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#167980 08/24/07 03:35 PM
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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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try 450.3(B)

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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
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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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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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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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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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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


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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.
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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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