Question:  What would be the ideal humidity inside a Southern California home during the summer and cooling with central air conditioning?

Reply: Actually if you go below 50% RH you will feel quite dry, in fact too dry. The other consequence is that the static electricity in the home builds up as well at lower humidity levels, this can be a problem if you have carpet in your rooms
The advisable level is somewhere between 55 and 65%RH if you have the ability to control it, all the better.
A/C in dry climates tend to over dehumidify the air as the moisture in the air is condensed on the indoor evaporator coil and drained away….causing dry eyes etc and A/C in high humidity zones tend to add to the cooling capacity making you feel less comfortable.
Some people in dry climates have been known to put containers or bowls of water in their home to keep the dryness at a reasonable level.

Question:

I have a really small cabin that I use a portable air conditioner. It works good until the temps outside rise above 90 degrees. Anything in the 100′s, or 110, it can’ tkeep up. I live in a VERY dry climate with < 10% humidity. I was thinking of also getting a portable evaporative cooler and running them both at the same time. Do you think there would be any benefit? I would like to add some humidity to the air anyway..

Reply:

Without going into a lot of detail the short answer is no. Evaporative cooling and refrigerated cooling working in the same environment do not work. The 2 types of cooling do effectively cancel themselves out.
If you do have a humidity problem and wish to cool your cabin you should invest in a dedicated humidifier that supplies the required humidity and can also be regulated according to your settings.
Sounds also that you need to upgrade to an A/C unit that will do the job. My advice is to buy a unit that will perform better in high temperatures conditions, have a heat load calculation carried out on your premises, they are usually given free with a quote for a new unit.
FYI: The industrial computer rooms of the past have struggled with this conundrum for many years and found the only solution to be as I have stated above or installed A/C units that have built in humidification systems.
Hope this helps.