XMER PRIMARY&SECONDARY PROTECTION

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steve66

Senior Member
Re: XMER PRIMARY&SECONDARY PROTECTION

Linda:

Do your low voltage panels have a main overcurrent device in them?

Steve
 

jschultz

Member
Re: XMER PRIMARY&SECONDARY PROTECTION

Originally posted by jim dungar:
If Linda has several panelboards with main breakers which are located within 25ft of cable of the transformer, then a single secondary device is not required per 240.21(C)(3). In this instance, the sum of the ratings of the panelboard mains can not exceed the output rating of the transformer
....
240.4(F) does require conductors on the multi-voltage secondary of a transformer to have overcurrent protection. 240.21 lists the requirements for the location and sizing of this required protection. A grouping of panelboard main devices would meet 240.4(F) if they are located per 240.21(C).
EXACTLY MY POINT. SHE NEEDS OVERCURRENT PROTECTION ON THE SECONDARY SIDE OF THE TRANSFORMER. Where it needs to be installed and what size it can be are different questions. Unless you have one of the transformers mentioned
in 240.21(c)1 first paragraph, you will have overcurrent protection on the secondary side. Just because the transformer section(450) does not require it, you end up with it from the overcurrent section(240).
 

jschultz

Member
Re: XMER PRIMARY&SECONDARY PROTECTION

Originally posted by llrodriguez:
If secondary protection of a xmer less than 600 v is never required, then when would you ever apply method 2 primary/secondary protection of Table 450.3(B)??
so you can size the primary ocp larger than 125%
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: XMER PRIMARY&SECONDARY PROTECTION

Linda, method 2 is for when you do provide secondary protection, then you can change the size of the primary protection.

jschultz, the correct answer is transformers do not need protection - stop looking in 450, conductors do need protection start looking in 240.

One example: Industrial application, 75KVA 480V primary to 208Y/120 secondary transformer, output rated for 208A, conduit, ambient, and terminations per 75C column.

Per 240.4(F) conductors need protection. 240.21 says the protection does not need to be at the begining of the conductors. 240.21(C)(3) will allow (3) 25' long feeders of 350kcmil (310A max.) to (3) wall mounted 100A main breaker panelboards.

[ April 21, 2004, 09:07 AM: Message edited by: jim dungar ]
 
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