Are Wire Current Ratings Different in AC than in DC?
Well you didn't specify the frequency of the AC and I know you probably meant 50 or 60 Hz but there is such a phenomena as skin effect. As frequency rises the electrons tend to congregate in the outer skin of the conductor with very little conduction taking place along the centre section. This means that the wire's electrical resistance rises because the cross sectional area of the copper wire is under-utilized.

See this wiki article for skin effect. Also note that there is also a phenomena called proximity effect and this is very similar to skin effect but for parallel cables carrying the same current. 

Here's a pretty graph of skin effect depth versus frequency: -



At 50Hz, (red vertical line in graph), copper has a skin depth of about 10mm and this means that if you are trying to pass really large currents (and calculate that you need a copper wire of 40mm diameter), the middle 20mm is under-utilized and the cable may overheat a tad.

At 1kHz a copper conductor's skin depth is only 2mm. At 1MHz the depth is below 0.1mm. This is why circuits that use high frequency ac tend to adopt a "litz" wire approach. Litz wire

Proximity effect: -



Two conductors carrying current in the same direction create magnetic fields which force the electrons in each conductor to occupy the the most distant areas of each respective cable.