Conductors in a raceway

[joeyww12000]

Can someone clarify for me NEC regulation on conductors in a raceway. The code states that you cannot have more than 3 current carrying conductors in a raceway unless you derate. Does a nuetral count as current carrying? I would think so.... If so then you could only have one circuit per conduit which makes no sense so obviously my interpretation is wrong. Exactly what are the facts on this subject. When I look at my Uglys book I find the conduit fill for 3/4 EMT THWN is 16 #12 THWN conductors. Is conduit fill and the above statement of no more than 3 current conductors in a raceway not the same subject matter? Just a little confused.

 

[Dennis Alwon]

Read art. 310.15(B)(4) Sometimes they count and sometimes they don't. If it is a 2 wire circuit the neutral will count in a MWBC they often don't count but they can depending on the load being served.

 

[Dennis Alwon]

Also look at Table 310.15(B)(2)(a).

 

[infinity]

First you need to remember that you'll begin your derating from the 90 degree C column if you're using a 90 degree C conductor like THHN. So look at this example:

#12 THHN = 30 amps (310.16)
9 CCC in a raceway requires a 70% derating value
30 amps * 70%= 21 amps

So you can see that although there are 9 current carrying #12 conductors in the raceway they're derated ampacity is still above 20 amps. That allows you to use a 20 amp OCPD.

Another example:

3 sets of A/B/C/N/G-#12 THHN in a 3/4" EMT from a 208Y/120 volts system

Although this is 15 conductors only 9 are considered CCC so with their derated value of 21 amps that can still be protected by a 20 amp OCPD. As you can see this is how you can have a raceway that is one conductor short of maximum fill and where the derating plays no part in changing the final ampacity of the 9 circuits installed.

 

[jeremysterling]

Rob,

Thank you very much for clearing up my confusion about Table 310.15(B)(2)(a). I, too, have struggled with how to calculate this and was frustrated like the OP. I had given up and decided that 2 boats were enough per pipe and I'd be OK. Now I see how to use the table.

Like the OP, I was trying to figure how to knock out 42 3/4" holes in a panel!!!

I heard an inspector say, "Big pipe means big wire, not more wire." He's partially right. But now, I can show him the math!! Thanks again.

 

[iwire]

Nice post Rob.

What he posted holds true for 14, 12 and 10 AWG in most cases.

However things change rapidly as you go up in conductor sizes. Don't start running feeders together.

Lets say you have two, three phase, 200 amp feeders running to the same place. Run separately 3/0 CU will do the job. Combine them and you would need 250 Kcmil CU.

 

[peter]

Horizontal Eight,
"3 sets of A/B/C/N/G-#12 THHN in a 3/4" EMT from a 208Y/120 volts system... Although this is 15 "
Excellent explanation, but just a quibling point: Usually just a single Ground wire would be pulled in this situation; thus resulting in a total of 13 wires.
~Peter