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M Bagley 10-03-2012 10:24 AM

voltage drop
 
I have a shop behind my house with power feeding from my house panel. I have 3 #6 wires going from my home panel to my shop. At my home panel I have 117 volts on each leg and the same in the shop. When I turn on the lights in the shop one leg goes to 102 volts and the other goes to 133 volts in the shop panel. I have swapped the #6 wires around and it doesn't change. I swapped breakers in the shop panel and it only reverses the readings on the 2 sides in the shop panel. All grounds are hooked up correctly. I am at my wits end with this. When I turn on the shop lights, which by the way are florescent, only two bulbs come on. I can plug up a skill saw into another circuit and turn it on and all the lights come on and brighten. In addition the distance from the house to the shop is about 150 feet and the #6 wire is aluminum and underground.

Jim Port 10-03-2012 11:11 AM

The voltage flucuation is a classic sign of a loose neutral. Check your connections. If this happens at the house also you may need to get the power company involved.

AllanJ 10-03-2012 11:15 AM

Aluminum? Underground?

Keep in the back of your mind that the wires could be "rusted out" in the middle. It is not unusual for woodchucks or other underground/surface animals to chew the insulation of buried cables.

ddawg16 10-03-2012 12:16 PM

Repeat the above tests but make the measurement at your load center. This will tell you if the issue is up stream or down stream.

I had exactly the same problem at my house a few years ago....I was measuring the voltage at me load center...

Called the PoCo.....they came out...wife gets a knock on the door..."Mam....we will be replacing your transformer".

Neighbors were haveing the same issue...they thought it was their problem....they thanked me after that....

M Bagley 10-03-2012 02:56 PM

Changed out the load center in the shop and still the same problem. I bonded the neutral and the ground bars per local code and no change. I have tested the voltage at the house panel both with a load and without a load and there is no change. I have 118 volts on both legs. When I swap the wires in the shop panel to the other side of the panel the low voltage stays on the same side but when I change the load to the other side the low voltage changes sides and the other side of the panel shows 133 volts. The more load i put on the one side the higher the voltage goes on the other.

ddawg16 10-03-2012 02:59 PM

Have you made the same check at your breaker? The one in the main load center at the house...

You have to keep working back until you find the problem

Glennsparky 10-03-2012 03:38 PM

Like Jim Port said, loose neutral. Aluminum wire is finicky. Aluminum "rust" is not visible and not conductive. Wire brush the Al wire at every connection. Use antioxidant grease, NoAlOx or similar. Torque the connection to spec.

AllanJ 10-03-2012 04:36 PM

For just a few minutes, bond the ground and neutral in the shop subpanel while leaving them bonded at the main panel. With some things running, measure the voltages again. This could uncover a problem with the neutral going between the house and the shop.

I take it you measured the voltages at the house with different loads turned on and found everything okay.

M Bagley 10-03-2012 08:56 PM

The panel in the shop has the ground and neutral bonded. The panel in the house is an older one and it is the same connections. I have measured the panel at the house with a load on at the shop and it is a steady 118 volts while with the same load at the shop is 90-98 one the loaded side and anywhere from 130 to 145 on the other. If you load up both sides about equal then the voltage stays around 115 on both sides. This beats all I have ever seen. I am no practicing electrician but have done quite a bit of electrical work over the years.

Jim Port 10-03-2012 09:17 PM

Is the feeder to the garage a 3 or 4 wire? This still sounds like a loose or failing neutral.

ddawg16 10-03-2012 10:13 PM

Do this.....take a long piece of wire....16g is fine......even smaller is fine....long enough to reach from the main load center to the garage...

DO NOT connect it to the sub panel in the garage

Connect one end to the neutral buss in the main load center.....take the other end and connect it to one of your meter leads....

Now, repeat your tests at the sub panel....except your now going to measure the voltage between your main load center and your sub panel neutral as well as your two hot legs.

dmxtothemax 10-03-2012 10:17 PM

You are certainly lossing power somewhere,
Have you measured the voltage at the supply end(in main panel)
and compared this to the voltage at the shop end ?
If the voltage drop is minimal at supply end,
then this indicates for sure that the problem lies
in the connecttions/cable in between.

Was this a problem in the past ?
Or has it ssuddenly got worse ?

Aluminium cable joints are well know for coming loose overtime.
Cause the cable expands as it heats up.
So over time joints tend to work loose.

Check and redo every joint and connecttion.
Be careful be safe !

If that doesn't work,
then the cable itself might be damaged or faulty.

mpoulton 10-04-2012 02:20 AM

You have a loose neutral connection or failing neutral cable between the house panel and the shop subpanel. Based on your measurements, this is a definitive diagnosis - that is the problem. The underground cable may be bad, or it could be a connection issue at one end or the other.

I notice that you mentioned that the neutral and ground were not bonded in the subpanel originally. They MUST be bonded. It's not a "local code" issue, it's a critical safety issue. Your grounding system is worse than useless without that connection - it's a hazard. Since you have a 3-wire feeder to the subpanel, the system relies completely on that neutral-ground bond to protect against ground faults. Without it, you have a very dangerous situation. If you find that you have to replace the underground cable, the current code requires that you use a 4-wire feed instead of 3-wire. With a 4-wire feed, the ground wire back to the main panel (where neutral and ground are already bonded) takes the place of the N-G bond in the subpanel, so you do not bond them in the subpanel. But as long as your feeder is 3-wire, you MUST bond them in the subpanel.

AllanJ 10-04-2012 09:26 AM

Quote:

Originally Posted by ddawg16 (Post 1023378)
Do this.....take a long piece of wire....16g is fine......even smaller is fine....long enough to reach from the main load center to the garage...

DO NOT connect it to the sub panel in the garage

Connect one end to the neutral buss in the main load center.....take the other end and connect it to one of your meter leads....

Now, repeat your tests at the sub panel....except your now going to measure the voltage between your main load center and your sub panel neutral as well as your two hot legs.

This is not meaningful unless the loads in use out in the shed during the voltage tests are connected to this long piece of wire instead of the garage subpanel neutral for the return current.

M Bagley 10-04-2012 02:49 PM

Thank you everyone for the help. I finally fixed the problem. It WAS the neutral somewhere underground. We ran a temporary neutral and the problem is gone. Checked connections at both ends and still had the problem with the old neutral. Only thing left to do is bury a new wire. Probably going to replace all the wires, if one went bad it's just a matter of time till another one does. Again, thank you.


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