NEC Chapter 9 Table 1

[FaradayFF]

Hi guys,

This might be a stupid question, but for the 1 wire column in Chapter 9, Table 1 for allowed conduit percent fill, does 1 wire represent literally 1 conductor or can it be a 1 multiconductor cable?

Thanks,
EE

 

[LarryFine]

This might be a stupid question, but for the 1 wire column in Chapter 9, Table 1 for allowed conduit percent fill, does 1 wire represent literally 1 conductor or can it be a 1 multiconductor cable?

Not stupid at all, and yes, it can be a single cable. An oval cable should be treated as a round cable equal to the larger dimension.

 

[charlie b]

Welcome to the forum, FaradayFF. I agree with Larry that here, as in most situations, there are no stupid questions.

Please allow me to offer a challenge (and others, please let the OP answer first). Can you tell us why the allowable fill area for 2 wires is less than the allowable fill area for 3 conductors?

 

[FaradayFF]

Welcome to the forum, FaradayFF. I agree with Larry that here, as in most situations, there are no stupid questions.

Please allow me to offer a challenge (and others, please let the OP answer first). Can you tell us why the allowable fill area for 2 wires is less than the allowable fill area for 3 conductors?

Thanks, Charlie.
I would say it has to do with jamming ratio and more than 2 conductors in a conduit more closely approximating a round cable in shape, which should be easier to pull, thus allowing more fill area per conduit.

 

[FaradayFF]

Not stupid at all, and yes, it can be a single cable. An oval cable should be treated as a round cable equal to the larger dimension.

Thanks for the reply, Larry. I actually meant Table 2..

 

[charlie b]

I would say it has to do with jamming ratio and more than 2 conductors in a conduit more closely approximating a round cable in shape, which should be easier to pull, thus allowing more fill area per conduit.

It does have to do with jamming ratio. The limit for 2 conductors is lower because it is harder to pull 2 conductors than it is to pull 3 conductors, and that is because 2 conductors are more likely to create a jam.

 

[Carultch]

It does have to do with jamming ratio. The limit for 2 conductors is lower because it is harder to pull 2 conductors than it is to pull 3 conductors, and that is because 2 conductors are more likely to create a jam.

Jamming is an issue that comes with 3 wires, rather than 2. It has to do with being unlucky enough for the conductors to form a straight line, instead of packing in a triangle formation. It is especially likely to happen if the middle wire pushes down on the two outer wires, and spreads them toward this straight line. If the diameters add up to the inner diameter or a little larger, it is likely to happen.

The reason 2 wires have a smaller allowable fill ratio, is that 2 is a special case of "circle packing in a circle", where the case of 2 circles is the least efficient at filling the outer circle.

Circle packing - Wikipedia

en.wikipedia.org

The fill limits come from the virtual circle that encloses the wires, being about 75% of the diameter of the conduit interior.

 

[Djelite]

Why does mike’s authors comment say that chapter 9 table 1 applies a 40% limitation on conductors of DIFFERENT size...... understanding the nec 2017 page 346 article 300.17

 

[don_resqcapt19]

Why does mike’s authors comment say that chapter 9 table 1 applies a 40% limitation on conductors of DIFFERENT size...... understanding the nec 2017 page 346 article 300.17

While that table applies to any combination or one or more conductors in a raceway, he probably says that because, in most cases you will use the tables in Annex C for the condition where the conductors are of the same size. Where the conductors are of different sizes you have to do a calculation, based on other tables in Chapter 9.

 

[Djelite]

Ok that’s still not chapter 9 table 1. Conductors of different size are calculated with table 5 chapter 9

 

[don_resqcapt19]

Ok that’s still not chapter 9 table 1. Conductors of different size are calculated with table 5 chapter 9

You still have to use Table 1 along with Tables 4 and 5. Table 1 sets the actual percentage fill rule, Table 4 gives you the area of the raceways, and Table 5 gives you the area of the conductors.

 

[tortuga]

Not stupid at all, and yes, it can be a single cable. An oval cable should be treated as a round cable equal to the larger dimension.

I have always used the circular mills of the oval. (Edit I think thats Note 5)