Note: This document is intended to be helpful information only, detailed reference should be taken from an appropriate Standards Agency's and Organizations or Manufacture sources.

An IP number contains two numbers (i.e. IP65) in most instances which relate to the 
level of protection provided by an enclosure or housing.

The first number relates to protection from solids as follows:

0: No Special Protection
1: Protected against solid objects up to 50 mm in diameter
2: Protected against solid objects up to 12 mm in diameter
3: Protected against solid objects up to 2.5 mm in diameter
4: Protected against solid objects up to 1 mm in diameter
5: Dust protected
6: Dust tight

The second number relates to protection from liquids as follows:

0: No special protection
1: Protected against dripping water
2: Protected against dripping water when tilted up to 15^o from normal position
3: Protected against spraying water
4: Protected against splashing water
5: Protected against water jet spray
6: Protected against heavy jet spray
7: Protected against the effects of immersion
8: Protected against submersion

Example: IP66 = Dust tight and protected against heavy water jet spray

NEMA (National Electrical Manufacturers Association) ratings can be approximately
compared to those of the IP system as shown below.  Other factors such as corrosion
protection are involved in the NEMA system, please refer to official documentation for details.

NEMA 1 = IP10
NEMA 2 = IP11
NEMA 3 = IP54 
NEMA 4 = IP56