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#167980 - 08/24/07 12:35 PM XFMR Secondary Protection
windmiller Offline
Junior Member

Registered: 12/05/06
Posts: 43
Loc: Alaska
What is the article that allows the secondary of a transformer to be protected by the primary?

Thank you,

Windmiller
_________________________
We all live under one King

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2014 / 2011 NEC & Related Books and Study Guides
#167981 - 08/24/07 12:42 PM Re: XFMR Secondary Protection [Re: windmiller]
Yoopersup Offline
Member

Registered: 03/04/03
Posts: 826
Loc: Michigan
try 450.3(B)

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#167984 - 08/24/07 02:38 PM Re: XFMR Secondary Protection [Re: windmiller]
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Originally Posted By: windmiller
What is the article that allows the secondary of a transformer to be protected by the primary?

Thank you,

Windmiller


Well that depends on the transformer, many transformer can not use the primary to protect the secondary.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#167986 - 08/24/07 03:49 PM Re: XFMR Secondary Protection [Re: iwire]
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
And even if the secondary of the transformer is protected by the primary OCPD, you still have to look to 240.21(C) for the protection of the secondary conductors.
Don
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Don(resqcapt19)

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#168140 - 08/27/07 05:08 PM Re: XFMR Secondary Protection [Re: resqcapt19]
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
I agree with Don and Bob- It's possible to proptect the xformer secondary and the secondary conductors with the overcurrent protection on the primary but we need more details about the xformer and the associated conductors on both sides of the xformer.
_________________________
George Little

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#168155 - 08/28/07 07:02 AM Re: XFMR Secondary Protection [Re: George Little]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Don't forget 240.4(F), this and 240.21(C)(1) are the only places that specifically allows secondary conductors to be protected by the primary.

450.3 doesn't care about conductors it is only concerned with the transformer.

The rest of 240.21(C) requires over current protection on the secondary but it is allowed to be at the end of the conductorsa instead of at the source.

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#168203 - 08/28/07 07:36 PM Re: XFMR Secondary Protection [Re: JBD]
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
Look at it this way, A 1PH transformer with either a 240/120 or a 120/240 secondary providing power to a 1ph3w load requires secondary protection.
A 3ph transformer that supplies power to a 3ph4w load must have secondary protection.
A 1ph transformer that simply feeds a 1ph2w-load need only to have primary protection is the pri. OCPD is sized correctly according to art 450.
It is the same for a 3ph transformer, which simply feeds a 3ph3w load.
In both of these examples the secondary current is proportional to the primary current so a pri. OCPD can protect the secondary.
Not so with a 1ph-3w or a 3ph-4w secondary. Think of it this way, if it is possible for the secondary to provide power where there are unbalanced L-N unbalanced loads if primary only over current protection was provided it would not see the unbalanced load and would not provide adequate transformer overload protection.

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#168214 - 08/29/07 10:20 AM Re: XFMR Secondary Protection [Re: Dave T]
windmiller Offline
Junior Member

Registered: 12/05/06
Posts: 43
Loc: Alaska
Hi,
So if I come from a 480 to 120/208 xfmr to a 3P/4W panel i need to go to a fuse or breaker ahead of the panel and not to a MLO panel?

This is getting interesting.

Thanks

Windmiller
_________________________
We all live under one King

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#168224 - 08/29/07 02:33 PM Re: XFMR Secondary Protection [Re: windmiller]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
 Originally Posted By: windmiller
Hi,
So if I come from a 480 to 120/208 xfmr to a 3P/4W panel i need to go to a fuse or breaker ahead of the panel and not to a MLO panel?

This is getting interesting.


This is why it is dangerous to give "how is it done" advice on the internet.

By bringing a panel into the discussion, you have just added a new requirement to look at 408.36(D).


Edited by JBD (08/29/07 02:34 PM)

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#168243 - 08/29/07 08:21 PM Re: XFMR Secondary Protection [Re: JBD]
Yoopersup Offline
Member

Registered: 03/04/03
Posts: 826
Loc: Michigan
Then again it depends on the Classification of the Panel board . 408.30(A) Then 408.36.
450.3(B)
240.21
408.30(A)
408.36
all must be reviewed to get the total picture.
2005 NEC

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