Protection for low voltage transformers

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corvalan

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Question A: Can the main overcurrent protection device (OCPD) of a panel fed by the secondary conductors of a transfomer be considered the secondary protection for the transformer?

Question B: Can a primary fuse that is NOT connected directly to the primary terminals of the transformer (but connected to it through a cable) qualify as primary protection?

Just to clarify my quuestion.

1. I am not asking about conductor protection.

2. I am not asking if the primary or secondary OCPD is properly sized.

3. I am asiking if the fuse in the primary side of the transformer qualifies as a primary OCPD even though it is not cnnected directly to the terminals of the transformer and this fuse is part of a feeder circuit of the upstream panel.

4. I am asking if the main OCPD for the downstream panel conected to the transformer qualifies as a secondary OCPD for the transformer even though it is not connected directly to the secondary terminals of the transformer but there is a cable between the transformer secondary and the main OCPD for the downstram panel.

Thanks.
 
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ron

Senior Member
A: The cable between the primary protection and the transformer primary terminals can be as long as you like, no limit.
B: The secondary protection can be the main cb of a downstream panelboard as long as the cable is within the length limitations of Article 240.
 

jim dungar

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Location
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PE (Retired) - Power Systems
3. I am asiking if the fuse in the primary side of the transformer qualifies as a primary OCPD even though it is not cnnected directly to the terminals of the transformer and this fuse is part of a feeder circuit of the upstream panel.

4. I am asking if the main OCPD for the downstream panel conected to the transformer qualifies as a secondary OCPD for the transformer even though it is not connected directly to the secondary terminals of the transformer but there is a cable between the transformer secondary and the main OCPD for the downstram panel.

Absolutely. It is impossible to connect an OCPD directly to a transformer's terminals, some amount of conductor is always required.

Ron has provided information on the length of these conductors.
 
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