XFMR SECONDARY PROTECTION

[SPierce]

I guess I'm confused about when you need overcurrent protection on the secondary side of a transformer rated less than 600v. Table 450.3(B) gives you two protection methods, "Primary only" & "Primary and Secondary". If I choose primary only, do I still have to follow the rules set forth in 240.21(C)(1) through (C)(6), which tell me when I need secondary protection? I guess what I'm asking is how do these two code sections apply to each other? Any help in getting me to understand this better is appreciated.

 

[jumper]

Here are a few links links that may help.

http://ecmweb.com/nec/code-basics/electric_transformer_secondary_conductors/

http://ecmweb.com/mag/electric_transformer_installation_made/

 

[raider1]

Keep in mind that the rules in 450 apply to the secondary windings of the transformer only. The rules in 240.21(C) apply to the secondary conductors of the transformer from the secondary windings to the termination on the overcurrent protective device.

Chris

 

[SPierce]

How do we install protection for the transformer secondary windings?

 

[augie47]

How do we install protection for the transformer secondary windings?

very carefully

You follow the rules in 240.21(C) mainly. They prescribe minimum sizes, lengths of conductors and termination rules.

 

[jim dungar]

How do we install protection for the transformer secondary windings?

You don't directly. When the secondary conductors are protected by 240.21(C) and the primary is sized per 450.3, then the secondary windings are considered to be protected. Remember the protective device from 240.21(C) can be considered as the 'optional' secondary protective device in 450.3.

 

[templdl]

In my opinion it's Simple, With a 1ph transformer if you have a 2wire secondary such as a simple 12v or 240 which would be a 2 wire secondary you can use primary protection only because the current of the primary and secondary a proportional. Its the same with a 3ph3w secondary of a 3ph transformer.
The minute that you add a neutral such as a 1ph3w 120/240v secondary of a 1ph transformer and a 3ph4w of a 480y/277 or 208y/120 then all bets are off. It's because of that neutral where the proportionate part is no longer true because it is entirely possible to overload on side of a 120/240 1ph transformer a phase of a 3ph4w secondary transformer and that overload will not be seen by the primary.
If the 1ph3w and 3ph4w transformers were perfectly balanced then primary protection would work but that's never going to happen such that the neutral current ends up to be zero.

 

[augie47]

In my opinion it's Simple, With a 1ph transformer if you have a 2wire secondary such as a simple 12v or 240 which would be a 2 wire secondary you can use primary protection only because the current of the primary and secondary a proportional. Its the same with a 3ph3w secondary of a 3ph transformer.
The minute that you add a neutral such as a 1ph3w 120/240v secondary of a 1ph transformer and a 3ph4w of a 480y/277 or 208y/120 then all bets are off. It's because of that neutral where the proportionate part is no longer true because it is entirely possible to overload on side of a 120/240 1ph transformer a phase of a 3ph4w secondary transformer and that overload will not be seen by the primary.
If the 1ph3w and 3ph4w transformers were perfectly balanced then primary protection would work but that's never going to happen such that the neutral current ends up to be zero.

I agree, but think it is important to add the "no secondary OCP required" rule needs to take into account the ratio of primary/secondary voltages and size of the primary OCP device.

 

[templdl]

I'm glad that you brought that up Augie47. Since they are proportional a transformer that may have a current rating that has a 1:4 with the primary being 1/4 of the secondary the primary OCP rating would be based upon 1/4 of the secondary FLA.