transformer conductors

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msteiner

Senior Member
Location
Maryland
When sizing transformer primary conductors, do you use the transformer rating (FLA x 1.25) or the rating of the OCPD? For example, if my primary overcurrent device is sized at 250% of transformer-rated current, do the conductors need to be sized accordingly?
 

lauraj

Senior Member
Location
Portland, Oregon
Re: transformer conductors

240.4 - Protection of Conductors, answers your question. The conductors must have an ampacity large enough to be protected by the overcurrent device.

Example: If we had a primary current of 100 A, primary OCPD would be 100 A x 250% = 250 A. The smallest THHN conductor we could install would be a 4/0 AWG.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: transformer conductors

In general the above answer is correct, but under certain circumstances, the primary wire is allowed to be sized smaller than the primary OCP. For example, see 240.21(B)(3). You may have a combined lenght of 25' of wire not protected at its ampacity on the primary and secondary.

Steve
 
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