Cable Fill Ratio-NEC

[BoltedFault]

I have a question regarding the NEC and the conduit fill ratio. The NEC recommends the cable fill ratio remains under 40% for two or more cables. What is the reasoning to limit the allowable fill ratio to 40%? What problems could occur if the 40% fill ratio is exceeded?

 

[davidbeach]

In the NEC world you have a Code Violation. In the NESC world I believe conduit fill is an engineering judgment. The NEC tries to make general rules that hopefully apply in nearly all cases (just below 40% fill can cause jamming problems) but these general rules don't always work well.

A good example is the four 90[°] bend rule. Some pulls are impossible to pull with only 180[°] worth of bends and others can pull just fine at 720[°]. But 360[°] is the rule.

 

[EEJaime]

As davidbeach stated the fact that this is in the NEC is enough for some AHJ's to consider this as a universal absolute. We even see this fill rule applied to low voltage cables by some AHJ's. This has to do both with installation as davidbeach noted as well as with heating of cables. Packing cables tightly together will increase the trapped heat and make insulation failures more likely. As you can see from the tables, a single conductor fill can be up to 53%. The FPN's and Notes to Tables after Table 1 of Chapter 9 of the NEC are also interesting.

 

[DonPhillips]

The concept behind the cable fill reduction is to account for heat dissipation to protect the insulation on the conductors. The two or more conductors reference is for current carrying conductors.

Don Phillips

http://worthingtonengineering.com

 

[BoltedFault]

Thanks for the responses. I suspected the fill ratio limitation had to do with heat dissipation, however I wanted to confirm it. I couldn't find the info anywhere, and I didn't want to accept something without knowing the reasoning behind it.