Ser cable ampacity

[Smart $]

The circuit is 100', 10' is 10%. What am I missing?

I'm missing something. I see using the higher ampacity for 10 ft or 10% of the circuit length

Consider the meaning of the entire highlighted phrase and how it is affected by the underlined portion.

Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance
equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.

 

[texie]

I'm in Smart$ camp on this and that is the way I've understood it.
But now may I complicate the question? Does the exception apply only in the middle of a run or can it be at either end? The reason I ask is that I'm of the understanding that the reason for this allowance is that the conductor beyond the transition point acts as a heat sink. If it was at either end you would have the heat sinking only on one side.

 

[GoldDigger]

But just to mess with everbody's minds:
If and only if the exemption applies, the entire circuit gets figured at the higher ampacity.
Now, who shaves the barber?

Tapatalk!

 

[Smart $]

I'm in Smart$ camp on this and that is the way I've understood it.
But now may I complicate the question? Does the exception apply only in the middle of a run or can it be at either end? The reason I ask is that I'm of the understanding that the reason for this allowance is that the conductor beyond the transition point acts as a heat sink. If it was at either end you would have the heat sinking only on one side.

It is always to one side, as each transition is determined separately.

 

[Smart $]

But just to mess with everbody's minds:
If and only if the exemption applies, the entire circuit gets figured at the higher ampacity.

So what if there are three sections of the run with different ampacities?

 

[infinity]

The requirement is 10% (or 10', whichever is less) of the adjacent portion's distance.

If you have a 100' run with 10' at the lower ampacity, the adjacent portion can at most be 90'. 10% of 90' is 9'. 10' exceeds the lesser 10% limitation.

Using the following variables:
d_h = distance at higher ampacity
d_l = distance at lower ampacity

d_t = d_h + d_l
d_l < d_h ? 10%

Substitute second equation into first...

d_t < d_h + d_h ? 10%
d_t < d_h ? 110%

Transposing, we get...

d_h > d_t ? 110%

The relationship of d_l to d_t can also be determined by transposing the second equation...

d_h > d_l ? 10%, or d_h > d_l ? 10

...and substituting into first equation...

d_t > d_l ? 10 + d_l
d_t > d_l ? (10 + 1)
d_t > d_l ? 11

NOTE: Substitute less/greater than or equal character for their "red" counterparts above. I would use the actual characters, but they just get parsed by forum programming to a ?.

This was added to the NEC in 1990. As convoluted as the NEC can be no one would think that such a complex calculation would be needed when a simple 10% limit of the overall circuit length would be sufficient to keep the conductors from overheating. The lack of a better written exception is part of the confusion. The CMP decreased the length from 15' to 10' just to err on the side of caution. Here's the original proposal. Sounds like a wording change should be coming in 2017.

Log # 2999
6- 58 - (310-15(c), Exception-(New)): Accept
Secretary's Note: It was the action of the
Correlating Committee that this proposal be
reconsidered and correlated with the action on
Proposals 6-105 and 6-110. This action will be
considered by the Panel as a Public Comment.
SUBMITTER: Frederic P. Hartwell, Amherst, MA
RECOMMENDATION: Add an exception as follows:
Exception: Where two different ampacities apply to
adjacent portions of a circuit the higher ampacity
shall be permitted to be used beyond the point of
transition a distance equal to 10 feet (3.05 m) or i0%
of the circuit length figured at the higher ampacity,
whichever is less.
SUBSTANTIATION: This proposal will permit more
productive use of the new ampacity tables by allowing
for short transitional lengths without the necessity of
splices at the point of transition.. Although generally
applied to underground to raceway in air transitions,
the proposal also recognizes, for example, messenger
cable to raceway transitions. The guiding principle is
that the part of the circuit in an ambient that
transfers heat more. readily (and thus permits a higher
ampacity) can and will act as a heat sink for the
adjacent part of the circuit. The submitter has been
told that 15 feet has been widely used in industry; the
proposal errs on the conservative side.
PANEL ACTION: Accept.
VOTE ON PANEL ACTION: Unanimously Affirmative.

 

[Dennis Alwon]

It is 10% or 10' whatever is less. Thus a 100' run with 10' going thru a different ampacity only has 90' at the higher ampacity. 10% of 90 is 9'. 9' is less than 10' therefore if 10' went thru the lower ampacity then you would have to derate.

 

[infinity]

It is 10% or 10' whatever is less. Thus a 100' run with 10' going thru a different ampacity only has 90' at the higher ampacity. 10% of 90 is 9'. 9' is less than 10' therefore if 10' went thru the lower ampacity then you would have to derate.

I'm not so sure that this was the intent of the original code change but agree that as written it would be applied in that way.

 

[augie47]

O.K. let's take it back to the OP.
In the '11 Code, SE cable in insulation must be installed at it's 60? ampacity.
I have a 60 ft run of SER cable, the 1st 5 ft leaving the panel is in insulation.
Since less than 10% and less than 10 ft is in insulation, can I use the 75? ampacity for the cable ?

 

[wwhitney]

As convoluted as the NEC can be no one would think that such a complex calculation would be needed.

For the case of a circuit with just two sections of different ampacities, you can use the exception when the lower ampacity section is 1/11 of the total circuit length or less (not to exceed 10 feet). You don't have to do the algebra each time.

For circuits with more than two sections, as I read it the exception applies to each transition point. So say I have a 220' run, with the central 20' at a lower ampacity, and the 100' on either side at a higher ampacity. Can I apply the exception twice to use the higher ampacity for the entire circuit?

Cheers, Wayne

 

[GoldDigger]

No.

Tapatalk!

 

[Dennis Alwon]

O.K. let's take it back to the OP.
In the '11 Code, SE cable in insulation must be installed at it's 60? ampacity.
I have a 60 ft run of SER cable, the 1st 5 ft leaving the panel is in insulation.
Since less than 10% and less than 10 ft is in insulation, can I use the 75? ampacity for the cable ?

I see it as 55' of cable in the higher ampacity and 5' in the lower ampacity. Since 10% of 55 is 5.5 then IMO the 5' is less than 10% allowed so you are good to go at the higher ampacity.

Basically no matter what the distance may be 10' is the max allowed at the lower temp rating. A 300 foot run with 11 feet in insulation would need de-rating

 

[Smart $]

...For circuits with more than two sections, as I read it the exception applies to each transition point. So say I have a 220' run, with the central 20' at a lower ampacity, and the 100' on either side at a higher ampacity. Can I apply the exception twice to use the higher ampacity for the entire circuit?

Yes.

 

[Smart $]

Basically no matter what the distance may be 10' is the max allowed at the lower temp rating. A 300 foot run with 11 feet in insulation would need de-rating

Depends on where the 11' is at in the run. If at one end or the other, your statement is correct. But if it is within the run somewhere, the determination is performed twice, once at each transition.

Say there is 80' to one side and 209' to the other. The former transition would allow higher ampacity "extension" up to .8'. The latter transition would allow higher ampacity "extension" up to 10' (less than 10%). This too would require derating because only 10.8' total is permitted to have "extension".

Now say there is 120' to one side and 169' to the other. The former transition would allow higher ampacity "extension" up to 10' (less than 10%). The latter transition would allow higher ampacity "extension" up to 10' (also less than 10%). This allows using the higher ampacity because the permitted "extensions" overlap.

 

[GoldDigger]

Yes.

Put a box with a device, or even just a splice at the middle of that 20' run and I might agree with you. Otherwise not.

Tapatalk!

 

[Smart $]

Put a box with a device, or even just a splice at the middle of that 20' run and I might agree with you. Otherwise not.

Consider how a device or splice would change anything (or not). Also see my most recent reply to Dennis... then comment again

 

[wwhitney]

Say there is 80' to one side and 209' to the other. The former transition would allow higher ampacity "extension" up to .8'. The latter transition would allow higher ampacity "extension" up to 10' (less than 10%). This too would require derating because only 10.8' total is permitted to have "extension".

10% of 80' is 8'. So you want your example to be, say, 7' to one side and "more than 100'" on the other side, to get a limit of 10.7' for the extension.

Cheers, Wayne

 

[GoldDigger]

My basic analysis is that the exception is not framed in terms of extensions or transitions, but rather in terms of portions of a circuit. The fact that there is a "transition" at each end does not permit the size of the lower ampacity segment to exceed 10 feet, period.
If there is some division of that segment into two distinct segments, whether that has a physical effect or not, might justify applying the exception separately to each 10' piece.

Tapatalk!

 

[Smart $]

10% of 80' is 8'. So you want your example to be, say, 7' to one side and "more than 100'" on the other side, to get a limit of 10.7' for the extension.

Cheers, Wayne

My bad. So the first determination would be permitted because total "extension" is 18'.

 

[Smart $]

My basic analysis is that the exception is not framed in terms of extensions or transitions, but rather in terms of portions of a circuit. The fact that there is a "transition" at each end does not permit the size of the lower ampacity segment to exceed 10 feet, period.
If there is some division of that segment into two distinct segments, whether that has a physical effect or not, might justify applying the exception separately to each 10' piece.

I disagree.

The exception starts, "Where two different ampacities apply to adjacent portions of a circuit, ..." This cannot include the entire run where there are two or more non-adjacent portions.

Then the exception continues, "...the higher ampacity shall be permitted to be used beyond the point of transition, ..." Note the singular form of point.

I see no exclusionary conditions for a run with two or more transitions.