- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: parallel feeders
A quick and dirty answer. Skin effect ratio is the ratio of AC resistance to DC resistance in a conductor, and is a function of the geometric shape of the conductor and the proximity of one conductor to another as well as the frequency. The result of skin-effect is to reduce the effective area of the conductor.
At normal power frequencies the skin effect ratio is neglible for small conductors. It first becomes a somewhat usable ratio around 266 kcmil at 60Hz.
One of my reference books has the following ratios for copper and aluminum conductors at 60Hz and 65C:
300Kcmil - 1.01
400Kcmil - 1.01
500Kcmil - 1.02
600Kcmil - 1.03
700Kcmil - 1.03
750Kcmil - 1.04
I find it hard to believe the allowable ampacity tables in the NEC have not taken the skin-effect ratio into account. This is not one of the correction factors mentioned in the NEC.
A quick and dirty answer. Skin effect ratio is the ratio of AC resistance to DC resistance in a conductor, and is a function of the geometric shape of the conductor and the proximity of one conductor to another as well as the frequency. The result of skin-effect is to reduce the effective area of the conductor.
At normal power frequencies the skin effect ratio is neglible for small conductors. It first becomes a somewhat usable ratio around 266 kcmil at 60Hz.
One of my reference books has the following ratios for copper and aluminum conductors at 60Hz and 65C:
300Kcmil - 1.01
400Kcmil - 1.01
500Kcmil - 1.02
600Kcmil - 1.03
700Kcmil - 1.03
750Kcmil - 1.04
I find it hard to believe the allowable ampacity tables in the NEC have not taken the skin-effect ratio into account. This is not one of the correction factors mentioned in the NEC.