240.21(c)

[ASG]

I was reading the following thread:

http://forums.mikeholt.com/showthread.php?t=93372

and it perplexed me. 240.21(C) is prefaced with saying you can have no OCPD if (1) through (6) applies. So I would think if your secondary panel is 100 feet from the transformer with a main circuit breaker, you wouldn't need a fused disconnect or circuit breaker within 10 or 25 feet of the transformer. What am I missing?

 

[david luchini]

240.21(C) says that you don't need overcurrent protection "at the secondary" of the transformer as long as the secondaries are installed in accordance with (C)(1) thru (C)(6).

(C)(1) thru (C)(6) specify where the OCPD for the secondary conductors should be installed (ie, within 10 feet or within 25 feet) instead of at the secondary of the transformer.

Overcurrent protection is still required for the transformer secondary conductors, but 240.21(C) allows the OCDP to be at a location other than the point at which they receive their supply.

 

[charlie b]

I agree with David. Take a look at the opening sentences of 240.21. It tells us that every conductor has to be protected against overcurrent, and the protection must be at the point at which the conductor gets its power. Then it says, "except . . . ," and goes on to spell out the conditions under which the overcurrent protection can take place at the end of the run, instead of at the beginning.

 

[ASG]

So does that mean if I have an OCPD at the primary in line of sight of the transformer I don't need one within 25 feet of the transformer?

 

[david luchini]

So does that mean if I have an OCPD at the primary in line of sight of the transformer I don't need one within 25 feet of the transformer?

The location of the primary OCPD is not relevant to the required secondary OCPD. The secondary conductor's OCPD must be installed in a location described in (C)(2) thru (C)(6).

For example, the secondary conductor OCPD must be within 10' of secondary conductor length per 240.21(C)(2) or 25' of length per (C)(3) or (C)(6) as applicable, or outside the building or inside, nearest the point of entrance for outside secondary conductors per (C)(4), regardless of conductor length.

 

[charlie b]

That is covered in 240.21(C)(1). Most of the time, the answer would be no. Keep in mind that there are separate requirements for protecting the transformer windings and for protecting the conductors on the primary and secondary sides.

 

[ASG]

The location of the primary OCPD is not relevant to the required secondary OCPD. The secondary conductor's OCPD must be installed in a location described in (C)(2) thru (C)(6).

Isn't the primary OCPD "where the conductor gets its power" as charlie stated?

 

[david luchini]

Isn't the primary OCPD "where the conductor gets its power" as charlie stated?

The point the "secondary conductors" receive their supply is at the transformer secondary.

The point the "primary conductors" receive their supply is at the primary OCPD