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Mabou 10-28-2007 12:47 AM

Unexpected behavior?
Hi Guys,

I cracked into a junction box in the basement with the intent of tapping in to power up a small light that I just installed. The junction box is used to run a couple of random light circuits on the first floor, nothing heavy hitting.

Anyway, I shut off the breaker to the circuit, and used my current sensor to double check that the junction box was indeed powerless.

I twisted the cap off of the neutral cluster of wires in the junction box and noticed small sparks jumping around on the ends. At the same time, my friend upstairs told me that the kitchen lights were flickering.

The junction box had no power running to it and the kitchen lights are on a separate circuit breaker.

Just to let you know, I double checked with the voltage sensor tool that there was no voltage in any of the wires in the junction box.

I have done a lot of electrical work in the past (though I am definitely not pro), I have never see sparks jump between neutral leads, on a circuit that is shut off.

Is this normal behavior?


slakker 10-28-2007 01:22 AM

I'm not pro electrician either, but it sounds like the return path for the circuit for the light in the kitchen is through a neutral in that junction box. Bizzare why they would have routed the circuits that way if that's the case....

frenchelectrican 10-28-2007 03:42 AM

What the sisuation you got there what we will called that term is MultiWire Branch Circuit.

it is common for resdentinal , commercal , industrail area.

what happend when you { OP } did disconnected the white wire on the MWBC there are few things will happend one ,, sure you will get the light flicker of course second thing .,, anytime you shut off one breaker or fuse and the other circuit still on it can hit ya back thru the netrual wire.

and the main golden rules with MWBC is make sure not only one circuit is off but other circuit there as well if you see two or more diffrent hot wire colours like black and red wires there that will tip you off to watchout [ sometime it not always be a MWBC at all it canbe used for 3 way switch useage or split circuit set up on the repectales ]

and if some reason if you lift the netural on the MWBC anything on 120 volts will go up high as 240 volt and smoke anything right the heck out of it

[ belive me i did see alot worst than this on commercal side which we deal much higher voltage as well 480Y/277 volts circuits are spooky one to dealt with it ]

but there some case someone ran the circuit and get the netural hook up at wrong location [ i did see that once a while but not too often ]

Merci, Marc

Andy in ATL 10-28-2007 05:34 AM

Although I do carry a current sensor in my pouch, I learned the hard way that they WILL lie to you.

ALWAYS confirm the power is off using a digital or analog meter. This is accomplished by turning off ALL circuits you think might be in the box, then CAREFULLY removing the wire nuts from the wires, (always in this order, Hot and then neutral) turn circuit back on, check with meter the hot to neutral, hot to ground, neutral to ground. When putting circuits back together go neutral first, then hot.

Only have circuits on when testing and ALWAYS wear safety glasses. As my French buddy said, you can easily FRY stuff in your house when you break the neutral connection with one half of a multi-wire branch circuit still hot. The neutral wire, contrary to popular belief, can and will KILL you dead!!!

There should be a hot wire in that box. There is a possibility that the white wires that you think are neutral are actually hot. Be extremely careful from this point foward. Hinky wiring is dangerous wiring.

HouseHelper 10-28-2007 11:02 AM

It is either a MWBC as Marc has pointed out, or an illegally shared neutral that is unfortunately quite common in older homes. Locate the breaker that controls the kitchen lights (the ones that went out when you disconnected the neutrals). You can look inside the panel (you have to take the cover off) to see if these two breakers are connected to black and red wires, respectively. If so, you have a MWBC. If not, you have an illegally shared neutral that should be corrected.

Mabou 10-28-2007 07:48 PM

Excellent replies, thanks gentlemen. I will look into it more deeply.

Take care,

BigJimmy 10-28-2007 08:57 PM


Originally Posted by Andy in ATL (Post 70276)
Although I do carry a current sensor in my pouch, I learned the hard way that they WILL lie to you.

Righto, Andy. The non-contact style rely on the user to create a coupling to ground in order to function. Stand on a fiberglass ladder with some well insulated shoes, and, well, you get the idea. Funny thing is that DVM's will lie like 5 year olds too. In fact, today I had an interesting situation where I had an outlet in my second floor kitchen that didn't work (yes, second floor, don't ask! :) ). Voltage at the recept terminals read 81VAC with my Fluke 87. When I traced everything back to panel, I expected there to be something else mixed in with the circuit (In the two years I've lived in my house, I've found more creative electrical than anyone would want to know about. Switched neutrals, anyone??!) but found that this particular recept was fed directly from a dedicated 20A breaker (kitchen small app. br. circuit). Circuit was run as MC cable and voltage reading from breaker terminal to neutral bus was 124VAC. I pulled out the old cable and replaced with a new one. When I was done, I did a continuity check on the old cable and found that the neutral was open. A visual check revealed where my HVAC guy had unknowingly (I'd like to think, anyway) cut through the armor and (luckily) opened only the neutral wire. So, I was seeing ghost voltage, probably enough leakage from open neutral to armor to read what I was seeing. When I read 124VAC from hot to ground at the recept, I should have known better.

I work in railroad signaling which is based on ungrounded systems. Whenever my meter tells me that full voltage is present, I always double check with an incandescent test light. Loading the circuit always tells the truth!

I always enjoy reading your posts, Andy, esp. the exchange going on with Honker and the spa!


Andy in ATL 10-28-2007 09:45 PM

Phantom voltage can be spooky. A solenoid "wiggy" type tester is really the best for go nogo testing. When I first started out the 30-40 VAC I'd get on various wires would have me convinced I was F'ing up... It is all about experience.

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