Transformer protection

Status
Not open for further replies.
C

cppoly

Senior Member
Location
New York
Is this misleading for transformer overcurrent protection:

NEC section 450.3 refers you to the transformer overcurrent protection tables 450.3(A) & 450.3(B). The tables list different overcurrent protection percentages based on primary only protection or a combination of primary and secondary protection. No where in these tables (in the title or in the key notes) does it say that secondary protection is required for all transformers (except 1-phase, 2-wire transformers and 3-phase delta-delta, 3-wire transformers per section 240.4 (F)).

I came across section 240.4 (F) just recently. But I thought it was misleading to go to the transformer tables in 450.3 and think that the information in these tables was enough to make a decision on whether or not to provide primary only protection simply following the percentages listed in the tables.

Example: 50A load, 3-phase, 4-wire wye transformer. Go to table 450.3(B) and select 125% primary only protection with secondary protection not required. But it IS required according to section 240.4 (F).

Shouldn't there be a note here in these tables stating this? Am I overlooking something?
 
As Iwire said:

You may not need to protect the transfromer secondary itself, but you must always protect the conductors connected to the secondary.
 
As long as you follow the rules of Art 450, the transformer is protected. For example, most transformers with primary overcurrent protection set at 125% are considered to be protected with any or no secondary protection.
But, as already been stated, other than as prescribed in 240.4(C) providing proper transformer protection does not necessarily provide conductor protection.
As in your example with a 3 phase 4 wire wye secondary, an overloaded unbalanced current could occur on the secondary which would not result in the primary OCP opening.,
 
Thanks this is helping. I see the distinction between protecting a transformer vs protecting conductors. But, I don't understand if article 240.4 is telling you that protection on the secondary for conductors is a must, wouldn't you have to use the percentages in article 450.3 for primary & secondary protection?
 
jim dungar said:
As Iwire said:

You may not need to protect the transfromer secondary itself, but you must always protect the conductors connected to the secondary.
Click to expand...

This is something that has always caused me confusion. Just so I'm clear, wouldn't 240.21(C)(2) allow us to have unprotected secondary conductors of any transformer ,not just those transformers specified in 240.4(F), as long as those conductors complied with the requirements of 240.21(C)(2)?
 
jim dungar said:
As Iwire said:

You may not need to protect the transfromer secondary itself, but you must always protect the conductors connected to the secondary.
Click to expand...

Properly sized conductors will be protected. One common mistake is made that the conductor is sized to the nameplate - normal - current, not to the maximum allowed by the transformer protection. There is no reason to size the transformer protection to the maximum allowable limit IF the conductors connecting it to the supply or load can-not carry that current.
 
weressl said:
Properly sized conductors will be protected. One common mistake is made that the conductor is sized to the nameplate - normal - current, not to the maximum allowed by the transformer protection. There is no reason to size the transformer protection to the maximum allowable limit IF the conductors connecting it to the supply or load can-not carry that current.
Click to expand...

Except for inrush. The primary breaker has to be large enough to handle the inrush of energizing the transformer. It was my understanding that this is even more of an issue with energy efficient transformers.
 
Status
Not open for further replies.
Top