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Transformer Overcurrent Protection #89457 09/16/04 07:31 AM
Joined: May 2004
Posts: 3
M
mistermike Offline OP
Junior Member
I'm getting differing opinions on the interpretation of Note 2 to Table 450.3(B). I have a 150 kVA, 480-208Y/120 V indoor dry-type transformer feeding three main breaker panelboards. The question comes from the phrase in Note 2 "shall not exceed the allowed value of a single overcurrent device". Is that value for multiple overcurrent devices (a) strictly a maximum of 125% of transformer rated current, or (b) permitted to be as high as "the next higher standard rating" of Note 1 of Table 450.3(B). If "value" means (a), the sum of the three main breakers is limited to 520 A, but if "value" means (b), I can go to 600 A.

2017 / 2014 NEC & Related Books and Study Guides
Re: Transformer Overcurrent Protection #89458 09/16/04 09:39 AM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
Mike,
The value is per note 1, in my opinion. This permits the rounding up to a total of 600 amps for the three overcurrent protective devices. Don't forget that the conductors between the transformer and the panles must be protected per 240.21(C).
Don


Don(resqcapt19)
Re: Transformer Overcurrent Protection #89459 09/16/04 11:41 AM
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Ryan_J Offline
Moderator
There is an interesting change to this in the 2005.


Ryan Jackson,
Salt Lake City
Re: Transformer Overcurrent Protection #89460 09/16/04 04:27 PM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Quote
There is an interesting change to this in the 2005.

Ryan is such a code teaser. [Linked Image]

Come on and give it up. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Transformer Overcurrent Protection #89461 09/17/04 08:21 AM
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Ryan_J Offline
Moderator
240.21(C) in the 2005 now says that you cannot use 240.4(B) on the secondary side.


Ryan Jackson,
Salt Lake City
Re: Transformer Overcurrent Protection #89462 09/17/04 01:23 PM
Joined: May 2004
Posts: 162
C
cpal Offline
Member
If I'm reading note two correctly it appears to only apply if the primary overcurrent protection is sized larger than 125% of the pri FLA?? In that are compelled to additionally protect the secondary winding.

If the pri OCPD is sized to protect the transformers pri. per 450.3 (B) [pri protection only} , then OCP for the secondary winding is not required.

You may have multiple taps from the secondary of the transformer (as mentioned 240.21 (C) ) to several loads the secondary conductors must be protected per that section and article 240.
If your pri overcurrent protection exceeds 225 A (480/.83)I think Note two is applicable.??


RYAN could you indicate the paragraph in the 05 that references 240.4 I'm looking at the pre-print and appear to be missing it!!

charlie

[This message has been edited by cpal (edited 09-17-2004).]

Re: Transformer Overcurrent Protection #89463 09/17/04 03:24 PM
Joined: May 2004
Posts: 3
M
mistermike Offline OP
Junior Member
Thanks for all of the feedback. In the end, the local AHJ said we had to stick strictly with 125%; next larger size denied. Sounds like the 2005 NEC will make all of the discussion go away.

Re: Transformer Overcurrent Protection #89464 09/19/04 12:52 PM
Joined: Aug 2003
Posts: 1,374
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Ryan_J Offline
Moderator
From the 2005:

240.21(C) Transfomrer secondary conductors. Each set of conductors feeding seperate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.


*(C)(1)through(C)(6) are the 10', 25', etc., rules, similiar to the 2002.


Ryan Jackson,
Salt Lake City
Re: Transformer Overcurrent Protection #89465 09/19/04 07:29 PM
Joined: May 2004
Posts: 162
C
cpal Offline
Member
Thanks Ryan

The reference to 240.4 (B) was accepted in Comment stage and not in the pre-print.

charlie


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