[460 Delta]

Ok, what useful purpose does that paper provide?

Asking for a friend.

Jobs for people in a paper mill.

 

[samgregger]

Ok, what useful purpose does that paper provide?

Asking for a friend.

It allows the exterior insulation to be more flexible because it does not stick to the conductor insulation.
Think stripping NM vs UF

 

[Ronan]

So, putting it all together.

Everything You Need to Know to Derate NM cable, with respect to bundling & not in a raceway:

• When we bundle NM cables that have a cumulative number of current-carrying conductors greater than 3, with lengths greater than 24", we then derate. An example is two 12/2, which has four current-carrying conductors, when the neutrals are counted.
• We use the 90°C column in Table 310.16: Because 334.80 Ampacity says to use the column for derating & 334.112 Insulation says NM "conductor insulation shall be rated at 90°C". This means you start to derate from 30 amps when derating 12 AWG.
• But we can't derate any lower in ampacity than what's in the 60°C column. 334.80 Ampacity
• 110.14(C) limits us to a final ampacity that is no greater than the 60C column for that conductor. "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 termination, conductor, or device."
• 240.4(D), (which is from the asterisked footnote of 310.16) has nothing to do with derating. It is used for conductor overcurrent protection limitations.
• One NM cable with at least 4 current-carrying conductors is derated, because Table 310.15(C)(1) Note says so. "Number of conductors is the total number of conductors IN the raceway or cable."
• When we're derating other insulated conductors, such as THWN, we derate from their respective 60°, 75° column from Table 310.16.

When to count neutrals as a current-carrying conductor: 310.15(E)(1)-(2)

• A normal 120V 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
• In a circuit for a 240V dryer with two hots, the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC.

An example of derating
Four, bundled, 10/2 Romex going through the attic to power four 208V AC units that require 20 amps each.

• Each AC unit gets two hots, no neutral. Total CCCs is eight.
• Table 310.15(C)(1) says they are derated to 70% of original ampacity.
• Table 310.16, 90° column shows 10 AWG has ampacity of 40 amps.
• 40 amps x .70 = 28 amps. So we're good to go.
• But, if we add in the derate due to temperature from Table 310.15(B)(1)(1), which is 110°, we get a multiplier of 87%.
• 40 amps x .70 (bundling) x .87 (temp) = 24 amps, so we're still good to go.

If I did the temperature derating correctly, it's interesting to see the multiplied effects of both temperature and bundling.

Is this about right, for derating Romex not in a pipe?

 

[Almost Retired]

after you have been in the resi trade for a while , you will automatically know that you dont need to worry about doing the calcs a majority of the time
just upsize the wire and run it, furthermore never make a habit of bundling romex over a distance
there is normally room to leave a space when you staple in the attic

how ever .... it is good to know and understand how to do it correctly
in conduit work it can be more complicated and from time to time it will come in handy

 

[Dennis Alwon]

So, putting it all together.

Everything You Need to Know to Derate NM cable, with respect to bundling & not in a raceway:

• When we bundle NM cables that have a cumulative number of current-carrying conductors greater than 3, with lengths greater than 24", we then derate. An example is two 12/2, which has four current-carrying conductors, when the neutrals are counted.
• We use the 90°C column in Table 310.16: Because 334.80 Ampacity says to use the column for derating & 334.112 Insulation says NM "conductor insulation shall be rated at 90°C". This means you start to derate from 30 amps when derating 12 AWG.
• But we can't derate any lower in ampacity than what's in the 60°C column. 334.80 Ampacity
• 110.14(C) limits us to a final ampacity that is no greater than the 60C column for that conductor. "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 termination, conductor, or device."
• 240.4(D), (which is from the asterisked footnote of 310.16) has nothing to do with derating. It is used for conductor overcurrent protection limitations.
• One NM cable with at least 4 current-carrying conductors is derated, because Table 310.15(C)(1) Note says so. "Number of conductors is the total number of conductors IN the raceway or cable."
• When we're derating other insulated conductors, such as THWN, we derate from their respective 60°, 75° column from Table 310.16.

When to count neutrals as a current-carrying conductor: 310.15(E)(1)-(2)

• A normal 120V 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
• In a circuit for a 240V dryer with two hots, the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC.

An example of derating
Four, bundled, 10/2 Romex going through the attic to power four 208V AC units that require 20 amps each.

• Each AC unit gets two hots, no neutral. Total CCCs is eight.
• Table 310.15(C)(1) says they are derated to 70% of original ampacity.
• Table 310.16, 90° column shows 10 AWG has ampacity of 40 amps.
• 40 amps x .70 = 28 amps. So we're good to go.
• But, if we add in the derate due to temperature from Table 310.15(B)(1)(1), which is 110°, we get a multiplier of 87%.
• 40 amps x .70 (bundling) x .87 (temp) = 24 amps, so we're still good to go.

If I did the temperature derating correctly, it's interesting to see the multiplied effects of both temperature and bundling.

Is this about right, for derating Romex not in a pipe?

I don't believe you need to double de-rate. 40 X .7= 28 amps and 40x .87= 34.8 amps all we need to do is use the lower of the two. Thus 28 amps is where you would be at and as long as the load is 28 amps or less we can still use a 30 amp breaker

 

[Ronan]

I don't believe you need to double de-rate. 40 X .7= 28 amps and 40x .87= 34.8 amps all we need to do is use the lower of the two. Thus 28 amps is where you would be at and as long as the load is 28 amps or less we can still use a 30 amp breaker

Mike Holt showed that we need to use both temperature and bundling percentages, at 13 min 8 seconds in. In the example he shows how easily 115 amps can drop down to 80 amps. So it makes me wonder how many electricians are risking things by simply ignoring temperature derating.

 

[Dennis Alwon]

Mike Holt showed that we need to use both temperature and bundling percentages, at 13 min 8 seconds in. In the example he shows how easily 115 amps can drop down to 80 amps. So it makes me wonder how many electricians are risking things by simply ignoring temperature derating.

Yes and every test and example I have seen will do a double de-rate but I remember one of the big magazine writers came on to Mike Holt forum and posted a question. I respond with the typical double de-rating and he and another member showed where that was incorrect. I wish I could find that thread as I have been looking for it. Maybe @A Little Short can find it or at least show me that I am misunderstanding.

 

[Ronan]

Yes and every test and example I have seen will do a double de-rate but I remember one of the big magazine writers came on to Mike Holt forum and posted a question. I respond with the typical double de-rating and he and another member showed where that was incorrect. I wish I could find that thread as I have been looking for it. Maybe @A Little Short can find it or at least show me that I am misunderstanding.

It's really frustrating that new electricians are expected to know the NEC well enough to take the journeyman's test, yet there is no one-stop location for a reliable pedagogy to actually learn what is expected of us. I literally have more books than anyone else I work with, along with the two most recent NEC handbooks and the Mike Holt lessons on DVD. I sift through the code close to every day and get made fun of when I ask specific questions (because nobody seems to be able to answer the questions I have) and get answers along the line of, "Well hell, we just throw axle grease on it and call it a day," or "We just get a bigger pipe and call it a day".

Personally, it makes sense to me (on a gut level) that if you have a high temperature on top of bundling, then derating aught to be multiplied, since both issues are present. But, I'm more wrong than I am right. Thanks for your input Dennis. Please let me know if you find the answer. I'll keep looking in the NEC as well.

 

[Ronan]

So I just found this.

 

[Dennis Alwon]

I get it. I have always done it that way especially for tests. I am just saying that I remember that I had done it wrong and all the examples given were incorrect based on a section in the code that I cannot find.

 

[Almost Retired]

I get it. I have always done it that way especially for tests. I am just saying that I remember that I had done it wrong and all the examples given were incorrect based on a section in the code that I cannot find.

there is always the context of a section of code
a statement permitting something in one article does not necessarily apply to all articles
could the situation you almost remember, have been one of those type deals maybe?
could it have been in 2023 only ?

 

[Dennis Alwon]

there is always the context of a section of code
a statement permitting something in one article does not necessarily apply to all articles
could the situation you almost remember, have been one of those type deals maybe?
could it have been in 2023 only ?

I think I am afflicted with CRS. Can't Remember S_ _ _

 

[Almost Retired]

I think I am afflicted with CRS. Can't Remember S_ _ _

dont feel bad
i definitely have it too

sometimes it will be the next before i can think of what i had in mind
sometimes it will be days later when something finally jogs my memory, and i never would have remembered it without the jog

 

[Dennis Alwon]

@Ronan @Almost Retired

I was only half baked. This is what I was thinking about

210.19(A)(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors shall
be sized to carry not less than the larger of 210.19(A)(1)(a)
or (b).
(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.
(b) The minimum branch-circuit 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.

 

[Ronan]

@Ronan @Almost Retired

I was only half baked. This is what I was thinking about

2023 is a little different. There is no (A)(1)(a) or (A)(1)(b).

And though I just skimmed the section, the only thing I see directly relating to derating scenarios is the Exception,and even then it doesn't seem to mention dealing with either/or/both temperature & bundling.

 

[Almost Retired]

your quote @Dennis Alwon

(a) does not mention derating for any reason, so that does not apply to the question
b) does not explain how to use either or both ratings, so no help or clarification to the question

so far all i have seen says you need to use both together

 

[Dennis Alwon]

your quote @Dennis Alwon

(a) does not mention derating for any reason, so that does not apply to the question
b) does not explain how to use either or both ratings, so no help or clarification to the question

so far all i have seen says you need to use both together

Yes in the example given you are correct that both de-ratings must be used. The section I quoted was what had me confused as I remember something about double derate. Of course, this does not fit the original problem and this section only applies to continuous loads in that if 125% is larger than the temp. ampacity you just pick one. In other words, if you had a 40 amp motor then we all know we need to multiply by 125% which lands us at 50 amps

If the 40 amp load has a ampacity factor of .7 then 40/.7 = 57 amps so our conductor needs to be sized for 57 amps

In the past, some would use the 125% = 50 and then take the 50/.7 = 71 amps. That is not correct

 

[Almost Retired]

Yes in the example given you are correct that both de-ratings must be used. The section I quoted was what had me confused as I remember something about double derate. Of course, this does not fit the original problem and this section only applies to continuous loads in that if 125% is larger than the temp. ampacity you just pick one. In other words, if you had a 40 amp motor then we all know we need to multiply by 125% which lands us at 50 amps

If the 40 amp load has a ampacity factor of .7 then 40/.7 = 57 amps so our conductor needs to be sized for 57 amps

In the past, some would use the 125% = 50 and then take the 50/.7 = 71 amps. That is not correct

i do see your point there and agree with that

 

[Ronan]

One last question. For conductors in the 75° column, are they too limited in derating to no higher than results in the 60° column, like Romex is?

 

[Dennis Alwon]

One last question. For conductors in the 75° column, are they too limited in derating to no higher than results in the 60° column, like Romex is?

No, nm cable is a different animal. If you ran conduit with 90C wire then you would use the 90C column for de-rating but no higher than the 75C column. Look at 110.14(C)