Conduit Fill

[drktmplr12]

Can anyone shed light on the history of why for 2 cables the permitted fill is 31% and over 2 cables its 40%? :?

 

[sblk55]

conduit fill

conduit fill

My guess would be cross section. If your 2 cables is over 31 % would they fit in the raceway?

 

[drktmplr12]

To put it another way, if I have a two cables that takes up 16% of the conduit and one that takes up 8%, its permitted. But if I only have two cables that take up 16% it is not.

 

[Dennis Alwon]

Can anyone shed light on the history of why for 2 cables the permitted fill is 31% and over 2 cables its 40%? :?

I think Smart$ did a drawing of this a few years back showing how 2 wires took up more room than 3 wires.

 

[infinity]

It has to do with the two conductors being side by side as opposed to say 3 conductors where they're in more of a triangular configuration. I believe that the NEC calls this the jam ratio.

 

[tom baker]

yes that is correct on jam ratio, there is a note at the bottom of the fill table on that

 

[Carultch]

Can anyone shed light on the history of why for 2 cables the permitted fill is 31% and over 2 cables its 40%? :?

The standard conduit fill area ratios generally represent about 75% of the inner diameter of the conduit being used. When 2 identical circles packed in a circle, the most you can fill it is 50% by area. With 3 identical circles, they can pack in a triangular arrangement, and fill 65% of the space in a tight surrounding circle. More than 3 identical circles can pack tighter than this, up to 80% for 19 wires, but the NEC doesn't bother taking advantage of this fact for larger quantities than three.

Observe that 65%/50% is the same ratio as 40%/31%. Three wires at 40% fill the conduit to the same diametral ratio as 2 wires at 31%.

 

[drktmplr12]

The standard conduit fill area ratios generally represent about 75% of the inner diameter of the conduit being used. When 2 identical circles packed in a circle, the most you can fill it is 50% by area. With 3 identical circles, they can pack in a triangular arrangement, and fill 65% of the space in a tight surrounding circle. More than 3 identical circles can pack tighter than this, up to 80% for 19 wires, but the NEC doesn't bother taking advantage of this fact for larger quantities than three.

Observe that 65%/50% is the same ratio as 40%/31%. Three wires at 40% fill the conduit to the same diametral ratio as 2 wires at 31%.

this makes perfect sense. thanks for the insight! :thumbsup: