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electrifier, Master Electrician
Category: Electrical
Satisfied Customers: 1633
Experience:  Electrical contractor and electrical inspector for over 24 years with phone and networking expertise
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I have lost power to my detached garage. It has 100 amp service

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I have lost power to my detached garage. It has 100 amp service from the main panel at the house. Two hot wires and one neutral go through a 1.5" conduit underground to a subpanel in the garage. At the main panel, I have 120 volts (after the 100 amp breaker) and it will power a light bulb. At the subpanel in the garage, I have 120 volts but it will not power a light bulb. I am using a volt meter to check for voltage, and a 25 watt light bulb to test for a load. Can you help?

electrifier :

Is everything in the house OK electricity wise?

electrifier :

What happens to the voltage when you turn on a load? Have you checked it with the power turned on?

Customer: replied 6 years ago.
The house electricity is fine. It has 200 amp service and feeds 2 subpanels inside the house and all circuits are operating. The main panel is just outside the back door, and inside are 2 main breakers - the 200 amp that feeds the house and the 100 amp that feeds the detached garage. Neither breaker has flipped off.

On the 100 amp breaker, on the output side (where the 2 hot wires are hooked that go to the detached garage, where they go into the conduit that goes underground to the detached garage), there is good voltage and good current. At the subpanel in the garage, there is still "voltage" but not enough current to even run a 25-watt lightbulb. Sometimes it will light the bulb with a very dim light, but if I add more load, even that current drops off.

The way I am testing this is first with a voltmeter with one lead on common and the other lead on each of the hot terminals. Then I do the same with 2 leads to a small light bulb. Before taking these measurements, I completely removed the breakers from the subpanel in the garage so that there would be no chance of any load drawing.

Something is happening between the house and the detached garage.
Is the wiring from the house to the garage aluminum or copper?
Is this wiring in conduit all the way or is it direct burial with conduit only to protect whee it emerges from the ground?
Can you see the type of wire insulation? Is it THHN or RHW or another type of insulation?
Customer: replied 6 years ago.
The wiring is aluminum, about 3/8" thick twisted with black jacket. The neutral is smaller. The house and garage were built in 1995. It is in conduit where it enters the ground and where it emerges out of the concrete floor in the garage. I don't know about the underground part. I am on my way out the door right now and will be back in 3-4 hours. I'll look at the markings on the wire when I return.
If you pull on one end and it moves at the other end the conduit would be continuous. If it feels like you are pulling against a tree chances are it is direct burial. Let me know when you find out the jacket and we'll go from there.
Customer: replied 6 years ago.
The detached garage is 60 feet away from the house. There is no play in the wires. So I dug down below the panel on the outside of the house and about 18" deep the conduit curves to horizontal and then ends. The wires are direct burial.

The 2 hot wires are black plastic jacketed SOUTHWIRE USE-2 600V XLP AWG2. There were many other markings that were mostly obscured, but something like this: 60 MILS SLK XLP SLN-NLP INSULATION 600 VOLTS AFR D36 (OR 1136) XLC TYPE USE2 AL TYPE SOUTHWIRE-W CULD AWG2. It probably has about 7 strands.

The common wire was smaller with very little markings. It is black plastic jacketed with 4 yellow stripes. It says AWG 4.
Hi, I'm Mike. The Electrifier is offline at this time. If you want I can assist you or you can wait for him. This is only an assist and when the answer is given I would expect that you accept it from Electrifier.
Customer: replied 6 years ago.
I am wondering if I have lost my electrician??
No you are online and the Electrifier isn't at this time again. I'll step in for him if you want.
Customer: replied 6 years ago.
Mike, the power is off in my detached garage due to some kind of problem between the main panel on the outside of my house and the subpanel in the garage. Are you able to review the notes between me and electrifier up to this time?
Yes I did and your discovery that the feeder is direct buried cable suggest that the feeder has failed. When you uncovered it was there a sand envelope around it? Was there a 2 foot loop at the bend in the PVC? Where are you located?

Edited by Mike G. on 10/18/2010 at 8:47 PM EST
Thanks Mike for helping while I was away.

It is common for failure of direct burial aluminum conductors given the chance of a pinhole in this outside jacket allowing moisture to enter the wire. The wire then turns to aluminum oxide, a powder, that does not conduct electricity. If you dig and find the failed portion of the wire you will find a bulge in the wire and maybe it's split open with white powder showing where there used to be aluminum.
What you must now do is replace this 60' feeder from the house to the garage. I suggest burying PVC conduit from one building to the other and pulling in the 4 wires required by the code. These can be either XHHW aluminum or THHN copper conductors. Price could be a factor here as copper is much more than aluminum. You need 2 hot wires, a neutral and a ground inside the conduit. You must not bond neutral and ground in the garage panel as they must remain separate after your main service panel. Ground rods are required at this garage building. If you have any additional questions please feel free to ask me here. I'll be glad to assist you further with this project even if you have accepted my answer. I'm here to see you succeed. Thank you for using Just Answer.
electrifier, Master Electrician
Category: Electrical
Satisfied Customers: 1633
Experience: Electrical contractor and electrical inspector for over 24 years with phone and networking expertise
electrifier and 3 other Electrical Specialists are ready to help you
Customer: replied 6 years ago.
The conduit in the detached garage is metal and goes into the ground. I assume that this creates the "ground rod" to which you refer, as the subpanel is physically attached to the conduit. I will dig up the line as you suggest, and install a new conduit and wires. I just hope that the original contractor was intelligent enough to run the garage conduit to the edge of the driveway. Otherwise I will be busting up concrete. Suggestions?
The code requires the conduit be run outside any concrete floor or drive. Hopefully it complied with the code.
The conduit is not a ground rod. You will need to drive 2 ground rods at the garage with a continuous grounding electrode conductor, placing the ground rods at least 6' apart. 8' apart is the best for 8' ground rods.