How far away does a NCVT trigger?

Can a Non-Contact Voltage Tester beep when it's 4 feet away from live wiring?

Hi Mike,

Using my non-contact voltage tester yesterday I was amazed that it lit up and howled as I was walking through a room holding the unit at about shoulder height. (4' below ceiling). There were exposed live wires at the ceiling from a light fixture being replaced. Does this mean any danger was present just walking through the room?

Thanks - Bill B.

Hey Bill B.

I’ve seen this phenomenon a few times. It can occur when you have a large surface that’s electrically energized at 120 Volts, or a small surface that’s energized with thousands of Volts.

Example 1: Large Surface at 120 Volts

This can be an RV with a lost ground connection that’s energized with 120 Volts AC. I’ve had a NCVT beep from up to 3 feet away testing a 40 ft RV trailer that I intentionally energized with a 120-Volt Hot-Skin. While a bit scary, that 120 Volts is not going to jump the 3 feet gap. But touching a hot-skin RV with your bare hand can be very dangerous.

I’ve also experienced your exact scenario in my 101 year old house when I was replacing the Knob & Tube wiring. Since the hot and neutral wires are widely separated from each other there’s no cross-current nulling effect, and a NCVT tester can beep from a few feet away. I also discovered several old lighting fixtures that weren’t grounded (because there were no ground wires required in 1923), so the entire chandelier was energized with a low-current hot-skin Voltage. So yes, my NCVT beeped from several feet away. Not really dangerous since the available fault current was probably only micro-amperes, but something to be aware of.

Example 2: Small Surface at 11,000 to 17,000 Volts

One of my colleagues is a retired lineman for the local power company and he often carried a NCVT while looking for downed power lines in the woods after a storm. Since there’s a lot of branches and leaves on the ground it’s possible to walk right onto a live wire if you’re not super careful. He says that a NCVT has saved his bacon a few times since it will beep up to a dozen feet away from a high-voltage power line laying on the ground. I’ve not tried this experiment myself, but it makes sense.

Hope this helps

No, the electricity isn’t going to jump several feet (or even inches) to you from an energized hot-skin with 120-volts. But the NCVT is just doing its job!

Let’s play safe out there… Mike