Based on the 2020 NEC.
Q. What are the Code rules for bundling adjustment to conductor ampacity?
See the answer below.
A. Where four or more current-carrying conductors are contained within a raceway that is longer than 24 in., the ampacities contained in Table 310.16 must be adjusted (reduced) in accordance with Table 310.15(C)(1).
Where multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in., the ampacities contained in Table 310.16 must be adjusted (reduced) in accordance with Table 310.15(C)(1).
The neutral conductor might be a current‑carrying conductor, but only under the conditions specified in Sec. 310.15(E). Equipment grounding conductors are never considered current carrying [Sec. 310.15(F)].
Conductor ampacity adjustment (reduction) is required when four or more current-carrying conductors are bundled together because heat generated by current flow is not able to dissipate as quickly as when there are fewer current-carrying conductors.
Let’s run through two examples to show you how to meet this requirement.
Example #1
Question: What is the adjusted ampacity of four current-carrying 12 THWN-2 conductors in a raceway? (Figure)
Answer: Adjusted Conductor Ampacity = Table 310.16 Ampacity × Bundled Ampacity Adjustment Factor from Table 310.15(C)(1)
12 THWN‑2 is rated 30A at 90ºC [Table 310.16]. The adjustment factor for four current‑carrying conductors is 80% [Table 310.310.15(C)(1)]. Adjusted Ampacity = 30A × 80% = 24A
Example #2
Question: What is the adjusted ampacity of eight current-carrying 12 THWN‑2 conductors in a raceway?
Answer: Adjusted Conductor Ampacity = Table 310.16 Ampacity × Bundled Ampacity Adjustment Factor from Table 310.15(C)(1)
12 THWN‑2 is rated 30A at 90ºC [Table 310.16]. The adjustment factor for eight current‑carrying conductors is 70% [Table 310.310.15(C)(1)]. Adjusted Ampacity = 30A × 70% = 21A
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
