Table 450.3(B) OCPD for Transformers (1000V or less)

[fifty60]

The way I read this table is that for a transformers that has a primary current of 9A or less, if I size the primary OCPD at 167% rounded down, then I do not need any secondary protection. But, if do provide secondary protection I can increase the size of the primary OCPD to 250%.

How common is it to see transformers with only primary side protection? For small dry type transformers that are 3KVA or less, is it common to see these without any secondary side OCPD?

 

[david luchini]

How common is it to see transformers with only primary side protection? For small dry type transformers that are 3KVA or less, is it common to see these without any secondary side OCPD?

Even when 450.3(B) allows primary protection only, you will likely still need an OCPD on the secondary side to protect the conductors per 240.21(C).

When the secondary OCPD is protecting the conductors only per 240.21(C), it is not limited to a percentage of transformer rated current.

 

[fifty60]

Thanks for clarifying that for me.

 

[Smart $]

You did not specify the transformer configuration. If there is no secondary neutral, it is possible to have primary only protection and secondary conductors can be considered protected by the primary ocpd per 240.21(C)(1).

 

[augie47]

my answer would be the same a david's so it still may not clarify it for you.
The Table in 450 addresses transformer protection In addition to meeting the requirements of Art 450, you must also meet the requirements of Art 240(C). In many case, Art 240(C) requires secondary protection for conductor protection so it's often a mute point as it relates to Art 450.

As smart$ points out there are situations where 240(C) allows the primary to provide secondary protection.
As to your "how often" question, I can only say in my situation a majority of the installs require Art 240 secondary protection. Perhaps 25% of 9 amp installs do not.

 

[Smart $]

... Perhaps 25% of 9 amp installs do not.

When we limit the discussion to control power and other small transformers, that percentage goes way up. Even doorbell and HVAC-control transformers fall into this category, though most everyone overlooks them.

 

[augie47]

When we limit the discussion to control power and other small transformers, that percentage goes way up. Even doorbell and HVAC-control transformers fall into this category, though most everyone overlooks them.

I don't disagree with that. I had actually placed many of those in the "current less than 2 amp" category when i guesstimated the 25%.

 

[jim dungar]

Another condition, of primary protection only, occurs when the secondary OCPD is greater than 125% of FLA, as might occur when rounding up to the next common device. As an example, the secondary is installed to support a 75 kVA future supply but, for temporary work, only a 30kVA unit is provided.

 

[Pierre C Belarge]

Article 450 is protection of the Transformer.
240.21(C) is protection of the conductor .

There is also, 408.36(B)
The exception to this Section number is very restrictive.