Thermal Resistance (RHO) De-rating Factor

[vheinz]

Tables 310.77-.86 are based on RHO=90. Does anyone know a quick derating factor for these ampacities listed in these tables for a RHO=120.

I tried to use Article B.310.15(B)(7) with the figure B.310.1, but I must not be using the Interpolation chart correctly.

All I am trying to do is convert the tables listed above so that they apply to RHO=120. Thanks.

 

[charlie b]

Re: Thermal Resistance (RHO) De-rating Factor

Figure B310.1 is of no value to your specific need. It presumes that you know the ampacity of a configuration that uses a RHO value of 60 and a Load Factor (LF) of 50%. It then provides a method for converting that information to an ampacity for a similar configuration, but one with a RHO value of 120 and a Load Factor (LF) of 100%. This figure cannot help you because your question starts with RHO = 90 (not 60) and LF = 100% (not 50%).

I?m afraid there is no ?quick derating factor.? If the configuration of your interest does not correspond to any NEC Table, and if you need to adjust the ampacities from the tabulated values by any means other than those described in the NEC (e.g., for ambient temperature or for more than 3 current-carrying conductors), then I believe you are in the realm of ?engineering supervision.? There are several software packages that have the formulas built into them. The least expensive that I have personally used has a cost on the order of $2000.

Are you sure that you need to make this type of ampacity adjustment? Is you installation to be buried under desert sands?

 

[bob]

Re: Thermal Resistance (RHO) De-rating Factor

Check Annex B Table B.310.7. Why do you need RHO
at 120?