Wire & Terminal Temp Rating

[steris]

Hi All,

I'm working with NEC 310.16. Most wire terminals that I've encountered are rated for 75*C. I understand that that many people will use 90*C wire because then you apply derating factors for ambient temperature and number of wires in a single conduit without having to drastically increase the wire gauge. The way I understand the wire temperature ratings is this: 2 gauge copper conductor THHN wire carrying a load of 130 amps in a 30*C ambient environment will operate at 90*C. Further, if THHN wires are derated because of higher ambient temperature and used to the full derated spec then the conduction will still reach 90*C. My question is this: why is acceptable to hook a conductor operating at 90*C up to a terminal rated for 75*C? I assume I am misunderstanding something basic here...

Best,
Steris

 

[DRWeig]

First, the temperature specified is the ambient temperature, not the temperature of the conductor.

You can hook up any wire of any temperature rating to any terminal of any temperature rating. It's just a wire and a terminal.

You must use the lower of the temperature ratings for your derating factors. In your case, the derating (for both conductor and terminal) must be considered at 75°F. You will also have to consider the limit of 80 percent of conductor ampacity if your load is continuous.

Look at 310.15(B), 2nd paragraph. If your ambient is 30°C, you need 1/0 wire at 80 percent derating if your load is 130A..

Best to you,

Goober Dave

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[steris]

Hi Dave,

Thanks for the reply. Can you please point me to the code section that requires 80% limit for continuous load? Thanks again!

 

[DRWeig]

Sure, steris,

210.19(A)(1)

Best to you,

Goober Dave

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[steris]

Thanks Dave! Just one more followup: If the load is say 100 amps continuous and after all the derating for temperature, number of conductors and continuous load, the wire ampacity is say 125 amps would it be code acceptable to size overcurrent protection for any value between 100-125?

 

[DRWeig]

Yes, overcurrent protection is to protect the conductors. Look at 240.4 in the NEC. However, you may need a higher-trip breaker to accommodate motor starting, transformer inrush, etc... Look in the appropriate section like 430 for motors.

Best to you,

Goober Dave

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[steris]

Dave - Thanks again for all the help!