EE is requiring us to use 60C conductor ampacity for THWN-2 & THHN sizing calculations, stating 110.14(C)(1).

[JimmysLimeade]

Not sure if the title makes sense here but I am designing a project in Illinois that needs an EE stamp. They have redlined a few of our plans stating the wire size needs to be sized up because we have to use the 60C column in 310.15 for conductor ampacity. Here are a few of the different situations where we have this issue:

And when I asked him about it, all he said was this "See NEC 110.14(C)(1). You can only use the 90 degree C for derating calculations. Since your calculation came out higher than the 60 degree C column, you have to use the 60 degree column for the ampacity."

Could someone help explain this to me because I don't understand how this works. I might be missing some info so if you need extra clarification let me know.

 

[wwhitney]

You should refer the EE to 110.14(C)(1)(3) and also demonstrate that "the equipment is listed and identified" for 75C conductor connections at both ends of the conductors in question. Much (most?) equipment is so listed and identified, so that is often taken for granted, but the EE is correct that it needs confirming.

Cheers, Wayne

 

[infinity]

If you're using a 90° C conductor like THHN then you can use the 75° C ampacity as long as the terminals are rated for 75° C. The limits in Article 240 for #14-#10 would still apply.

 

[Strathead]

It looks like you were trying to use the 90 degree table for the top example. It is rare to have 90 degree rating at both ends, so that is rare.

 

[jaggedben]

(Does Illinois require EE stamps for simple low voltage solar installations? I'm sorry to hear that.)

 

[winnie]

Just to consolidate what others have said:

Even though you are using 90C conductors, you are not able to use the 90C ampacity, because of the temperature limitations of other components in the system. In particular you are limited by the temperature ratings of the terminations that the conductors land on.

Depending on the amp rating of the terminations, the temperature rating defaults to 60C or 75C, but usually equipment is marked and you can use the 75C amapacity.

It sounds like both you and the EE are wrong.

Your table also says '4/0 AWG welding cable'; you will want to confirm that this cable has an appropriate NEC acceptable rating, and that your terminations are rated for fine stranded wire.

-Jon

 

[JimmysLimeade]

Just to consolidate what others have said:

Even though you are using 90C conductors, you are not able to use the 90C ampacity, because of the temperature limitations of other components in the system. In particular you are limited by the temperature ratings of the terminations that the conductors land on.

Depending on the amp rating of the terminations, the temperature rating defaults to 60C or 75C, but usually equipment is marked and you can use the 75C amapacity.

It sounds like both you and the EE are wrong.

Your table also says '4/0 AWG welding cable'; you will want to confirm that this cable has an appropriate NEC acceptable rating, and that your terminations are rated for fine stranded wire.

-Jon

Sounds about right, Ill look into the termination ratings of the equipment and go from there.

Thank you for all the help and clarification!

 

[ron]

Sounds about right, Ill look into the termination ratings of the equipment and go from there.

Thank you for all the help and clarification!

Keep in mind that the termination rating itself is not the whole story. If you try to go above the 75 deg rating, the terminations along with the equipment assembly need to be tested above 75 deg. There are many/most situations that the terminations are 90 deg, but the equipment has only been tested to 75 deg.