Residential Electricity

January 2, 2008 10:25 AM

Residential Electricity: Is it possible to measure current flow through romex in a non-invasive manner?

I am trying to identify which of 5 wires is on a particular circuit. The wires enter and exit this certain space in such a manner that I cannot determine their source or their destination. The wires are typical (for US) household romex 120v/15a. I want to cut one of these wires and put on outlet on that line. If I could measure current non-invasively, I could turn off the circuit at the breaker box and then I identify the wire I want. Is there a tool to do this? If not, any other suggestions?

posted by allelopath to home & garden (16 comments total)

My stud finder has a live wire detector so you could turn off all but one circuit and find it that way.
posted by zeoslap at 10:37 AM on January 2, 2008

You could use a non-contact voltage detector to identify the wire you want. But please discuss your idea with an electrician before you get started. There are many non-obvious ways to compromise the safety of the circuit.
posted by kc8nod at 10:40 AM on January 2, 2008

Yes.

What you need is a clamp-on ammeter.
posted by c0nsumer at 10:44 AM on January 2, 2008

Yes!

Well, my help is dependent on your definition of "invasive". If you have access to the wire iteself (meaning that you dont have to cut into a wall or pull out a socket, there's a dead simple way:

Use a clamp-on AC Ammeter.

Or, you can use an AC voltage detecting pen tool - though this is only useful at telling you which line is active (most helpful for figuring out which line is on a particular circuit, if you don't mind turning a selection of your circuits on and off a few times).

These tools are cheap and available almost anywhere that sells your basic digital multimeter. Both are useful additions to the toolbox of any curious person.
posted by terpia at 10:47 AM on January 2, 2008

Crap, sorry for not checking for similar answers on preview.
posted by terpia at 10:48 AM on January 2, 2008

A clamp-on ammeter won't work in this situation. If you clamp the meter around the romex, you will measure the current on both the hot and neutral wires Those currents are always (under normal circumstances) equal and opposite, so the meter will always read zero, no matter how much current is being drawn. You could split open the romex to clamp the meter around one wire, but that's probably undesirable in this case.
posted by kc8nod at 10:55 AM on January 2, 2008

what kc8nod says.
posted by rmd1023 at 11:33 AM on January 2, 2008

oh, one helpful suggestion instead of me just being negative girl -- if you are okay with shutting off the circuit in question, putting a tone generator fox and hound style cable identifier on the circuit and then see if you can determine if any of your wires are on that circuit that way. at that point, i'd then try skinning the romex and testing with a clamp-on ammeter. and then, again, make a small cut on the insulation of the hot leg and neutral leg and check with a voltage tester before you actually cut the wire.

cutting a live piece of romex with electrician's pliers leads to small half-moon divots blown out of the edge of the cutting pliers. and a big boom. and a flash. and sometimes, what looks like bits of copper spattered on the inside of your pliers.
posted by rmd1023 at 11:39 AM on January 2, 2008

You want a non-contact meter. Don't get the craftsman, get the green one they sell at Lowe's. It's about $14. Get the heavier load one too---it's like $2 more expensive and is waaaay better. It just looks for induction, but it does it with a blinky light and alarm. I did this a lot when we moved into our new building here, as one junction box might contain a hot red, black, green, and white wire, and sometimes carrying 220v across 3 wires with no ground. Fun!
posted by TomMelee at 11:44 AM on January 2, 2008

TomMelee:
Do you mean this:
Greenlee® Non-Contact Voltage Detector
http://www.lowes.com/lowes/lkn?action=productDetail&productId=158088-72068-GT-11&lpage=none
posted by allelopath at 12:21 PM on January 2, 2008

That's the one. Pay close attention, one's got like a 200v rating and the other has about a 600v rating, I got the 600, but so I can do 220's and 240's.
posted by TomMelee at 12:35 PM on January 2, 2008

If you just want to now if there is current flow, you can use a compass. The needle will swing away from N near the wire, if there is current flow. This is true (but falls off with distance more quickly) even if there are two wires carrying opposite current.

This will not, of course, give you an actual number for current flow, but should do the trick just to see if it is off.

Also, once you install the new outlet, you will have to have your handiwork inspected by a friendly IBEW member before your house is up to building code, I believe.

Good luck!
posted by noble_rot at 3:31 PM on January 2, 2008

Also, pardon my atrocious spelling. now -> know
posted by noble_rot at 3:32 PM on January 2, 2008

Also (heh), since it is AC, the compass needle will vibrate, not swing. There, I am finished.
posted by noble_rot at 3:45 PM on January 2, 2008

I have my doubts about a compass working. My reasoning being that a compass is influenced by the entire planet, and I'm testing one wire, which may have no current, but a wire 2 inches away might have current and influence the compass.

Anyway, I have purchase the tool recommended by TomMelee and will try it out this weekend.
posted by allelopath at 8:48 AM on January 3, 2008

I should have posted this yesterday, I see, but I think you could do this by getting a pair of headphones of any sort, wrapping aluminum foil around the phone plug to short it out (thereby completing the circuit for the purpose of inducing voltage across the earphones), and holding a single straight piece of phone cord in one hand up against each Romex in turn (don't wrap it around to avoid the canceling effect k8nod astutely mentions). The one where the 60 cycle hum is loudest will be the one with current running through it.
posted by jamjam at 10:37 AM on January 3, 2008