It won't necessarily cause respiratory depression, it depends on your tolerance. 30mg hydrocodone is not a huge dose for someone with some tolerance - 30mg hydrocodone = 20mg oxycodone (roughly) - have a look at the chart in hydro's link as she suggested or in our signatures and work out what the equivalent dose of hydrocodone is. Someone with no opiate tolerance should tread more carefully of couse.
It is always best to start low and work up, and don't take the charts as 100% accurate - they are a guide only - but they should give you some idea of what effect 25/30mg hydrocodone will have on you.