125% of the transformers primary rated current and therefore no secondary protection will be required per 450.3(B). Do others agree that if primary breaker is within 125% or primary rated current than no secondary protection is required?
 
I agree with your assessment, except that the primary c/b would be allowed to be no greater than 70A (not 75A) for primary only protection.

You could either change the primary c/b to 70A, or the secondary c/b to 175A to make the installation comply with 450.3(B).
 
fmtjfw said:
How many secondary conductors? If there are more than 2 then you cannot protect the secondary with a primary OCPD
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:? "Primary Only" or "Primary and Secondary" Transformer protection per Table 450.3(B) is not dependent on the quantity of secondary conductors.
 
mull982 said:
One more transformer protection quesiton for the day :)

I'm looking at an appliation where there is a 45kVA transformer 480V-pri, 208V-sec. 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. 125% of this transformers secondary rated current is 156A (125A * 125%). The 200A secondary breaker exceeds this 156A value and therefore does not provide adequate required secondary protection.

My recommendation would be to change the 80A primary breaker to a 75A breaker which will be within 125% of the transformers primary rated current and therefore no secondary protection will be required per 450.3(B). Do others agree that if primary breaker is within 125% or primary rated current than no secondary protection is required?
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Is it a delta primary and wye secondary? If so, you will need both primary and secondary. 240.21(C)(1)
 
pfalcon said:
Only Delta-Delta permits "primary only" protection. Otherwise the secondary protection is required. Presumably (arg) this is a 480/208 Delta-Delta transformer though.
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This is not correct. Per 450.3(B), "Primary Only" Protection of a transformer is permitted for any type of transformer (delta-delta, delta-wye, wye-delta, wye-wye) where the primary OCPD does not exceed 125% of the primary current (or the next size up) for primary currents of 9A or more (or 167% for currents less than 9A, or 300% for currents less than 2A.)

The OP's question is about Transformer Protection, not secondary conductor protection.
 
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
 
jumper said:
208V delta? :?
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Um, yeah, sorry. :dunce:

david luchini said:
This is not correct. Per 450.3(B), "Primary Only" Protection of a transformer is permitted for any type of transformer (delta-delta, delta-wye, wye-delta, wye-wye) where the primary OCPD does not exceed 125% of the primary current (or the next size up) for primary currents of 9A or more (or 167% for currents less than 9A, or 300% for currents less than 2A.)

The OP's question is about Transformer Protection, not secondary conductor protection.
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What are you wiring where the transformer doesn't have secondary conductors?

240.21(C)(1) said:
... and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
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The protection may not be for the sake of the transformer but ... what's the point of a transformer without secondary connections?
 
pfalcon said:
What are you wiring where the transformer doesn't have secondary conductors?
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Huh? I don't understand the question.

pfalcon said:
The protection may not be for the sake of the transformer but ... what's the point of a transformer without secondary connections?
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Who said the transformer doesn't have secondary connections? The OP is an application of a 45kVA transformer with an 80A primary OCPD and a 200A secondary OCPD.

mull982 said:
I'm looking at an application where there is a 45kVA transformer 480V-pri, 208V-sec. Primary breaker is an 80A breaker and secondary main breaker in panel on secondary is a 200A breaker.
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This OP's question is about Transformer Protection per 450.3(B). There is nothing in the post about conductor sizes or conductor protection.

The installation clearly does not meet the requirements for either "Primary Only" protection or "Primary and Secondary" protection in 450.3(B).

The deficiency could be corrected by reducing the primary ocpd to 70A to comply with "Primary Only" transformer protection (even though this is a delta-wye transformer,) or it could be corrected by reducing the secondary ocpd to 175A to comply with "Primary and Secondary" transformer protection.
 
ron said:
Why mix the guy up. Even if it is delta - delta or an auto-transformer, then you still are going to protect the downstream wire anyway.

Just put primary and secondary.
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I don't think you read the original post all the way through. He HAS an OCPD on both the primary (80A) and the secondary (200A). His question is about protecting the transformer, not the wire. The simpler solution would be to change to primary OCPD to protect the transformer as "Primary Only" protection per 450.3(B).
 
ron said:
Why mix the guy up. Even if it is delta - delta or an auto-transformer, then you still are going to protect the downstream wire anyway.

Just put primary and secondary.
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I don't think that is mixing him up. Many are already mixed up on these installations because they don't understand that the rules in 450 apply only to the protection of the transformer itself and that the rules in 240 apply only to the protection of the conductors that are connected to the transformer. You need to comply with both. Many users just look at 450 and forget about 240. In most cases the conductor protection required by the rules in 240 will protect both the conductors and the transformer, but the reverse is not true.
 
david luchini said:
This OP's question is about Transformer Protection per 450.3(B). There is nothing in the post about conductor sizes or conductor protection.

The installation clearly does not meet the requirements for either "Primary Only" protection or "Primary and Secondary" protection in 450.3(B).

The deficiency could be corrected by reducing the primary ocpd to 70A to comply with "Primary Only" transformer protection (even though this is a delta-wye transformer,) or it could be corrected by reducing the secondary ocpd to 175A to comply with "Primary and Secondary" transformer protection.
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emphasis added

You're really going to tell him that a 70A primary-only fuse will correct the deficiency in compliance with Table 450.3(B) Note 2? Knowing that it's a D-Y transformer requiring secondary overcurrent protection?
 
pfalcon said:
emphasis added

You're really going to tell him that a 70A primary-only fuse will correct the deficiency in compliance with Table 450.3(B) Note 2? Knowing that it's a D-Y transformer requiring secondary overcurrent protection?
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The secondary of the transformer does not need protection. The secondary conductors need protection. These are two different issues.
The rules in 450 cover the protection of the transformer itself.
The rules in 240 cover the protection of the primary and secondary conductors.
You have to satisfy both sets of rules.
 
pfalcon said:
You're really going to tell him that a 70A primary-only fuse will correct the deficiency in compliance with Table 450.3(B) Note 2? Knowing that it's a D-Y transformer requiring secondary overcurrent protection?
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Yes, reducing the primary OCPD to 70A from 80A will correct the deficiency in compliance with Table 450.3(B). Nothing in 450.3(B) says that a D-Y transformer requires secondary overcurrent protection.

The OP mentions that there is already a 200A OCPD on the secondary side to provide secondary conductor protection. Reducing the primary OCPD from 80A to 70A is all that is required to make the installation compliant.
 
don_resqcapt19 said:
The secondary of the transformer does not need protection. The secondary conductors need protection. These are two different issues.
The rules in 450 cover the protection of the transformer itself.
The rules in 240 cover the protection of the primary and secondary conductors.
You have to satisfy both sets of rules.
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emphasis added

david luchini said:
Yes, reducing the primary OCPD to 70A from 80A will correct the deficiency in compliance with Table 450.3(B). Nothing in 450.3(B) says that a D-Y transformer requires secondary overcurrent protection.

The OP mentions that there is already a 200A OCPD on the secondary side to provide secondary conductor protection. Reducing the primary OCPD from 80A to 70A is all that is required to make the installation compliant.
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emphasis added

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.

The OP asked if the install was legit. Not just the transformer. 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. You did not "make the installation compliant" by only adjusting the primary.
 
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