Article 450 Transformers - Primary and Secondary Overcurrent Protection - HELP

[chevywaldo]

What are the criteria to determine if overcurrent protection is required on "both" the primary and secondary side of a transformer (600 Volts and less) ?

Table 450.3(b) shows the Maximum Ratings for overcurrent protection devices. That said, I can't find anything that describes when one should use overcurrent protection on both primary and secondary.

Table 450.3(b) shows overcurrent protection on the "primary" side in every scenario.

The column named "Protection Method" shows 2 choices. Primary Only and Primary and Secondary.

So what are the criteria for determining of secondary protection is needed?

thanks.

 

[augie47]

As far as transformer protection as covered by Art 450, the transformer needs to be protected at 125% (current 9 amps or more).
That protection can be provided on the primary or on the secondary side.
If you have 125% protection on the secondary then the primary protection can be as high as 250%.
If you have 125% protection on the primary side, no transformer protection is needed on the secondary side,
(Note 1 applies as noted).
For transformers less than 9 amps, the percentages change.

It is important to note that Art 450 is about transformer protection,. For selection/protection of the conductors you need to go to Art 240.
Also, if a panel is involved, note 408.36

side note: Please update your profile to show your relation to the electrical industry (contractor, electrician,estimator, engineer etc.)

 

[chevywaldo]

As far as transformer protection as covered by Art 450, the transformer needs to be protected at 125% (current 9 amps or more).
That protection can be provided on the primary or on the secondary side.
If you have 125% protection on the secondary then the primary protection can be as high as 250%.
If you have 125% protection on the primary side, no transformer protection is needed on the secondary side,
(Note 1 applies as noted).
For transformers less than 9 amps, the percentages change.

It is important to note that Art 450 is about transformer protection,. For selection/protection of the conductors you need to go to Art 240.
Also, if a panel is involved, note 408.36

side note: Please update your profile to show your relation to the electrical industry (contractor, electrician,estimator, engineer etc.)

ok thanks. Does it say that protection can be on the Primary "OR" Secondary side?

I didn't see that anywhere. Was confusing.

 

[augie47]

Primary always needs protection.
If it's 125% then you don't necessarily need secondary protection (as far as transformer protection..you may still need conductor protection)
If the primary is >125% and not greater than 250% then you need 125% secondary protection.

 

[chevywaldo]

Primary always needs protection.
If it's 125% then you don't necessarily need secondary protection (as far as transformer protection..you may still need conductor protection)
If the primary is >125% and not greater than 250% then you need 125% secondary protection.

Hmmmm.......................?

 

[augie47]

Hmmmm.......................?

Not sure what else I can add but if you still have questions I will try....