2 x Wantai Stepper Motor 42BYGHW811 on A4988 - Ramps 1.4

Hi

i am new to 3d printing and added a second Wantai Stepper Motor (NEMA 17) to the Z axis. I added Heatsinks to all A4988 drivers and installed a 50mm fan to the ramps case. What is the max current i can set on A4988 driver without burning them up ? Wantai 42BYGHW811 Stepper Motors are 2.5A(U) max.

Please assist

The reason why i would like to raise max current is that the left stepper motor seems not to get enough power. I connected the z stepper motors to the 2 z connectors on the ramps 1.4 - current is 0.60. The only difference is that the left stepper motor has a longer and thinner cable. Can this be my problem ?

Don't exceed 1.8, that will be the maximum.