Transformer Primary Side Overcurrent Protection

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MAster12

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Boston, MA, US
I am sizing the primary side overcurrent protection for a 75KVA step-down transformer 480-120/208

Table 450.3(B) provides the MAXIMUM rating of overcurrent protection on the primary side as being 125%. Does this mean the overcurrent protection can be rated less.

Is a transformer considered a continuous load?

75,000/480/1.732 = 90A - Can I use a 100A breaker/fuse as my primary side protection or does the code require 125% - 90A x 1.25 = 113A (125A next size up)

I know... very basic question
 

david luchini

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Connecticut
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I am sizing the primary side overcurrent protection for a 75KVA step-down transformer 480-120/208

Table 450.3(B) provides the MAXIMUM rating of overcurrent protection on the primary side as being 125%. Does this mean the overcurrent protection can be rated less.

Is a transformer considered a continuous load?

75,000/480/1.732 = 90A - Can I use a 100A breaker/fuse as my primary side protection or does the code require 125% - 90A x 1.25 = 113A (125A next size up)

I know... very basic question

The Code requires that the primary OCPD be sized for the load on the feeder. It permits the primary OCPD to be sized up to 250% or 225A.

The transformer is not considered a load.
 
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