I don't like that there is 240 volts between the garbage disposal and hot water maker though. Why didn't they just use a tandem breaker for the two circuits?That duplex receptacle has the garbage disposal and instant hot plugged into it
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How does a certified electrician make this mistake?
- Thread starter sorka
- Start date
240V between receptacles on the same duplex device is not a problem. Certainly no worse than a 240V receptacle. And using an MWBC saves a wire (materials) and reduces voltage drop (energy loss).
Cheers, Wayne
What I've never understood is if they share the same neutral going back and this isn't a 240 volt appliance, then it means the load on each circuit is being combined on the neutral, right?
Yes, the neutral is shared. What is key is that the hots are on the opposite phase. This means the neutral loads are not additive, they are perfectly out of phase. You don't want this on a tandem breaker, that would put the hots on the same phase. The shared neutral is why they are on a common trip. The neutral could could be carrying current from either circuit. You want both circuits to be off if you are working on it.
My house originally had a lot of shared neutrals, it was built during a copper shortage. The disposal/dishwasher situation is quite common and an efficient way to bring two circuits in one spot.
When my Powerwall installer relocated my Dishwasher and Waste Disposal, they messed up and put them on breakers that were no longer adjacent, and I guess either forgot, or did not know they needed to be tied together. I never noticed myself for a year or two, and only recently found out. They are now out of business, so I ended up fixing it myself. They did some really nice clean wire counting etc, but this sloppy and potentially dangerous.
Yes, this is something to be vigilant about. The circuits with the shared neutral need to be on opposite poles (bus bars), and normally are adjacent to enforce it. If you have these circuits and add, or have added, breakers to your panel, make sure that the breakers stay adjacent. Definitely something to watch for in main service panel upgrades or swaps.
All the best,
BG
They are currently required to have handle ties (simultaneous manual disconnect), so they have to be adjacent.
Cheers, Wayne
I suppose one easy test is to check the voltage across the two hots to make sure it's 240v.
Ahhh! Now I get it. Pretty slick! So the loads on the neutral are actually subtractive. If the two hots happen to be drawing equal power there would be no current in the neutral, right?
yes, an earlier draft of my post mentioned but I thought it would be too confusing.
Ah yes, I wrote "common trip" in a post earlier when the proper term was handle ties.
To be rather pedantic: with AC, it's not quite enough to say "drawing equal power" to ensure that the currents will cancel in the neutral. You need the current waveforms to be actual opposites of each other to cancel in the neutral. E.g. identical loads.
There a number of ways that could fail to be the case even when the real power delivery is the same: one load could be differently non-linear compared to the other, so that they have different current harmonics. Or the loads could have different power factors, in which case the lower power factor load would have a higher current. Even when the currents are the same (rather than the power), they could be out of phase (again, different power factors), and so not fully cancel.
So I guess you can say that for linear loads of the same power and current (which implies same power factor), the neutral current will be zero (assuming symmetric connections so the neutral conductor is still at the neutral point).
Cheers, Wayne
Does this mean this if you had two identical loads plugged in and could turn them on at exactly the same time that the neutral wouldn't have any current going through it at all. Does this mean that you could disconnect the neutral as long as you left the appliance neutrals connected to each other and the two appliances would essentially act as voltage splitters each taking 120 volts from the 240 volt circuit?
Yep. I understand. I was thinking about that as I typed but thought it too technical to explain. Thanks.
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Yes.
But without the neutral connection, the current through both loads is forced to be the same. So if the loads differ at all, if one load want to draw more current, it will end up seeing less than 120V, and the other load will see more than 120V. [The load that wants to draw more current has a lower impedance, so with equal currents it has a lower voltage drop.]
Cheers, Wayne
Right. I get that. I was proposing it as hypothetical perfect balanced case to make sure my understanding was correct.
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