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Transformer secondary protection

A

anbm

Senior Member
Location
TX
Occupation
Designer
4.16KV-480V transformer feeds a 480V panel located in a different room. The 480V panel has a main breaker.
1) Can the 480V panel's main breaker be used to protect both transformer secondary conductor and also panel load?
2) If the panel and transformer above are located in same room but 30-40 feet away from each other - within sight,
does the xfmr still need secondary conductor protection, which code section requires this protection? Any exemptions?
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
1) Yes, if the requirements in 240.21(C) are met.
2) Yes, protection in required, see 240.21 and in particular 240.21(C).

Cheers, Wayne
 
A

anbm

Senior Member
Location
TX
Occupation
Designer
wwhitney said:
1) Yes, if the requirements in 240.21(C) are met.
2) Yes, protection in required, see 240.21 and in particular 240.21(C).

Cheers, Wayne
Click to expand...
No exceptions for my case. Still wondering why the panel main CB cannot be used to protect xfmr secondary conductor if the CB is sized properly...
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
anbm said:
mot sure, seems the xfmr is always protected by primary CB no matter what...
Click to expand...
Transformer protection, primary conductor protection, and secondary conductor protection are 3 possibly separate issues. Transformer protection requires compliance with 450.3. Primary conductor protection is per 240.3 as usual, although there is a tap rule in 240.21(B). Secondary conductor protection is per 240.21(C). For the case of a single set of secondary conductors, there are only 2 OCPD involved, one on the primary side and one on the secondary side, so the 3 requirements will overlap.

Cheers, Wayne
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
anbm said:
mot sure, seems the xfmr is always protected by primary CB no matter what...
Click to expand...
It is not about protecting the transformer, which as you point out is done on the primary.

The issue is protection the conductors on the transformer secondary. The rules locating and sizing the secondary side protection are found in 240.21(C), as noted in post #2.
 
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