I often create blog posts that I can link to in my Inspection Reports to provide further information about an issue. Typically this information would go beyond what would be necessary in the report itself.
These posts might be used to support or elaborate on discussions with the buyer during the inspection. This can save time at the inspection that might otherwise involve long winded explanations of complicated issues. I can give them the quick version of the story at the time of inspection and the more detailed version online.
A very common inspection issue that usually needs further explanation are, “Multi-wire Circuits.” It is usually best to discuss the issue simply and briefly; and, let them know that there will be access to a more detailed explanation provided in the report.
What exactly is a Multi-wire Circuit?
In order to save time and money the electrician will sometimes pull just one wire with two hot conductors and and neutral conductor instead of two wires that each have a hot conductor and a neutral conductor.
A pretty common multi-wire circuit would be to the Dishwasher and Garbage disposal. These two appliances need their own circuit so the electrician will run one cable to the sink location where it can then be split for the two appliances. Within this one cable there will typically be a black wire (perhaps for the disposal) and a red wire (perhaps for the dishwasher). This wire will also have one white neutral wire and the bare ground wire. The Neutral wire is being “shared” by the two appliances—and that is what it means to be a multi-wire circuit.
This will now get a little complicated—so pay careful attention.
To fully understand these types of circuits, we have to understand the relationships between the sizes of the wires and the amperage they can carry and may be asked to carry–as well as the breakers involved in protecting those wires from overheating. Lets keep using the example of the dishwasher/disposal multi-wire circuit. Each of these circuits is typically a 15 amp circuit. For 15 amp circuits we usually run 14-3 with Ground (red, black, & white #14 wires, plus the ground wire). Each circuit will be protected by a 15 amp breaker. During operation of these appliances, each appliance under worse case scenarios would be capable of drawing 15 amps—before the circuit breaker tripped.
Hang on to these concepts for a moment.
Now we have to discuss the “bus bars” in the panel.
In most modern panels the two hot conductors come into the panel from the Utility Company and connect to the Main Breaker which distributes power to the two bus bars. These two conductors and bus bars are 180 degrees out of “sync” and can be understood by visualizing two waves running together where the trough of one wave is opposed by the crest of the other wave. Thus these waves are able to travel on the same wire without creating any problem (as in the top half of the picture below).
Here is where the trouble can start—when the two circuits share the SAME bus bars (as in the bottom half of the picture above). The neutral wire can now be required to carry the possible full load of both circuits on the same wave length—-30 amps—-and the wire is only rated for 15 amps. Insulation starts to melt from overheating and the risk of an electrical fire increases when this happens.
To make it easier to wire breakers in the electrical panel the bus bars are configured like fingers such that every other breaker space is a different bus bar. Most electrical panels have breakers that increase the capacity of the panel by having two breakers in each of these spaces—-this is usually where the mix-up occurs and the two hot conductors end up on the same bus bar.
In the next picture we can see just such an installation. The bottom left two breakers occupy one “space” for that particular finger of the bus bar.
In the next picture, the transparent overlay is an approximation of the underlying fingers of the bus bar for the left side of the panel that show how every other space is energized by that bus bar. The areas on the right hand side that have no overlay correspond to the fingers of the other bus bar.
This next picture of a typical panel shows pretty clearly the fingers of the different bus bars.
This defect is a relatively easy for the electrician to fix. It is usually just a matter of reorganizing the wires or the breakers so that the wires are not energized from the same bus bar. It will likely take longer to re-label the breakers than it will to make the repairs.
I am not a big fan of these multi-wire circuits for the reasons cited in this post—as well as another problem that can occur that doesn’t need to be discussed in relation to this topic. Because these multi-wire circuits are very common, we have to know how to inspect them and make sure that they are safe.
There is now wire cable available that has a second neutral in the bundle that eliminates this problem—14-4 with ground (the cable has two neutral conductors, two hot conductors and a ground wire).
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Fabulous explanation and I’m glad you’re the inspector and I’m the agent. I have other things to look out for. Always nice to see you on the job.
Thanks Glenn and the same goes for you.
Just wanted to thank you for taking the time to write this. Very helpful!
You are welcome—glad you found it useful
Thank you for a great explanation to a difficult concept.
Hello Charles, thanks for your great blog. Only one thing I’d like to see you add if this is going to be read by many people, the dangerous situation of uncoupled breakers used in multi-wire circuits. This used to be common, and is still done by amateurs.
Any chance you could add this? I run into this frequently and it can be difficult to explain to people why it’s dangerous.
Bruce, excellent idea—will add it or do an addendum post.
Great explanation.
The two legs of a 120/240 system, the red and The black can be represented by a wave drawing. But they cannot travel on the same wire at the same time. If a hot red wire touches a hot black wire there will be a huge arc and hopefully both circuit breakers will trip. I think you are trying to talk about neutral returned current. After current is used by an appliance neutral return current travels through the white wire back to the source. In the situation where there is a shared neutral, the black return current and the white return current are both connected to the same white wire so they interact with each other. This is Called “balance”. If the current is equal the black and red balance each other And there is no net flow on the neutral white wire. If the current is not equal there is an unbalanced neutral load which travels on the white wire back to the source. If both hot wires are connected to the red leg by mistake you could have an overloaded neutral which is the danger you talked about. Also if the shared neutral is disconnected or broken you could feed 240 V into a 120 V appliance and destroy it.
I am not quite sure what you are trying to say with some of this. In a 240 volt circuit, one that does not have any 120 volt components that would require a neutra, there would be no neutral wire or neutral current to balance. The current flows through the appliance on one leg and returns to the transformer on the other hot leg. This can happen because the two legs are 180 degrees out of sync with each other.
Not always.
Take a clothes dryer for example. The heater is 240vac but the timer and control circuits are 120vac. Without a white return line the timer circuits will not run.
No always what? A dryer with a 120 volt receptacle is not the type of multi-wire circuit this article is about.
Not always what? A dryer with a 120 volt receptacle is not the type of multi-wire circuit this article is about.
When you said “the black return current and the white return current” I think you meant “the black return current and the red return current”.
If both hot wires are connected to the same bus and a situation such as the dishwasher heating water and the disposal jammed with a locked rotor then the neutral white wire could possibly be overloaded and start a fire.
If the hot wires are connected each to a different buss and the white wire fails then one of the appliances could be subject to voltages much higher than 120vac and could start a fire.
Both of these possibly fatal outcomes are due to the greed of the electrician and can easily be eliminated by using completely separate wires or 14-4 in this situation.
On a slightly different greed driven subject, I have recently had three instances of overheated receptacles caused by “stab-in” connections. One simply shut off power down the string, another was hot enough to turn the white insulation black, and the third one actually spit fire into the room. IMHO “stab-in” connections should never be used.
NMJ
I cannot find anywhere where I said “the black return current and the white return current.” I kind of agree with you about the back-stab receptacles but I have not been able to force damage one that was back-stabbed in a shop test.
Not to re-necropost but.. They don’t interact other than the residual heat from resistance because the return from both circuits on the neutral will be out of phase with each other. The problem arises since being a two pole breaker if the neutral comes loose at the panel or a junction and something is plugged in on both circuits they will be effectively across red to black at 220v and sparks may fly or someone inexperienced takes apart a junction and they take apart the neutrals before the hots while live although you still need something plugged in on both circuits to complete the path from black to red. Make your connections good and don’t let monkeys play with electricity and you should be ok.