What is a 2 wire branch circuit?

Are multiwire branch circuits becoming antiquated for most new installations, or are they still being used in most circuit applications? Changes have been made in the last few National Electrical Code (NEC) editions to ensure circuits supplying critical loads—such as those provided for operating room and critical-care equipment in hospitals, exhaust fans for uninterruptible power supply system battery rooms, and emergency and other life safety equipment—will have individual nonmultiwire branch circuits. Disconnecting the power at one circuit breaker supplying a bank of lights at a patient bed location should not interrupt power to lights for other patient bed locations. Critical equipment and lighting cannot be indiscriminately disconnected from power if that shut down will affect other similar circuits.


Over the past decade, the electrical industry has been moving toward the use of individual branch circuits with dedicated neutrals for each ungrounded circuit conductor to ensure that each circuit can be independently disconnected and a failure or faulty operation in one circuit will not adversely affect another. By requiring each multi-wire branch circuit to be provided with a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates, such as at the panelboard or load center, a change to 210.4(B) in the 2008 NEC prompted the industry to move away from multiwire branch circuits. Remember, a multiwire branch circuit “consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.”


This change prompted further adjustment for healthcare facilities in Article 517. In general-care patient bed locations covered by 517.18(A) and critical-care bed locations covered by 517.19(A), a new sentence states, “the branch circuit serving patient bed locations shall not be part of a multi-wire branch circuit.” Restricting the branch circuits from being multiwire branch circuits ensures that disconnecting the power to one circuit will not affect the operation of any other. In other words, each branch circuit supplying general- or critical-care patient bed locations, as well as other critical circuits, must be individual branch circuits with a separate neutral for each ungrounded conductor and cannot be a multiwire branch circuit using multiple circuit breakers tied together either in the field or by the manufacturer.


Wherever a multiwire branch circuit occurs, various sections of the NEC, such as 225.33(B), 230.71(B) and 240.15(B), permit two or three single-pole switches or breakers on the multiwire branch circuits that are capable of individual operation—one pole for each ungrounded conductor and one multipole disconnect—provided these devices are equipped with identified handle ties or a master handle to disconnect all ungrounded conductors. Disconnecting all of the ungrounded conductors simultaneously provides disconnection of the circuit’s neutral current and affords additional safety while working on the multiwire branch circuit by disconnecting both ungrounded and grounded currents.


Another problem that can occur with multiwire branch circuits—or with any circuit requiring a neutral—is the loss of a neutral conductor in the circuit. With an individual branch circuit that consists of an ungrounded conductor and a neutral (grounded) conductor, the loss of the neutral means the circuit is no longer operational. With a multiwire branch circuit, if the ungrounded conductors have the same load on each ungrounded conductor, there will be balanced loads on all of the ungrounded circuits, so the loss of a neutral may not be as noticeable since there will be very little, if any, imbalance.


However, as loads are decreased on a particular ungrounded conductor in the multiwire branch circuit, the loss of a neutral will be very noticeable. Rather than having all of the loads from the ungrounded conductors balanced and in parallel with each other with the neutral carrying the unbalanced current, the ungrounded conductors will be in series, with current in the series ungrounded circuit being the same and voltage drop being different, thus, having more wattage on the series circuit with the most impedance. Some lights will be dim, some appliances will run much hotter than others, and the imbalance will not have a path back on the neutral.


Ensure that any load is supplied by the appropriate circuit, either circuits with an ungrounded conductor and a grounded (neutral) conductor where the branch circuit is essential, or by multiwire branch circuits where the load is not as essential.

Your home's electrical system begins with main service wires that enter your home from an overhead utility line or underground feeder wires and connect to the main service panel, usually located in a utility space or garage.

Many smaller homes may not have the option of a utility space or garage, in which case the main service panel may be located on any accessible outside wall. It may also be located outside but only if it's a weatherproof panel type.

The electrical system hardware belongs to a power utility company up to the meter base and meter. Beyond that point, the homeowner owns the wire and equipment. From the main service panel, the current is divided into individual branch circuits, each of which is controlled by a separate circuit breaker.

A branch circuit is part of the electrical system that originates at the main service panel and feeds electricity throughout the structure. There are 120-volt branch circuits that supply power to standard outlets and fixtures, along with 240-volt circuits that power major appliances.

The main service panel is controlled by the main circuit breaker that serves as the primary disconnect for the power supply to the main service panel. This is normally a 100- to 200-amp two-pole circuit breaker providing current at 240-volts and feeds it to two 120-volt hot bus bars running down vertically through the panel.

Below the main circuit breaker, there are two rows of smaller circuit breakers, and it is these that form the beginning of the individual branch circuits that run to all areas of your home to provide power. These individual breakers will be either 120-volt breakers, tapping into only one of the hot bus bars in the panel; or they will be 240-volt breakers that connect to two of the 120-volt bus bars.

Thus, your branch circuits will be either 120-volt circuits—which feed all the standard outlets and lighting circuits; or they will be 240-volt circuits—which feed circuits that feed major appliances, such as an electric clothes dryer, an electric range, and central air conditioning units.

Both 120-volt and 240-volt branch circuits can vary in the amount of power they deliver—a quantity measured by amperage. Branch circuits for 120-volt circuits are usually 15-amp or 20-amp circuits, although occasionally they will be larger than that. For 240-volt circuits, the amperage is more often 30-, 40-, 50-, or 60-amp.

The amperage of each branch circuit can be read by the printing on the lever of each circuit breaker. The wires attached to that circuit must also be sufficient to handle the load of the branch circuit; attaching wires that are too small for the circuit amperage poses a definite danger of fire. The ratings of individual wire gauges are as follows:

• 15-amps: 14-gauge copper wire
• 20-amps: 12-gauge copper wire
• 30-amps: 10-gauge copper wire
• 45-amps: 8-gauge copper wire
• 60-amps: 6-gauge copper wire
• 80-amps: 4-gauge copper wire
• 100-amps: 2-gauge copper wire

Normally this is not an issue, as the original circuits in your home are likely wired correctly. However, anytime a circuit is being extended, it's critical that the new wiring is the appropriate gauge for the circuit amperage. It is a common DIY mistake to wire with incorrect gauge size.

There are several different types of branch circuits in your home. 

• Dedicated appliance circuits. These serve only one appliance and are often required by code. They can be 120- or 240-volt circuits and serve appliances such as electric ranges, dishwashers, refrigerators, garbage disposers, air-conditioners, and clothes dryers. Normally, any appliance that has a motor will require a dedicated circuit.
• Lighting circuits. These are what they sound like—circuits that serve general lighting needs in rooms. Normally, a lighting circuit will serve several rooms, and most homes will have several. One advantage of separating the lighting circuits from the outlet circuits is that each room will be left with some means of lighting them if one circuit is shut off. While working on the lighting circuit, for example, a plug-in lamp can be used to illuminate the space.
• Outlet circuits. These are circuits that serve only general-purpose plug-in outlets. They can be specific to a room or a group of rooms. A second-story in a small house, for example, may have one or two outlet circuits that serve multiple rooms.
• Room circuits. Depending on how the home has been wired, sometimes the circuit layout has all lights and outlets in a room served by individual circuits.