Multi-wire Circuits the Movie

Multi-wire circuits can be confusing to people. 

How multi-wire circuits function, and problems related to them, are important to understand for electrical safety.  In a multi-wire circuit, the neutral (grounded conductor) is shared between two circuits.  Most circuits have their own neutral.  Generally speaking, multi-wire circuits perform just fine as long as the rules of installation are not violated.

Each leg of the multi-wire circuit must terminate on a different bus bar. 

Because bus bars are 180 degrees out of sync with each other, the neutral current can travel on the neutral wire safely.  If they were to terminate on the same bus bar, the current from the two circuits gets added together.  This is because the circuits are no longer 180 degrees out of sync with each other.  Wiring the circuits this way can result in overloading the neutral.  The amount of current leaving the breaker however, would not be more than normal and the circuit breaker would not trip even with the overheating wire.

The following read-along, video-blog attempts to discuss some of these issues.  Please pause or rewind the video where necessary to suit your own level of understanding and learning curve.  The demonstration board is set up to do many more experiments than what this post is about, so pay more attention to the overlays in the presentation and ignore all the rest.  Perhaps some of the other components will show up in a future post.

Charles Buell, Real Estate Inspections in Seattle

 

Multi-wire Circuits

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 “phase” 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).

How the waves of a muti-wire circuit work

How the waves of a muti-wire circuit work

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.

Two breakers on the same bus bar

Every other pair of breakers, starting from the bottom, is on the same 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.

Bus bar fingering

Bus bar fingering

This next picture of a typical panel shows pretty clearly the fingers of the different bus bars.

Bus bar fingers in an electrical panel

Bus bar fingers in an electrical panel

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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