Messenger wire 310.17 vs 310.20

[salgant]

While reviewing a single line from a client, I see a typical single string 20A conductor on a #10AWG wire back to the combiner box. The installer is using a messenger wire system. I have always used 310.17 for this, however my coworker pointed out that I should be using 310.20, which would require all wire in messenger systems to be #8AWG minimum.

I questioned what ambient temp we should actually be basing the calculations on as the customer's engineer did not specify. In addition, they may be using CAB which can break wires out into groupings of three, but we have no indication that they are doing that.

The big difference in tables are:
1. 310.17 ambient temp of 30°C. 310.20 ambient temp of 40°C.
2. No more than three single-insulated conductors.

My actual question is, what table do I actually use?

 

[pv_n00b]

I've seen this conversation a lot over the years. One of the problems is that "free air" was not a defined term in the NEC until 2020 and using the free air table resulted in smaller and less expensive conductors. So there was motivation to use it and hold one's nose.
The only problem with that is that multiple conductors in CAB are not in free air. "Free air" is clearly defined in 100 and if the conductor does not have open airflow around the conductor it's not in free air. That means the use of 310.17 is not applicable to CAB unless the conductor is by itself. Using 310.20 is correct for CAB systems.

 

[salgant]

Thanks for the feedback pv_noob. How Do I justify using 310.20 with the max 3 single-insulated conductors?
Under the notes it states to go back to 310.15C for more than 3 conductors, however 310.15C table specifically states that it applies to 310.16-310.19.

 

[pv_n00b]

Thanks for the feedback pv_noob. How Do I justify using 310.20 with the max 3 single-insulated conductors?
Under the notes it states to go back to 310.15C for more than 3 conductors, however 310.15C table specifically states that it applies to 310.16-310.19.

There is some mislabeling in the NEC. I go by the instructions for table 310.20 and assume it is correct. It's interesting that no one has corrected this.