Tap Rules - Mutual Exclusive?

[gbstand]

Are the Article 240 "tap rules" mutually exclusive?

For example, if you had an outside feeder tap conductor that was less than 25 feet in length that originated outside but terminated inside upon entering a building, would both 240.21(B)(2) and 240.21(B)(5) apply?

 

[david luchini]

Either could apply. Figure out which one would be better for the installation.

 

[Carultch]

Are the Article 240 "tap rules" mutually exclusive?

For example, if you had an outside feeder tap conductor that was less than 25 feet in length that originated outside but terminated inside upon entering a building, would both 240.21(B)(2) and 240.21(B)(5) apply?

You get to choose which tap rule to apply, and you no longer have to think about the context rules in the other rules. So whichever is best for your situation, is what governs.

As an example, the 10 ft rule specifically states that it must be in a raceway, while the 25 ft rule gives the option to either use a raceway or "protect from physical damage" in another method. Most people would put either kind of tap in a raceway anyway, without even thinking about this rule.

Suppose you planned a 200A tap off of a 600A feeder, that only has to travel 10 ft to get to the 200A OCPD/disconnect. By default, it is already meeting the 1/3 requirement of the 25ft rule. You are meeting the general conditions of both rules, but which list of context rules apply? The answer, we get to follow either list of context rules, so if you protected the tap conductors from physical damage in a method other than a raceway as the 25 ft rule allows, it would be acceptable in this example. But if you were only making a 100A tap off this same feeder with only 100A worth of wire, it would have to be in a raceway to follow the 10 ft tap rule.

 

[gbstand]

Suppose a design with an "outside tap of unlimited length" [240.21(B)(5)] that originates outside the building, routed/protected from physical damage, and terminates into a single overcurrent device inside the building. Further, suppose that the tap was greater than 10 feet, but less than 25 feet [240.21(B)(2)]. Finally, suppose the feeder was rated 380A (500 MCM) and the tap was rated 150A (1/0 AWG).

240.21(B)(2) requires the tap to have an amapacity of 33.33% * 380A = 126.7A. The design fails under the ampacity requirement.
But 240.21(B)(5), unlike the 10 ft or 25 ft "rule," has no minimum tap ampacity requirements. The design meets this ampacity requirement.

From a code perspective which provision should apply in your judgment?

 

[gbstand]

Edit: Suppose feeder tap rated 115A (2 AWG), not 150A (1/0 AWG).

 

[david luchini]

240.21(B)(2) requires the tap to have an amapacity of 33.33% * 380A = 126.7A. The design fails under the ampacity requirement.

The 240.21(B)(2) tap conductor is required to have an ampacity not less than 1/3 of the feeder OCPD, not the feeder conductor ampacity. Tahts would be 133.3A

If you needed a 110A tap from your 400A feeder, and 240.21(B)(2) would require a #1/0 conductor while 240.21(B)(5) would require a #2 conductor, which provision would you use?

 

[kwired]

With outside feeder tap supplying a building you also have 225.32 limiting amount of entry into the building, you will have your overcurrent device near the entry of the building, especially in places that have set a specific entry length as their own rule for service conductors they are likely to carry that same distance to this rule.

 

[gbstand]

The 240.21(B)(2) tap conductor is required to have an ampacity not less than 1/3 of the feeder OCPD, not the feeder conductor ampacity. Tahts would be 133.3A

Agreed. My error. Thanks for the correction. Supposing a 400A feeder OCPD 33.33% * 400 = 133.3A. The minimum 240.21(B)(2) feeder tap ampacity would be 1/0 AWG (150A).

If you needed a 110A tap from your 400A feeder, and 240.21(B)(2) would require a #1/0 conductor while 240.21(B)(5) would require a #2 conductor, which provision would you use?

This is reaching the crux of matter. So a design perspective would favor 1/0 AWG; but an installation-cost perspective would favor #2. These two perspectives are what motivated this question. The only reason that specifying #2 conductor under 240.21(B)(5) is an option is because the tap is "located the building . . . .except at the point of load termination." If we consider that if the same tap was located inside the building then a 1/0 AWG would be required without question. It strikes me that the minimum ampacity provision for tap conductor less than 25 feet is there for a reason and should be applied here.

With outside feeder tap supplying a building you also have 225.32 limiting amount of entry into the building, you will have your overcurrent device near the entry of the building, especially in places that have set a specific entry length as their own rule for service conductors they are likely to carry that same distance to this rule.

Precisely. Note that 240.21(B)(5) requires meeting these considerations.

These are the reasons why I personally would apply the minimum tap ampacity requirements of 240.21(B)(2) and the "service" requirements of 240.21(B)(5). The problem I am grappling with is the apparent mutual exclusivity of the tap rule provisions. Hence the initial question: Are the rules mutually exclusive?

 

[kwired]

Agreed. My error. Thanks for the correction. Supposing a 400A feeder OCPD 33.33% * 400 = 133.3A. The minimum 240.21(B)(2) feeder tap ampacity would be 1/0 AWG (150A).



This is reaching the crux of matter. So a design perspective would favor 1/0 AWG; but an installation-cost perspective would favor #2. These two perspectives are what motivated this question. The only reason that specifying #2 conductor under 240.21(B)(5) is an option is because the tap is "located the building . . . .except at the point of load termination." If we consider that if the same tap was located inside the building then a 1/0 AWG would be required without question. It strikes me that the minimum ampacity provision for tap conductor less than 25 feet is there for a reason and should be applied here.



Precisely. Note that 240.21(B)(5) requires meeting these considerations.

These are the reasons why I personally would apply the minimum tap ampacity requirements of 240.21(B)(2) and the "service" requirements of 240.21(B)(5). The problem I am grappling with is the apparent mutual exclusivity of the tap rule provisions. Hence the initial question: Are the rules mutually exclusive?

I don't get why you are asking, the outside portion has no limitations, once you enter a building you need to have proper overcurrent protection, as worded it leaves exact distance allowed inside up to AHJ. but still requires something near the entry point. Nothing prohibits a 12 AWG tap from a 400 amp feeder to run 100 feet to a utility shed with a single 20 amp circuit once you hit that shed (if that single circuit is otherwise permitted in the situation)

 

[gbstand]

I don't get why you are asking, the outside portion has no limitations, once you enter a building you need to have proper overcurrent protection, as worded it leaves exact distance allowed inside up to AHJ. but still requires something near the entry point. Nothing prohibits a 12 AWG tap from a 400 amp feeder to run 100 feet to a utility shed with a single 20 amp circuit once you hit that shed (if that single circuit is otherwise permitted in the situation)

The case that you outline above is one where there tap conductor is greater than 25 feet. The case that motivated the initial question and under consideration, however, is one where the tap conductor is less than 25 feet, originates outside the building, but terminates inside.

In the case under consideration, the question that naturally arises is whether either 240.21(B)(2) or 240.21(B)(5) should apply to the tap or whether both 240.21(B)(2) and 240.21(B)(5) can/ should apply to the tap. Practically,

If an "either/or" perspective is taken then it means it would be acceptable to have either a 1/0 AWG, sized and installed in accordance with 240.21(B)(2), or #2 sized by load and installed in accordance with 240.21(B)(5). This view treats the tap rules as mutually exclusive.​

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If a "both/and" perspective is taken it means that both the minimum tap ampacity of 240.21(B)(2) requirements would apply and the "service" requirements of 240.21(B)(5) would apply. This view treats the tap rules as not mutually exclusive.​

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[wwhitney]

If an "either/or" perspective is taken then it means it would be acceptable to have either a 1/0 AWG, sized and installed in accordance with 240.21(B)(2), or #2 sized by load and installed in accordance with 240.21(B)(5).

This.

The general rule is the OCPD has to be at the point of supply of the conductor. Each tap rule is an exception to this, if you meet all the associated conditions. As soon as you meet the conditions for one rule, the arrangement is NEC compliant, and the other rules don't matter.

Cheers, Wayne

 

[david luchini]

If an "either/or" perspective is taken then it means it would be acceptable to have either a 1/0 AWG, sized and installed in accordance with 240.21(B)(2), or #2 sized by load and installed in accordance with 240.21(B)(5). This view treats the tap rules as mutually exclusive.

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I think either/or was the answer given in post #2 and post #3.​

 

[kwired]

Tap rules are basically permissions, whatever rule is lesser restrictive that can also meet the needs of the application can apply, sometimes more than one rule can be complied with, does it really matter which one you applied if both are able to be met?

If outside the restrictions are pretty wide open by comparison to the other tap rules and this permission kind of makes the other rules meaningless to even look at.

Once you enter a building you are no longer an outside tap though but at same time there is not so specific wording in code of just how far one can enter before you must have an overcurrent protection device.

Or are you asking if one can use unlimited rule for outside portion and then use the 10 or 25 foot rule for those distances once entering if you otherwise comply? I still think art 225 will require it to be "nearest to point of entrance" or however your AHJ determines what that may mean.

 

[gbstand]

Thanks for the responses.

I think either/or was the answer given in post #2 and post #3

Based on those responses as well as others I am persuaded that from a code perspective 240.21(B)(2) and 240.21(B)(5) are mutually exclusive. And further persuaded in general that as that as soon as the conditions of one tap rule are met the arrangement is NEC compliant.

However, from a design perspective the adequacy of design must be considered in addition to compliance with the code. It still seems that the minimum ampacity should be considered because the outside tap conductor is less than 25 feet. As noted above, the only reason that specifying #2 conductor under 240.21(B)(5) is an option is because the tap is "located outside the building or structure, except at the point of load termination." If we consider that if the same tap was located completely inside the building then 240.21(B)(5) could not apply and a 1/0 AWG would be required under 240.21(B)(2) without question. It strikes me that the minimum ampacity provision for tap conductor less than 25 feet is there in the code for a reason. (My suspicion is that the 1/3 minimum ampacity requirement is intended to ensure a tap has an adequate thermal damage curve during a short-circuit condition the upstream feeder OCPD must open for. Perhaps someone could provide the technical basis behind the 1/3 * upsream OCPD requirement.) It seems that if a 1/0 AWG were specified above the 2 AWG the reason could not be that both 240.21(B)(2) and 240.21(B)(5) are applicable, but rather based on design discretion the minimum ampacity for tap conductor less than 25 feet is required in addition to 240.21(B)(5) requirements to ensure adequacy.

Tap rules are basically permissions, whatever rule is lesser restrictive that can also meet the needs of the application can apply, sometimes more than one rule can be complied with, does it really matter which one you applied if both are able to be met?

If both 240.21(B)(2) and 240.21(B)(5) rules were able to be met it would not matter which was applied. But to know that both rules are able to be met means that the requirements of both rules must first have been considered. The case here is where either 240.21(B)(2) or 240.21(B)(5) could potentially apply based on the application, but if 240.21(B)(2) were applied the 1/3 minimum ampacity requirement would not be met.

Once you enter a building you are no longer an outside tap . . .

240.21(B)(5) indicates that an outside tap conductor is one that complies with all the listed conditions and is "located outside of a building or structure, except at the point of load termination." Based on the I would slightly differ from the view that an outside tap conductor is no longer classified as one upon entering the building and would submit that once a conductor meets the conditions to be an outside tap conductor it would remain one until it terminated into a single OCPD inside the building.

. . . though but at same time there is not so specific wording in code of just how far one can enter before you must have an overcurrent protection device. . . . I still think art 225 will require it to be "nearest to point of entrance" or however your AHJ determines what that may mean.

I would agree that for an outside tap conductor coming into a building the OCPD is required "nearest the point of entrance" and the AHJ would determine what that may mean, but I am not certain that Article 225 would be applicable to an outside tap conductor. It seems that once classified as an outside tap conductor under 240.21(B)(5) then that provision would only apply. Because 240.21(B)(5) does not reference Article 225, I am not sure I see how Article 225 becomes applicable. This seems to go back to mutual exclusivity---can we apply both 240.21(B)(5) and Article 225? I would submit the "nearest the point of entrance" is a requirement not under Article 225 but rather under 240.21(B)(5) condition (4)(b) or (4)(c). Condition (4)(b) would require that the disconnecting means (which under condition 3 contains the OCPD or is immediately adjacent to it) for the tap conductors is installed at a readily accessible location . . . inside, nearest the point of entrance of the tap conductors." Or, condition (4)(c) that alternatively allows the tap conductors to be "installed in accordance with 230.6, nearest the point of entrance of the tap conductors."

 

[kwired]

That distance allowed to enter is the same rule as for service conductors. Only thing with NEC is it doesn't give an exact distance for either an outside feeder or service conductors. Local jurisdictions often have a limit they will allow, from reading posts here there apparently are a few places that don't let you enter a building at all, but most seem to be somewhere in the three, five or even ten foot range. You don't need to consider 1/3 or 1/10 ampacity if you still comply with whatever maximum entry is allowed on an otherwise outside feeder tap.

 

[gbstand]

That distance allowed to enter is the same rule as for service conductors. Only thing with NEC is it doesn't give an exact distance for either an outside feeder or service conductors. Local jurisdictions often have a limit . . .

I agree. 240.21(B)(5) Condition (3) mirrors 230.91 for service conductors and 225.32 for outside feeders. 240.21(B)(5) Condition (4) mirrors 230.70(A)(1) and 230.6 for service conductors and 225.32 for outside feeders. Similar to service conductors and outside feeder conductors, the specific distance an outside feeder tap conductor could enter the building would be governed by local jurisdictions formally interpreting what "nearest point of entrance" means under 240.21(B)(5) Condition 4 and limiting the distance (or denying entrance all together as you mention).

Although certainly not definitive, the phrase in 240.21(B)(5) "located outside of a building or structure, except at the point of load termination" suggests that the distance an outside tap conductor enters into a building should be limited because the outside tap conductors should be located outside except at the point of termination, implying the outside tap conductor should come through the wall and then terminate.

You don't need to consider 1/3 or 1/10 ampacity if you still comply with whatever maximum entry is allowed on an otherwise outside feeder tap.

I would agree with this from a code perspective. When a less than 25 ft. tap conductor enters a building from outside it seems that 240.21(B)(5) best fits the application because of the service-entrance-like requirements found in Condition (3) and (4). Because the tap rules are mutually exclusive once 240.21(B)(5) is chosen to apply, 240.21(B)(2) cannot. Once the arrangement met all the conditions specified in 240.21(B)(5)---which does not include a minimum ampacity requirement--- it would be code compliant.

However, as mentioned above, a design adequacy concern arises (in my mind) from considering that if the same less than 25 ft. tap conductor and associated upstream OCPD were used in an application inside a building where only 240.21(B)(2) could be applied the 1/3 minimum ampacity requirement would have to be satisfied. This suggests that the outside tap conductor size should be considered from a design perspective. Conservatively verifying or specifying that an outside tap conductor has an ampacity greater than the calculated load (in the case above 1/0 AWG vs #2 AWG) would be based on design adequacy considerations, not because both 240.21(B)(5) and 240.21(B)(2) mutually and prescriptively applied (as I thought could be done prior to this post discussion).