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1. You do not derate from the 30A figure. You use the actual temperature rating of the wire, which is the 90 deg C column for new THHN conductor NM cable.

#10 NM cable is derated from 40 amps. And you do not derate by 35%. You derate TO 35%.
Even at a 35% adjustment you CAN use a 15A breaker since we round UP to the nearest standard breaker size.
40 x 35% = 14A = 15A breaker

2. Originally Posted by TedL
Sounds like some bad math.#10 would be 30 amp without derating. 30x.65=19.5 amps, which would allow a 20 amp breaker.

It is 35% not 65%

3. Originally Posted by Speedy Petey
15A breaker since we round UP to the nearest standard breaker size.
40 x 35% = 14A = 15A breaker
Thanks Speedy Petey that rounding up rule is one I forget once in a while.

Still the caculation does come out to less than 15A. I still say the wiring routes taken are very bad practice.

4. Originally Posted by codeone
According to 310.15(B)(2)(a) more than 41 current carrying conductors have to be derated by 35% Which means you could not even put a 15A breaker on those #10's. in the first picture on the left if I counted correctly. Hard to see actually how many conductors are there.
I didn't dig out my copy of the code, but took Codeone's statement "by 35%" at face value. I just did the math check.

I did remember the "next larger standard overcurrent device"
.

5. Just for debate we need to look at Article 334.80 of the NEC . Looks like it rules out the next highest breaker with two words.

334.80 Ampadty. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60°C (140°F)
conductor temperature rating. The 90°C (194°F) rating
SHALL be permitted to be used for ampacity derating purposes,
PROVIDED the final derated ampacity does not exceed
that for a 60°C (1400 P) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11

Now I'm not saying some AHJ's dont allow this, however it is something to consider. It may be just bad wording to give the intent. That happens sometimes.

6. ## math

There is a BIG difference between derating BY 35% and derating TO 35%, and it appears most of the "mathemeticians" here are derating TO 35%. I might not want them retagging sale merchandise if it is supposed to be 35% OFF the original price, and they tag it as 35% OF the original price.

7. English can be hard to follow sometimes.

Heres the derating chart.

Table 310.15(B)(2)(a) Adjustment Factors for More Than
Three Current-Carrying Conductors in a Raceway or Cable
--------------------------------------------------------

Percent of Values in Table
310.16 through 310.19 as
Temperature if Necessary

---------------------------------------------------------
4-6 = 80
7-9 = 70
10-20 = 50
21-30 = 45
31-40 = 40
41 and above = 35

8. Well, 240.4 would seem to allow it (rounding up to the next larger standard overcurrent device), but I'm working from the 2002 NFPA pocket guide for residential installations.

Notice the title of the table Codeone posted.

"Percent of Values in Tables 310.16 through 310.19 as Adjusted"

It requires the conductors to be reduced to a maximum value OF the percentages found in Tables 310.16 through 310.19

The text of 310.15(A)(2) makes the statement that the lowest ampacity from a calculation is the ampacity of that entire conductor. There is also an exception to this rule if the distance is no more than 10 feet or 10% of the entire run.

We must also look at 334.80 for guidance when installing Non-metallic Cable.
In the last sentence of the second paragraph it clearly states that the exception to 310.15(A)(2) does not apply’

41 or more current carrying conductors of 12 American Wire Gauge would have a maximum ampacity of 10.5 amps.

Now without a doubt there are a lot of folks out there that will start screaming about 240.4(B) where an allowance is made to allow the next higher overcurrent device protect a conductor as long as the overcurrent device does not exceed 800 amps.

What these folks forget to do is read the entire section. There are three requirements that this conductor must adhere to in order to be allowed to go to the next higher overcurrent device.
The one which catches my eye and holds my attention is 240.4(B) (1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

So if these #12 are supplying receptacles then There is no conductor in either of these pictures on either thread that can supply receptacles anywhere in that building.

10. Originally Posted by jwelectric
The one which catches my eye and holds my attention is 240.4(B) (1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
Yup, you are right. I for one was just speaking from memory. I didn't go re-read the section.
To be honest I have never had to apply it in this way so it never occurred to me.

BTW- I was not screaming.

11. Originally Posted by codeone
Just for debate we need to look at Article 334.80 of the NEC . Looks like it rules out the next highest breaker with two words.

334.80 Ampadty. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60°C (140°F)
conductor temperature rating. The 90°C (194°F) rating
SHALL be permitted to be used for ampacity derating purposes,
PROVIDED the final derated ampacity does not exceed
that for a 60°C (1400 P) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.11

Now I'm not saying some AHJ's dont allow this, however it is something to consider. It may be just bad wording to give the intent. That happens sometimes.
I have to say, I completely disagree with this interpretation. I don't feel this has any bearing on the situation. The final outcome is LESS that the 60 deg C column, not exceeding it.

Bottom line is JW is right. If these circuits are for receptacle circuits then 240.4(B)(1) rules it out, nothing else.

12. Originally Posted by Speedy Petey
I have to say, I completely disagree with this interpretation. I don't feel this has any bearing on the situation. The final outcome is LESS that the 60 deg C column, not exceeding it..
Lets say I have 9 #12's That would be 35A x 70% = 24.5A If by your deff. this exceeds the 60 deg C column.

Now lets say I have 10 #12'S That would be 35A x 50% = 17.5A So this doesnt exceed the 60 deg C column. So can I go to a 20A breaker?

Now lets say I have 41 #12'S That would be 35A x 35% = 12.5A So can I go to a 15A or 20A breaker.

Or in these cases derating is to know weather you need to go to the next bigger wire size to carry the ampacity of your circuit to keep from overheating.

In the case of the #10'S you would still have overheating. And need to go to a #8.
Also exceeding means it will not have the ampacity needed.

Yes JW' s answer seals the case.

13. Well it doesn't much matter as it all has to come out and be replaced. It should be replaced with MC cable which should have been used in the first place

14. Ah yes! Would be the better choice of materials. Derating not as much of a problem. Safer for the public!

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