Backfeed question

[blugrs77]

I had to go to this job today that apparently another electrician screwed up. Here is the question. There are 2 120V circuits feeding a sign in a parking lot(top and Bottom), they share a neutral. one circuit keeps tripping. The 2 circuits are run through a contactor that is controlled by a photocell. When one circuit is energized and the other is off I get power on both wires. The wires are overhead wires at a Garden center so they are not color coded. I believe the wires are backfed and when both breakers are on one of the two trip. I read 120 to ground on both but not 208V across each. This leads me to believe they are on the same phase right? SO my question is how does this happen. If he crossed the neutral and a hot leg am I correct in my thinking that current is running through the sign and back out on the neutral tied to a hot leg and then back to the panel? So in essence the shared neutral of the sign and one hot leg are both being fed hot? Am I making sense> Any help would be great. I have never encountered this before as I am used to new construction.

 

[LarryFine]

Weclome to the forum!

If there's no voltage between the lines, the breaker shouldn't trip.

Will either breaker stay on by itself, or does the same one trip regardless of which is on first?

It sounds like you should start at the source end of the circuit, and make sure the supply is correct, then work your way downstream.

 

[blugrs77]

One breaker will hold. Its when you throw the other breaker on that the one trips...dead short? If I only throw one on then I get line voltage on both wires despite the fact that one of the breakers is off. With the new code they should probably be on a two pole breaker but they are not. I read 120V on both lines individually to ground but no 208V across the two despite them both being hot. The entire circuit is about 500 feet long with 3 points that are barrel crimped at three wooden poles. So I believe at one of the splice points he may have lost track of the neutral but if you crossed them would that cause line voltage to come back to the panel?

 

[blugrs77]

Also it is always the same breaker that trips when the two are both turned on. Sorry I forgot to put that in there.

 

[tallgirl]

Sounds like a dead short between two different phases. When #1 is open, the circuit is powered by #2. When #2 is open, the circuit is powered by #1. With the dead short, you always see 120V L1 or L2 to N. When you close both, there's a dead short and the more sensitive (or worn out ...) breaker trips.

Disconnect the wires in whatever junction box feeds the sign, open both breakers, then meter. You should get an open circuit, but I'm betting you see continuity between L1 and L2. You won't see continuity between L1 or L2 and N, because if there was, you wouldn't be able to close each breaker separately.

 

[cadpoint]

Since your touching it, put a dual breaker in or a tie bar between the two breakers!

...they share a neutral. ...

After you finish solving the 1st problem...

Welcome to the Forum ! :grin:

 

[hurk27]

What if a 240 volt load is also at the sign, and it is then the one becomes over loaded, it would also feed back through the other leg is there any lights also on this sign like to light the parking lot?,it is possible that the two circuits are tied together, but if one side of the sign does go out when the one trips, then it is more likely there is another load across the two hots such as a 240 volt load that is causing the one circuit to be over loaded, could be a bad 240 volt ballast, not sure whats all on the sign, but I would isolate the circuits at the sign then try to see if the breaker holds, if it does check to see if you have 240 volts at the sign, if so then if you do work on signs check for any 240 volt loads, if so then test these 240 volt loads to see if there is a problem, IE shorted ballast, bad fixture wiring? also two bad ballast wired across the two hots will also act like a 240 volt load if the neutral was disconnected but still connected between these two ballast.

best way is to isolate and concur, one circuit at a time, and map the circuits in your mind or on paper.

Oh and I'll bet you it will be the last thing you check.

 

[Dennis Alwon]

Here is my guess, If you have power on the circuit that tripped then I would say it is fed in a loop on opposite phases. So phase A feeds one part of the sign, then phase B feeds the bottom but is fed from 2 different circuits.

Now do both signs work when one breaker is tripped? That would be the tell tale sign-- no pun intended.

 

[blugrs77]

Just to add some more info. They are two 120v circuits. One feeds the top part of the sign and the other feeds the bottom part. If I throw on the breaker the top part of the sign works or at least the breaker holds. It's when I throw on the breaker for the bottom part of the sign that I run in to issues. The garden center had their sign guy come out and he says everything is working fine on his end. They also had another electrician come out and he says we must have crossed a wire somewhere in replacing the triplex cable. I didn't do the original work but the company I work for did so we have to fix it and since I am the on call guy this weekend I have to fix it. I am definitely missing new construction right now and not loving service work.
@tallgirl I hear you with the dead short but as everything was functioning properly prior to replacing the cable I am wondering how it could be dead shorting now. Once I get a lift and can actually troubleshoot more than just at the panel maybe it will shed some more light on the subject for me

 

[blugrs77]

@Dennis When you say fed in a loop can you explain a little more to me? How would this happen? I keep trying to draw it out and visualize it but the sign is huge and other then the overhead wires being bugged to the 3 wires coming out of the bottom of the top sign I can't get in the sign. Could it be wired in a loop accidentally when the other guy re wired the overhead conductors that the truck tore down? I am trying to picture the loop...like in a series circuit? I don't know why this is giving me so much trouble

 

[Dennis Alwon]

I am saying the bottom part of the sign is fed with 2 feds from opposite phases. If you turn both on you have a 220 short. If either circuit is on then the sign works. If that is the case then that is probably your problem. I have seen this in houses a few time where the electrician accidentally fed the cir. with 2 feeds from the same cir. In this case it won't trip but if they are opposite phases it will trip. Tallgirl basically is saying the same thing.

 

[hurk27]

I would go and isolate the connections at the sign, cap all three wires (for safety), go and turn on the power and tape up the photocell, return to the sign, check the voltage between each hot and neutral, and between the hots, to make sure you dont have the bottom hot switched with the neutral, if this happened then your guys have sent 240 volts through the ballast of the bottom portion of the sign, and most likelly fried the ballast and now that there shorted, the top of the sign is now working with reversed polarty. make sure when you take apart any wire you temperary mark with tape or something which wire are connected to which. so if the neutral and bottom hot was switched, you can follow it by the voltage readings you get in the last step above. not marking the triplex if all wire look the same is a big mistake, and can cost when stuff like this happens, HO sign ballast are not cheap but at least it is fixable.
Also document what you find so you can show your boss as to what happened, because those who are responcable need training so this wont happen again.


the way it should be wired
H___LB___N___LT___H

the way it was wired if N and H was swapped
H___LB___H___LT___N

LB= Bottom LT= Top of sign
L= load
H= hot conductors
N= neutral conductor

this is what it would look like if wired with switched hot and neutral top would work with reversed polarity, bottom would have 240 volts normal neutral would be middle conductor.

The biggest question I have is you said this sign is fed via a triplex, which would be two hots and a neutral, where is your EGC? is there a bare messager wire? if not you have a sign that can be a electrocution hazard and this needs to be address ASAP! grounding electrode can not be used inplace of an EGC period!

 

[blugrs77]

@hurk27 Yes there is a bare twist messenger wire so I guess the correct terminology would be a quadplex? The guy who put it up kept calling it a triplex and burnt it in my head. Thank you for the help everyone and if anything else comes to mind keep me posted. I have been so used to just wiring things new for the last 10 years I never really had to do much of this troubleshooting stuff. I thought I knew a good amount until I got in to this service stuff. I see more crazy wiring stuff now then I ever imagined.
I do know though that failing to mark your wires always leads to this and I am not sure why my coworker failed to do so. Nothing worse then trying to fix someone else's f'up especially when that f'up is 20ft in the air.

 

[hurk27]

@hurk27 Yes there is a bare twist messenger wire so I guess the correct terminology would be a quadplex? The guy who put it up kept calling it a triplex and burnt it in my head. Thank you for the help everyone and if anything else comes to mind keep me posted. I have been so used to just wiring things new for the last 10 years I never really had to do much of this troubleshooting stuff. I thought I knew a good amount until I got in to this service stuff. I see more crazy wiring stuff now then I ever imagined.
I do know though that failing to mark your wires always leads to this and I am not sure why my coworker failed to do so. Nothing worse then trying to fix someone else's f'up especially when that f'up is 20ft in the air.

Or having to face the owners of this sign to tell them they need new ballast in half of the sign that was caused by one of your company personnel, for the last 10 years service work was most of what I did for my last company before it shut down, and we also didn't have some of the brightest installers if you could call em even that, so it was a humbling experience to deal with the out come of their effects with customers, just never could get the owner of our company to understand why we needed to quit hiring every jo-blow who could twist two wires together and make a light come on. but I know where it was coming from, low cost employees. what he couldn't see was the cost of the repairs and replacements, it was costing him big time in the long run as well as the lost customers. but thats what happens when you run a company with blinders on, back then when we were running like gang busters it was easy to over look the details, and now details are so ever important and in the end we all wound up out of work, so now I have been working hard to build my own company up, and create a small living out of what has been dealt to us in this economy.

but as far as doing trouble shooting, and service calls, that was my most liked position, as being very hyperactive, I much liked moving from one location to the next, being on the go, not stuck in one place for weeks on end, and the harder the problem the better, also control design, system design, is one of my prides, building a control system from nothing but a sequence of operation is just heaven for me.

 

[gar]

101001-2313 EST

blugrs77:

hurk27 has probably described your problem.

However, your description is confusing. With a little more work at the panel you could answers some questions that would make remote troubleshoot easier.

If at the panel you disconnect the wire on the output side of the breaker that trips, then energize both breakers. From neutral to each breaker you should read approximately 120 V. What do you read between the two breakers? If it is the assumed three phase Y panel, then the reading should be about 208. Is it? If it is 208, then the problem is probably as hurk27 described.

There is the possibility you have to look for a short in the wiring somewhere.

If I understood the comments correctly the breaker for the top part of the sign always holds, that is never trips.

If the top breaker is not closed, then the bottom breaker can be closed and it remains not tripped. What part of the sign lights up and how bright with the top breaker off and the bottom breaker on? How much current flows from the bottom breaker? Then switch the bottom breaker off and the top breaker on and now what is the current from the top breaker?

.

 

[hurk27]

The only thing I would not recommend is sending anymore 240 volt current into this sign with the possibility of maybe that one ballast being shorted might have protected any others that might be paralleled on the feeds to the bottom of the sign, hitting it again with high amprage short circuit current might finally open the short taking out more ballast, just might save having to replace all the ballast on the bottom circuit, but it could have taken all the ballast out but why take the chance?


The method I would use would be to disconnect the triplex at the sign make sure both breakers are off, and disconnect any other loads that might also be on these two circuits, such as the photo cell or timer motor if they are also supplied by these two circuits, leave the neutral connected and EGC at the panel, at the sign test each conductor of the triplex to the EGC, the neutral will be the one that has continuty to the EGC since they are tied together at the MBJ in the service, then while at it, was it the one connected to the neutral of the sign wiring? now you know which two are the hots also and can just rewire it here, marking the neutral with white tape this time

 

[tallgirl]

I am saying the bottom part of the sign is fed with 2 feds from opposite phases. If you turn both on you have a 220 short. If either circuit is on then the sign works. If that is the case then that is probably your problem. I have seen this in houses a few time where the electrician accidentally fed the cir. with 2 feeds from the same cir. In this case it won't trip but if they are opposite phases it will trip. Tallgirl basically is saying the same thing.

I ran into this in a building in Katrinaland, except some "electrician" fed the second floor "backwards" from the first floor and then tied the two floors together (L1 on 1st floor looped to L2 on 2nd, L2 on 1st to L1 on second ...). Their solution was to pull the meter for the first floor since they mostly used the second floor.

It was a fun time in my life. Glad I don't go there so much anymore ...

 

[mxslick]

After reading through all this and getting a major headache, I see only one possible cause, which was alluded to earlier.

The OP needs to check the wiring at the feed end of the triplex....and wear gloves as he WILL find that the bare neutral has been connected to one of the hot legs. (And one of the hot legs is connected to the neutral.)

At least that' show it looks from here.

 

[sameguy]

not on gfci correct?

 

[billsnuff]

just for mxslick

just for mxslick