A few questions on adjustment factor of Romex while bundled

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sparkycoog

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This is a 3 part question so to help make my questions clearer, I put the main question in bold and tried to separate the question to make things easier. If it did the opposite, I apologize in advance.


Art. 334.80 allows only 2 wires in a bored hole where thermal insulation, firecaulk, etc will be used-before we have to apply the adjustment factors in table 310.15(B)3a . That part is easy, but at what point, are other NM wires bundled together required to have adjustments? Is it when the bundle essentially becomes a raceway - ie. exceeds 24 inches without air circulation?

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When calculating the adjustments, is there a time when the neutral conductors not counted? The rule on Neutral conductors states that a the neutral may or may not be a current-carrying conductor. In accordance with 310.15(B)(4)(a), a neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a). But, if the neutral is part of a three-wire circuit consisting of two phase conductors and the neutral conductor and it is supplied from a four-wire, three-phase, wye-connected system, the neutral must be counted as a current-carrying conductor.
The first part of that statement confuses me because unless we have a perfectly balanced circuit, there is never a time when a neutral conductor is NOT carrying current. What am I missing here?

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The actual calculation confuses me a little too. In particular, what the role of ambient temperature is here. I've read that Romex would be treated as having a rating of 90 degrees C. How exactly is that role used? Is it to use Ambient Temperature Corrections from table 310.15(B)(2)(a) the way it would be for a regular conduit, or is used to treat the conductors based on the 90 degree C ratings in table 310.104(A)? ie. each current carrying conductor in 12/2 Romex is now rated for 30 amps?

In other words, suppose you had 20 12/2 romex wires bundled in an airtight situation over 24 inches long (if that is in fact the trigger to apply adjustments), would each conductor now be 30 amps x 40% = 12 amps?


 
Dennis Alwon

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sparkycoog said:
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Art. 334.80 allows only 2 wires in a bored hole where thermal insulation, firecaulk, etc will be used-before we have to apply the adjustment factors in table 310.15(B)3a . That part is easy, but at what point, are other NM wires bundled together required to have adjustments? Is it when the bundle essentially becomes a raceway - ie. exceeds 24 inches without air circulation?
Click to expand...
No where does it state that you cannot install more than 2 nm cables thru a bored hole with thermal insulation. It does say, however, that you must derate over 3 current carrying conductor's. So If I install 4 12/2nm cables thru a bored hole with insulation I must derate. Nm cable is rated 90C so I can use 30 amp to start my derating. Table 310.15(B)(2)(a) or (3)(a) deping on what version of the NEC you are using, states that 8 current carrying conductor's must be derated 70%. 4- 12/2 nm is equal to 8 current carrying conductor. 30 amps times 70%= 21 amps so you could have 4 12/2 nm cables thru that hole.

And yes bundling starts at 24" length except that it starts immediately thru a bored hole where thermal insulation, chalk, etc is used.
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Dennis Alwon

Dennis Alwon

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sparkycoog said:

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When calculating the adjustments, is there a time when the neutral conductors not counted? The rule on Neutral conductors states that a the neutral may or may not be a current-carrying conductor. In accordance with 310.15(B)(4)(a), a neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a). But, if the neutral is part of a three-wire circuit consisting of two phase conductors and the neutral conductor and it is supplied from a four-wire, three-phase, wye-connected system, the neutral must be counted as a current-carrying conductor.
The first part of that statement confuses me because unless we have a perfectly balanced circuit, there is never a time when a neutral conductor is NOT carrying current. What am I missing here?
Click to expand...


A neutral will carry current on a balanced load but the amount of current carried by the 3 conductors on a multiwire branch circuit will never be higher than the 2 hot conductors.

Suppose A phase of that cir carries 15 amps and the "B" phase carries 12 amps. The neutral, as you know, will carry 3 amps. So lets add that up -- 15 + 12 + 3= 30 amps.

Now supposed both cir of the multiwire branch circuit carried 20 amps. Neutral has 0 amps and the other two conductors have a total of 40 amps. This will always be the max for a 20 amp multiwire branch circuit. In the previous example we had less amps and at no time will it be greater than 40 so in a sense the neutral doesn't count as a current carrying conductor even though it may carry current.

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Dennis Alwon

Dennis Alwon

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texie said:
I agree, but then you have the folks that want to invoke the 10% of the length rule where applicable. Do you think this is valid?
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I think it should be allowed but look at 334.80 and it states the provisions of 310.15 (A)(2) exception shall not apply
 
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GoldDigger

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Dennis Alwon said:
I think it should be allowed but look at 334.80 and it states the provisions of 310.15 (A)(2) exception shall not apply
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It looks like that is written to apply specifically to the firestop provision and therefore does not affect the "in thermal contact with insulation" derating in the same section. It seems reasonable to infer from that that it also does not affect the closely spaced NM bundle limitation.
So to the extent that the length of that circuit length (undefined, by the way) exceeds 240" the 24" limitation is proportionally increased too, but not to exceed ten feet.
 
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infinity

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One other note for derating NM cables through fire stopped or caulked holes the derating begins when you have three cables or more not 3 CCC's. It's possible to have two cables with 6 CCC's and no derating required. Same thing would apply to cables contacting insulation.

334.80 Ampacity.
Where more than two NM cables containing two or
more current-carrying conductors are installed, without
maintaining spacing between the cables, through the same
opening in wood framing that is to be sealed with thermal
insulation, caulk, or sealing foam, the allowable ampacity
of each conductor shall be adjusted in accordance with
Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2),
Exception, shall not apply.
Where more than two NM cables containing two or
more current-carrying conductors are installed in contact
with thermal insulation without maintaining spacing be-
tween cables, the allowable ampacity of each conductor
shall be adjusted in accordance with Table 310.15(B)(3)(a).
Click to expand...
 
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