How does one calculate the normal operating current of a load from the MCA rating, given that is all you have. Thanks

Do I find the normal operating current by taking the MCA and dividing it by 1.25? seems right....

It is not clear why you would want to do this. The short answer is no, you can't do that. For equipment that lists an MCA and MOP you size the conductor ampacity to match MCA and size the overcurrent protection to match MOP. This tells you nothing about the operating current because someone has already done all the math. MCA would normally be equal to 125% of inductive (motor) loads plus 100% of resistive loads if they are continuous but will be more for noncontinuous cases, substantially more with very short operating times where an engineered value would be given or where diversity plays a major role.

Look at the 3 methods under NEC for load calculations and then think about how you would work them "backwards". It quickly becomes obvious why you can't work them "backwards". If you can't get name plate values then the correct approach is to take readings over a period of time. This is actually the most accurate method but the most time consuming. It is often used industrially to take advantage of extra existing capacity in existing loads but must be done fairly accurately since otherwise inrush and starting currents can cause nasty surprises.

The term normal operating current is a little confusing. MCA is 125% of the largest load, plus 100% of the rest. The largest load could be either a motor, as in the case of a condensing unit, or could be resistance heat, in the case of an air handler. Starting currents, inrush, etc. have nothing to do with MCA