New subpanel, and now strange voltage readings


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Old 01-23-14, 02:14 PM
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New subpanel, and now strange voltage readings

Hey all,

I just installed a new subpanel to feed my kitchen. All of the circuits in the kitchen are new and are fed by this subpanel.

The only circuit breaker that is "on" at the subpanel is for my lights. The lights are on an independent circuit.

Here's my question: When the lights in the kitchen are off, the voltage across the hot and neutral wires on the rest of the circuits in the kitchen measures about 3V. When I turn the lights on, the voltage on the other circuits measures about 28V.

I'm using a digital fluke meter. Am I measuring actual voltage, or are these anomalous readings. If they are accurate, why is there voltage across these wires if the breakers are off?

Thanks.
 
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Old 01-23-14, 03:55 PM
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It's called phantom voltage. It's not uncommon to find that with a digital meter due to the physics at play. Basically it's voltage that is transferring inductively from one set of wires to another, and that is picked up because of the way digital meters work. While it may register volts, the current is only at the microamp level (1/1,000,000th of an amp). It will not even support a voltage reading by an analog meter (if you do the same test with an analog meter, it will read 0v).
 
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Old 01-23-14, 04:12 PM
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Awesome. That concept rings a faint bell, but I just wanted to be sure.

So just to satisfy curiosity: I'm assuming the voltage changes on the independant circuits when the lights are turned on because all the branches have common grounds and neutrals. But what difference does that make?

If it's way to heady, no need to go deeper.

Thanks!
 
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Old 01-23-14, 06:08 PM
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Load of a circuit.. heavier loads will register more voltage on nearby circuits just by sheer inductance.
Lighting ballasts can also cause mysterious voltage readings on dead circuits.
How close the wiring is for the two circuits and the longer the parellel distance the higher the voltage.
 
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Old 01-23-14, 07:17 PM
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I'm assuming the voltage changes on the independant circuits when the lights are turned on because all the branches have common grounds and neutrals.
Not exactly. Whenever you have two electrical conductors in parallel they will induce currents in each other. Research Electromagnetic Induction and Faraday's Law of Induction (Electromagnetic induction - Wikipedia, the free encyclopedia). The reason you are seeing varying voltages is because the current flowing through the wires is increasing/decreasing as you power more/less devices, respectively. Let's say wire 'A' is feeding your kitchen lights and wire 'A' is near wire 'B' and parallel to it (doesn't have to be perfectly parallel--but perfectly parallel will induce the most current). The current through wire 'A' generates a magnetic field and the magnetic field intersects wire 'B' which induces a current in wire 'B'. This causes your digital multimeter to register a voltage. If you monitor your meter and start unscrewing light bulbs, you should see the voltage on your meter decrease. If you also move the wires around inside the panel (I don't recommend this), you will also see a change. It is because of this phenomenon that communications wiring is twisted at right angles. When two conductors intersect at right angles, there is (theoretically) no electromagnetic induction between the wires. This is why, as a general rule of thumb, you should never run communications cabling (cable TV, internet, phone) parallel with power wires. They should always cross at right angles to each other, or if that is not feasible, as far apart as possible. By the way, this phenomenon only occurs for AC. DC current does not generate a magnetic field so there is no coupling between the wires.
 
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Old 01-23-14, 09:15 PM
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Originally Posted by mossman
By the way, this phenomenon only occurs for AC. DC current does not generate a magnetic field so there is no coupling between the wires.
Everything else was right except this.. DC current flowing through a circuit does absolutely have a magnetic field (otherwise DC motors and science fair electromagnets would not work!).. It is just different than the field created in an AC circuit. The alternation of AC current is needed for inductive coupling of the current between two parallel wires.. This is why transformers work. IE: electrons need to be moving back and forth on one wire to be able to induce movement in the electrons on the parallel wire.

Since DC only flows in one direction, it does create a polarized field around the wire (which is amplified and focused when wound around a core), but since it is not alternating back and forth, it can not induce electron movement in a parallel wire. That's why there are no DC transformers (and this is why Edison's dream of a DC power grid could never have worked).
 
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Old 01-24-14, 08:07 AM
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Everything else was right except this.. DC current flowing through a circuit does absolutely have a magnetic field
You are correct sir. What I should have said is there will be no coupling between the two wires when the signal is DC. To elaborate further, it is the changing magnetic field generated by an AC signal that induces an emf (electromotive force) on the second wire. DC does not induce emf because the magnetic field is static (not time-varying).
 

Last edited by mossman; 01-24-14 at 08:45 AM.
 

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