Transformer Primary Conductor Sizing / Protection

[richwaskowitz]

I have looked at all of the posts I could find on this subject but cannot find a concise answer to this direct question. When a power transformer has a primary OCPD set at some high value (in order to avoid nuisance tripping on inrush) of say 200% of the FLA, does that mean that the conductors between those fuses and the transformer must be sized so that they are protected from overload by that fuse even though the actual load on that transformer is limited by the secondary OCPD at 125%? To put numbers to this, a 480 Delta to 480 Wye 51 kVA 3 phase drive isolation transformer has an FLA of 64 A. The secondary is protected with an 80 A fuse (64 FLA x 1.25 = 80 A). A primary fuse set at 200% of FLA is desired. That would be 130 A. What size conductor is required between the 130 A fuse and the transformer primary? Are those conductors to be sized based on the FLA of 64 A (x 125%) or based on the 130 A fuse? Please cite the code reference.

 

[don_resqcapt19]

The primary conductors are required to have an ampacity equal to or greater than the rating of the OCPD that supplies those conductors.

 

[augie47]

Best Code reference is probably 240.4 "Conductors... shall be protected against overcurrent i accordance with their ampacity......."
&
450.3 Information Note which refers you to Art 240 for protection of conductors.

 

[electrofelon]

I would just add that the next size up rule can be used for the primary conductors. Also there is a probably not so often used provision where the transformer can be fed from a feeder tap (240.21(B)(3)).

 

[david luchini]

I'm going to go out on a limb and say you have a motor feeder...430.62 would apply.

 

[Sahib]

I'm going to go out on a limb and say you have a motor feeder...430.62 would apply.

That is for a MV transformer for which primary protection is against short circuit only. But OP seems to talk about protection of LV transformer and its supply conductors.

 

[david luchini]

That is for a MV transformer for which primary protection is against short circuit only. But OP seems to talk about protection of LV transformer and its supply conductors.

That is not correct.