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

Product ID: 272328882

Details

Brand
KLEIN TOOLS
Color
Multi
Item Weight
4.16 ounces
Power Source
Corded
Style
Receptacle

📊Backlit LCD for clear voltage & wiring readout

⏱️Precise GFCI trip time measurement

🛡️CAT II 135V safety rating for reliable testing

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AR$97899

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Description

⚡ Test Smarter, Work Safer — The Pro’s Choice for GFCI Confidence

TRIP TIME PRECISION - Measure GFCI trip times accurately to troubleshoot faster and avoid costly downtime.
AUTO HOLD CONVENIENCE - Lock readings effortlessly on hard-to-reach outlets—no more guessing or repeated tests.
BRIGHT LED INDICATORS - Quickly identify wiring issues with vivid green and red LEDs for instant visual confirmation.
ADVANCED FAULT DETECTION - Patent-pending tech spots open neutral & ground faults others miss—boost your safety game.
CRYSTAL CLEAR LCD DISPLAY - Instantly read voltage and wiring status with a large backlit screen, even in dim spaces.

The Klein Tools RT250 is a professional-grade GFCI outlet tester designed for North American 3-wire 120V receptacles. Featuring a large backlit LCD, it provides clear voltage readings, identifies common wiring faults including open neutral and ground, and measures GFCI trip times with precision. Its CAT II 135V safety rating and bright LED indicators ensure reliable, safe, and efficient electrical testing, making it an indispensable tool for electricians and serious DIYers alike.

Specifications

ASIN	B08QW7K1JJ
Best Sellers Rank	#1,343 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #3 in Voltage Testers
Brand Name	KLEIN TOOLS
Color	Multi
Customer Reviews	4.8 4.8 out of 5 stars (11,487)
Global Trade Identification Number	00092644693212
Included Components	2 x 1.5V AAA Alkaline Batteries and Manual
Item Dimensions	4.53 x 1.16 x 2 inches
Item Type Name	Klein Tools RT250 GFCI Receptacle Tester with LCD Display, for Standard 3-Wire 120V Electrical Outlets
Item Weight	4.16 ounces
Manufacturer	Klein Tools
Maximum Operating Voltage	120 Volts
Measurement Type	Voltmeter
Minimum Operating Voltage	120 Volts (AC)
Model	RT250
Part Number	RT250
Power Source	Corded Electric
Specification Met	CSA
Style Name	Receptacle Tester
UPC	092644693212
Unit Count	1 Count
Upper Temperature Rating	60 Degrees Celsius
Warranty Description	Please refer to attached warranty document for information

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