conduit fill for NM cable

[Ponchik]

Note 9 of Ch.9 states to treat the cable as one conductor.

When doing conduit fill calculation say for (2) 12-2 NM cables, the two cables are considered as (2) individual conductors and use the 31% column or treated as 4 conductors and use the 40% column?

TIA

 

[winnie]

Each separate cable gets treated as a single conductor. So with 2 cables you would calculate area as 2 individual conductors. The conduit is going to get big fast.

Say for 12/2 NM-B cable you would calculate conduct fill as if you had two conductor each 0.4" in diameter.

 

[Dennis Alwon]

Is the conduit a raceway or is it just a sleeve?

 

[Dennis Alwon]

The reason I ask is Note 2

• 
  Table 1 applies only to complete conduit or tubing systems and is not intended to apply to sections of conduit or tubing used to protect exposed wiring and cable from physical damage.

 

[Ponchik]

Each separate cable gets treated as a single conductor. So with 2 cables you would calculate area as 2 individual conductors. The conduit is going to get big fast.

Say for 12/2 NM-B cable you would calculate conduct fill as if you had two conductor each 0.4" in diameter.

so you would use the 31% column to calculate the fill?

 

[Ponchik]

Is the conduit a raceway or is it just a sleeve?

Let's assume it is complete system and not a sleeve,

It is not a real installation, I was reading through the ch9 and this hypothetical came to mind.

 

[Dennis Alwon]

You do know that with a complete system you wouldn't use nm cable, however if you did then 31%is correct for 2 conductors

 

[Ponchik]

You do know that with a complete system you wouldn't use nm cable, however if you did then 31%is correct for 2 conductors

Yes I know.

 

[infinity]

so you would use the 31% column to calculate the fill?

Yes, if you had one cable it would be 53% fill. More than 2 cables 40%. Raceway 24" or less 60%.

 

[Carultch]

Yes, if you had one cable it would be 53% fill. More than 2 cables 40%. Raceway 24" or less 60%.

For 2 conductors in general, this is a blindspot in the NEC, since two circles have a lot of empty space when packing in another circle, and that's where the 31% special case comes from. Following the logic of the 60% rule, I estimate that the equivalent limit for 2 wires should be around 45%.

The general rules are about 75% diametral fill, and the 60% rule is about 90% diametral fill for 3+ wires. So 2 wires would have this diametral fill at 45%.

What about oval packing? If you can still meet the 60% rule with just 2 oval-shaped cables, go for it. The fact that ovals pack better than circles, should allow you to take full credit for the 60% nipple rule. In a longer raceway, you have a lot less control over how they arrange, so in that case, you account for the full virtual circle of the larger "diameter" of the oval.