I am an electrician by trade, but in my area I can't advertise my services. Your 15a breaker will easily hold a 400w light.
P(watts)=E(voltage) * I(amps)
400=120*I
3.3333333= I
Your 400w bulb is only using 3.3 amps of power(figure on 4amps for ballast effeciency). Leaving plenty of room for more lights, plants, and sweet-sticky ganja!