Current carrying capacity of a conductor in AC & DC

What is current carrying capacity?

depends on what material? where are you using? underwater? underground? in air? or in vacuum? at what ambient? and many mother factors.

and also what quality are you looking for?

There is a difference due to skin effect. How much? depends on frequency of AC Current.More is frequency lesser is the ability.

What is skin effect? Google--------

Generaly speeking the DC current needs almost the same cross section as the RMS AC current needs.Especially at main frequency (60 or 50 Hz ).

but you need stranded for AC

No way . At 60 Hz it's no matter if the wire is solid or stranded ( at the same cross section of course )

Engineering Electromagnetics-Hayt

"In a power station a bus bar for AC at 60 Hz with a radius larger than 1/3rd of an inch (8 mm) is a waste of copper, and in practice bus bars for heavy AC current are rarely more than 1/2 inch (12 mm) thick except for mechanical reasons.

Skin depth (depth below the surface of the conductor at which the current density decays to 1/e : about 37% of the current density at the surface) is 8.5mm approx for 60 Hz, ie the current(density) at about 8.5mm below surface is only 37% of surface current.

So At 60 Hz it's no matter if the wire is solid or stranded !!!

If its is so, I think there was no reason that we use the stranded wires for all our cables (and also transmission line) Why to waste the $s ?

Thanks for your reply , but we are not talking about "transmission lines" , neither "power stations". The actual discussion is about the home power line , i think. Maybe i'm wrong.

sb is correct as far as large ampacity bus bars go; such busbars are often hollow sections of copper pipe for the very reason he cites. But absent that level of current, what he says is not a definite rule at all. I don't know how they do things in India, but in the USA house wiring is all solid. The reason that power cords for equipment are stranded is flexibility, not ampacity.

The only time stranded cable buys you ampacity over the same cross-section of solid wire is if you are at a very high frequency (pronounced skin effect) and each strand is separately insulated magnet wire. This is called Litz wire, and it forces the current to flow on each and every conductor, instead of just the outside circumference. But that kind of wire is usually reserved for rf circuits, such as reducing the resistance and increasing the "Q" of a tunable loop antenna used at frequencies in the tens of kilocycles to thirty megacycles.

Note : this is a homework question (that is why no specifice) and i gave a general reply - the effect of the skin effect exixt whether it is bus bar or home wiring.

But in home wiring we are not bothered simply because the

a) length is short so the drop in the effective specific resistance will not be significant.

b) Current carried is small.

c) The conductors are almost universally used at a very high factor of safety.

However it is not only trensmission/ geneartin, in the industries, the machines are also fed by stranded wires for the same reasons and also the earth strip is a flat strip and not a round bar or a rectangular one (not only in the earth pit but also on the areas outside pit).

Sorry, no skin effect in house wiring at 60 Hz. Skin depth in copper is about 8 mm at 60 Hz, and that is much thicker than anything besides the main feeders into the breaker box. And using stranded wire would have no effect on skin depth losses anyway, unless the stranded wires were individually insulated Litz wire (per my original post) which it most certainly is not in house wiring.

Another responder from India, rakesh_semwal , correctly identified the reason for using higher strength steel cores in power transmission lines - for strength, and because of skin effect, the higher resistance steel at the cable center does no harm.

And if the earth strip is flat that is for low inductance, which implies lightning safety issues; at 60 Hz there is no difference in ampacity between a round wire and flat strap of equal cross-section and length.

Beside current issue stranded wires can have a centre core wire made of some alloy steel to get higher tensile strength.

They will be the same. The difference will be the voltage drop value as the frequency increase for the ac copper conductor.