table 310.16

[keith gigabyte]

Trying to explain to a co-worker about the table and limitations of ampacity. I am under the assumption that max ampacity is based on weakest link. If our terminations; lugs terminal blocks etc are rated at 90 deg c then we can use 75 degree c column for THHN putting #6 at 75 amps.

We will leave conduit fill de-rating out of the equation for this entire explanation

If we have 60 deg c rated lugs terminal blocks etc we use 60 deg c column even though we are using THHN wire..putting #6 ampacity at 55 amps

I am referencing 310.15 (A) informational note 1 of 2020 code

 

[infinity]

Trying to explain to a co-worker about the table and limitations of ampacity. I am under the assumption that max ampacity is based on weakest link. If our terminations; lugs terminal blocks etc are rated at 90 deg c then we can use 75 degree c column for THHN putting #6 at 75 amps.

We will leave conduit fill de-rating out of the equation for this entire explanation

If we have 60 deg c rated lugs terminal blocks etc we use 60 deg c column even though we are using THHN wire..putting #6 ampacity at 55 amps

I am referencing 310.15 (A) informational note 1 of 2020 code

For the most part your calculations are correct but #6 THHN is only good for 65 amps at 75° C. For the #6 AWG at 60° C with an ampacity of 55 amps it's possible that you could use a 60 amp OCPD with the next size up rule in 240.4(B).

 

[kwired]

Most common situation is terminations are rated 75C and you must use 75C ampacity as the minimum conductor size. Next most common is older equipment that has 60 C terminations or using NM cable that must be sized based on 60C conductor ampacity.

Either case you still can use 90C conductor ampacity as a starting point for ambient temp or number of conductors in raceway adjustments, regardless of the adjustment outcome though, you still can not go smaller than the 60 or 75 degree conductor requirement for terminations.

 

[Srv52761]

I think another exception is if your conductor has two ampacities because of changing conditions, you can use the greater ampacity if the lesser ampacity section is shorter than 10’ and less than 10% of the other section.

 

[Dennis Alwon]

Here is the Informational Note being referenced

Informational Note No. 1: Table 310.16 through Table 310.19 are application tables for use in determining conductor sizes on loads calculated in accordance with Part II, Part III, Part IV, or Part V of Article 220. Ampacities result from consideration of one or more of the following:

1. (1)
   Temperature compatibility with connected equipment, especially the connection points.
2. (2)
   Coordination with circuit and system overcurrent protection.
3. (3)
   Compliance with the requirements of product listings or certifications. See 110.3(B).
4. (4)
   Preservation of the safety benefits of established industry practices and standardized procedures.