Transformer Secondary Rule

[david luchini]

This approach is similar to the Max. setting for Short Circuit & Ground Fault Protection of a Motor Circuit: 250% of the FLA for Branch Circuit OCPD Maximum rating; Conductors sized per FLA. (Table 430.52).

Obviously a transformer wouldn't fall under the rules for a motor such at T430.52. The transformer itself must have protection per Art. 450, and the conductors must have protection per Art. 240

If there were no Secondary OCPD(s), then the Maximum OCPD for the Primary Feeder would be 125% of the Primary FLA (Full Load Amperes).

I think you are confusing the OCPD protection rules for the transformer, per Art. 450 with protection for the primary and secondary conductors. The transformer must be protected with either primary only, or primary & secondary protection per Art. 450. The conductors must have OCPD per 240.4 "Protection of Conductors." A transformer that had "primary only" protection (at 125%) must still have an OCPD in the secondary for protection of the secondary conductors. See 240.21(C).

Personally, if this drawing was part of a Project I was compiling, I would size the Primary feeder's OCPD at maximum 175%, which would be 225 Amp max., while still using the 1/0 THHN Cu. Feeders. The Drawing is pointing out the maximum parameters only.

A 225A primary c/b with #1/0 cu primary conductors would also violate 240.4, just as the drawing does. 240.4 requires conductors to be protected in accordance with their ampacity specified in 310.15, except as permitted in 240.4(A) thru (G). T310.16 lists the ampacity of 1/0 as 150A. A 225A c/b will not provide proper protection to conductors with an ampacity of 150.

Side Note: Could you please denote which part of 240.4 you are referring to in the quoted text.
240.4(B) would be applicable to the Secondary Feeders, but not the Primary Feeders.

I am referring to the first part of 240.4: "Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)." Nothing in (A) thru (G) permits alternate protection for transformer primary conductors. Also, I think you have 240.4(B) backwards. It is NOT applicable to Secondary Conductors (see 240.21(C)) but it is applicable to Primary Conductors.

The #6 Cu. in reference is the Equipment Grounding Conductor, sized per the OCPD - as described in Table 250.122:
OCPD Not Exceeding 200 Amps; 6 Cu

Take a look at Art. 250.30 for Grounding Separately Derived A-C Systems. Specifically, 250.30(A)(1) describes the System Bonding Jumper (which you show connecting the ground bar to the neutral at the transformer,) 250.30(A)(3) describes the Grounding Electrode Conductor, and 250.30(A)(2) which describes the Equipment Bonding Jumper. I believe what you are calling an equipment grounding conductor is really the EBJ "run with the derived phase conductors from the source of the SDS to the first disconnecting means."

 

[don_resqcapt19]

Scott,
The rules in Article 450 don't have anything to do with the protection of the primary or secondary conductors. The rules in that article only cover the required overcurrent protection of the transformer windings. The protection of the conductors is covered in Article 240. In general the rules in Article 240 require that the conductors be protected at or below their ampacity. If you can make one of the tap rules in 240.21 apply, then fine, but it not very common that you can make the feeder tap rules apply to the primary feeder conductors.

As far as the equipment bonding jumper, it can't be an equipment grounding conductor at that point in the circuit. There is no overcurrent protection at that point that you can use to apply Table 250.122. The conductor in question is on the line side of the secondary overcurrent protection and is a bonding jumper and must be sized based on the size of the secondary conductors and Table 250.66.

 

[scott thompson]

Looks like it's time for me to end responding to this portion of the thread, as it is rapidly heading nowhere.

Besides, I did not plan on Thread-Jacking - however the Thread has been taken Hostage by me (I must have been hypnotized!).

Thanks to the both of you for your comments.

Scott