1) Never use your dog’s crate to punish them.

Your dog should consider his crate a happy, comfortable, and safe place.   

2) Never let your dog out of the crate while he is barking or whining.

Opening the crate then simply teaches him that if he makes enough noise, then he will be let out.   

3) Never leave your dog crated for longer than he is physically able to hold his bladder or bowels.

Dogs should not be left crated for more than this length of time (3-4 hours for puppies and non-housetrained adult dogs) without being taken out for exercise, playtime, and time to spend with you. 
Is Crate Training Cruel?  Crates allow dog owners the peace of mind of knowing their dog is safe when they are not there to supervise.  Dogs are known to be den animals. They like having a safe and secure place to call their own.