Current (amps) is voltage (volts) divided by resistance (ohms) or I=v/r. Breakers "trip" when there is too much current flowing through them. This can be caused by wiring that is too small for the application or a short (or a bad breaker, but since you just replaced that).
Cable Size:
The conductors have resistance and heat up when current runs through them. Properly sized conductors allow this heat to be dissipated. Long cable runs or cable runs which are tightly bundled with other cables can prevent this dissipation. Heat adds additional resistance which in turns causes more heat, eventually tripping the breaker to prevent a fire. For a 40 amp breaker, the cable should be a dedicated, 8 gauge wire.
Short:
A short (low impedance connection) can be between two conductors or between one conductor and ground (also called a ground fault). Shorts can be tough to track down when they are intermittent. This can be because the insulation was damaged (during installation, by rodents, etc). Moisture can deteriorate insulation or conductors.
It may only be present under certain conditions, so you can try this immediately following a trip. Shut both the breaker and the cutoff at the condenser (you want to isolate the cable), then use a meter to check for bleed between each of the conductors and each other conductor or ground. The neutral eventually connects to ground so it will have zero impedance. If you find there is a connection, you (or your electrician if you don't know what you're doing) will have to track it down and repair it.
It could be a clamp that is too tight where the line enters a jbox or panel, a staple that was driven too far and is biting into the insulation, a bend that is too tight, or a length that isn't supported well and subjected to vibration from a nearby motor .
Other possibilities:
At 40 Amps they'd be really expensive, but are you using a GFI or Arc-Fault breaker? Either of those could cause unwanted tripping.
Good luck with it.
jack g
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