nec 450.3(b) note 3

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mattawamkeag

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hello, i hope this isn't a re-post

i'm having trouble interpreting art. 450.3(b) note #3...
A transformer that comes with factory primary OCPD, may have a second primary OCPD installed as long as its not more then 6 times the rated primary current of the transformer, for transformers with 6% or less impedance. and if more than 6% - 10% impedance use 4 times the rated primary current.

is this the correct way to interpret this note?
-Thank you
 
No....

The first part uses the term thermal overload protection, which is not the same as overcurrent protection. Think automatic high-temperature cutout switch.
 
450.3(b) note 3

450.3(b) note 3

That clears note 3 up alot for me thank you.

So? Where note 3 applys in the table does this mean i have to round down to the next standard size OCPD. Beacuse note one does note apply?
-Thank you
 
mattawamkeag said:
That clears note 3 up alot for me thank you.

So? Where note 3 applys in the table does this mean i have to round down to the next standard size OCPD. Beacuse note one does note apply?
-Thank you
Click to expand...
Notes 1 and 3 are only applicable where noted...

Note 1 applies only to either...


  • the primary OCPD under the Primary only protection schema, where primary current rating is 9A or greater


  • the secondary OCPD under the Primary and secondary protection schema, when secondary current rating is 9A or greater, regardles of whether or not Note 3 applies to the primary OCPD.

Note 3 only applies to the primary ocpd under the Primary and secondary protection schema when its conditions are met. Under this schema, Note 1 is never applicable to the primary OCPD.
 
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