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At the outlet:

  • 40V - red to neutral (white)
  • 40V - black to neutral (white)
  • 230V - red to black
  • 110V - red to ground
  • 110V - black to ground

At the circuit breaker:

  • 230V - red to black
  • 110V - red to neutral bar
  • 110V - black to neutral bar

Do I need to run a new neutral wire?

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I can supply a picture of the meter reading if anyone can explain how to attach it. –  robin Nov 6 '13 at 0:23
    
Post the links and someone will edit in the pictures for you. You can post pictures once you have higher rep –  BlueRaja - Danny Pflughoeft Nov 6 '13 at 0:51
    
@BlueRaja-DannyPflughoeft How that can happen is that this is AC. The wire shows a resistance which drops 75VAC, and this is independent of polarity. There is no +115 and -115 in AC; both hot conductors are 115VRMS. Instantaneously, they have opposite polarity, but that's irrelevant here since 115 is an RMS average over time. –  Kaz Nov 6 '13 at 1:25
1  
Look for a burned, relaxed or loose connection in the socket first, then check the neutral connection in the breaker panel. –  Fiasco Labs Nov 6 '13 at 2:07
    
@robin click edit under your question, click the location within the body of your question where you'd like the image, press cont-g or the image picture (to the right of the {} icon above the question body), and select the file to add. There's no longer a rep requirement to add photos. –  BMitch Nov 6 '13 at 3:36

2 Answers 2

There is a problem with the neutral wire. I would start by making sure the screw in the breaker box to the dryer's neutral wire is secure, and that the wire is mechanically intact by firmly wiggling it at the neutral bar end.

Also, the neutral bus in the main breaker box should be bonded to ground—usually by a green screw like this: enter image description here

Check the bonding screw by unscrewing it a turn or two and then screwing it back in well. If this is not in a service breaker box, but an auxiliary panel instead, there must be no neutral to ground bond.

If the breaker end checks out okay, then look inside the dryer outlet box. (Turn off circuit breaker first.) Those have screws to secure the house wire into the outlet. Make sure the neutral wire (as well as the others) are mechanically sound and firmly attached to the socket.

If you measured the neutral inside the dryer, you should also check the appliance pigtail ("wall cord") for signs of wear.

If the ends of the neutral wire are okay, then it is time to measure the resistance of the wire from end to end. Unless the dryer is located within leads span of the breaker box, you will need to scrounge up a piece of wire to extend the lead range. It can be any gauge, but do measure its resistance so you can easily determine the additional resistance of the dryer wire. The dryer's wires should be 8 or 10 gauge so the resistance should be very low: 100 ft (30 m) should be no more than 0.10 ohms.

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You are not going to believe this. Your comments got me thinking about the loss of voltage and that the neutral line was broken somewhere (which was what I was afraid of. Having to run a new wire thru the ceiling, cutting holes etc). I went to the main and started tightening all of the screws into the neutral bar. The very last one was very loose (how does that happen?) I tightened it and now have 110 volts to both hots to neutral. Thanks for your expert replies. They got me thinking and you have saved me hundreds of dollars. Hats off to you !!!! –  robin Nov 6 '13 at 14:08
    
@robin: Glad to have been of service. It is very unlikely for a wire to break somewhere in the middle of a run. Almost all of the trouble is at the ends. Fortunately, those are highly accessible. (Maybe you could accept my answer?) –  wallyk Nov 6 '13 at 15:36

This situation suggests that the neutral conductor has a problem somewhere between the dryer outlet and the panel.

The conductor should behave as a very low resistance, which drops 0V (in particular in a multimeter test, where very little current is permitted to flow due to the very high resistance of the meter).

Of course, the voltage between the neutral point at your panel and either of the hot wires is 115. But, only 40V of that is being dropped by your meter: the other 75 is dropped by the neutral wire. Your meter's resistance is forming a 40:75 voltage divider with the neutral wire's resistance. And since your meter's resistance is very high (probably 10 million ohms or more), it means that the wire's resistance is on the same order: it is close to twice the resistance of your meter (40:75 is almost 40:80 or 1:2).

In practical terms, it means that the neutral connection is broken somewhere. Perhaps the wire itself is broken, or a hookup. Even though the connection is broken, current is able to find a way to flow through an alternative path, which has a very high resistance. This current shows up as a voltage of around 75V which subtracts from 115 to leave 40 across your meter. The voltage is simply from Ohm's Law: V = IR. Some tiny current I, flowing through a huge resistance R, happens to make 75.

Example with diagram:

enter image description here

Here, the multimeter is symbolized as a 10 megohm resistor. The bad neutral wire is modeled as a 20 megohm resistance. This means that AC voltage between point A and B (what the multimeter measures!) will be 115 x (10/(10 + 20)) = 38.3V. If the neutral connection weren't faulty, the meter would take the full voltage and show 115. This is all exactly same if we disconnect the meter from point B and connect it to point C instead: the other AC phase. Between the two phases, there is no fault: if we measure between B and C, we get 230 because they are opposite phase, and since ground isn't faulty, then from ground (such as point D) to ether B or C, there is 115 VAC.

The resistances are just representative. Your meter and the faulty connection could both have a much higher resistance.

A final point is that the voltage measured in a circuit where there is a high impedance is real, but the specific value is not meaningful. That specific voltage value only exists while the meter is connected to the circuit. Another multimeter could read very different value. The multimeter is designed under the assumption that the resistances in the circuit which it measures are small compared to that of the multimeter, so that the multimeter's resistance dosn't disturb the voltages in the circuit.

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