240.21(B2) Tap Rule??

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jclint07

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As I read this code section, for tap conductors under 25' to be considered/ used as tap conductors then all 3 conditions have to comply. But doesn't condition #1 contradict condition #2?

Condition #1- tap conductors ampacity is not less than 1/3 the rating of the overcurrent protection device.
(So then overcurrent protection device is allowed to be OVERSIZED for the conductor ampacity?)

Condition #2- tap conductors terminate in a circuit breaker or set of fuses that LIMIT the load to the ampacity of the tap conductors.
(So then overcurrent protection device must be sized FOR conductor ampacity?)

This NEC 2008 code section is confusing to me. Any help would be greatly appreciated.
 
The rule applies to two different overcurrent protection devices (OCPD). Subsection240.21(B)(2)(1) refers to the OCPD ahead of the tap; it protects the tap conductors from short circuit, and ground fault. Subsection 240.21(B)(2)(2) refers to the OCPD ahead of the load; it protects the tap conductors from overload.

See the definition of overcurrent in Art 100.

I wish the principals of overcurrent protection were more clearly spelled out. Note: I?m only discussing conductors in the next paragraph.

Overcurrent protection for short circuit and ground fault must be between the source and the conductors to be protected. Overcurrent protection for overload may be anywhere between the source and the load in series with the conductors to be protected. In some cases the load may not even be subject to overload and only short circuit and ground fault protection is actually necessary.

Since we commonly, but not necessarily, use a single OCPD for all three overcurrent conditions, we sometimes get the impression that all OCPDs must provide for all three.
 
It clearly states "...tap conductors comply with all the following:" Not or. Am I missing something or am I interpreting this code section incorrectly?

Scenario #1 - Permanent back up generator has conductors installed, from genny to service equipment, that are too small for generators overcurrent device rating. Argument has been made that since genny is less than 25' from service equipment, conductors only need to be sized for 1/3 it's overcurrent device rating. But since it is a generator, wouldn't Section 445.13 apply, including the exception?
 
rbalex said:
Jim,

In the case, of 240.21(B)(2), I believe all three conditions mut be met.
Click to expand...

Absolutely all of the conditions must be met.
poor question reading skills:ashamed:


For the OP.

Condition 1 limits the minimum size of the protective device.
Condition 2 limits the minimum size of the conductors.
 
jclint07 said:
It clearly states "...tap conductors comply with all the following:" Not or. Am I missing something or am I interpreting this code section incorrectly?

Scenario #1 - Permanent back up generator has conductors installed, from genny to service equipment, that are too small for generators overcurrent device rating. Argument has been made that since genny is less than 25' from service equipment, conductors only need to be sized for 1/3 it's overcurrent device rating. But since it is a generator, wouldn't Section 445.13 apply, including the exception?
Click to expand...

445.13 applies to the conductors from the generators windings to the first overcurrent protective device. If the generator has an overcurrent device built into the generator then 445.13 would not apply to the conductors from the generators overcurrent device to the transfer equipment.

What you have is a feeder between the generator to the transfer equipment. 240.21(B) could apply provided that the conductors are sized to carry the calculated load and they terminate in a single circuit breaker that is sized to protect the conductors.

Are you terminating in an automatic transfer switch or a manual one?

Chris
 
jclint07 said:
It clearly states "...tap conductors comply with all the following:" Not or. Am I missing something or am I interpreting this code section incorrectly?

Scenario #1 - Permanent back up generator has conductors installed, from genny to service equipment, that are too small for generators overcurrent device rating. Argument has been made that since genny is less than 25' from service equipment, conductors only need to be sized for 1/3 it's overcurrent device rating. But since it is a generator, wouldn't Section 445.13 apply, including the exception?
Click to expand...
No tap rule applies in this case; 445.13 does. (The Exception is difficult to achieve without an extremely sophisticated governor)
 
So far, no onboard OCPD has been discussed, so tap rules don't apply - yet. If there is an onboard OCPD, I agree with Chris - if you append his last statement with "...a single circuit breaker that is sized to protect the conductors from overload."

 
rbalex said:
So far, no onboard OCPD has been discussed, so tap rules don't apply - yet. If there is an onboard OCPD, I agree with Chris - if you append his last statement with "...a single circuit breaker that is sized to protect the conductors from overload."
Click to expand...

Correct, I was just paraphrasing the requirements from 240.21(B) for feeder tap conductors.

Chris
 
Rbalex - Yes, the generator does have an ocpd installed on it. So 240.21(B2) does apply, correct? If so, then that goes back to my original post/question, which I still do not clearly understand. Slow learner this morning I suppose.

Raider1 - Generator feeder conductors terminate in an automatic transfer switch, which powers entire service load.
 
jclint07 said:
Rbalex - Yes, the generator does have an ocpd installed on it. So 240.21(B2) does apply, correct? If so, then that goes back to my original post/question, which I still do not clearly understand. Slow learner this morning I suppose.

Raider1 - Generator feeder conductors terminate in an automatic transfer switch, which powers entire service load.
Click to expand...
As such, tap rules can/do apply.

Condition 1 is regarding the [feeder] tap conductors ampacity relative to the feeder overcurrent device rating (the genny ocpd at the supply end). You do not have feeder conductors per se... you have feeder terminals. Not explicitly stated, but most to all still consider your scenario a feeder tap installation.

Condition 2 is regarding the rating of the ocpd required at the load end of the tap conductors.
 
jclint07 said:
Rbalex - Yes, the generator does have an ocpd installed on it. So 240.21(B2) does apply, correct? If so, then that goes back to my original post/question, which I still do not clearly understand. Slow learner this morning I suppose.

Raider1 - Generator feeder conductors terminate in an automatic transfer switch, which powers entire service load.
Click to expand...
In the particular case of 240.21(B)(2), there are two (2) OCPDs required. OCPD (1) protects the conductors for short circuit and ground fault; OCPD (2) protects the conductors from overload.

IF and ONLY IF:
  • The conductors between the onboard OCPD and the ATS are less than 25?; AND
  • the onboard OCPD (1) rating or setting is less than 1/3 of the ampacity of those conductors; AND
  • those conductors terminate in an OCPD (2) at the ATS with a rating or setting that limits the load to the ampacity of those conductors; AND
  • those conductors are suitably protected physically.
THEN 240.21(B)(2) applies; otherwise the onboard OCPD must protect for all three overcurrent conditions (short circuit, ground fault and overload) and apparently it doesn?t.
 
Additional info I forgot to mention (my apologies)....

-Generator has a 250A OCPD installed within its enclosure, not seperate.
-Conductors from 250A generator OCPD to automatic transfer equipment are 3/0 THHN copper (225A ampacity).
-Terminals on generator OCPD are 75c degree rated. Now 3/0 THHN conductors can only be loaded to 200A instead of 225A.
-But argument is that 240.21(B2) applies and the 3/0 THHN copper conductors are fine.
I am trying to fully understand this code section.
 
rbalex said:
  • the onboard OCPD (1) rating or setting is less than 1/3 of the ampacity of those conductors
.
Click to expand...
Stated from that perspective, it should be: the onboard OCPD (1) rating or setting is not more than 3 times the ampacity of those conductors

The ampacity of the tap conductors is not less than
one-third of the rating of the overcurrent device protecting
the feeder conductors.
Click to expand...
 
Smart, the load end does terminate in a 200A disconnect. The 250A OCPD is in the generator.

Thanks everyone, for all the input. I clearly understand that code section better. Taps are obviously not my area of expertise.

But code book aside...

In theory, how can you get away with a generator supplying up to 250A of current (OCPD rating), on a 200 amp rated wire? In theory...no code references allowed.
 
jclint07 said:
Smart, the load end does terminate in a 200A disconnect. The 250A OCPD is in the generator.

Thanks everyone, for all the input. I clearly understand that code section better. Taps are obviously not my area of expertise.

But code book aside...

In theory, how can you get away with a generator supplying up to 250A of current (OCPD rating), on a 200 amp rated wire? In theory...no code references allowed.
Click to expand...

If I understand correctly, you stated that the 3/0 conductors terminate at a 200 ampere rated disconnect (which I will assume has 200 amp rated OCPD's). Even though the generator OCPD is rated at 250 amps the 3/0 will not see that much under normal conditions because of the 200 amp OCPD at the load end termination.

Pete
 
You do understand correctly, Pete. Yes the 200A disconnect is the overcurrent protection as well.

So if I understand you correctly, the generator could even have a 500A OCPD installed on it and in theory, it would not matter. The 200A disconnect/ocpd would trip long before conductors were ever overloaded. Seems pretty clear to me.
 
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