Was thinking about running 18/7 A/C control wire from my rocker switches to relays in engine bay. I have 4-5 switches. I want to know if the 18 ga. Wire from 12v power bar to switches will be able to handle the load from 4-5 switches be all on at once.

 

If that is solid wire, don't use it in a vehicle.
If the insulation isn't rated for automotive use, (temperature and chemical environment), don't use it.

Before you can know if 18awg is adequate, you need to know what amperage the circuits will have to carry.

 

What ronjenx said. You didn't give us enough info. Powering a few led's, no problem. Trying to power an inverter, supplemental heater, frigerator, 18 gage no good.

 

Just want to use from rocker switches to relays, that is it. I have 5 rocker switches and 5 relays. With the 18/7, I could run power to switches, a ground and the 5 switch to relay wires all in one wire bundle. I will use 16 or 14 from relays to led light bars and elocker

 

Ok, so you are only switching the relays on and off which shouldn't take much current at all. The relays themselves then have the heavier gage for the power draw. Seems to me that 18 gage will be fine.

 

I just want to add, for the sake of completeness (since it doesn't appear to be an issue for this particular install) that length of wire run versus voltage drop is also sometimes critical. For example, a 10 foot run for a 1.3 amp draw refrigerator of 16 gauge wire is too small. The refrigerator will get low voltage and shut down in order to preserve the battery all too soon. 12 gauge wire is recommended, even though that has way more current carrying capacity than what would at first blush seem necessary. Otherwise, the voltage drop makes the refrigerator think that the vehicle starter battery is weak, so it's trying its best to allow the vehicle to start. Another scenario is lighting: Drop 1.2 volts and you have a roughly 10 percent drop in lighting output for tungsten lights.