Is it ok to use this tester for the 20a power outlet (not gfci)?

A Very good question, since the device name starts with GFCI. Answer is YES, because despite the name, the device is mainly a regular outlet tester, checking cross wiring, missing wiring, and reporting voltages, all worthwhile tests for non-GFCI outlet. Also works with 15amp & 30amp with adapter. Thanks for the question.

Most of the outlets in the basement where i'm living say >6.00 when i test them. are they wired wrong? is it safe to use the outlets?

It is not safe to use those outlets. If an outlet is good, the green light on the tester should illuminate, and the display should register a voltage close to 120 volts.

Is this accurate enough to measure with some confidence which is the most upstream receptacle in a circuit for choosing gfci location?

Resolution accuracy inadequate for that task. Displayed digits determines resolution. In this case, meter accuracy will not be and can not be better than +/- 1 volt because digital resolution is always +/- 1 in the least significant digit. Example: If measuring a true, standardised 120 volts, meter might display 119V (-1), 120V or 121V (+1) if calibrated/adjusted correctly. You've got the right idea, but this meter displays only 2-1/2 digits (120V). You will need a meter with 3-1/2 (120.0V) digits displayed. Worst case resolution at 120 volts will then be ~0.1V (100 mV). You need to see those millivolts differences. Get one with stated accuracy and preferably calibrated. Cheers!

What will happen if you press the gfci test on a non gfci? will it do nothing. or will it trip the breaker in the panel, and i have to flip the switch

The amber lights on the tester will blink, nothing will trip.

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Klein Tools RT250 GFCI Outlet Tester with LCD Display, Electric Voltage Tester for Standard 3-Wire 120V Electrical Receptacles,Green/Red

Klein Tools RT250 GFCI Outlet Tester with LCD Display, Electric Voltage Tester for Standard 3-Wire 120V Electrical Receptacles,Green/Red

Product ID: 557085750

Details

Brand
KLEIN TOOLS
Colour
Multi
Item Weight
0.26 Pounds
Power Source
Corded
Style
Receptacle

📊Clear LCD readout

⏱️Precise GFCI trip time display

🛠️Detects open neutral & ground faults

S/.336

Duties & taxes included

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Sold byAmazon PeruDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

⚡ Test smarter, work safer — the pro’s choice for flawless electrical checks!

SPEEDY SAFETY CHECK - Trip time display lets you troubleshoot GFCI devices with professional precision.
HANDS FREE CONVENIENCE - Auto-Hold feature locks readings for those hard-to-reach outlets—work smarter, not harder.
INSTANT VOLTAGE CLARITY - Large backlit LCD ensures you never miss a detail, even in dim spaces.
TRUSTED PROFESSIONAL GRADE - CAT II 135V safety rating and Klein’s legacy mean reliability you can bet your career on.
FAULT FINDER EXTRAORDINAIRE - Patented detection for open neutral and ground faults keeps your projects fail-safe.

The Klein Tools RT250 GFCI Outlet Tester is a professional-grade electrical tester designed for North American 3-wire 120V outlets. Featuring a large backlit LCD for clear voltage and wiring fault readings, it uniquely displays GFCI trip times and detects open neutral and ground faults with patented technology. Its auto-hold function and bright LED indicators enhance usability, while a CAT II 135V safety rating ensures dependable, safe operation trusted by professionals since 1857.

Specifications

Asin	B08QW7K1JJ
Best Sellers Rank	#388,509 in Home Improvement (See Top 100 in Home Improvement) #582 in Multi Testers
Brand Name	KLEIN TOOLS
Colour	Multi
Customer Reviews	4.8 4.8 out of 5 stars (10,989) 4.8 out of 5 stars
Global Trade Identification Number	00092644693212
Included Components	2 x 1.5V AAA Alkaline Batteries and Manual
Item Dimensions	11.5 x 2.9 x 5.1 Centimeters
Item Length	4.5 Inches
Item Type Name	Klein Tools RT250 GFCI Receptacle Tester with LCD Display, for Standard 3-Wire 120V Electrical Outlets
Item Weight	0.26 Pounds
Item Width	2 Inches
Manufacturer	Klein Tools
Maximum Operating Voltage	120 Volts
Measurement Type	Voltage Tester
Minimum Operating Voltage	120 Volts
Model Number	RT250
Number Of Items	1
Part Number	RT250
Power Source Type	Corded Electric
Specification Met	CSA
Style Name	Receptacle Tester
Unit Count	1 Count
Upc	092644693212
Upper Temperature Rating	60 Degrees Celsius

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