In commercial applications, most lighting circuits are 20 amp.
It can be done.. would be fine, but in a residential situation, 14ga is easier to work with, 15 amp circuits would be divided into smaller groups of fixtures, in the case of a failure or short, the 15 amp circuit would blow faster meaning less heat at the failure and through the circuit.
If you never have a problem, it will not make a difference.
There are advantages either way. If 20amp circuits work better for you.. go for it. Just be sure to use 20 amp rated switches and dimmers rated for the wattage of the total of the fixtures. The total of the rated wattage of the fixtures should be 80% or less of the dimmer's rated wattage.
Dimmers are another consideration..
A 2000 watt dimmer is really only good for 1600 watts of rated light fixtures.
That 2000 watt dimmer may cost you $100.00 or more and will need to be kept separate from the other dimmers, or, they will have to be further derated.
600 watt dimmers are available everywhere, can actually handle 480 watts. Typically sell for around $15.00 for a good quality dimmer.
Much to consider.
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