Tap conductors

[Smart $]

This paragraph?



That seems to say the same thing as 110.14(C), that conductors with an insulation temperature rating higher than the temperature rating of the terminations can take advantage of the higher ampacity for correction and adjustment factors, but that the ampacity is still limited by the tabular value for the temperature rating of the terminations.


That's bridge under the water. This thread's a blank slate as far as I'm concerned.

I've always been a bit unclear on whether an installed conductor has a single ampacity, or whether it has different ampacities for different purposes, or what, so I would welcome any references you have on the topic.

Cheers, Wayne

Look at 215.2(A)(1) ...210.19(A)(1) also for grins and giggles.

Note the two-part determination of (a) and (b) thereunder. Part (a) is selection and coordination with 110.14(C). Part (b) is selection and coordination with 310.15(B) second paragraph of general statement.

I'm saying all that is necessary to comply with 240.21(B)(2)(1) is a part (b) determination. Actual circuit ampacity will have to comply with a full determination using 215.2(A)(1) parts (a) and (b).

Revisit the problem posed in the OP. 400A feeder mentioned so we will assume a 400A OCPD rating, as a 240.21(B)(2)(1) determination is based on OCPD rating not feeder conductor ampacity. To qualify, the tap conductor ampacity must be at least 1/3 of 400A or 133A.

#1 copper is rated 145A @ 90°C but only 130A @ 75°C. Let's say our load is not more than 125A noncontinuous or 100A continuous so we can use a 125A-rated OCPD and 75° terminations. Can you explain why OP'er cannot "safely" use #1 rather than going with #1/0?

 

[david luchini]

Once you use a 75° C termination of some sort on either end the conductor ampacity is now 75° C even if it has a 90° C insulation unless as you say you're using the higher ampacity for "ampacity adjustment, correction, or both". From Smart's last pdf page 4:

What Rob said.:thumbsup:

When you have 75°C terminals, albeit terminations, selection and coordination with 110.14(C) is technically establishing the minimum conductor SIZE, and as result, also places a cap on circuit ampacity... not conductor ampacity,

That's a strange reading of 110.14(C).

The temperature rating associated with the ampacity of a CONDUCTOR shall be selected and coordinated...

 

[Smart $]

What Rob said.:thumbsup:

That's a strange reading of 110.14(C).

Note it says "The temperature rating associated with the ampacity of a conductor..."

The subject is temperature rating... not ampacity of a conductor.

And immediately thereafter in the very next sentence (sorry for being overly and rhetorically redundant ), "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

 

[infinity]

The two taps rule specify that the tap conductor ampacity must be either 1/10 for the 10' rule or 1/3 for the 25' rule. When the conductor is 90° C it will have one ampacity until it's connected via 75° C connector or terminal. What sense does it make to require it to be either 1/10 or 1/3 and then be less than that when you connect it to a terminal?

 

[Smart $]

The two taps rule specify that the tap conductor ampacity must be either 1/10 for the 10' rule or 1/3 for the 25' rule. When the conductor is 90° C it will have one ampacity until it's connected via 75° C connector or terminal. What sense does it make to require it to be either 1/10 or 1/3 and then be less than that when you connect it to a terminal?

I believe it has to do with available fault current. But no matter what, the tep conductors are not protected by the supply OCPD. That's why it is called a tap to begin with. Protection is based on load and load end OCPD... just like a service. Looks to me like it's someone's bright idea that the longer the tap, the closer the ampacity of the tap conductor must be to the feeder OCPD rating... and the minimum rating appears to be chosen randomly.

Take the OP example of 400A feeder. If tap is not more than 10 feet, the tap conductor ampacity is required to be not less than 40A... but the moment the tap conductor is 11 feet long, it has to be at least 133A... ??? There is no physics behind the requirement.

 

[david luchini]

Note it says "The temperature rating associated with the ampacity of a conductor..."

The subject is temperature rating... not ampacity of a conductor.

It says the temperature rating ASSOCIATED with the AMPACITY of a conductor. Selecting the ampacity of the tap conductor based on the 90 deg rating exceeds the rating of the 75 deg terminal. That is a violation of 110.14 (C).

And immediately thereafter in the very next sentence (sorry for being overly and rhetorically redundant ), "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

I didn't see any mention of ampacity adjustment or correction in the OP.

 

[wwhitney]

Look at 215.2(A)(1)

Hmm, the 2014 version is more confusing than the 2011 version. Is it your reading that in the 2014 version the phrase "allowable ampacity" refers to the tabular values without any correction factors? That reading would make more sense to me if part (b) didn't also use the phrase "allowable ampacity". But based on the 2011 version, I guess that is what the 2014 version is trying to say.

Note the two-part determination of (a) and (b) thereunder. Part (a) is selection and coordination with 110.14(C). Part (b) is selection and coordination with 310.15(B) second paragraph of general statement.

I don't see that correspondence at all. Based on the 2011 version, what I see is that 215.2(A)(1) as saying is:

(a) The tabular ampacity value should exceed 125% continuous plus non-continuous.
(b) After correction and adjustment, the ampacity value should exceed continuous plus non-continuous.

None of these discusses the 110.14(C) temperature limitations, so my default assumption would be that those 110.14(C) limitations apply to both (a) and (b).

Can you explain why OP'er cannot "safely" use #1 rather than going with #1/0?

I can't possibly explain that, as I have no idea where the 25' figure and the 1/3 ampacity figure for the tap rule came from.

I guess my basic assumption coming into this discussion is that whenever the word 'ampacity' is used without modifiers, it refers the result of the full 310.15(B) calculation. 310.15(B) references 110.14(C), so the temperature termination limits would apply.

Cheers, Wayne

 

[jaggedben]

I believe it has to do with available fault current. But no matter what, the tep conductors are not protected by the supply OCPD. That's why it is called a tap to begin with. Protection is based on load and load end OCPD... just like a service. Looks to me like it's someone's bright idea that the longer the tap, the closer the ampacity of the tap conductor must be to the feeder OCPD rating... and the minimum rating appears to be chosen randomly.

Take the OP example of 400A feeder. If tap is not more than 10 feet, the tap conductor ampacity is required to be not less than 40A... but the moment the tap conductor is 11 feet long, it has to be at least 133A... ??? There is no physics behind the requirement.

There you go using reason and logic. :happyno: I'll support you though.

The only sense I can make of the tap rules regarding ampacity is that if you have a longer conductor with perhaps more resistance then it should be larger in order to be surer to clear a short-circuit fault. The math can only be described, charitably, as a generalization, if that's the case. The OCPD at the load end has to protect the conductors from an operating current overload. Anyway, the point is...

If the only purpose of the percentage requirements is to trip an OCPD in its fast acting functionality for a short circuit, I don't see what difference the conductor insulation rating is likely to make. Let's say 4awg is likely sufficient to clear the fault by tripping the magnetic (not thermal) component of an upstream circuit breaker, while 6awg is not. It's seems mistaken to suggest that 90C rated 6awg would be as good for the job as 60C 4awg based on ampacity tables.

If there's a more scientific basis for the tap rules I'm open to being enlightened. But it seems to me the rules were written with a variety of concerns in mind besides scientific sense.

 

[Smart $]

It says the temperature rating ASSOCIATED with the AMPACITY of a conductor. Selecting the ampacity of the tap conductor based on the 90 deg rating exceeds the rating of the 75 deg terminal. That is a violation of 110.14 (C).
...

Let me state it as simply as possible. 110.14(C) establishes the maximum current a circuit is permitted to carry so as not to exceed the temperature limitation imposed at the circuit's terminations. It is, per se, a termination ampacity determination, not a conductor ampacity determination. Do not confuse the two.

 

[Smart $]

Hmm, the 2014 version is more confusing than the 2011 version. Is it your reading that in the 2014 version the phrase "allowable ampacity" refers to the tabular values without any correction factors? That reading would make more sense to me if part (b) didn't also use the phrase "allowable ampacity". But based on the 2011 version, I guess that is what the 2014 version is trying to say.

I view the term allowable ampacity as the table values. However, not being a defined term, many express a misunderstanding (as I see it) without knowing it. My view is established through years of interpretation.

I don't see that correspondence at all. Based on the 2011 version, what I see is that 215.2(A)(1) as saying is:

(a) The tabular ampacity value should exceed 125% continuous plus non-continuous.
(b) After correction and adjustment, the ampacity value should exceed continuous plus non-continuous.

None of these discusses the 110.14(C) temperature limitations, so my default assumption would be that those 110.14(C) limitations apply to both (a) and (b).

I know Code doesn't state this explicitly, but a 110.14(C) compliance determination uses 125% continuous plus 100% noncontinuous. I learned this to be the way it is done over the years based on authoritative publications. I've submitted proposals to get a reference to 110.14(C) included in what is now part (a) to no avail.

I can't possibly explain that, as I have no idea where the 25' figure and the 1/3 ampacity figure for the tap rule came from.

But yet you are saying the ampacity criteria must be based on terminations...?

I guess my basic assumption coming into this discussion is that whenever the word 'ampacity' is used without modifiers, it refers the result of the full 310.15(B) calculation. 310.15(B) references 110.14(C), so the temperature termination limits would apply.

See my most recent reply, as of this posting, to David

 

[Smart $]

... I'll support you though.

...

That's a refreshing notion.

 

[david luchini]

Let me state it as simply as possible. 110.14(C) establishes the maximum current a circuit is permitted to carry so as not to exceed the temperature limitation imposed at the circuit's terminations. It is, per se, a termination ampacity determination, not a conductor ampacity determination. Do not confuse the two.

That's simply nonsense. Nothing in 110.14(C) establishes the maximum current a circuit is permitted to carry.

110.14(C) says that the TEMPERATURE RATING associated with the AMPACITY of a conductor shall be selected and coordinated so as not to exceed the LOWEST TEMPERATURE RATING of any connected termintation, conductor or device.

If you select #1AWG with a 145A ampacity at 90 deg in order to meet the 1/3 requirement of the 400A OCPD for the tap conductors, then the TEMPERATURE RATING associated with the AMPACITY of the conductor you have selected EXCEEDS the rating of the termination of the c/b that the tap conductors terminate on. That is a violation of 110.14(C). It's a simple as that.

 

[Smart $]

That's simply nonsense. Nothing in 110.14(C) establishes the maximum current a circuit is permitted to carry.

110.14(C) says that the TEMPERATURE RATING associated with the AMPACITY of a conductor shall be selected and coordinated so as not to exceed the LOWEST TEMPERATURE RATING of any connected termintation, conductor or device.

If you select #1AWG with a 145A ampacity at 90 deg in order to meet the 1/3 requirement of the 400A OCPD for the tap conductors, then the TEMPERATURE RATING associated with the AMPACITY of the conductor you have selected EXCEEDS the rating of the termination of the c/b that the tap conductors terminate on. That is a violation of 110.14(C). It's a simple as that.

I'm not saying ignore 110.14(C) entirely. Just for qualification under 240.21(B)(2)(1).

I look at the criteria of 240.21(B)(2)(1) as being selected and coordinated with 215.2(A)(1)(b)...

(1) The ampacity of the tap conductors is not less than
one-third of the rating of the overcurrent device protecting
the feeder conductors.

(b) The minimum feeder conductor size shall have an
allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factors.

...and 240.21(B)(2)(2) as being selected and coordinated with 215.2(A)(1)(a)...

(2) The tap conductors terminate in a single circuit breaker
or a single set of fuses that limit the load to the ampacity
of the tap conductors. This device shall be permitted
to supply any number of additional overcurrent devices
on its load side.

(a) Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
minimum feeder conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.

 

[david luchini]

I'm not saying ignore 110.14(C) entirely. Just for qualification under 240.21(B)(2)(1).

I don't see anything that allows you to ignore 110.14(C) for qualification under 240.21(B)(2)(1).

 

[Smart $]

I don't see anything that allows you to ignore 110.14(C) for qualification under 240.21(B)(2)(1).

I don't see anything that specifically compels us to either. Remember we are in an article concerning OCP. The requirements for actual conductor ampacity are in Article 215.

 

[david luchini]

I don't see anything that specifically compels us to either. Remember we are in an article concerning OCP. The requirements for actual conductor ampacity are in Article 215.

And the requirements for the temperature rating associated with the ampacity of a conductor are in Article 110. REQUIREMENTS for Electrical Installations.

 

[wwhitney]

I've submitted proposals to get a reference to 110.14(C) included in what is now part (a) to no avail.

So what were the panel statements in response to your proposals?

Cheers, Wayne

 

[Smart $]

So what were the panel statements in response to your proposals?

The latest...

Committee Action: Rejected
Resolution: The NEC is intended for use by trained persons. Attempting to incorporate all of the installation rules into each specific application area can create unintended restrictions and potentially not include other requirements that drive the safe installation of the electrical system

I think it's a boilerplate resolution.

 

[Smart $]

And the requirements for the temperature rating associated with the ampacity of a conductor are in Article 110. REQUIREMENTS for Electrical Installations.

As I said, I'm not suggesting 110.14(C) be ignored. But obviously my selection and coordination method is different than yours.