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Old 11-08-2008, 10:30 AM   #1
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Where is my power going??


I seem to be loosing power when I plug anything in to my outlets.

I have a detached garage that has a 100amp box that is fed from the main house 200amp box. In the 100amp box I have two circuits one 20amp for the lights and one 30amp for the outlets.

A couple days ago only half of my lights in the garage (2 rows of Florescent) would come on and the garage door opener was not working.

I got out the multi-meter and started troubleshooting. I had 120vac on each leg of the power coming into the 100amp box but when I put a load on it (any load) it drops to 40vac or less?????

Anyone run accross this before?

Jeff

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Old 11-08-2008, 11:49 AM   #2
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Where is my power going??


High amperage connections need to have the lugs tightened with a torque wrench to the manufacturer's specifications for the breaker, panel, etc. If this was not done, your wires may be coming loose.

Aluminum wiring needs to have anti-oxidant "goop" applied before installation. If this was not done, the wires may be oxidizing at the connections.

Check your main connections going to the garage...

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Old 11-08-2008, 12:05 PM   #3
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Where is my power going??


P.S. And of course if you are not comfortable doing the above, call an electrician.

And of course before working on any wiring (if you know how to do this work), turn off the main power first. Then verify that power is off to the whole house by turning on lights, etc.

"Turn off power, then verify power is off!"
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Old 11-08-2008, 12:18 PM   #4
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Where is my power going??


All of the above, plus. . .

Quote:
Originally Posted by Jeff Gilchrist View Post
but when I put a load on it (any load) it drops to 40vac or less
So there is a connection with 80v across it and a few amps through it.
It should be pretty hot.

It could also be a wiring problem in that a load (a lamp or something) is somehow wired in series with the rest of the loads, instead of all the loads being across the 120v line.

Speaking of which, if it's in more than one place in your house it could be a high resistance in the neutral connection.

BTW, I wouldn't use my real name on the Internet.
Stuff about me is already splattered all over CyberSpace, and I've been relatively careful.

Last edited by Yoyizit; 11-08-2008 at 12:21 PM.
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Old 11-08-2008, 04:56 PM   #5
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Where is my power going??


When you say it drops to 40 volts, are you saying from 240 or from 120?
If from 120, do this.
Turn on a relatively heavy 120 volt load (a microwave for example, or 120 volt heater) and measure the volts across each leg to neutral at the panel. If one went down (as before) but the other went up, you have a bad neutral.
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Old 11-08-2008, 07:47 PM   #6
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Quote:
I had 120vac on each leg of the power coming into the 100amp
Ah, but.....did you have 240V between them?

The 120V may have been backfeed. ALWAYS test phase to phase.
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Old 11-09-2008, 06:52 PM   #7
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Thanks for all the help everyone!

FYI I learned my limited electrical skills through necessity because of several tours in the middle east. I am in no way or would ever consider myself a skilled electrician. I really do appreciate the help of real professionals.

I tried tightening all terminal connections--no help.

I removed the power leads at the 100amp box in my garage. I am getting 121v on one lead and 109v on the other.

I hooked up the lead showing 121v to the box and it will run my plugs and lights no problem.

I hooked up the lead showing 109v and it will not run anything, when I put a load to it the voltage drops to 40v +-5.

I tested the voltage at the supply point (200amp box in main house) 122v at both leads. To me it looks like there is a problem somewhere between my 200amp box and the 100amp sub-panel in the garage.

The house and garage are only 3 yrs old and the wiring is aluminum. I checked for oxidation but all ends look good.

I'm good as long as I don't need 220v, problem is, I will need it in the summer to operate my pool pump.

Before I start digging this line up I wanted to check here one more time.

Thanks,
Jeff
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Old 11-09-2008, 07:59 PM   #8
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It does sound like a faulted hot conductor.

Aluminum wire is notorious for that. If water gets on the wire it tends to turn to white powder.

No j boxes between source and garage???

Direct buried cable??
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Old 11-09-2008, 08:28 PM   #9
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Quote:
Originally Posted by Jeff Gilchrist View Post
I hooked up the lead showing 109v and it will not run anything, when I put a load to it the voltage drops to 40v +-5.
How much load? What kind of load? The normal 1/4 ohm source impedance of outlets should only drop 2.5v under a 10A load.
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Old 11-10-2008, 07:27 AM   #10
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Quote:
Originally Posted by Jeff Gilchrist View Post
...I tested the voltage at the supply point (200amp box in main house) 122v at both leads. To me it looks like there is a problem somewhere between my 200amp box and the 100amp sub-panel in the garage...
At the supply point, I assume this would be a double pole breaker for the garage?

Did you use the screws on the breaker for this test?

Did you also measure the bare wires coming out of the screw down lugs on the breaker?

If there is a bad connection there, the screws/lugs would give you a good reading, but the wires coming out of the screws/lugs would give you a different reading.
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Old 11-10-2008, 08:17 AM   #11
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Where is my power going??


Switch the wires and if the problem switches you can start digging.
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Old 11-11-2008, 12:33 PM   #12
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Where is my power going??


The below won't really help the Op with his posted problem, but it is a code violation and a safety issue....

Quote:
Originally Posted by Jeff Gilchrist View Post
In the 100amp box I have two circuits one 20amp for the lights and one 30amp for the outlets.
Im surprised no one else called the OP on this. No general purpose outlet circuit should be connected to a OCPD rate more than 20 amps. This 30 amp breaker must be changed out for no more than a 20 amp breaker. This depends on the size of the smallest wire being used in this circuit (if its 14 gauge it needs a 15 amp breaker, 12 can go up to 20 amps). Also, does the circuit the lights run on have 12 gauge wire running for the entire circuit? I suspect its all 14 gauge wire...
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Old 11-11-2008, 12:44 PM   #13
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Where is my power going??


What would you plug a 225 amp welder or 7.5HP air compressor into?
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Old 11-11-2008, 01:05 PM   #14
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What would you plug a 225 amp welder or 7.5HP air compressor into?

A dedicated circuit.
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Old 11-11-2008, 06:03 PM   #15
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Quote:
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A dedicated circuit.
Exactly!

The Op is clearly talking about general use outlets in the garage, not a dedicated circuit for a single outlet/use.

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