Cable Derating factor and Temperature

Status
Not open for further replies.
E

Electriman

Senior Member
Location
TX
Greetings,

I just wondering if we have multi wires more than three running in the same conduit, when we are applying derating factors should we derate the ampacity that the cable is rated for or should we use the temperature the its terminals are rated for? For example, I have a cable that is rated for 90 degree c. But cable terminal is rated for 75 degree C. That cable can handle 30 A at 90°C but only 21 A at 75°C so I need to derate at 70% which at 90 can handle 21 A and I can use a 20A breaker whereas at 75°C is almost 15A.

Does code allow us to use cable ratings at 75° when we are dealing with less 100A applications if the terminal are rated fro 75°?
 
Electriman said:
Does code allow us to use cable ratings at 75° when we are dealing with less 100A applications if the terminal are rated fro 75°?
Click to expand...

Yes.

But I couldn't quite follow this...

Electriman said:
I just wondering if we have multi wires more than three running in the same conduit, when we are applying derating factors should we derate the ampacity that the cable is rated for or should we use the temperature the its terminals are rated for? For example, I have a cable that is rated for 90 degree c. But cable terminal is rated for 75 degree C. That cable can handle 30 A at 90°C but only 21 A at 75°C so I need to derate at 70% which at 90 can handle 21 A and I can use a 20A breaker whereas at 75°C is almost 15A.
Click to expand...



 
david luchini said:
Yes.

But I couldn't quite follow this...
Click to expand...

Sorry for confusion. I am using an example. I have 9 current carrying wires #12 in a conduit. The wires are XHHW so they are rated for 90°C. The wires are connected to terminals that are rated for 75°C. Per NEC, the adjustment factor is 70%. So my question is what is the cable ampacity (25*.7=17.5A or 30*.7=21A)? Can this cable fed from a 20A CB?
 
Electriman said:
Sorry for confusion. I am using an example. I have 9 current carrying wires #12 in a conduit. The wires are XHHW so they are rated for 90°C. The wires are connected to terminals that are rated for 75°C. Per NEC, the adjustment factor is 70%. So my question is what is the cable ampacity (25*.7=17.5A or 30*.7=21A)? Can this cable fed from a 20A CB?
Click to expand...

Since you mentioned XHHW as a 90° C conductor I'm guessing that you're not referring to a wet location, for a wet location XHHW is a 75° C conductor so you would derate from 25 amps and not 30 amps.
 
Status
Not open for further replies.
Top