Did you perhaps up-size the A/C, say from 13500 to 15000? Even if you didn't, if the compressor in the old one was getting "tired" with its valves starting to leak, a new A/C might draw more amps than the old worn one did. That happened to me at the house. Replaced old A/C with a new one rated for LESS amps and it tripped breakers the day we installed it. Ran and got new, problem solved.
Don't install a Hard Start Kit till you see if there's one already there. They differ in effectiveness too. The cheapest ones use a PTCR device (Positive Temperature Coefficient Resistor) to get the extra start capacitor out of circuit. PTCR does this by heating up. That means it won't help much if at all if you short cycle the compressor with an Off/On cycle. Because the PTCR has to cool to get the start cap back in circuit. The better Start Kits use electronics or a relay for what they call "potential sensing."
