[RE-wrenches] Portable power station for surge loads

[Ray Walters]

Yes, T2 is the conductor temp.  I use 25C for wire in underground 
conduit that around here is close to earth temp, 15C.  Just depends on 
how much current is flowing, but the idea of doing volt drop calcs with 
the wires running at max temp seems over kill in many cases.  75C means 
that you are 1C away from exceeding the temp rating of many of the 
connector lugs.

Ray Walters
Remote Solar

On 3/22/2026 8:06 PM, Corey Shalanski via RE-wrenches wrote:
> Hi Ray,
>
> RE. the /NEC/ Chapter 9 Table 8 resistance adjustment formula: I 
> believe temperature T2 is the operating temperature of the conductor - 
> not necessarily ambient temperature... unless I'm mistaken?
>
> Corey Shalanski
> Jah Light Solar
> Portland, Jamaica
>
>
> On Sat, Mar 21, 2026 at 7:20 PM Ray Walters <ray at solarray.com> wrote:
>
>     I remember doing some measurements on a long wire run, and being
>     surprised that the actual voltage drop was much less than the
>     calculations.  That's when I found a small note at the bottom of
>     the NEC Table 8 conductor properties used for official VD
>     calculations.   The resistance is listed at 167 F (75C), so if you
>     use the adjustment equation in Note 2, it is much less VD, and
>     corresponds to reality.  For underground conductors, I might use
>     25C.  Use common sense.
>
>     The formula is R2=R1 (1+ a(T2-75))
>
>     I made a spread sheet about 15 years ago, that uses that formula
>     for doing wire sizing.  Happy to share, if anyone is interested.
>
>     Ray Walters
>     Remote Solar
>
>
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