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#167980 - 08/24/07 03:35 PM XFMR Secondary Protection  
windmiller  Offline
Junior Member
Joined: Dec 2006
Posts: 43
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

2017 / 2014 NEC & Related Books and Study Guides

#167981 - 08/24/07 03:42 PM Re: XFMR Secondary Protection [Re: windmiller]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
Michigan
try 450.3(B)


#167984 - 08/24/07 05:38 PM Re: XFMR Secondary Protection [Re: windmiller]  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
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

#167986 - 08/24/07 06:49 PM Re: XFMR Secondary Protection [Re: iwire]  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
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


Don(resqcapt19)

#168140 - 08/27/07 08:08 PM Re: XFMR Secondary Protection [Re: resqcapt19]  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
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

#168155 - 08/28/07 10:02 AM Re: XFMR Secondary Protection [Re: George Little]  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
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.


#168203 - 08/28/07 10:36 PM Re: XFMR Secondary Protection [Re: JBD]  
Dave T  Offline
Member
Joined: Jan 2005
Posts: 155
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.


#168214 - 08/29/07 01:20 PM Re: XFMR Secondary Protection [Re: Dave T]  
windmiller  Offline
Junior Member
Joined: Dec 2006
Posts: 43
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

#168224 - 08/29/07 05:33 PM Re: XFMR Secondary Protection [Re: windmiller]  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
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).

Last edited by JBD; 08/29/07 05:34 PM.

#168243 - 08/29/07 11:21 PM Re: XFMR Secondary Protection [Re: JBD]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
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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