National Electrical Code (NEC) typically defines rooms and areas based on the "intended" use of the area. They do not provide a definition for a "storage room", but they do provide a definition for a "clothes closet".
Clothes Closet. A non-habitable room or space intended primarily for storage of garments and apparel.
To answer the rest of your question, we'll have to take a look at Article 110. Since section 110.26 of the NEC is a bit long, quoting the entire text here may not be considered "fair use" so instead I'll summarize it for you.
National Electrical Code 2008
ARTICLE 110 Requirements for Electrical Installations
II. 600 Volts, Nominal, or Less
110.26 Spaces About Electrical Equipment. Sufficient access and working space shall be provided and maintained about all electrical
equipment to permit ready and safe operation and maintenance of such
110.26(A)(1) Depth of Working Space.
This section says the depth of Working Space must be between 3 and 4 feet, "measured from the exposed live parts or from the enclosure or opening if the live parts are enclosed". In a typical residential situation, the depth of Working Space will be 3'.
110.26(A)(2) Width of Working Space.
This section says the width of Working Space must be the width of the equipment or 30", whichever is greater. And that there should be enough Working Space to allow the equipment door to open 90 degrees.
110.26(A)(3) Height of Working Space.
The Working Space must be clear from "the grade, floor, or platform" to the height of 6 1/2', or the height of the equipment whichever is greater. There is an exception to this which says:
In existing dwelling units, service equipment or panelboards that do not exceed 200 amperes shall be permitted in spaces where the headroom
is less than 2.0 m (61⁄2 ft).
This section also says that any other equipment associated with the electrical installation"located above or below the equipment, cannot extend more than 6" beyond the front of the equipment.
110.26(B) Clear Spaces. Working space required by this section shall not be used for storage. When normally enclosed live parts are
exposed for inspection or servicing, the working space, if in a
passageway or general open space, shall be suitably guarded.
Which means the equipment can be in an area used for storage, but the Working Space cannot be used for storage. The basic idea is if you draw a box on the floor in front of the equipment 30" wide and 36" deep, and then extend that box up 6 1/2', you'll have your "Working Space". As long as you keep this Working Space clear, you should have no problems. However, there are a few more things to consider.
110.26(C) Entrance to and Egress from Working Space.
110.26(C)(1) Minimum Required. At least one entrance of sufficient area shall be provided to give access to and egress from working space
about electrical equipment.
You'll have to check local building codes to determine what the "sufficient area" is, and then make sure you keep this area clear too.
Another important consideration is lighting.
This is a section that is often overlooked in residential situations, and is often satisfied simply by having lights in the room where the service panel is installed. Personally, I take this section seriously. I recommend installing a dedicated circuit, consisting of a single switch and a single luminaire. Then installing it just above the equipment, as to provide adequate light for anybody working on the equipment. Putting the luminaire on a dedicated circuit allows you to deenergize other circuits, while maintaining good lighting while servicing the equipment.
It's a good idea (though a bit overkill), to also provide emergency lighting near the equipment. Installing a simple fixture like this near the equipment, ensures that you won't have to fumble in the dark to flip breakers if the power ever goes out.
This unit can be installed on the same dedicated equipment luminaire circuit. That way even if you have to switch off the whole panel to do work, you'll still have light.
- Keep an area 30" wide, 36" deep, and 6 1/2' high in front of the equipment clear (Working Space).
- Provide "sufficient area" for entrance to and egress from the equipment (see local building codes to determine "sufficient area").
- Make sure the area has enough lighting.
- Rooms are defined by intended use.