Transformer secondary protection

[101010]

Does section 240 21 C1 tell you you cannot use 2403b when sizing conductors for secondary protection? With this said you cannot feed a 400 amp breaker with 500mcm? Delta wye transformer.

 

[david luchini]

Does section 240 21 C1 tell you you cannot use 2403b when sizing conductors for secondary protection? With this said you cannot feed a 400 amp breaker with 500mcm? Delta wye transformer.

Yes, you are reading that correctly. You would need 600mcm (min.) to feed a 400A c/b on a D-Y transformer secondary.

 

[ron]

I don't agree that 240.21(C)(1) tells you that for a D-Y trans. It says you can't rely on the primary protection only. I think note 1 listed in table 450.3(B) allows round up.

 

[jumper]

I don't agree that 240.21(C)(1) tells you that for a D-Y trans. It says you can't rely on the primary protection only. I think note 1 listed in table 450.3(B) allows round up.

450.3(B) note 1 allows you to upsize the breaker for the xfmr, not for the secondary conductors.

240.21(C) seems pretty specific:
The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.

 

[texie]

I don't agree that 240.21(C)(1) tells you that for a D-Y trans. It says you can't rely on the primary protection only. I think note 1 listed in table 450.3(B) allows round up.

But 240.21(C) says 240.4(B) shall not be permitted. It does not distinguish transformer type. 600 MCM is required. I hope so, I've seen this on exams before.

 

[ron]

It was the typo in the OP that screwed me up "2403b". I was hung up on a section I couldn't find rather than the intent of the question. You are correct.

450.3(B) note 1 allows you to upsize the breaker for the xfmr, not for the secondary conductors.

240.21(C) seems pretty specific:
The provisions of 240.4(B) shall not be permitted
for transformer secondary conductors.

 

[jmanderson]

In the example out of Mikes book below, is there any limitations to the distance from the transformer secondary to the panel with the MCB in it?

 

[david luchini]

In the example out of Mikes book below, is there any limitations to the distance from the transformer secondary to the panel with the MCB in it?

In the illustration shown, the secondary length would be limited to 25 ft per 240.21(C)(6) (or (C)(3) if it was an industrial installation.)

If the conditions of 240.21(C)(6) weren't met, the length would be limited to 10 ft per 240.21(C)(2).

 

[jumper]

In the illustration shown, the secondary length would be limited to 25 ft per 240.21(C)(6) (or (C)(3) if it was an industrial installation.)

If the conditions of 240.21(C)(6) weren't met, the length would be limited to 10 ft per 240.21(C)(2).

I mean this with respect sir.

I swear you must have a built in radar that if someone even whispers transformer, you show up with the correct answer.

 

[david luchini]

I mean this with respect sir.

I swear you must have a built in radar that if someone even whispers transformer, you show up with the correct answer.

Knock it off...People will start calling me "Radar." I'm about the same height and wear glasses.

 

[jumper]

Knock it off...People will start calling me "Radar." I'm about the same height and wear glasses.

Yes sir, Mr O'Reilly.

PS. I still watch the show.