Voltage Drop equation?

[Smart $]

Good education here on how different folks look at simple problems differently.

Original question does not give the temperature, so 2.2 vs 2.84 correct if a more probable 20C assumed vs 75C.


Or is the NEC so sacred that the chap 9 is one size fits all ?

Not quite sure how to answer that, but using Chapter 9 Table 8 or 9 values assumes the conductor is operating at 75°C.

Table 8 Note 2 provides the formula for temperature correction of the resistance value, but the temperature is that of the conductor resulting from thermodynamics, a combination of ambient temperature, self-heating from the current, dissipation of heat through the conductor insulation, the type and size of conduit and its dissipation of heat, proximity to other conductors, and perhaps other factors. What it amounts to is having to perform a Neher-McGrath calculation for just a little gain in accuracy.

 

[Carultch]

Not quite sure how to answer that, but using Chapter 9 Table 8 or 9 values assumes the conductor is operating at 75°C.

Table 8 Note 2 provides the formula for temperature correction of the resistance value, but the temperature is that of the conductor resulting from thermodynamics, a combination of ambient temperature, self-heating from the current, dissipation of heat through the conductor insulation, the type and size of conduit and its dissipation of heat, proximity to other conductors, and perhaps other factors. What it amounts to is having to perform a Neher-McGrath calculation for just a little gain in accuracy.

Anixter has a table I like to use, that has 60C, 75C, and 90C values of resistance.

The general strategy I go by, is that I use the 75C column if I'm using the minimum local size wire, and I use the 60C column if I've had to do any upsizing to curtail voltage drop.

 

[Smart $]

...
The general strategy I go by, is that I use the 75C column if I'm using the minimum local size wire, and I use the 60C column if I've had to do any upsizing to curtail voltage drop.

FWIW, temperature rise is roughly proportional to the current squared.

 

[Carultch]

FWIW, temperature rise is roughly proportional to the current squared.

So given 50A through a #8 wire in 30C ambient, you might expect an operating temperature of 75C. I know it will really be lower by a safety factor, but that is its 310.15(B)(16) ampacity at this temperature limit.

Now reduce the load to 25A, and instead of a temperature rise of 45C, it now has 1/4 of the temperature rise to about 42C. Is this what you are telling me?

 

[Smart $]

So given 50A through a #8 wire in 30C ambient, you might expect an operating temperature of 75C. I know it will really be lower by a safety factor, but that is its 310.15(B)(16) ampacity at this temperature limit.

Now reduce the load to 25A, and instead of a temperature rise of 45C, it now has 1/4 of the temperature rise to about 42C. Is this what you are telling me?

Yes.

As a way of confirming, consider the formula for conductor heat loss I²R. For there to be more heat loss, there has to be more heat... which in essence means higher conductor temperature (and increased resistance).