Table 310.15(B)(16)

[2Broke2Sleep]

I know this table is permitted to be used to up to 2000V, but that seems to be ironic if we use 120v @ 20A = 2400VA and 277v @20A= 5540VA, so on and so forth. Am I missing something fundamental here???

 

[Carultch]

I know this table is permitted to be used to up to 2000V, but that seems to be ironic if we use 120v @ 20A = 2400VA and 277v @20A= 5540VA, so on and so forth. Am I missing something fundamental here???

You "pay for" voltage with plastic, you "pay for" current with metal. This is why it is strategic to use higher voltages for longer distance circuits, since plastic is cheap and metal is expensive.

The reason is that you need a plastic (or other insulating) barrier to keep current from flowing between the wires in the circuit, and the more voltage you have, the more "effort" you have attempting to cause this flow to happen. With current by contrast, you need as much mass of conductor in parallel to distribute the current throughout the material for reducing the temperature gained from generating heat to within safe limits.

The same wire size and type carrying 20A will generate just as much heat per unit length, whether the 20A are driven by 120V or 277V. The total amount of copper or aluminum is based on what is needed for that wire to safely diffuse that heat. In theory, a wire built for a 120V circuit could have a thinner insulation than a wire built for 277V. But in practice, most common wire types that you use in applications governed by the NEC, will standardize on insulation capable of withstanding 600V and less. It is not economically practical to manufacture diverse voltage ratings of wire insulation below 600V. Plus it helps avoid mistakes by standardizing on one master insulation voltage rating.

 

[jcleavit]

Think of it like water. The voltage is like pressure. The amperage is like the pipe cross sectional area. You can raise the pressure to get more gallons per minute(VA) without changing pipe (wire) size.

 

[Carultch]

Think of it like water. The voltage is like pressure. The amperage is like the pipe cross sectional area. You can raise the pressure to get more gallons per minute(VA) without changing pipe (wire) size.

That's not actually how the water analogy works, because the flow rate in gallons per minute is analogous to the current in amperes.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html

 

[kwired]

Think of why the HV conductors to a transformer are smaller then the LV conductors though both are carrying same load (with only difference being minor inefficiencies).

 

[Ingenieur]

That is an ampacity vs temp chart
ampacity based on area and material limited by various temperature rises
the temp limits are based on what the insulation can tolerate without damage or performance degradation

Wire temp rise ~ i^2 x R
R = p x L/A = wire resistance
p = resistivity and is a function of the material, consider it constant for Cu
A = area
L = length, consider it per unit so it can be ignored

So temp rise ~ i^2 / A
so as area goes up the temp goes down
so voltage has very little to do with i limitation of the conductor

the 2000 v is more a result of mfg standards and some additional impact when v gets into the higher ranges