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Derating Bundled NM Cable (Romex)

Short answer: yes, you need to count all the current carrying conductors in each cable. I guess as an inspector, you could assume all the circuit conductors are current carrying and put the burden on the inspectee to demonstrate otherwise if applicable.

Cheers, Wayne
 
On 12/2 or 14/2, you count the nuetral and the hot, but not the ground wire because it is not current carrying. That means two conductors per cable. If you have 12/3 or 14/3, the neutral doesn't count either because it only carries unbalanced load from the two hots, so you only count the two hot wires - meaning that you still have two conductors per cable.

The TL;DR is that if you have more than 4 cables bundled of 14/2, 14/3, 12/2, or 12/3, you will have eight current carrying conductors, and that will be a bundling violation unless you do something unorthodox with your overcurrent protection.
 
BN4537 said:
On 12/2 or 14/2, you count the nuetral and the hot, but not the ground wire because it is not current carrying. That means two conductors per cable. If you have 12/3 or 14/3, the neutral doesn't count either because it only carries unbalanced load from the two hots, so you only count the two hot wires - meaning that you still have two conductors per cable.

The TL;DR is that if you have more than 4 cables bundled of 14/2, 14/3, 12/2, or 12/3, you will have eight current carrying conductors, and that will be a bundling violation unless you do something unorthodox with your overcurrent protection.
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So that gets adjusted off of the 90 degree column?
 
The derating factor is based on the ampacity in NEC Table 310.16. This may be higher than the breaker rating if the insulation is rated over 60° C. You can have (6) #14 75° C conductors or (9) 90°C conductors and still use a 15 A breaker.
 
jar546 said:
So that gets adjusted off of the 90 degree column?
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Yes. You adjust the ampacity based on the value in the 90 degree column as you are allowed to do per 334.80. If the resulting number will still work with a breaker sized off the 60 degree column, you are good.

Example: You have 5 cables of 14/2 NM cable bundled together. That would be 10 current carrying conductors. Table 310.15(C)(1) (2023 NEC) states that if you have between 10-20 conductors, you reduce ampacity to 50%. 14 awg wire is 25 amps in the 90 degree column. You take 25 amps and multiply it by the 50%, which equals 12.5 amps of current capacity. The 60 degree column says that 14 awg should be protected at 15 amps.

Since the 12.5 amps of capacity we calculated is less than the 15 amps in the 60 degree column, it would be a violation to put 5 bundled 14/2 NM cables on a 15 amp or larger breaker, because 334.80 says that NM cable must be protected at it's 60 degree capacity, even though it's listed as 90 degree wiring.
 
BN4537 said:
If you have 12/3 or 14/3, the neutral doesn't count either because it only carries unbalanced load from the two hots, so you only count the two hot wires - meaning that you still have two conductors per cable.
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That's true if your 12/3 or 14/3 is carrying an MWBC, or if two of the conductors are travelers in a lighting circuit (per the footnote to Table 310.15(C)(1)). But if you had, say, a neutral, a constant hot, and a switched hot, then that counts as 3 CCCs. [Even if both hots are protected by the same breaker, which is a flaw in the NEC that for some reason the NFPA did not want to rectify when I proposed a change in the footnote to Table 310.15(C)(1).]

Paul Sweet said:
The derating factor is based on the ampacity in NEC Table 310.16. This may be higher than the breaker rating if the insulation is rated over 60° C.
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NM-B cable is required to have 90C rated insulation, so when derating modern NM cable, you can always use the 90C column as your starting point.

BN4537 said:
Since the 12.5 amps of capacity we calculated is less than the 15 amps in the 60 degree column, it would be a violation to put 5 bundled 14/2 NM cables on a 15 amp or larger breaker
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Not necessarily. It's a violation if the circuit is supplying more than one receptacle for cord-and-plug connected portable loads. If it is not, then 240.4(B) applies, and it's fine to protect 12.5A ampacity conductors at 15A.

Cheers, Wayne
 
Words matter.
(B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above theampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.

There is two ways to interpret (1). It could be assumed that a conductor could not supply a branch circuit with more that one receptacle.

It could also be assumed that any load other than a single receptacle is likewise, not permitted.

It's the words "more than" that leads to the conundrum. Multiple receptacles is obviously more than 'one receptacle' but so is a range, an AC condenser, etc.
 
ICE said:
There is two ways to interpret (1). It could be assumed that a conductor could not supply a branch circuit with more that one receptacle.
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I think the wording is pretty clear. If the branch circuit supplies more than one receptacle for portable cord and plug connected loads, you may not use 240.4(B). If it does not, it passes 240.4(B)(1) and you may proceed to check the remaining requirements. Note that multiple cord and plug connected fixed-in-place loads are fine, only receptacles for portable loads are an issue.

Note also for the 12.5A ampacity #14 NM example under discussion, I should also have stated that the calculated load needs to be at most 12.5A, per 210.19(A)(1)(b). And for 210.19(A)(1)(a), the continuous portion of the load needs to be at most 15A / 125% = 12A.

Cheers, Wayne
 
wwhitney said:
I think the wording is pretty clear.
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Hold it up to a mirror. The wording is clear to both trains of thought. The arcane premise that multiple receptacles that are open to use by portable loads is somehow a greater concern than a load of unknown description raises questions.... as in perhaps that's not where they went with this.
 
ICE said:
The arcane premise that multiple receptacles that are open to use by portable loads is somehow a greater concern than a load of unknown description raises questions.... as in perhaps that's not where they went with this.
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Not following--the "portable cord-and-plug connected loads" are the unknown loads. Any fixed-in-place cord-and-plug connected loads, or hardwired loads, are known and need to be reflected in the Article 220 load calculation, and hence 210.19(A)(1)(b) requires they not overload the circuit. It is the unknown portable cord-and-plug connected loads that are a potential issue.

For whatever reason, 240.24(B)(1) says that one of those is OK, while two or more are not. The choice could just as well have been made to not allow any such loads, but it wasn't.

So the use of "more than one" in 240.24(B)(1) definitely means "two or more," and not "anything other than just one of the following type."

Cheers, Wayne
 
I would ask Grok but I don't want to crash Elon's mainframe. I would be willing to bet that the wording could be improved.
 
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