Current carrying conductors?

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petey_c

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A question was posed to me yesterday about de-rating conductors and conduit fill. The question was, three 240VAC circuits, and three 120VAC circuits (and grounds) in EMT. They calculated 9 current carrying conductors. I said twelve: six for the three 240VAC circuits, three for the 120VAC hots and three for the neutrals. The conduit fill I think we worked out (3/4" EMT, 16 permissible, 9 as a "magic" number). I was sure that this had been covered before but, when I tried to do a search for "current carrying conductors" (in quotes) it said the words were too common. Thanks, Pete
 
petey_c said:
A question was posed to me yesterday about de-rating conductors and conduit fill. The question was, three 240VAC circuits, and three 120VAC circuits (and grounds) in EMT. They calculated 9 current carrying conductors. I said twelve: six for the three 240VAC circuits, three for the 120VAC hots and three for the neutrals. The conduit fill I think we worked out (3/4" EMT, 16 permissible, 9 as a "magic" number). I was sure that this had been covered before but, when I tried to do a search for "current carrying conductors" (in quotes) it said the words were too common. Thanks, Pete
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I count 12 CCCs here also.

FYI, the magic number of 9 CCCs only works for #14 and #12 IIRC.
 
If each of your 120v circuits has it's own neutral, I count 12 also.
In most cases, if the 3 120v circuits shared a common neutral then you would only have 9 ccc's as the shared neutral would only carry the unbalanced load. Be sure to check the specifics in 310.15(B)(4) {'08} before applying this in general.
 
If this is a 1?, 120/240 volt system the best you can do is 10 CCC's. You could share one neutral with two of the 120 volt circuits.
 
infinity said:
If this is a 1?, 120/240 volt system the best you can do is 10 CCC's. You could share one neutral with two of the 120 volt circuits.
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Thanks. Bob. Should have differentiated between 3 phase and single phase. I'm really off my game today. Better quit posting.
 
Hi all:

My first post after lurking forever...please be gentle. :)

I'm not sure how to include test from a prior post other than to cut and paste... but, as "jumper said:

"FYI, the magic number of 9 CCCs only works for #14 and #12 IIRC..."

My question is: what is the "magic number" you referr to. Are you referring to the discrepancy between the values arrived at Chapter 9, table 4 and those of Annex C?

Thanks

T-minus 22 days till test day -(NC limited)~!
 
Welcome Scott. The magic number 9 refers to the number of conductors you can install in a conduit or group together before it affects the ampacity of a circuit. Table 310.15(B)(2) tells us we must de-rate 70% for 9 ccc. If #12 is good for 30 amps then 70%X30 is 21amps. This does not affect the situation. 10 CCC would make it 50% and a 15 amp circuit.

BTW Scott my PC is in the hospital with a virus now......
 
Dennis - I was thinking this was similar to what we were discussing at the Electrical Institute last week. The way you can come out with different values for conduit fill based on whether or not one uses Ch.9 T4 or Annex C.

But, I'm still confused. Anything over 3 conductors affectes the ampacity. 310.15 (B)(2) starts from there...over 3 CCC's. I still don't get the "magic number." jumper was saying: "we worked out (3/4" EMT, 16 permissible, 9 as a "magic" number). " I calculate 16 permissible using both Ch.9 T4 and Annex C (#12THHN's - 3/4"EMT)...so, what is special about 9?

Good luck on the PC...
 
9CCC does not affect conduit fill. It affects the de=rating that must be done when you have more than 3. If you are using #12 or #14 wire then the ampacity is not affected till you get over 9 CCC because the 90C rating of those conductors are 10 amps higher than the actual usable ampacity.

As I stated earlier #12 with 9 CCC is 21 amps so the ampacity of the conductor can still be allowed with a 20 amp breaker. Using #14 at 25 amps with 70% de-rating you may still use a 15 amp breaker.

Once above 9CCC with #12 and 14 awg you must change to a smaller over current protection device(OCPD). This is why 9 is magical for these wire sizes
 
Hello Scott, one other thing to note is that the "magic number" does not work if your initial derating of a conductor does not start at the ampacity in the 90C column.


Final ampacities must also meet the limitations of 110.14(C).
 
...9CCC does not affect conduit fill. It affects the de=rating that must be done when you have more than 3.

ah haaaaaaaaa! I finally see. I was getting tunnel vision and not discerning between conduit fill and de-rating. All is clear now. It's an interesting point made about the magic number! Thanks!
 
sbrehler said:
...9CCC does not affect conduit fill. It affects the de=rating that must be done when you have more than 3.

ah haaaaaaaaa! I finally see. I was getting tunnel vision and not discerning between conduit fill and de-rating. All is clear now. It's an interesting point made about the magic number! Thanks!
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I love the feeling when the light bulb finally clicks in my head when I am trying to understand something. Glad we could help and good luck on the exam.
 
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