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

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
0.26 Pounds
Power Source
Corded
Style
Receptacle

⚡Detects open neutral & ground faults

🔋Backlit LCD for clear voltage readout

⏱️Precise GFCI trip time display

21683 Ft

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Description

⚡ Power up your safety game with Klein’s smartest GFCI tester yet!

SPEEDY SAFETY CHECKS - Instant trip time readout 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.
FAULT FINDER EXTRAORDINAIRE - Detects common wiring issues plus patent-pending open neutral & ground faults for ultimate safety.
CRYSTAL CLEAR VOLTAGE INSIGHT - Large backlit LCD ensures you never miss a detail, even in dim spaces.
TRUSTED LEGACY MODERN PERFORMANCE - From a 160+ year family-owned brand, engineered for pros who demand reliability.

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 wiring faults including open neutral and ground, and displays GFCI trip times for precise troubleshooting. With an auto-hold function and over 160 years of Klein’s trusted craftsmanship, this compact, lightweight tester is a must-have for electricians and serious DIYers aiming for flawless electrical safety.

Specifications

ASIN	B08QW7K1JJ
Best Sellers Rank	#1,489 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #3 in Voltage Testers
Brand	KLEIN TOOLS
Brand Name	KLEIN TOOLS
Color	Multi
Customer Reviews	4.8 out of 5 stars 11,348 Reviews
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
Item dimensions L x W x H	4.53 x 1.16 x 2 inches
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	Receptacle Tester
Style Name	Receptacle Tester
UPC	092644693212
Unit Count	1 Count
Upper Temperature Rating	60 Degrees Celsius
Warranty Description	See Manufacturer Website

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