Derating conductors

[mull982]

Hello

I am fairly new to the whole method of sizing conductors for various installations. I was recently working on a project that required me to size conductors to power a continuous 100 hp motor, and I was told that I needed to derate the conductors b/c my load was a continuous motor.

This is the first time I heard of derating conductors and I was wondering if anybody could give me more information on this subject, such as when and for what do derate conductors for. Is there anywhere I could look such as possibly online or in the NEC for quidelines on how to derate conductors. I would appreciate any help with this matter.

Thank You

Mull982

 

[haskindm]

There are many times that you will need to derate conductors.
1. We derate for continuous loads.
2. We derate for high ambient temperatures.
3. We derate for more than three current carrying conductors in a conduit.
4. We derate for excessive voltage drop.
5. All motors require that the branch circuit conductors have an ampacity of 125% the calculated amp load of the motor - so I guess you would call that derating.
You need to spend some time in the first four chapters of the NEC to familiarize yourself with these concepts.