Annex B Tables B.310.1 & B.310.3

[short-circuit52]

Ok... maybe someone can clear this up. There is very little mentioned about Annex B Tables B.310.1 & B.310.3 to be used for ampacity ratings. Table B310.1 states it is used for two or three insulated conductors, within an overall covering (jacket) in a raceway in free air. I would assume from the table description this refers to 3c tray cable type wire. Table B310.3 specifically lists type TC, MC, MI, UF and USE cables. in free air. The ampacities are lower than tables 310.16 & 310.17. Maybe someone explain the purpose of Annex B Tables B310.1 & B310.3:-?

 

[chris kennedy]

Maybe someone explain the purpose of Annex B Tables B310.1 & B310.3:-?

Welcome to the Forum. Your explanation can be found in B.310.15(B)(1). NECHB commentary will follow in blue.

B.310.15(B)(1) Formula Application Information.

This annex provides application information for ampacities calculated under engineering supervision.

The data in Annex B are based on calculations using the Neher?McGrath conductor ampacity formula. The use of this method to calculate conductor ampacity is permitted under engineering supervision per 310.15(A)(1) and 310.15(C). The formula is found in 310.15(C).
Although conductor ampacities calculated using this formula may exceed those found in a table of allowable ampacities, such as Table 310.16, the limitations for connecting to equipment terminals specified in 110.14(C) have to be followed. For equipment 600 volts and under, the ampacities used with equipment terminals are based on Table 310.16. See the commentary following 110.14(C) and 310.15(C).

 

[short-circuit52]

Thanks. I did read the B310.15(B)(1). It's my understand that annex B is used primarily for underground cables. Although it did say it can be applied to most installation. Comparing it to the conductor ampacity tables 310.16 & 310.17, table 310.1 is slightly more conservative.