I have to wonder if most homeowners, and/or home do-it-yourselfers, and/or "professional" contractors (!) know that electrical wiring sheathing color is relevant and important.
The question is asked because so often I see outdoor wiring that uses incorrect wiring, or is not protected properly, such as with a conduit.
Take this wiring attached to this deck.
We see two colors, gray and white.
This location is wrong no matter what the color - it is right beside a staircase, exposed to weather and sunlight, and the connectors are made for indoors.
As to color, gray wiring is made to be used underground, and its sheathing is more resistant to sunlight, oil and water. It should still be attached in areas where it is better protected.
I don't have a problem so much here with the gray wiring, although I'd like it elsewhere under the deck.
But the white wiring is not made for this application. It's made for indoors!
Its installation here is a sure sign of unprofessional work. That means there was no permit. It connects numerous new lights installed around this deck.
It originates in a light fixture, improperly installed itself, which is itself connected to another light fixture I think inside the house. I say that because the white wire enters the house through a hole in the ledger board which attaches the deck to the house.
Outdoors you always want to see gray sheathing, protected by intelligent placement, properly supported and originating professionally.
My recommendation: it's the little things that count! And a knowledge of what and where. Colors usually mean things. Sizes usually mean things. Locations usually mean things. And as to outdoors, gray sheathing is the way to go. Look to make sure all that is correct!
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
