This article is about the multiwire branch circuit wiring technique in home electrics. It is not common. This panel might appear like that you would find at any home, but there is a key difference that a knowledgeable inspector should recognize and explain.
But first, let’s cover some basics to set the stage.
Conductor -> Wire -> Cable
A conductor is the metal that carries electricity. That metal is copper or aluminum. A wire is a bare conductor or a conductor covered with a plastic insulator. So, whether we say conductor or wire, it is the same for practical purposes. Two or more wires covered in a sheathing or jacket is a cable as shown.
Home electrical systems are mostly single strand insulated copper wires in 2-, 3-, or 4-wire cables. For this topic, we focus on 3-wire cable.
Typical use of 3-wire cable
3-wire cables are most often used in one of these ways:
- In 240 Volt outlets like a stove or a dryer where the two hot wires connect to a single appliance. The two hot wires always connect to separate phases at the main panel so that each carry 120 Volt for a combined 240 Volt. Most homes have a few of those. It is easy to envision: one cable, one purpose.
- In 120 Volt outlets like ceiling fan & light combos. One wire is for the switched light; the other wire is for the switched fan. It is also easy to envision: one cable, two purposes that are closely-related.
- In 3-way light switches. One wire is for the switched light; the other wire is a carrier wire to the companion 3-way switch to the same light. You can envision this if you squint: one cable, one purpose using two switches.
So that leads us to the point of this article: non-typical use of 3-wire cable.
Multiwire Branch Circuit
A multiwire branch circuit is a different way to use a 3-wire cable. The National Electric Code (NEC) defines it as a single electrical cable with two circuits that have a voltage between them and that share a common neutral. There are two takeaways from that definition:
- The first is that one cable is deployed as two circuits (in conventional use, one cable is one circuit). The two hot wires from the same cable go to different outlets which may or may not be in the same room. For example, the black wire is for the living room circuit while the red wire is for the dining room circuit. Those two rooms share the neutral wire but are on separate breakers.
- The second is that the two circuits must have a voltage between them, i.e., they must terminate on separate phases in the main panel to ensure least flow on the neutral wire.
- The house will have fewer cables – instead of two 2-wire cables, it uses one 3-wire cable. When there are fewer cables, there are fewer wires connecting into the panel. Fewer wires in the panel declutters it.
- To prevent overloading the neutral conductor, the two hot conductors must be connected to opposite phases in the panel. Surely the original installer knew that, but when circuits are moved around during changes, the two hot conductors could end up on the same phase. That condition will overload the neutral wire which might not even be in sight in the panel.
- If the neutral wire is ever disconnected, different voltages will result at the two circuits. That condition will fry equipment. You can delve deeper in this article where an expert explains how it happens: Understanding the Dangers of Multiwire Branch Circuits.
- To de-energize one cable entirely, you must turn off two separate breakers.
- It adds complexity and potential risk for minimal saving in wiring.
Don’t panic if your home or prospective home has Multiwire Branch Circuits. If a smart Home Inspector like Chester County Home Inspections identified your electrical system as utilizing this technique, then any and all electrical work must be done by a certified electrician. If you are buying the home, you may wish to have a licensed electrician certify that the wiring is still proper.
A core principle at Chester County Home Inspections is that we educate our clients, whether verbally at the inspection, through our detailed reports, or through specialty articles like this. Schedule online now, or inquire online, or call or text (484) 212-1600.