so a 40a will work and hopefully not explode?
Correct, the amp rating for ECS’s is a do not exceed number.
All amp ratings in the RC world are do not exceed. It perfectly fine to use less amps than than what it is rated for.
Start with the motor, pick a prop that will not cause the motor to pull more amps than the motor is rated for. Next pick an ESC that can supply more amps than the motor is drawing. It’s good to have a bit of overhead, the servos & Rx also draw a few amps. Finally pick a battery that can supply the amp needed.
With batteries the amp capacity is calculated. The formula is mAh/1000 x C = A
Example a 2,000 mAh 20C battery can supply 40 amps. (2,000/1,000 x 20 = 40)
Sometimes a battery will have 2 C ratings, the higher one is a burst rate, what the battery can do for a short burst, 10-15 seconds. The lower one is what you want to use, it’s a continuous use rate.
If you exceed the amp rating of any component, you risk burning it out.