I've spent way too many hours studying derating, only to find that anything & everything I thought I'd learned has turned out to not be necessarily true. Can y'all correct me on what I think I now know?

Ambient temperature aside (since I haven't studied it yet) with respect to bundling and not in a raceway, this is how we derate NM cable:

• We use the 90°C column in Table 310.16 (because 334.80 says so). Is this because all NM is THHN or just because code says so?
• We derate from that column, but we can't go any lower in ampacity than what's in the 60°C column (because 334.80 says so).
• And for gauges that are 10 AWG to 18 AWG, we use the 60°C column and 240.4(D) which states 18 AWG 7 amps, 16 AWG is 10 amps. For some reason those two aren't listed in the 60°C column in Table 310.16.
• If we happen to have just one NM cable with at least 4 current-carrying conductors, we derate it, because Table 310.15(C)(1) says so. "*Number of conductors is the total number of conductors IN the raceway or cable, including spare conductors." I've never seen 12/4 NM, but I figure it probably exists.
• When we bundle and not space NM cables (for lengths greater than 24") we then derate.

If all of this is correct, my questions are:

• If we have three 12/3 conductors (not in a raceway) that are bundled, is that considered a count of six current carrying-conductors? Since we count each current-carrying conductor in a single NM cable.
• If we have a single 12/3 Romex, that's not going to be derated, is the ampacity we expect to use in the 90°C column of Table 310.16 (which is 30 amps) or is it the 60°C column which is 20 amps? I've always worked with 12 AWG was 20 amps, so now I question everything I ever thought I knew.
• When we're derating other insulated conductors, is Table 310.16 pretty straight forward, like THWN is 75°C column, or is it as convoluted as derating NM cable?

#1:
three 12-3 romex bundled could be 6 or 9 current carrying
depends on whether they are multi wire branch circuits, or some other special case

but yes usually 6

#2 & #3:
you are confusing ampacity with the maximum allowed OCPD
the maximum allowed OCPD of 16 thru 10AWG is listed in 240.4d regardless of insulation
however, modern romex uses THHN inside the jacket

the ampacity of a conductor begins with the max temp value of the insulation
could be 90, 75, 60, or other

 

there are actually only two reasons to derate, ambient temp and no. of current carrying conductors (CCC)

i use strictly THHN because it is the highest max temp, and the most common in stock at the store
(or XHHW for alu for the same reasons)
if you have to derate for any of several reasons you start from the 90*C list (IF that insulation has that value)
and continue to apply the derating multipliers in any order

you could end up above or below the 60*C column ampacity
you can never put a higher OCPD than allowed
but you may need to put a lower one if there is enough derating

derating is done for bundling romex, and or ambient temperature (for instance an attic)
or no. of conductors in conduit, on a roof or factory high temp area

for residential wiring: bundling or attic temps are seldom putting you below the 60*C ampacity, almost no one figures it

for conduit, there is a point where a bigger pipe and bigger wires begins to lose value, it ends up costing more than smaller pipes with fewer wires

for instance a maximum of 9 - 12AWG-CCC in 3/4" is my rule of thumb for multiple conductors (plus grnd is 10, plus not counted neutrals)
the multi-conductor derate takes you down to 21 amps

if an ambient derate is needed, i figure that first and then figure the max CCC derate i will use and then size the pipe accordingly

in other words i try to keep 12AWG on a 20A breaker, 10 on a 30A, etc.

 

ETA there is also a temperature limitation for the listed temp of the the termination at Both ends
both the breaker and the wire nut or lug where that conductor terminates

 

Nm cable is constructed with 90C conductors. Section 334.112

334.112 Insulation. The insulated power conductors shall be
one of the types listed in Table 310.104(A) that are suitable for
branch-circuit wiring or one that is identified for use in these
cables. Conductor insulation shall be rated at 90°C (194°F).

Since the conductors are 90C we can de-rate from the 90C column based on 110.14(C) but limits us to a final ampacity that is no greater than the 60C column for that conductor.

(C) Temperature Limitations. 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. Conductors
with temperature ratings higher than specified for terminations
shall be permitted to be used for ampacity adjustment,
correction, or both.

Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

 

Nm cable is constructed with 90C conductors. Section 334.112



Since the conductors are 90C we can de-rate from the 90C column based on 110.14(C) but limits us to a final ampacity that is no greater than the 60C column for that conductor.



Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

This is good! Thanks for the input. Will read up on it tonight.

 

Isn't the neutral considered a current carrying conductor? So three 12/3 wires have nine current carrying conductors and three ground wires

 

Did you even look at the post directly above yours? Neutral is not always a current carrying conductor.

 

85% of the electricians will never use the heat derating. I do infrequently in Arizona.
But I never plan on running wire that is that close to the rated amperage.
Where most people would run 4/0 for a 200 amp riser, we here in the desert know it has to be 250 mcm. Or the 75 degree column. The old days of purchacing and installing THW or TW are gone. Now days everything available is THHN or XHHW. I do not concern myself with wire I can not purchase easily.

 

To the OP: If you are going to glean anything at all from this thread (and the other) it should be this;

85% of the electricians will never use the heat derating. I do infrequently in Arizona.
But I never plan on running wire that is that close to the rated amperage.

Not saying you shouldn't understand the process.

 

@Ronan if you have 9 or less current carrying conductor's in a bundle then the derating will not affect the ampacity. For example, #12 is rated 30 amps at 90C. 9 current carrying conductor's has a factor of 70%. Therefore, 30 amps x 70% = 21 amps. The #12 is still good for 20 amps. It is not often that you have more than 9 current carrying conductor's in a bundle or in a raceway. That 10 the wire is the killer because it brings it down to 50%

 

I understand that, and thanks. I've heard someone else say the same thing, and it makes me feel better about running multiple/bundled/stapled 10/2 Romex together, in attics for rooftop AC units (before I even learned about derating). I hope that the additional increase in temperature won't push the conductors past their limit.

I'm trying to understand the derating calculation process. It wasn't until this morning, after watching another video, that I realized that 1) a single Romex cable can be derated, and 2) that Romex uses the 90°C column. Not to mention learning about spare conductors yesterday. None of the guys I work with, including journeymen or foremen, know about any of this stuff, and there doesn't appear to be any decent pedagogy anywhere that lays out how to do it; and YouTube channels just add to the confusion. It really shouldn't be this difficult to learn the processes of something we're expected to either know or look up in the NEC.

So I appreciate your input and clarifications.

 

Why does Mike Holt say, "Conductor adjustment and correction is based on the 90°C column," when there are other conductor types in the 60°C & 75°C columns? If they're in those columns, how is derating only done in the 90°C column? And his panelist Boyd repeats this at 11:12, "The majority of the time that I mention the 90° column is to say, 'the only reason it's there is so you know what to look for when you go to temperature and bundling adjustments."

 

See 334.80 Ampacity (NM)

60 Degree C ampacity is used.

90 Degrees C for Temperature and bundle adjustments, but final adjustments cannot exceed 60 degrees C data.

Considering the paper material inside the NM-B (Romex) cable, the 60 degree C temperature rating seems appropriate.

 

The flash point of kraft paper is about 450F if that had anything at all to do with anything.

 

Degrade the paper cellulose, not ignite it. Higher temperatures accelerates aging the paper and making it become more brittle with time. That Kraft paper can be in a house for 50+ years. Limiting the current (heat) is good thing.

 

450F would be roughly 232C

 

450F sounds better. F451 almost sounds like a movie title!

Wonder if there's a connection?

 

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

 

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

 

So I just found this.

 

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 ?

 

@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 @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.

 

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

 

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

 

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)

 

for single conductors in conduit (not romex)
10AWG and under is not allowed to be used over 60
all other sizes are used according to their insulation
the insulation determines the temp rating of the conductor
you derate from that

if your breaker and end terminations are 60*C you cannot use the conductor above 60*C
if your breaker and end terminations are 75*C you cannot use the conductor above 75*C

however it is becoming more common to find breakers rated 75*C
to determine what the breaker is rated for look on the side
60*C is not always marked, all others are always marked (i have seen older breakers marked 40*C)

most supply houses only sell individual conductors of any size whether cu, or alu with 90*C insulation

 

Just when I thought I'd nailed it.

I thought only Romex wasn't allowed to be derated over 60° column. Per 334.80 "The 90° rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that of the 60° rated conductor." Now you're saying that all the different 10 AWG - 18 AWG insulation types in both 75° & 90° columns can't be derated above the 60° column? Because I'm not finding that comment anywhere else in the book for any other cable or insulation type.

 

2020 NEC
table 310.16
there is an asterisk * beside each of the small conductors
at the bottom of the table, in the notes the * refers to 240.4(D)

 

But previously when I mentioned 240.4(D) and the asterisk, you said, "you are confusing ampacity with the maximum allowed OCPD". So I stayed away from it and focused on ampacity and the 60° column.

 

step 1: you always derate from the insulation max degree column (including for ambient and no. of CCC)

step 2: after that you size your derated conductor to the load

step 3: then you size your breaker (which may be 60 or 75*C) to the derated conductor

step 4: if it is romex of ANY AWG, use the 60 degree column to size your breaker (there are cables that are not romex and 60 does not apply to them)
if it is 10AWG and under refer to 240.4D no matter what type of cable, or single conductor, there is normally no exception to this rule

once you get your head around this it will be the same thing over and over and be commonplace for you

and btw stop talking about 75*C nobody uses those insulations anymore (you are developing a habit that will hinder you)
you cant buy (without special order and it will cost more) anything but 90; not even HD or lowes stock anything but 90
so getting in the habit of derating from 75 will cause you trouble
get in the habit of derating from 90

 

once you get your head around this it will be the same thing over and over and be commonplace for you

and btw stop talking about 75*C nobody uses those insulations anymore (you are developing a habit that will hinder you)
you cant buy (without special order and it will cost more) anything but 90; not even HD or lowes stock anything but 90
so getting in the habit of derating from 75 will cause you trouble
get in the habit of derating from 90

My god there is some gold in these comments! And you’re right, I’m having trouble wrapping my head around the final part of 240.4(D) and derating and the coincidence of 60 degree column. Will review tonight and drill it into my head. I would have been totally lost without you two. Thanks!!
But I have to ask, is this info clearly laid out anywhere? I’ve watched numerous videos and highlighted just about everything in my code book, and Googled extensively, but I haven’t found anything that puts all of this together to decrypt the NEC.

 

LAMOOOOO
Nothing is put together clearly in the NEC
you have to get about 6 place holders and a scratch pad

starting from the article in question: read and make a synopsis
go to every reference and make notes or a synopsis of each one
sit down and study all the synopses till you feel like you have it
then go back thru the book and reread it all until you know you have it

this process can sometimes take days of letting it sink in and becoming familiar to you
____

think of 240.4.D as a table with a copper column and an alu column
(laid out just like 310.16 except this is for breaker/OCPD sizes and Not for ampacity and Not for insulation)
if you need to, get the scratch pad and draw it out to look at,
believe it or not this will actually help you learn it more easily
i personally dont bother with trying to remember anything smaller than 14AWG

on the other hand the asterisks in 310.16 remind you to use the 60 column and you already know that if it is romex it is automatically the 60 column regardless of AWG

there is no relationship between240.4.D and 310.16 and the 60 column, consider it a coincidence and dont think about it again

you are welcome for the help
i remember how hard it was for me to get anything out of the code when i was younger
but that was before i developed a way to pull everything together in one spot so that i could read it thru without losing my train of thought hunting references

about 10 yrs ago i actually spent about a month teaching myself the basics of solar from the code
that is where i developed my system
you can learn a lot of things from the code, even though it says it is not intended to teach anything

 

Stop over thinking it.
You start with the 90 degree column ampacity for your ampacity before derating with NM-B.
Then derate just as you would with a bunch of conductors in a raceway.
Remember to count all of the current-carrying conductors in the bundle.
If your calculated derated ampacity is equal to or higher than the 60 degree column ampacity, then you use the 60 degree column ampacity by default.
If your calculated derated ampacity is lower than the 60 degree column ampacity, then you use your calculated ampacity.

YES, derating for romex in MOST residences is a complete non issue.
For example, if you have four NM-B 12/2's in a bundle, then you would have eight current-carrying conductors.
Eight current-carrying conductors is a 70% derate.
You start with the 30 amps in the 90 degree column.
30 amps x 70% = 21 amps.
At this point you are still HIGHER than the 20 amps listed in the 60 degree column.
So 20 amps it is.

Life is good.