0

My breaker #1 (115VAC) is connected to my dishwasher. My breaker #2 (115VAC) is connected to my garbage disposal.

Why do I get 38.5VAC on the dishwasher line, even when breaker #1 is off? When breaker #2 (Garbage Disposal) is also off, I get 0 VAC on dishwasher. Why?

Harper - Reinstate Monica
  • 276,940
  • 24
  • 257
  • 671
BEBOT
  • 9
  • 1
  • 3
    Does this answer your question? [Unexpected voltage with switch turned off](https://diy.stackexchange.com/questions/110092/unexpected-voltage-with-switch-turned-off) – JPhi1618 Feb 25 '20 at 16:41
  • 2
    Please do not use all caps on public forums. It is hard for most people to read even if it might be easier for you. – JPhi1618 Feb 25 '20 at 16:42
  • @jphi1618, there are a lot of phantom voltage answers with accepted answers I would choose one of those but phantom voltages are common in most wiring methods where parallel runs of cable in close proximity, multi wire branch circuits have required handle ties for decades so if one was off they both would be off, even a short section of cable in close contact is enough to induce a voltage on a non energized conductor for the answers below.+ – Ed Beal Feb 25 '20 at 17:29
  • 2
    agreed Ed. A while back I was working (helping) in a commercial building with long runs in conduit. While the circuit I was working on was clearly turned off, there was still enough induced current to give enough of a shock that I couldn't work on the circuit without rubber gloves. I put an incandescent bulb on it just to make sure it was just induced voltage (it was bc voltage when to zero). Just saying this bc induced voltage can be a bigger issue in some instances. – George Anderson Feb 25 '20 at 17:44

3 Answers3

1

Almost certainly an induced voltage. Is this part of a Multi Wire Branch Circuit (MWBC)? If you have black, red & white wires in the cable it's probably a MWBC. Common to have induced voltages in them.

George Anderson
  • 22,321
  • 3
  • 31
  • 76
1

You have a multi-wire branch circuit feeding these two appliances. This means that it's a single /3 cable, with two hots and a shared neutral. Since the two hots run together in the same cable, you are getting capacitive "crosstalk" between the two wires. This is very weak, and you can only see it because you're using a DVM. DVMs are very sensitive.

The bigger problem is the breakers

And the biggest risk is in phasing the multi-wire branch circuit incorrectly, which will overload the neutral wire. The surest way to get that right is to use a factory provided handle-tie, either built in to a 2-pole breaker, or made by the factory to tie two individual breakers. The handle-ties are keyed so they won't fit on the breaker in a mis-phase situation. 2-pole breakers are much easier to obtain. Note that you will need a 2-pole breaker if you ever want to install GFCI protection.

Second, it should not be possible to turn only half the circuit off. That is precisely to protect you from getting nailed like you almost did. The 37 volts wouldn't have harmed you, but if you interrupted the neutral, that would've! Bigtime. Interrupting a neutral on a live circuit can kill you, that's why neutral wires have insulation.

So, when you use a 2-pole breaker or factory handle-ties to tie the two breakers together, that takes care of that also.

Again, the reason not to tie them with a nail is because a nail will let you tie, say, the two halves of a duplex breaker, which is something you should not do. That would cause precisely the overloaded-neutral scenario I mentioned.

Harper - Reinstate Monica
  • 276,940
  • 24
  • 257
  • 671
  • Harper, what do you mean by "interrupting a neutral..."? – clwhoops44 Feb 25 '20 at 20:26
  • Hey Harper: Great, thorough resp. as usual. But I have one issue with this resp. and others that refer to phantom voltages as "capacitive". I believe the correct term is "inductive". Capacitors can store power, transformers can't. Wires running very close to each other act like transformers inducing (hence the term inductive) power from one wire to another via electromagnetism. – George Anderson Feb 25 '20 at 22:34
  • @GeorgeAnderson yup, I used to think that too, until I was corrected by many people here. They may all be wrong, but I have not been willing to sit down and do the research to prove it. – Harper - Reinstate Monica Feb 25 '20 at 22:58
  • 1
    @clwhoops44 I mean say, there's a neutral in the box coming and going on a wire nut, so you need to add a pigtail wire so you separate the wire nut to add the wire. You just interrupted neutral. Worse if the supply and onward neutrals are on the two receptacle screws, *which they better not be in a MWBC*, and you go to change the recep. Obviously what's in your head is "They're neutrals, they're harmless" not at all. – Harper - Reinstate Monica Feb 26 '20 at 02:23
  • Harper thank you for the explanation. I want to make sure I understand correctly the part about neutrals on the receptacle screws in a MWBC. It's not a problem for receptacles that are after the split, correct? For example, I have 3 wire cable running from my panel to a junction box where it is split into two circuits. The receptacles on those circuits are previously wired on the screws, not pigtails. Do I need to make any changes? – clwhoops44 Feb 26 '20 at 16:33
  • 1
    @clwhoops44 correct, the pigtailing is only required where 2 or more hots are present. – Harper - Reinstate Monica Feb 26 '20 at 16:48
  • @GeorgeAnderson -- electrical engineer here, and you're a bit off -- both capacitors and inductors can store power, and this type of coupling is indeed capacitive. Inductors store energy in a magnetic field, which they create by passing current through a coil of wire. Capacitors store energy in an electric field, which they create by applying voltage to two plates separated by an insulator. Wires in a cable (that isn't coiled) behave like (weak) capacitors, because they're basically long thin plates separated by an insulator. – Nate S. Mar 22 '21 at 15:34
  • Thanks for the resp @NateS. I also have electrical engineering training and while I certainly agree with you regarding capacitors storing electricity, I just don't see how an inductive circuit can STORE electricity. Clearly power can be induced across long runs of parallel wiring as long as one of the legs is hot, but not store it. Once the power is off, there can be no induction and no power. If what you say is true, every transformer would need to treated like a capacitor, which they are not. – George Anderson Mar 22 '21 at 15:53
  • @GeorgeAnderson, it's true though, inductors store energy. Not the same way as capacitors, which is why they're not treated the same, but we *do* need special considerations on inductive circuits: the switches need to be beefier to handle more arcing, since the stored magnetic energy will create some crazy high voltages for a moment after the circuit is broken. It's like the opposite of inrush current -- the inrush is because you're charging up the magnetic field, and then when you break the circuit, that stored energy will tend to want to arc unless you give it somewhere else to go. – Nate S. Mar 22 '21 at 16:00
  • @GeorgeAnderson, Wikipedia goes over all the math behind inductive energy storage, if you're interested: https://en.wikipedia.org/wiki/Inductor#Energy_stored_in_an_inductor – Nate S. Mar 22 '21 at 16:01
  • @NateS.That article was over my head! LOL. But thanks anyway. I do get the concept of inrush current and instantaneous crazy peaks, esp. it large stuff (which I have no experience with), I've watched you-tube vids on large transformer start-ups, crazy! In my high school electronics class some of us bad boys (me included), would charge up a capacitor to a couple hundred volts and then toss it to an unsuspecting fellow student. ...gave him a pretty good zap! Stupid stuff. Also, signs on resistors saying "caution 100,000 ohms" ! – George Anderson Mar 22 '21 at 17:05
  • @GeorgeAnderson Difference being, capacitors use their stored energy to resist changes in *voltage*.... inductors use their stored energy to resist changes in *current*. If you have 5A going through an inductor, and you suddenly try to change that to 0A, the inductor will use its stored energy to try to keep the DC current at 5A. Now a cap will increase current to hold voltage... an inductor will increase voltage to hold current. Increase voltage quite a lot! Thousands of volts, which can sustain an arc as contacts part. That's why breakers are so beefy. – Harper - Reinstate Monica Mar 22 '21 at 17:24
  • @Harper-ReinstateMonica Agreed, but isn't that instantaneous. A capacitor can hold a charge for a long time...often used for backup power for electronics that need to save settings across a power outage. I don't believe transformers can do that. Surely there are powerful dynamics when switching, but If the power is out, I don't believe inductive loads store energy, capacitors can. – George Anderson Mar 22 '21 at 18:00
  • @GeorgeAnderson Build a circuit with DC+ going through 2 MOSFETs in series, to a large coil (say electromagnet), then back to neutral. In between the 2 MOSFETS insert a freewheeling diode between there and neutral. (a diode oriented so it will never flow power). Turn both MOSFETs *on*... let the magnetic field build... and then, turn both MOSFETS off simultaneously. Since no energy is stored, the circuit will simply be quiescent, no current should flow, right? So what made the inner MOSFET explode *after* they both shut off? – Harper - Reinstate Monica Mar 22 '21 at 19:17
  • @Harper-ReinstateMonica You and Nate obviously know a LOT MORE about electronics than I do! But we aren't talking DC here, it's AC, sure a large electromagnet might store a charge for a bit, but nothing like a capacitor. In all my work with transformers (low voltage, isolation transformers, buck up-buck down transformers) I've never had an issue with stored power in a transformer. Guess I just don't have the experience you guys do. We'd better be careful here or we'll get snipped by a moderator for telling us to move this to chat! LOL! – George Anderson Mar 22 '21 at 21:39
  • @GeorgeAnderson Ah, because you were working in AC. The stored-energy effects are most obvious when working in DC. Still there in AC, but they're a lot more routine. Still, it's why an AC contactor has a lower motor rating than resistive rating. – Harper - Reinstate Monica Mar 23 '21 at 00:14
  • @Harper-ReinstateMonica risking getting snipped by moderators as being conversational or chat, one other thing I learned here (and I've learned A LOT), is that high power DC circuits are much more difficult to control than AC, bc the voltage never goes to zero as in an AC circuit. Of course none of this applies to this DIY page, but I really enjoy the discussions with people, even if off topic, that are a lot smarter than me! Take care and DIY on!\ – George Anderson Mar 23 '21 at 03:25
0

In a lot of homes in the US, the circuits for your dishwasher and garbage disposal are fed to a single duplex receptacle under the sink, which has the two hot wires coming from separate breakers to the separated receptacles, but they share the same neutral wire (called a Multi Wire Branch Circuit, or MWBC). What you are reading, because you most likely have an inexpensive digital meter, is just the capacitive coupling though the motor windings and starting capacitors of those motors because they are sharing the neutral. If you had used an older meter with an inductive coil on it or a more expensive digital one that has proper filtering, the meter burden would have taken that voltage reading to zero. Bottom line, don't worry about it.

JRaef
  • 14,724
  • 1
  • 17
  • 39
  • 1
    I must disagree with part of this answer. There's no way that "coupling though the motor windings and starting capacitors of those motors " would create a phantom voltage. The phantom voltage is generated simply because wires running in very close proximity on AC current are acting like a transformer. Simple electromagnetic principles. I do agree that highly sensitive meters can mislead a lot of people. Even a tiny "load" across the measured wires can help interpret the readings correctly. – George Anderson Feb 25 '20 at 23:45