You would know when the humidity of your crawlspace is above the recommended limit and a fluctuation of temperature. These conditions can cause and increase in fungal and mold growth underneath your home!
Once your crawlspace is sealed, it is recommended that the insulation not be reinstalled during the drying process. This is to allow the dehumidifier to work properly without having to do double work in removing moister in the air and wood as well as moisture trapped in the insulation.
Yes, it is very important to have dedicated electrical lines installed by a professional electrician to ensure that the devices work properly.