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#206106 - 05/22/12 03:45 PM 450.3(C)
Steve T Offline

Registered: 02/14/01
Posts: 312
Loc: Oak Park, IL, USA
Since 450.3 says "Overcurrent protection...comply with...(A), (B), or (C).", if I am providing overcurrent protection per (B), then I am not required to provide primary fuse protection for my indoor voltage transformer, correct?

If I choose to only follow (C), I can pick any size fuse I want because there is no reference to a table in (C), correct?

Or should 450.3 read "...(A) or (B) and (C)? What's the intent?


Steve T

2014 / 2011 NEC & Related Books and Study Guides
#206107 - 05/22/12 04:13 PM Re: 450.3(C) [Re: Steve T]
gfretwell Offline


Registered: 07/20/04
Posts: 8724
Loc: Estero,Fl,usa
A "voltage transformer" is not a power transformer.
It is a precision transformer used in protective relay systems. Different deal altogether.
For power transformers you are either 450.3 A or B depending on voltage.

I agree they could have a more informative FPN here.
Greg Fretwell

#206109 - 05/22/12 04:41 PM Re: 450.3(C) [Re: gfretwell]
Steve T Offline

Registered: 02/14/01
Posts: 312
Loc: Oak Park, IL, USA
Thanks Greg. That sounds right based on the FPN, but without any explicit definitions, a search for 'voltage transformer' on the net brings up a lot of stuff. Anyone have the original substantiation on this section? Or a link to a manuf. web site that shows a voltage transformer as intended in the code section? Thanks again. An Engineer is going to be happy smile

#206111 - 05/22/12 08:32 PM Re: 450.3(C) [Re: Steve T]
gfretwell Offline


Registered: 07/20/04
Posts: 8724
Loc: Estero,Fl,usa
I found this article that has some info on them
Greg Fretwell

#206116 - 05/23/12 03:07 PM Re: 450.3(C) [Re: Steve T]
Tesla Offline

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
Class 450.3 (A) = OVER 600 Volts...

So forget it. That's industrial territory.

Class 450.3 (B) = LESS THAN 600 Volts...

This is the category for dry-type transformers we all know and love.

Class 450.3 (C) = Voltage Transformers...

Which is what you've described it as.

EITHER WAY you're required to provide OCPDs...

See 408.52: " protected by standard overcurrent devices rated 15 amperes or less."

So, you don't get to pick a rating out of the air.


BTW, I don't think that your transformers are 450.3(C) Voltage Transformers. I'll bet that they are a part of some low voltage lighting set-up.

That makes them 450.3(B) transformers, in my eyes.

Why? They're not switchboard devices with potential coils, pilot lights, etc.

They're power supplies for some low voltage loads. -- Hence 450.3(B).

Edited by Tesla (05/23/12 03:10 PM)


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