Shed Power Oddity

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Old 02-01-15, 08:38 AM
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Shed Power Oddity

My brother just moved into a new-to-him home. It has a shed about 100 feet from the house. Inside, there is an ancient electrical box with a blade switch. The switch controlled a light, and there is an outlet in an old, round box beneath the box. This stuff appears to be decades old. There was the skeleton of an electrocuted mouse in the box, too. There were Christmas lights strung all over the place - newer ones - so we assume that there was power out there.

There are 2 wires feeding the box from an underground conduit, look to be 12 or 10 gauge. One's black, the other is white. There is no ground (!). The wires originate in a box in the basement that are on the same circuit as the well pump (on a 20A breaker).

Setting the ground issue aside for a moment....my brother installed a new 20A switch and outlet. He brought the black and white wires to either side of a new outlet, then switched the hot (black) via the switch and ran the neutral directly to the lights.

The light comes on very dim and the outlet won't even charge his cell phone. When he checked the voltage, he's got 123V in the basement and 118 out in the shed. I would consider this normal loss for that run. The question is why does it appear there's very little current (amperage)?

Back to the ground - what's the correct way to introduce a ground to this situation? I assume a grounding rod and a proper subpanel in the shed?
 
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Old 02-01-15, 09:04 AM
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What was the voltage while the light was on dimly?

Measure between the two receptacle slots or between the gold screw and silver screw on opposite sides of the receptacle.

The wires could have been corroded in the middle underground. There may have been enough conductivity to show 118 volts on your meter (which draws a few milliamperes) but not enough conductivity to deliver full power to the lights (perhaps 100 watts or a little under one ampere).

If a problem resulgint in a high resistance spot where none should be then a voltage difference will occur at tht spot. Let's say that 40 volts were lost there. That leaves 80 volts for the light. The amount of voltage dropped at the high resistance point depends on the number of amperes attempting to flow through; more amperes equals more lost volts. Ohm's Law: volts across any two points in the circuit for example the two sides of the corroded spot on the wire, equals amperes times the resistance between the two points.

The light is also a high resistance item. If everything were working correctly there would be almost no resistance in the black wire, significant resistance in the light bulb filament (at glowing temperature) and almost no resistance in the white wire. Almost all of the 120 odd volts would be correctly dropped (and measured as such) at the light which glows brightly.
 

Last edited by AllanJ; 02-01-15 at 09:53 AM.
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Old 02-01-15, 09:04 AM
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The voltage drop you are experiencing would be considered ok for 100', but normally you will only see voltage drop when there is a load on the circuit. However it is well within tolerance so that is not your issue.

With 118 volts you lights should be working normal. Dim lights are an indication that he wired something in series instead of parallel. The issue is not lack of current. Current will flow when there is a load and voltage present. Have him recheck his wiring, or post here what has been done (pictures) and we can help figure it out.

The only "real" way to fix the ground is to pull a new ground wire. If this was run in RMC then the conduit would be the ground. Since it is an existing installation, I would recommend installing a GFCI device at the first point of entry to the shed.
 
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Old 02-01-15, 09:38 AM
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what's the correct way to introduce a ground to this situation? I assume a grounding rod and a proper subpanel in the shed?
No. A ground rod* only provides reduction of atmospheric charges. It can not be substituted for a EGC (Equipment Grounding Conductor) which provides a low impedance circuit back to the breaker to clear faults (trip the breaker in case of a short to the metal case of a device or appliance).

As noted if you have RMC (conduit thick enough to be threaded) that could serve as a ground but if it is EMT (thin wall metal conduit) it is probably rusted into underground and can't be used as ground.

*The ground rod is part of your GEC, Ground Electrode Conductor system but soil conductivity varies too much for it to be safely used as an equipment ground (EGC).
 
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Old 02-01-15, 10:05 AM
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The only way to know for sure what the issue is.... is to make two voltage measurements in the shed. One is done.... open circuit. Now we need to wait to hear the loaded voltage with the bulb connected.
 
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Old 02-01-15, 11:07 AM
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Well I went over there. It looks like he did everything right. It does not look like there is any conduit at all. I can see where the wires leave the basement through the cinder block wall and where they pop up through the sill plate in the shed. They're old, thickly insulated wires - just 2 of them. I suspect the problem is corrosion somewhere along the line. The previous owner said that it used to work, but hasn't for some time. There was still 118V at the decrepit switch box and outlet, but the light bulb won't light up and cell phones won't charge.

My brother is going to disconnect it at the junction box in the house for now. Once the ground thaws we'll trench some conduit and tie it into the breaker panel (which appears to be fairly new) properly.

Thanks for all the replies.
 
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Old 02-01-15, 05:07 PM
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How much does the voltage between the receptacle slots change as you flip the light switch on and off?
 
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