Transformer size/OCP question

[ggunn]

For OCP, Table 450.3(A) is for transformers over 1000V and Table 450.3(B) is for transformers 1000V or less. Is a transformer with a MV (>1000V) primary and a 480/277V (<1000V) secondary governed by 450.3(A) or 450.3(B)?

 

[electrofelon]

For OCP, Table 450.3(A) is for transformers over 1000V and Table 450.3(B) is for transformers 1000V or less. Is a transformer with a MV (>1000V) primary and a 480/277V (<1000V) secondary governed by 450.3(A) or 450.3(B)?

It could be a little clearer in the table description, but in 450.3(A) note the secondary side has a column for over and under 1000V. Now what about a transformer with a LV primary and a MV secondary?

 

[ggunn]

It could be a little clearer in the table description, but in 450.3(A) note the secondary side has a column for over and under 1000V. Now what about a transformer with a LV primary and a MV secondary?

So the over vs under 1000V refers to the primary side voltage? You're right; it should be clearer.

 

[jim dungar]

So the over vs under 1000V refers to the primary side voltage? You're right; it should be clearer.

450.3 deals with protecting the primary side of the transformer.
Primary side protection maximum values may be impacted by optional secondary side protection.

 

[electrofelon]

450.3 deals with protecting the primary side of the transformer.
Primary side protection maximum values may be impacted by optional secondary side protection.

So you are saying table B would cover, say, a single phase 240 to 12470GRDY/7200 step up transformer?

 

[jim dungar]

So you are saying table B would cover, say, a single phase 240 to 12470GRDY/7200 step up transformer?

Yes.

Note that both tables (A) and (B) have entries where secondary protection is listed as Not Required'.

 

[electrofelon]

Yes.

Note that both tables (A) and (B) have entries where secondary protection is listed as Not Required'.

Okay, I will have to cogitate on it and review the wording. My first thought is that I don't see why secondary protection not being required in certain cases would imply anything about which table to use for the aforementioned transformer.

 

[jim dungar]

Okay, I will have to cogitate on it and review the wording. My first thought is that I don't see why secondary protection not being required in certain cases would imply anything about which table to use for the aforementioned transformer.

You choose the table based on the primary voltage. You choose the column based on the secondary conditions.

 

[electrofelon]

You choose the table based on the primary voltage.

After reviewing the book, I think you are right but they should still state that more clearly.

 

[Tulsa Electrician]

cogitate
Had to look that one up.