I recently had a new pump 1 1/2 hp installed for my sprinkler system which replgaced a 1hp. It was connected to a 15 amp circuit. With the new pump, the breaker kept popping. It was suggested to change this to a 20 amp. We did, using an electrician.(there is a label on the pump that says max amp 19.9/9.95. This breaker keeps popping also. Had another electrician out several times and we are still having the same problem. It doesn't always pop but most of the time. Can someone give me some direction?

You need to move the circuit to an amperage rating that will allow for the "In rush current" needed by the pump.

Is this dedicated circuit that the pump is on? Be advised, if you move the amperage rating higher that 15 amps, the cable from the breaker to the pump must be changed accordingly.

For example, a 14 gauge cable can take no higher than 15 amp. A 12 gauge can take no higher than 20 amp. A 10 gauge cable can take no larger than 30 amps.

I suggest you contact the manufacturer of the pump to make sure that you are using the correct amperage.

Or you may try using a 25 amp breaker with a 10 gauge cable and a 20 amp receptacle.

You failed to mention the voltage of the motor. What I am going to assume is that 19.9/9.95 is the low and high voltage currents. e.g 19.9 A for 120 and 9.95 for 240.

Motor loads are sized differently when dedicated.

Can you write down everything on the nameplate and/or take a picture using go advanced/manage attachments?

There may be things like LRA and FLA as well as HP.

It's very important to know the voltage and to a lesser extent the distance away from the panel.

Also what wire gage is currently being used?

It is permissible to size the wire smaller when a dedicated motor load is used.

If the sprinkler controls are on this circuit, they can also contribute. Any idea of the power requirements for the controls?

As donf said, the in-rush current is tripping the breaker.

With a 1.5 HP motor, I would be concerned also with the cost of replacement and thus knowing if the motor is thermally protected can be of use.

If the motor is 120 volts, the wire size shall be rated to handle 20 + 25%, or 25 amps, which is #10 wire.

The breaker can be up to 2.5 times larger than the 20 amps rating from NEC Table 430.138, or 50 amps, and yes... here it comes... on the #10 wire.

If it is 240 volts, it needs #12 wire with a 30 amp breaker.

Best to tell your electrician to re-read Article 430 of the NEC.

Several electricians on site did not realize that a 19.5A motor starting under load could trip a 20A breaker??

That label is telling you that when the pump is wired as a 240 volt motor it draws 9.5 amps. Usually changing motor voltage is very easy, sometimes just moving a plug inside the wiring compartment. If this is a dedicated line changing it to a 240 volt line is also very easy. Have an electrician do these things and your problem will go away. Use a 15 amp 2 pole breaker.

Motor maybe, but there are controls issues. Just upping the motor HP might be the wrong thing to do. Other changes may need to be made. The contactor MAY be the wrong size.

Converting to 240 may be way too difficult especially using the original electrician.

Find a reputable electrician. Anyone vaguely familiar with motors would know that a motor drawing 20 amps running would trip a breaker on startup.