Feeder ampacity thru insulated 2x6 wall: 60° or 75° column?

[jvitez]

Damn convoluted electrical codes...... I'm in Canada, so under the CEC. The easily accessible information is for the NEC though, so I'd really appreciate clarification.

Running a subpanel feeder in free air allows use of the 75° column, but if run within insulation it's the 60° column. So for a 100 ft run, if it penetrates an insulated 2x6 stud wall horizontally for 5.5 inches, do you have to derate the entire run and use the 60° column? Or not??

 

[pattenp]

If in the US I'd say not. The NEC here is not clear as to how much of the run needs to be run within insulation before the 60C rule kicks in. I think it's reasonable if the wire makes a straight shot through the wall that the 75C can be used. But that's just my feeling.

 

[jvitez]

Yeah, it would make logical sense that 5.5 inches of a 100 ft run is rather unlikely to overheat enough to cause a fire even if run at full ampacity. But with these codes you never know.

Is this an issue for us under the CEC any fellow Canucks?

 

[Aceman]

Read 310.15(A)2 including the exception.

 

[pattenp]

Read 310.15(A)2 including the exception.

That's good to know. Had to read it a couple of times for the legalize to sink in.

 

[Aceman]

It took me a couple times the first time I read it too...

 

[tfi racing]

Which table are you trying to interpret?Always use the column that corresponds to the insulation value as marked on the conductor,then you apply the necessary derating factors(%) to that number.It is very rare to buy anything that is not 90 degree rated anymore,you will rarely stray from that column.

 

[Charles (in GA)]

It took me a couple times the first time I read it too...

I still don't understand what it is saying. Could you elaborate?

Charles

 

[jvitez]

Double damn! Aceman, you da man!! That's exactly what I was looking for.

Charles:

"310.15 Conductor Ampacity

(A) General Requirements

The ampacity of a conductor can be determined either by using the tables in accordance with 310.15(b), or under engineering supervision as provided in 310.15(C).

FPN No. 1: Ampacities provided by this section do note take voltage drop into consideration. See 210.19(A) FPN No. 4, for branch circuits and 215.2(D) FPN No. 2, for feeders.

If a single length of conductor is routed in a manner that two or more ampacity ratings apply to a single conductor length, the lower ampacity must be used for the entire circuit. See 310.15(B).

Exception: When different ampacities apply to a length of conductor, the higher ampacity is permitted for the entire circuit if the reduced ampacity length does not exceed 10 ft and its length doesn’t exceed 10% of the length of the higher ampacity."




So for my scenario, I'd have to size the wire to that in an insulated space not free air, as part of it goes through the stud wall. This single wire (well, actually a 3 conductor cable) would have two ampacities, the part within the insulated wall, and the rest in free air.

The exception answers my question: 5.5 inches within an insulated wall is less than 10 ft, and less than 10% of the proposed 100 ft run.

Many thanks everyone!!

 

[pattenp]

I believe the 10% is calculated against the length of the circuit portion that is of the higher amp capacity or in this case the length of the portion not within insulation, not the total circuit length.

 

[jvitez]

A subtle point pattenp, but only for my numbers. It could certainly trip someone up with different lengths. Thanks for the clarification.

So for my numbers, the exemption holds as 5.5 inches is less than 10ft, and 5.5 inches is less than 10% of (100 ft -5.5 inches)=99 ft 6.5 inches.

I could see it not being allowed if someone was running a short total length with some of it vertically inside an insulated wall.

 

[pattenp]

A subtle point pattenp, but only for my numbers. It could certainly trip someone up with different lengths. Thanks for the clarification.

So for my numbers, the exemption holds as 5.5 inches is less than 10ft, and 5.5 inches is less than 10% of (100 ft -5.5 inches)=99 ft 6.5 inches.

I could see it not being allowed if someone was running a short total length with some of it vertically inside an insulated wall.

I'm not totally sure I'm correct on the way to calculate, but in your case it doesn't mount to a hill of beans. This is all NEC, did you find the same in the CEC?

 

[jvitez]

Now that I have my ducks in a row I'll call our area's inspector and get his official word. I'm going to price out some cable too.

 

[Charles (in GA)]

Double damn! Aceman, you da man!! That's exactly what I was looking for.

Charles:

"310.15 Conductor Ampacity

(A) General Requirements

The ampacity of a conductor can be determined either by using the tables in accordance with 310.15(b), or under engineering supervision as provided in 310.15(C).

FPN No. 1: Ampacities provided by this section do note take voltage drop into consideration. See 210.19(A) FPN No. 4, for branch circuits and 215.2(D) FPN No. 2, for feeders.

If a single length of conductor is routed in a manner that two or more ampacity ratings apply to a single conductor length, the lower ampacity must be used for the entire circuit. See 310.15(B).

Exception: When different ampacities apply to a length of conductor, the higher ampacity is permitted for the entire circuit if the reduced ampacity length does not exceed 10 ft and its length doesn’t exceed 10% of the length of the higher ampacity."




So for my scenario, I'd have to size the wire to that in an insulated space not free air, as part of it goes through the stud wall. This single wire (well, actually a 3 conductor cable) would have two ampacities, the part within the insulated wall, and the rest in free air.

The exception answers my question: 5.5 inches within an insulated wall is less than 10 ft, and less than 10% of the proposed 100 ft run.

Many thanks everyone!!

Problem is, you and I have different wordings in the code.

Mine 2011 code, 310.15(A)(2) reads:

(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be
used.
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.

Mine is a direct copy and paste out of my PDF version of the code.

I still don't understand what my copy is trying to say.

Charles

 

[pattenp]

Problem is, you and I have different wordings in the code.

Mine 2011 code, 310.15(A)(2) reads:

(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be
used.
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.

Mine is a direct copy and paste out of my PDF version of the code.

I still don't understand what my copy is trying to say.

Charles

Charles... look at page 120 of this PDF.

http://www.mikeholt.com/instructor2/img/product/pdf/1259687476sample.pdf

 

[Charles (in GA)]

Yes, I understand that, but the PDF of the code I have says something stupid, I don't understand...........

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.

Read it and tell me what it says, it does not say what has been quoted by others here.

Also, everything here says 10% of the circuit length (meaning the TOTAL circuit length) while Mike Holt (and a lot of people on the net take exception to much of what MH spouts) says 10% of the length of the higher amp rated section of the circuit.

This is MH's "quote" of the exception paragraph included in the code, which does not agree with the quote I posted.

Exception: When different ampacities apply to a length of conductor,
the higher ampacity is permitted for the entire circuit if
the reduced ampacity length doesn’t exceed 10 ft and its length
doesn’t exceed 10 percent of the length of the higher ampacity.

Its very possible that I have a "defective" copy of the code, as I have found many typos in it, lack of necessary punctuation, capitalization, lack of spaces between words, etc, that make reading a document full of legalese difficult at best. My copy is a download directly from NFPA and has a "stamp" on the front page with my number on it and my name at the top of each page, but that still don't make it "right", it just means they are too busy generating advertising copy of **** to mail out for books, classes, and other garbage to bring in revenue (its all about the money, huge money) to bother to proof read the actual code for errors.

Charles

 

[pattenp]

Charles my NEC 2011 book has it written the same wacky way as yours. The Mike Holt version to me is just his plain English interpretation of it. I believe the point of transition is the point at which the higher amp capacity goes to the lower amp capacity and beyond means in the direction of the lower amp capacity. You need to look at it from going high to low and once you reach that transition point you can use the higher amp capacity beyond that point over the lower amp rating.

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.