I was wondering if anyone knew of a tool, exact name please, for determining the size of a wire for instances where no lable is visible? Some of these 4, 6 and 8 get hard to tell apart, especially in older wiring or wires with thicker insulation.
I have been searching the internet and I can’t find anything.
Until they are comfortable guaging wire some inspectors will go somewhere (big Box store< etc.) and buy a short piece of each wire size with the size on it to carry with them on inspections. 
I had an electrician friend supply me with samples when I was starting up.
Hi Larry, you must have sat in on one of my classes as that is just what I teach. wire guages are available but after many years of hearing horror stories of wires detaching from breakers and breakers becoming detached from busses I stopped recommending them.
Regards
Gerry
I have carried samples for years just in case. I’m pretty good at wire sizing, however sometimes, even with samples in hand, it gets difficult. I currently have to figure out if a wire is #4 or #6. The wire is about 30 years old and has very thick insulation which makes it look a lot bigger than it is.
Anyhow, I was hoping there is some sort of tool for helping with the sizing.
Do you know exactly what the tool is called? or who carries one?
When I first started out, I made this - I still use it, from time to time.
For anyone who might want a sizing tool, I finally found one.
Item #E659
Would not the simple wire stripping tool work? It does stripp the wire casing and not the wire, right? Tells you what size right on the tool.
Just a thought.
Marcel 
That would be:
Looks pretty tough to use that safely a panel, though…
Lookie…
The only SURE way to get it right is with practice and experience. The slip over or side guages simply can be wrong as insulation varies from type to type. The best method is to visit the supply house or supply store and get familiar with the different sizes…and I STRONGLY suggest making a CHEAT tool as Micheal has made…really helps you visually when looking in the enclosure.
Good advice, I noticed one guy pull out a 4" aluminum disk, with notches cut into it, and he went diving into a hot panel to ‘gauge’ live wires. I dove out of the room. 
tom
Now see…I can picture that…CLEAR…panel Grenade…and TOM dives on the floor in a fetal position…
To add to the visual, imagine me wimping “I don’t want to die, I don’t want to die”. 
I found what he tried to use. http://www.romazone.com/images/wire_gauge_tool.JPG
As you can see you need to slide it over the bare conductor. With live breakers, and close together, you can short out several wires at one. Wow what a pretty shade of blue.
tom
I carried a plastic caliper for a while and a card that had the sizes.
