Calculations on Ampacity

[grscc3]

How do they calculate the ampacity of a conductor. Simply, how come that a conductor with twice the area of other doesn't carry double amperage.

 

[bphgravity]

One method for seeing how it is done is shown under Section 310.15(C) of the NEC. Annex B gives examples.

In regard to your second question, Google the term 'Skin Effect'. This will help understand the dynamics of current flow through a condcutor.

 

[charlie b]

I seem to recall that the original values of ampacity were determined by experimentation, not by calculation. In answer to your second question, current tends to follow the outside of a conductor. Doubling the total area will not double the portion of the conductor through which current flows.

 

[jim dungar]

Conductor ampacity has much more to do with heat transfer (surface area) than with cross sectional area. Proximity effect (unequal distribution of electrons in conductors parallel to each other) and mutual heating also affect heat transfer. At 60Hz, skin effect is negligible for conductors smaller than 300kcmil.

 

[jghrist]

Conductor ampacity has much more to do with heat transfer (surface area) than with cross sectional area. Proximity effect (unequal distribution of electrons in conductors parallel to each other) and mutual heating also affect heat transfer. At 60Hz, skin effect is negligible for conductors smaller than 300kcmil.

Agreed. And doubling the area only increases the surface area of the cable by sqrt(2).