Voltage Not 120 Through Fixture? - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Voltage not 120 through fixture?

09-26-2009, 03:46 PM   #1

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Voltage not 120 through fixture?

So, I've read about phantom voltage, and floating grounds, and I'm still confused at what I'm seeing. I have a wire coming directly from the panel and a wire going to a ceiling light fixture. I connect the blacks then measure the voltage across the white wires and i was expecting 120v, but I see 80 or so volts varying +-5. Shouldn't I see 120 volts? Thanks.

ps, I get 120volts across the black and white wire coming from the panel.

09-26-2009, 04:36 PM   #2
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Quote:
 Originally Posted by dunnlow So, I've read about phantom voltage, and floating grounds, and I'm still confused at what I'm seeing. I have a wire coming directly from the panel and a wire going to a ceiling light fixture. I connect the blacks then measure the voltage across the white wires and i was expecting 120v, but I see 80 or so volts varying +-5. Shouldn't I see 120 volts? Thanks. ps, I get 120volts across the black and white wire coming from the panel.
If it is an incandescent light, you should see 120 V, but if it is a fluorescent light, it can be different.

09-26-2009, 04:46 PM   #3
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Quote:
 Originally Posted by dunnlow So, I've read about phantom voltage, and floating grounds, and I'm still confused at what I'm seeing. I have a wire coming directly from the panel and a wire going to a ceiling light fixture. I connect the blacks then measure the voltage across the white wires and i was expecting 120v, but I see 80 or so volts varying +-5. Shouldn't I see 120 volts? Thanks. ps, I get 120volts across the black and white wire coming from the panel.
I can't tell exactly what you're measuring from the description, but it sounds like you're inserting the meter in series with the fixture? That should show 120V, assuming there are lamps in the fixture. If the lamps are CFL that might cause some of the problem, but still doesn't really explain it. Maybe if you describe your setup better it would make more sense.

09-26-2009, 07:56 PM   #4

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Quote:
 Originally Posted by mpoulton I can't tell exactly what you're measuring from the description, but it sounds like you're inserting the meter in series with the fixture? That should show 120V, assuming there are lamps in the fixture. If the lamps are CFL that might cause some of the problem, but still doesn't really explain it. Maybe if you describe your setup better it would make more sense.

Yes, I suppose in series. Cable comes from panel, I connect the black to the black going to the light. Then I put one lead from my voltmeter on the white wire from the light and put the other lead on the white going to the panel. If I connect the two white (neutral) wires, the light turns on. BTW, the ground wires are connected.

The light fixture is a plain jane incandescent light fixture mounted in the ceiling and poking through a drop ceiling. (there has been a compact fluorescent bulb in the fixture since we moved in a few months ago)

It seems like from what I seeing so far, it is agreed that I SHOULD be seeing 120 between the two neutral wires. Does this mean the fixture needs to be replaced?

Thank you again.

09-27-2009, 12:53 AM   #5
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Quote:
 Originally Posted by dunnlow Yes, I suppose in series. Cable comes from panel, I connect the black to the black going to the light. Then I put one lead from my voltmeter on the white wire from the light and put the other lead on the white going to the panel. If I connect the two white (neutral) wires, the light turns on. BTW, the ground wires are connected. The light fixture is a plain jane incandescent light fixture mounted in the ceiling and poking through a drop ceiling. (there has been a compact fluorescent bulb in the fixture since we moved in a few months ago) It seems like from what I seeing so far, it is agreed that I SHOULD be seeing 120 between the two neutral wires. Does this mean the fixture needs to be replaced? Thank you again.
You definitely should be seeing 120V. Do you see 120V between hot and neutral in that box, with the light connected and operating? If so, then don't worry about it.

09-27-2009, 06:24 AM   #6

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Quote:
 Originally Posted by mpoulton You definitely should be seeing 120V. Do you see 120V between hot and neutral in that box, with the light connected and operating? If so, then don't worry about it.
Sorry, I'm not sure what you mean. If I connect the white wires in the configuration i described above, the light turns on. Where would I measure to see the voltable "between hot and neutral in the box?"

Also, would you please explain (or point me to an explanation of) why I shouldn't worry about that case if it measures 120 with the light on.

Thanks again very much.

09-27-2009, 11:33 AM   #7
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Quote:
 Originally Posted by dunnlow Yes, I suppose in series. Cable comes from panel, I connect the black to the black going to the light. Then I put one lead from my voltmeter on the white wire from the light and put the other lead on the white going to the panel. If I connect the two white (neutral) wires, the light turns on. BTW, the ground wires are connected.
Take the bulb out and you will read zero volts.

You cannot measure voltage in series PERIOD. You must read voltage in parallel. Connect the lights white to the source white. Connect the lights black to the source black. Measure from black to white you read 120 volts. PERIOD. The way you are measuring this is wrong. The way you are connecting your meter leads is the problem. You have one lead on the source black and one lead on the light white. Now do you see? You have now put a resistor in a series circuit in a sense. I am not going to give you a class on series and parallel circuits. Just measure VOLTAGE in parallel with everything in your house and you will be correct each time.

Try this:
Install a bulb thats double the wattage of the one you have. Test like you where doing before. You will read 1/2 the voltage you were reading before.
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