# Derating Question



## Gamit (Dec 30, 2009)

Currently I am taking a prep course for my contractor’s license and am a little unclear on what column to use when derating. My teacher is telling me to always use the 90 degree column when derating,is this correct? Here is a example question where he said to use this column.

Question: Allowable Ampacity
4 AWG THW Copper
In conduit with 3 other CCC of same size
Ambient Temp 98 degrees F

This is the only information given for the question
THW does not exist in that column only in the 75 degree column

I understand how to apply the CF and AF just dont know what colum to use.

Thanks:blink:


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## user4818 (Jan 15, 2009)

Yes, you start at the 90 degree column and apply your derating factors, working your way down. For your example problem, you start with what you're given. If that's a 75 degree conductor, you start there and work down.


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## Dennis Alwon (May 9, 2009)

Gamit, lets say you have a piece of NM cable. We ll know that NM has 90c wire in it but art. 334 tells us that NM must be used at 60C. 

Now we have NM run with a bunch of other NM cables and for whatever reason we must derate. We may start at the 90C col and derate but we cannot ever use OCP that would be higher than the 60C col (after derating).

In a nutshell, if I derate from the 90C and the ampacity is higher than the 60C column I must use OCP at the 60C column


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## Gamit (Dec 30, 2009)

Peter D said:


> Yes, you start at the 90 degree column and apply your derating factors, working your way down. For your example problem, you start with what you're given. If that's a 75 degree conductor, you start there and work down.


THW is a 75 degree conductor but I'm told to use the 90 degree column.

So your saying
85 amps x 80% = 68
68 x .88 (CF)= 59.84 amps is the answer?

These numbers are from the 75 degree column


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## user4818 (Jan 15, 2009)

Gamit said:


> THW is a 75 degree conductor but I'm told to use the 90 degree column.
> 
> So your saying
> 85 amps x 80% = 68
> ...


Sorry, I was wrong. Dennis gave the correct answer. The final ampacity cannot exceed the listed rating of the conductor.


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## Gamit (Dec 30, 2009)

My answer choices are:

60
75
80
85


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## NolaTigaBait (Oct 19, 2008)

60 sounds right.


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## Dennis Alwon (May 9, 2009)

Gamit said:


> My answer choices are:
> 
> 60
> 75
> ...


Okay, if you have thw then you start at the 75C column and derate from there. Use your CF and your more than 3 ccc in a conduit to derate. Tell us what you get.


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## Gamit (Dec 30, 2009)

dennis alwon said:


> okay, if you have thw then you start at the 75c column and derate from there. Use your cf and your more than 3 ccc in a conduit to derate. Tell us what you get.


59.8


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## NolaTigaBait (Oct 19, 2008)

> If u see kay tell her I want her


Is that from the Godfather?


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## NolaTigaBait (Oct 19, 2008)

Gamit said:


> 59.8


And the closest answer issssss....


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## Dennis Alwon (May 9, 2009)

Gamit said:


> 59.8


Good now look at 220.5 (B) and 240.4(B). 240.4(B) would be for future questions.


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## Dennis Alwon (May 9, 2009)

NolaTigaBait said:


> Is that from the Godfather?


If it is I didn't know it-- I thought I made it up. I guess you deciphered it.


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## NolaTigaBait (Oct 19, 2008)

Dennis Alwon said:


> If it is I didn't know it-- I thought I made it up. I guess you deciphered it.


I thought I heard Michael Corleone say that in he movie....I may be wrong though.


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## Gamit (Dec 30, 2009)

Dennis Alwon said:


> Good now look at 220.5 (B) and 240.4(B). 240.4(B) would be for future questions.


Thanks for the imput,my biggest concern was which column to start from and because the question was using TWH you start from 75 degree


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## Dennis Alwon (May 9, 2009)

Gamit said:


> Thanks for the imput,my biggest concern was which column to start from and because the question was using TWH you start from 75 degree


Be careful with the 90C col. You can start from there but you can never install a breaker at that ampacity since you must use the weakest link in the system. Breakers are only rated 75C (Most of them) so you can rarely use 90C col. for the OCPD. You can start there.


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## NolaTigaBait (Oct 19, 2008)

Dennis Alwon said:


> Be careful with the 90C col. You can start from there but you can never install a breaker at that ampacity since you must use the weakest link in the system. Breakers are only rated 75C (Most of them) so you can rarely use 90C col. for the OCPD. You can start there.


But for derating purposes since he has thw wire, you would derate from the 75 column.


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## Dennis Alwon (May 9, 2009)

NolaTigaBait said:


> But for derating purposes since he has thw wire, you would derate from the 75 column.


Yes, I was hypothetically speaking for a problem that would use 90C. I should have been more clear. sorry


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## NolaTigaBait (Oct 19, 2008)

Dennis Alwon said:


> Yes, I was hypothetically speaking for a problem that would use 90C. I should have been more clear. sorry


Gotcha. It just kinda threw me off for a sec.


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## Murphy (Dec 10, 2009)

the answer is 60.. 

85 A x .88 CF = 74.8 

six conductors is .8 cf = 59.84

always the 75 degree column when the question is not specific.


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## NolaTigaBait (Oct 19, 2008)

Murphy said:


> the answer is 75..
> 
> 85 A x .88 CF = 74.8
> 
> always the 75 degree column when the question is not specific.


Theres 4 current carrying conductors.


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## Dennis Alwon (May 9, 2009)

NolaTigaBait said:


> Theres 4 current carrying conductors.


Very poorly worded but I believe that to be the case also. It says a #4 with 3 other CCC so we must assume that they are all CCC with the info given.


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## Gamit (Dec 30, 2009)

Dennis Alwon said:


> Very poorly worded but I believe that to be the case also. It says a #4 with 3 other CCC so we must assume that they are all CCC with the info given.


Yes you are correct


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## Murphy (Dec 10, 2009)

ha yea.. oops. i noticed after i posted and i changed my answer to 60 but apparently didnt work.


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## electures (Oct 23, 2008)

Gamit said:


> Currently I am taking a prep course for my contractor’s license and am a little unclear on what column to use when derating. My teacher is telling me to always use the 90 degree column when derating,is this correct? Here is a example question where he said to use this column.
> 
> Question: Allowable Ampacity
> 4 AWG THW Copper
> ...


Who is the instructor?


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## BWA (Jan 20, 2010)

Let me interject a different example from an example situation in my position, with questions I'd like answered.

My relevant field is maintenance, so I run into a lot of electrical fixup, and some wiring that is getting old. On occasion, the fixup gets more involved. In the case we're looking at right now, a car bumped hard into the building, knocking the wall in, and I want to know what the electrical installers are going to run into.

There are various options to installing circuits: (when I say "60C" or something like that, I mean "60 degrees centigrade" since there is no ASCII degree symbol, even tho many extended character sets have it)

Possibility 1:

Old Bryant Class NI Cat#4-8-70F load center, use with Class NI Bryant Type BR, BRO or QP Breakers, box total allowed 70AMP 120/240VAC (suitable for use as service equipment when not more than 6 main disconnecting means are provided), with

New Eaton Cutler-Hammer CAFCI 20AMP breaker for use in Cutler-Hammer, Westinghouse, Challenger & Bryant Loadcenters, Cat# BR120CAF, 10 KAIC MAX RMS Sym, 40C - CTL - 60/75C Wire, 120 V ~, assembled in Dominican Republic, with

6 new single THHN 12AWG copper current carrying conductors in new 1/2 EMT in-wall going thru crawl space, and
6 new single THHN 12AWG copper current carrying conductors in new 1" PVC conduit in-wall going thru crawl space

Estimated maximum temperature in area for most years would reach 104F at most, but I'd say once a decade it might get up to 107F max for 2 or 3 days.


So, limiting factors:

96-104F (36-40C) ambient temperature derating = .96 (96%) for THHN 90C in conduit ("Raceway") based on ambient of 30C (86F) (Ugly's 2008 p73)

6 CCC derating, 4-6CCC = .80 (80%) (Ugly's 2008 p78)

Going to 90C column (Ugly's 2008 p73) for THHN, I see 12AWG copper at 30AMPs. So far, we have deratings of .96 (ambient temp) and .80 (6 CCCs) equaling a multiple of .768 so far, but do we need to also derate for the 40C terminating equipment (Cutler Hammer CAFCI breaker)? Repeating from above, it says "40C - CTL - 60/75C Wire" on it. I do not know what "CTL" means (circuit terminating lug? control?). But more importantly, it says 40C, with 60/75C wire. I see on another breaker (see below) that it is good for:

Al/Cu
60/75 C

So, what 60/75C means is Aluminum 60C, copper 75C. Since we're using all copper wire, we go with 75C out of "60/75C" markings in this context. So, returning to my above-mentioned Cutler-Hammer 20AMP CAFCI breaker, it says "40C - CTL - 60/75C Wire". Thus, would we start at the 75C column, which for THHN 12AWG in conduit is in 90C column but we jump to the column to the left? (Thus yielding 25AMP * .768 from above = 19.2 AMPS, BUT that's wrong since the ambient temperature derating is actually .88 in that column so it would be .80 * .88 * 25AMP = 17.60 AMPS?) So the answer would be 17.60AMPS?, and the 20AMP CAFCI is over-amped, and either we need to (1) drop the CAFCI amps to 15A, or (2) replace the 6 CCCs and associated grounds with 10AWG wire (which means a max of 5 10AWG THHN conductors in 1/2 EMT so that wouldn't work, or a max of 9 10AWG THHN conductors in 1" PVC, so with 2 per 1phase 120VAC circuit (6 CCCs) and 1 ground each on the PVC = 9, which is the max, or for the 1/2" EMT we can omit the ground wire since the EMT is grounded and bonded right but would still max at 5 CCC 10AWGs so we'd need to turn one circuit into a neutral with opposite phases of 240V (120V to neutral) circuit)? OR is there some other way to calculate all that? OR, is that all wrong, and we need to look at the "40C" rating of the breaker and derate for that? What does "40C" on a breaker mean, or more specifically, what does ""40C - CTL - 60/75C Wire" on a breaker mean? There's no 40C column for THHN in conduit in the table I looked at.



Possibility 2:

New HOM3040M200TC 200 Amp Homeline
30 Space/40 Circuit Max. Load Center with
Equipment Ground Bar Kit, with:

New breaker HOM120CAFIC single pole CAFCI 20AMP 120V 60Hz 10 000 AIR/A no.l, issue # DP-3640, Al/Cu 60/75C, with:

New 12AWG copper THHN 6 current carrying conductors in new 1" PVC in-wall going thru crawl space, and 12AWG copper THHN 6 current carrying conductors in new 1/2 EMT in-wall going thru crawl space

Once again, we start off the top:

Derating factor for 6 CCCs is .80

Derating factor for ambient temperature for relevent conductor group (THHN in conduit ("raceway")), assuming 90-104F is good, is .91 for 90C THHN copper

Then, the breaker is listed at 75C for copper. So, do we start with the THHN (90C) column since we're using THHN, or with the 75C column since we're using the breaker listed at 75C? If it's the 90C column, we go 12AWG -> 30AMP in table (Ugly's 2008 p73) * .80 (6CCCs) * .91 (96-104F) = 21.84AMPS, so the 20AMP CAFCI is good. But, if it is the 75C column, we go 12AWG -> 25AMP in table * .80 (6CCCs) * .94 (96-104F) = 18.8 AMPS, meaning the max CAFCI amperage is 15AMPs unless we up the AWG (see above for upping AWG)? If we up the AWG to 10AWG copper (which we'd only do if we needed to), then starting from 75C it goes 35AMP from table * .80 (6CCCs) * .88 (104F) = 24.64AMPs, so 20AMP CAFCI is OK. OR, am we supposed to use 90C (40AMP in table) * .80 (6cccs) * .91 (104F) = 29.12AMPs, 20AMP is ok. Is 104F good for San Jose, California, or do we use 105-113F (!)? This is crawl space and interior, so no chance of going above in shade outside ambient temperature at 6 feet height plus about 3degF for stupid tenants who can't open and close windows at the right times of day. Let's be pessimistic and say the code people say we have to use 113F (!) with 75C column for the 75C breaker, so for 10AWG that would be 35AMP in table * .80 (6cccs) * .82 (113F) = 22.96AMPs, so definately good there. Going to 10AWG copper 6 current carrying conductors (+ 3 grounds = 9 which is the max) would be a pain in the rear end for the 1" PVC, and damn near impossible for the 1/2 EMT since it says max 10 AWG copper wires is 5, so we'd have to remove one circuit from that bring it to a 4CCCs circuit so the equation would still be .80, and so that'd be pessimistically 75C terminator 35AMP from table * .80 4cccs * .88 104F = 24.64AMPs, or if we use 105-113F, * .82 = 22.96Amps.

Possibility 3:

Above example Bryant load center with an old Bryant breaker listed at 40C with above example conduit and wiring. There's no 40C column for wire in Ugly's 2008 page 73.


The questions I have are simple:

1. How do we calculate the derating(s) for a circuit which is connected to a 40C rated breaker? (Bryant BR 2020 Type BRD, Bridgeport, U.S.A., NI circuit breaker, Insp. Issue FP-63, E32424 120/240 VAC 2 pole unit, 40 degrees C)

2. How do we calculate the derating(s) for a circuit which is connected to a 75C rated breaker (copper)? Must we use the 75C column in the table for THHN even though THHN is in 90C column?

3. Do we use 96-104F, or 105-113F, in San Jose, California? San Jose is a valley, meaning no coastal fog, but it is the first range in, so it isn't as hot as the San Juaquin Valley which is two mountain ranges in and quite a bit hotter.

4. Rounding. After applying the deratings, do we round? If not, then 19.99AMPS=15 AMP breaker. If yes, then how? If we get an AMP number of 17.60 AMPS, do we round down to 15AMPs breaker or up to 20AMPs breaker? If we get 19.2 AMPS, do we round down to 15 AMPs breaker or up to 20AMPs breaker? If we get 16.4 AMPS, do we round down to 15 AMPs breaker or up to 20AMPs breaker?


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## Magnettica (Jan 23, 2007)

> How do we calculate the derating(s) for a circuit which is connected to a 75C rated breaker (copper)? Must we use the 75C column in the table for THHN even though THHN is in 90C column?



You're going to use the 90º column for derating only, unless everything (circuit breakers, terminals, insulation) is also rated for 90º. Most stuff is not rated for 90º so THHN ampacity rating falls into the 75º column. 



> Rounding. After applying the deratings, do we round? If not, then 19.99AMPS=15 AMP breaker. If yes, then how? If we get an AMP number of 17.60 AMPS, do we round down to 15AMPs breaker or up to 20AMPs breaker? If we get 19.2 AMPS, do we round down to 15 AMPs breaker or up to 20AMPs breaker? If we get 16.4 AMPS, do we round down to 15 AMPs breaker or up to 20AMPs breaker?


Round up for your overcurrent protection size for everything up to 800 amps. 240.4 (B)


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## BWA (Jan 20, 2010)

Magnettica said:


> You're going to use the 90º column for derating only, unless everything (circuit breakers, terminals, insulation) is also rated for 90º. Most stuff is not rated for 90º so THHN ampacity rating falls into the 75º column.
> 
> 
> 
> Round up for your overcurrent protection size for everything up to 800 amps. 240.4 (B)


Thank you! That definitely cleared this up a lot. (P.S., whew, everything's good.) So for the 1/2" EMT with 6 current carrying conductors of type THHN 12AWG copper, I go to 90degC column for the THHN 12AWG copper, *.80 4-6 CCC's, and *.87 for a conservative heating of 113F in walls & crawl space of living space, yielding 20.88AMPS, so 20AMP breaker is OK with respect to the wires in conduit. Then, with respect to those already-known-to-be-OK-wires-in-conduit, those wires connected to a breaker rated at 75 degress C for copper, I would use the 75C column, match the size and metal type of wire that I already OK'd for the conduit part of the run, which is 12AWG copper, and see that I'm going to be OK at 25 AMPS with my 20 AMP breaker.

Regarding 240.4(B), I cannot use that, because it won't apply to my situation because I don't meet the first criteria: this is definately a multioutlet branch circuit with cord loads; it's a typical residential scenereo.

Elsewhere I read about 110.something or similar # that says if you don't know the ratings of the terminating equipment, you use the 60C column in some cases, the 75C column in other cases. So, even in that scenereo, 12AWG copper brings us to 25AMPs in both 60C and 75C columns, so we're good there too.

This still leaves the question: a 40C rated breaker means what? If I rub out the "40C" on the breaker, then do I get to use the 101. or 110 or whatever it is and declare it "unknown" and just use 60C conservatively (only this one sentence is sarcastic)? There's no 40C column or footnotes in my Ugly's.

I tried getting NEC 2008 online, and since I change computers every 6 months, that's a no-go. The library is closed, and Amazon delivers in under two weeks. Sigh.


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