NEC Table 9 Design Criteria Question

[nightfox1925]

I have one more question, it may sound stupid to others, but I will give it a try for my understanding. The NEC Table 9 is entitles as "Alternating Current Resistance and Reactance for 600V cables, 3Phase, 60Hz, 75 Deg C, Three Single Conductors in Conduit"

Are these installation and temperature conditions to be statisfied in order to use the Ohms to neutral impedances in our V drop calculations?

If I have more than three current carrying conductors in the raceway, or say I chose a 60 Deg C or 90 Deg C conductor temperature, will these impedances still going to be valid?

Thanks

 

[davidbeach]

They will still be close enough. Resistance varies with temperature and will always be given for a stated conductor temperature; hot wire gives the most voltage drop and cold wire should be used for fault calcs but generally isn't. Reactance is a function of geometry of the conductor in question and the surrounding conductors. I'd just use the Table 9 values for all low voltage work and not worry about the minor variations.

 

[tommom]

Some people correct the resistance values to 25 degrees C when doing short circuit calcs - that's conservative and doesn't change the answer much.

 

[RyanJackson]

Night Fox hits on the real problem with Table 9, and why I think it may easily go away in the 2011 NEC. The problem with Table 9 is that it doesn't tell you what to do if you don't meet the exact criteria of the Table. There was a change in 90.3 in the 2008 that tells us that Tables are only valid as referenced elsewhere in the Code. Nowhere in the Code does it refer to Table 9. This makes the TAble technically unusable. Watch for proposals in the 2011 to either refer to the table in a FPN in 210.19/215.2, or watch for proposals to make it an annex or delete it altogether.