Confused about 240.21(C)

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unsaint33

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240.21(C) states "set of conductors .... shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6)." What does the "at the secondary" mean?

If I ran 20' of secondary conductors to a panelboard with 200A MCB, and the 200A MCB meets the requirement of 450.3 (transformer secondary protection), then, isn't the overcurrent protection by the 200A OCPD located "at the secondary?"

From the way I understand, 240.21(C) allows me not to have the secondary OCPD, but then at least 240.21(C)(3)~(6) says, the secondary conductor shall terminate to a single CB (or one set of fuses). Isn't the single CB considered to be secondary OCPD? I thought 240.21(C) rules allowed me not to have secondary OCPD.
 
unsaint33 said:
240.21(C) states "set of conductors .... shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6)." What does the "at the secondary" mean?

If I ran 20' of secondary conductors to a panelboard with 200A MCB, and the 200A MCB meets the requirement of 450.3 (transformer secondary protection), then, isn't the overcurrent protection by the 200A OCPD located "at the secondary?"

From the way I understand, 240.21(C) allows me not to have the secondary OCPD, but then at least 240.21(C)(3)~(6) says, the secondary conductor shall terminate to a single CB (or one set of fuses). Isn't the single CB considered to be secondary OCPD? I thought 240.21(C) rules allowed me not to have secondary OCPD.
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"The secondary conductors terminate in a single circuit breaker or set of fuses..."

This is letting you know that under these options within 240.21(C), you can do so IF they END UP on a circuit breaker. So is that breaker a "secondary OCPD"? Technically no, not for THESE conductors because they are ahead of the breakers. But what they are saying is that it is ALLOWABLE, given the other stated conditions, because the only realistic way of overloading them is through that breaker anyway, AND the transformer winding is protected by virtue of the PRIMARY protection on it..
 
Jraef said:
"The secondary conductors terminate in a single circuit breaker or set of fuses..."

This is letting you know that under these options within 240.21(C), you can do so IF they END UP on a circuit breaker. So is that breaker a "secondary OCPD"? Technically no, not for THESE conductors because they are ahead of the breakers. But what they are saying is that it is ALLOWABLE, given the other stated conditions, because the only realistic way of overloading them is through that breaker anyway, AND the transformer winding is protected by virtue of the PRIMARY protection on it..
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I thought the question was”where exactly does a tranny secondary begin?.

The answer is at the secondary tranny taps, x1,x2,x3, etc....

No?
 
Sorry again about the unclarity of my original question. I thought the OCPD that the 2ndary conductors terminate to can be the OCPD that also protects the 2ndary conductors. I totally missed the part that that OCPD is downstream of the 2ndary conductors.

So, if my 2ndary conductors are to be longer than 25', then where do I need to install a fused disconnect or OCPD to protect the 2ndary conductors? Right at the 2ndary terminals inside the transformer? Or Can I just add a set of fused disconnect or a box with OCPD within 25' of the transformer 2ndary?
 
unsaint33 said:
Sorry again about the unclarity of my original question. I thought the OCPD that the 2ndary conductors terminate to can be the OCPD that also protects the 2ndary conductors. I totally missed the part that that OCPD is downstream of the 2ndary conductors.

So, if my 2ndary conductors are to be longer than 25', then where do I need to install a fused disconnect or OCPD to protect the 2ndary conductors? Right at the 2ndary terminals inside the transformer? Or Can I just add a set of fused disconnect or a box with OCPD within 25' of the transformer 2ndary?
Click to expand...

Once you land your secondary conductors on a single breaker, or disconnect with a single set of fuses, the conductors on the load side of this device now classify as feeder conductors, rather than transformer secondary conductors. Then you can run them as long as you need.

Essentially, this section is telling you to put your overcurrent/disconnect device immediately adjacent to the transformer, so that the conductors don't need to run more than 10 ft or 25 ft, depending on which option you'll follow. You can run multiple secondary conductors off of a single transformer, and they each would terminate at their own OCPD.

Feeder conductors have overcurrent protection at their source. Transformer secondary conductors do not The exception is in part 1 of 240.21(C), when the transformer topology allows the primary OCPD to "protect the secondary by proxy" as I call it.

What I mean by "protect the secondary by proxy", to explain through an example, is that a 200A OCPD on a 480 primary effectively becomes a 400A OCPD on a 240V secondary. Multiply primary OCPD by the primary-to-secondary voltage ratio, and it becomes the effective secondary OCPD. Single phase 2-wire secondaries and 3-wire delta-to-delta qualify, everything else does not. Fault currents have to line up winding-to-winding for it to qualify, with no possibility of re-distributing and going unseen.
 
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