Ampacity for fun

Old 01-12-02, 02:48 PM
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Ampacity for fun

This question is purely academic. The situation I present does not actually exist. I'd just like to discuss how such calculations are made.

Hypothetically, I want to put a bunch of 12/2 NM-B cables through a 2" EMT conduit. The first question is how many 12/2 cables I'm allowed to put through this 2" EMT.

We head to Chapter 9 of the 2002 NEC for the answer. Table 4 says that a 2" EMT conduit has a total area of 3.356 square inches. Table 1 says that I can only fill this cable to 40% if I have over 2 conductors. Back to Table 4 says that 40% of the conduit is 1.342 square inches.

12/2 NM-B cable is 1/2" wide. Note (9) says I must treat this as a 1/2" round cable. Using pi*r*r, I compute the cable has a cross section of 0.196 square inches. Dividing 1.342 by 0.196 says I can put 6.83 cables in this conduit. Note (7) tells me that since the decimal is above 0.8, I can round this up to 7.

So the answer to the first question is seven. So far so good?

Now the second question is what size breaker I'm allowed to put on these seven 12/2 cables. So we go to Section 310 of the 2002 NEC.

Table 310.15(B)(2)(a) gives me adjustment factors for more than three current-carrying conductors in a raceway. Since I have seven 12/2 cables, I figure I have 14 current-carrying conductors. The table tells me that for 10 to 20 conductors, I just use only 50% of the ampacity determined by tables 310.16 through 310.19.

So now I head for table 310.16. This table tells me that THHN wire (which is what 12/2 NM-B is made from) has an ampacity of 30 amps when I have not more than three conductors. I will assume an ambient temperature of 86 degrees F so that I need not make an ambient temperature correction. Taking 50% of the 30 amps gives me 15 amps. Since 15 amps is less than the 60 degree rating of 12-gauge copper, I will use the 15 amps.

So the answer to the second question is 15-amp breakers.

Did I do this correctly? Were there any factors or exceptions that I overlooked?
Old 01-13-02, 04:00 AM
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John, I am not amazed or surprised in any manner. I truly expected you to be that accurate in your calculation. Loved the break down and can think of no other codes that you would have to hit. Looks perfect to me.

Great job

Old 01-13-02, 12:10 PM
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What about a raceway within a raceway?

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