Don’t you have to measure it at the exposed conductor itself, and not around the sheathing in order to determine the amperage rating of the conductor??
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Evening, David. Hope to find you well.
I could be incorrect but, for an accurate determination, when measuring house hold 120/240V AC low voltage wire diameter, you only measure the diameter of ‘the bare wire itself.’ Not including the jacket. Insulator Jacket thickness has evolved over time…
www.InspectionWireGauges.com sells set of gauges is designed to help aid inspectors and professionals in determining the size of most modern NM, SE, and THHN-THWN electrical wiring. 1: Copper Wire Gauges (Red Color) 1990’s to present. 2: Old Copper Gauges (Green Color) 1950’s thru 1980’s. 3: Aluminum Wire Gauges (Silver Color 0r White Color).
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Different insulation will have different thicknesses from other types of insulation.
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Some kits have two ends, for the bare wire (if exposed), or the insulated wire including assumptions about the insulation. The second is fairly useful in my 50-100 year old home area, given the variation over the decades. There are too many varieties.
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This kit measures the overall jacket size, not the bare conductor. There may be other tools that can accomplish this.
Yes. And this kit specifies this in the description / manual / instructions…
This is all in the description on [Inspector Outlet]…(Inspection Wire Gauges - Complete 7 Piece Set with Cord – Inspector Outlet).
Also keep in mind older wiring had a thicker insulation, therefore the gauges supplied will not work with many older wires.
Do not force gauges onto wiring or into tight spaces. Doing so can cause breakage or damage to wiring/devices. IF the gauge doesn’t fit with ease, then its either an older wire with thicker insulation or it not the correct size.
Some aged wiring may have larger jackets. Service wire is the most common. Some specific brands used thicker insulation.
This kit only measures outside jacket size 
With all that said, this is not a perfect measuring tool but is to be used as instructed by the manufacturer and is a basic reference for what it was designed to do. Use it at your own risk.
Edit: I didn’t initially catch this @ryoung7 but your statement below is incorrect. Residential 120/240 Volt AC is NOT considered low voltage.
It will likely get you close because as stated the insulation thickness may vary. Also most modern aluminum conductors have compact strands so even if you measured the actual bare conductor for the same size AWG the compact stranded conductor would be smaller than a normal stranded copper conductor. It looks like these gauges may take that into account since they’re different for copper and aluminum.
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In Canada, low voltage is defined as 12–600 volts.
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In the US less than 1000 volts is considered low voltage. Here’s an excellent reference that explains various voltage systems.
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Thank buddy. Thank you Robert. I was going to post that but hesitated, because I can get pushback being a Canadian member.
I stand corrected, my bad 
I never realized under 1000V in USA was considered low-voltage (or 600V in Canada).
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Not at all, David. We all learn something new in this industry. Its a plus!
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If less than 1000 volts is considered low voltage, then why does your reference say that 270V and 480V is high voltage in the U.S.?
277 volts is a high voltage commonly used in the US for commercial and industrial applications, says the Electrical Technology reference.
480 volts is a high voltage commonly used in the US for commercial and industrial applications, says the Electrical Technology reference.
Even the reference appears to be contradicting itself:
Voltage Classes according to NSI C84.1-2016
- LV = Low Voltage: <1000 V
- MV = Medium Voltage: >1kV & <100 kV
- HV = High Voltage: >100 kV & ≤ 230 kV
- EHV = Extra-High Voltage: >230 kV but <1000 kV
- UHV = Ultra-High Voltage: ≥1000 kV
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Could be just common slang. In a commercial building there is typically two main voltage systems 208Y/120 and 480Y/277. It’s common to refer to the 480Y/277 as high voltage and the 208Y/120 as low voltage. Theyre both under the umbrella of the ANSI definition of low vovltage.
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high and low voltage depends on context and who is saying it. if i say it (low voltage), it is a dc low voltage system for internet , phones, or doorbells, but if a commercial electrician says it, it probably means under 1000v ac. notice i said probably. i’ve heard electricians refer to it differently depending on what they were working on at the time, like at a power plant or an apartment building. so while there is an official definition from ansi and the nec, it varies in practice when folks refer to it as high and low depending on context and who it is.
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To me this means that I need to be cautious and cognizant about what I call high voltage or low voltage.
My takeaway is to steer clear of those terms ‘high voltage’ and ‘low voltage’ since they have subjective meanings to people.
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Robert was correct. Low Voltage is anything up to 1000V. Medium Voltage is 1kV-35kV. High Voltage is anything above 35kV.
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According to whose definition(s)?
Agreed. Its kinda like when we don’t want to say Major or Minor, its just a defect and for a HI environment, its just voltage.
ANSI, IEEE, NEMA, NEC, NESC, American Electrical Engineers Handbook, American Electricians Handbook, and almost everyone else.
ANSI/IEEE Standard C84.1-2020.
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