Under a personal insurance policy (PAP), there is normally an exclusion stating that there is no coverage for the use of an auto furnished or available for your regular use that is not owned by you, such as a company car. So when you drive the employer owned auto (for business or personal use), your personal auto insurance policy does not protect you. The same can be said for your spouse or other family members if they drive the auto. Normally the policy written in the name of the business would protect the employee but only during use of the vehicle for business purposes and not personal.

Extended Non-owned Coverage is a type of endorsement that can be added to a personal automobile insurance policy that covers the insured while driving any non-owned automobile on a regular basis, such as a business or company car.

This is a type of endorsement that provides broader liability coverage for specifically named people operating any non-owned automobile. Though the terms of the coverages can vary depending upon the terms of your policy, in general this type of coverage covers non-owned autos, use of autos to carry people or property for a fee, and individuals driving employer-furnished cars who do not own vehicles themselves.

One insurance company that offers this endorsement states that coverage is provided only for autos not owned, leased, hired, or borrowed by the named insured. Coverage includes autos owned by the insured's employees or members of their households, but only while used in the named insured's business or personal affairs.

An extended non-owned endorsement on your personal policy only includes liability coverages to the company vehicle and not physical damages of Comprehensive and Collision so typically the business policy of the employer would need to cover the vehicle for this. You should ask your employer though if you were to use these coverages if the business auto insurance provider would subrogate (come after you) for the cost of damages if physical damage coverages did have a claim against them while you were operating the car for personal use.