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#94438 - 07/29/05 08:18 PM Transformer fusing
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Can someone tell me what the last line of Note 2 to Table 450.3(B) is talking about? The Note 2 for Table 450.3(A) makes sense but not the one for Table (B). 2002 NEC.
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#94439 - 07/30/05 03:24 AM Re: Transformer fusing
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
They are almost identical.

Last sentence of note 2 Table 450.3(A)

"If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses."

Last sentence of note 2 Table 450.3(B)

"If both breakers and fuses are utilized as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses."

Am I missing something?


[This message has been edited by iwire (edited 07-30-2005).]
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Bob Badger
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#94440 - 07/30/05 05:20 AM Re: Transformer fusing
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Yes they are the same Bob, but the difference is Table (A) they give reference to a different % for fuses than for breakers in some cases. In Table (B) they don't differentiate between fuses and breakers???

Maybe I'm missing something??

[This message has been edited by George Little (edited 07-30-2005).]
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#94441 - 07/30/05 05:34 AM Re: Transformer fusing
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I get what you are pointing out now.

I looked in some older NECs and found that as recently as 1996 both tables had separate ratings for fuses and breakers.

The 1999 Table 450-3(b) changed to what we see now.

I guess there was a proposal to change the table but not the notes.

Looks like you could put in a proposal.

You are certainly a careful reader of the NEC.

Bob

[This message has been edited by iwire (edited 07-30-2005).]
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#94442 - 08/02/05 03:35 AM Re: Transformer fusing
jes Offline
Member

Registered: 07/12/02
Posts: 103
Loc: CT
Hello George,
FYIW, I believe these notes to have nothing to do with the percentages in the tables, just the aggregate sum of the OCP ratings. For example, at smaller OCP values let's say you had 6 devices (for 6 circuits) and a 60 amp single device limit on the xmfr. You could have 6 fuses at 10 amps each. 3 fuses at 10 amps and 3 breakers at 15 amps would not meet the requirement in the note. (10 amps is not a standard breaker rating in 240). You probably can come up with a situation also with motors or another load where the permitted settings for breakers and fuses are different.

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#94443 - 08/02/05 03:23 PM Re: Transformer fusing
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Okay JES, I can buy into that but there doesn't seem to be any documentation anywhere. Kinda a seldom needed bit of information. Only a Code Geek such as myself would even pursue it
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George Little

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