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: 756166332

Details

Brand
KLEIN TOOLS
Colour
Multi
Item weight
0.26 Pounds
Power source
Corded
Style
Receptacle

📊Backlit LCD for clear voltage & wiring readout

⏱️Trip time display for precise GFCI testing

🛡️CAT II 135V safety rating for reliable protection

€ 97.44

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Description

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

AUTO HOLD CONVENIENCE - Lock readings on hard-to-reach outlets—no more guessing or repeated tests on the job.
BRIGHT LED INDICATORS - Green and red LEDs provide immediate visual cues, speeding up diagnostics and reducing errors.
ADVANCED FAULT DETECTION - Patent-pending tech spots open neutral & ground faults others miss, elevating safety standards.
CRYSTAL CLEAR LCD READOUT - Instantly see voltage and wiring status with a large backlit display, even in dim spaces.
PRECISION GFCI TRIP TIMING - Measure exact trip times to troubleshoot GFCI devices faster and with confidence.

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 clearly displays voltage, wiring faults, and GFCI trip times. Its patent-pending detection system identifies open neutral and ground faults, while bright LEDs and an auto-hold function enhance usability. Rated CAT II 135V for safety, this compact, durable tester is trusted by electricians and managers alike to ensure electrical safety and compliance on every job.

Specifications

ASIN	B08QW7K1JJ
Best Sellers Rank	602,534 in DIY & Tools ( See Top 100 in DIY & Tools ) 705 in Circuit Testers
Brand	KLEIN TOOLS
Colour	Multi
Customer Reviews	4.8 4.8 out of 5 stars (10,941)
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 centimetres
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
Manufacturer Part Number	RT250
Maximum Operating Voltage	120 Volts
Measurement Type	Voltmeter
Minimum Operating Voltage	120 Volts (AC)
Model Number	RT250
Number of Items	1
Number of Packs	1
Power Source	Corded Electric
Specification Met	CSA
Style Name	Receptacle Tester
UPC	092644693212
Unit Count	1 1
Upper Temperature Rating	60 Degrees Celsius

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