240.21(C)(2)&(6) & 450.3 Clarification

[Rock86]

Alright, I don't know why I am feeling dumb about this. I recently had another post regarding a Transformer.

Article 240.21(C) tells me what I need for over current on the secondary side of a transformer, great, got it. Here is my question... I'm surveying a school, it has 3 transformers in a dedicated room with nothing but the transformers. Each is fed from circuit breakers in a 480V panel in the hallway outside of the room. Each transformer feeds a single panel with a main circuit breaker in a classroom. The means of OCP are the primary side breakers, and the secondary side main breakers on the panels. The secondary panels are much farther than 10ft or 25ft.

Should there be a disconnect (fused or unfused) within 10ft as per 240.21(C)(2) or because the conductors leave the room, the disconnect would fall under 240.21(C)(6)? or is the system kosher and I'm over thinking this?

Sometimes these codes make me feel like i'm trying to explain the laws for robotics from the movie I-Robot... "But not if it violates the first and second law"

 

[augie47]

Are the transformers located in a room meeting the requirements of 450 Part III for a "vault" ?

If not the following would apply:
You have to first look at 240.21(C) to see if the transformer secondary conductors are considered to be protected by the primary protection (applies to 2 wire -2 wire or delta-delta installs..not likely in this case,). If that does not apply then protection would be required per the appropriate 240.21(C) rule.
Other than outdoor secondary this would normally mean an OCP within 10 or 25 ft,

 

[Rock86]

You have to first look at 240.21(C) to see if the transformer secondary conductors are considered to be protected by the primary protection (applies to 2 wire -2 wire or delta-delta installs..not likely in this case,). If that does not apply then protection would be required per the appropriate 240.21(C) rule.
Other than outdoor secondary this would normally mean an OCP within 10 or 25 ft,

I can agree with that. Okay, then by having a fused disconnect within 25ft of the transformer, I in theory comply with 240.21(C)(6). I only say that because the conductors leave the room which violates 240.21(C)(2)

 

[augie47]

What Code cycle are you referencing.?? I can;t see where "leaving the room" plays a part in (C)(6)

 

[Rock86]

What Code cycle are you referencing.?? I can;t see where "leaving the room" plays a part in (C)(6)

2017... 240.21(C)(2) "If the length of the secondary conductors does not exceed 10 ft and comply with all of the following: (2) the secondary conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply."

 

[charlie b]

That has nothing to do with leaving the room. The secondary conductors in your case terminate at the main breakers of the three panels. Since they do not extend beyond that panel, this will meet the rule. However, you still have a problem with the length of the conductors. I believe you need to put something (I suggest an enclosed circuit breaker) in the room, immediately downstream of each transformer. After that, the length of the conductors no longer matters.

PLEASE NOTE: If you do this, then each of the N-G bonds have to take place inside the breaker enclosure. That also means you need to run an EGC from the enclosed breaker to the downstream panel, and you need to separate the N and G buses at each downstream panel.

 

[Rock86]

That has nothing to do with leaving the room. The secondary conductors in your case terminate at the main breakers of the three panels. Since they do not extend beyond that panel, this will meet the rule. However, you still have a problem with the length of the conductors. I believe you need to put something (I suggest an enclosed circuit breaker) in the room, immediately downstream of each transformer. After that, the length of the conductors no longer matters.

PLEASE NOTE: If you do this, then each of the N-G bonds have to take place inside the breaker enclosure. That also means you need to run an EGC from the enclosed breaker to the downstream panel, and you need to separate the N and G buses at each downstream panel.

That helps clear things up a little more. Thank you!