The demands we put on electricity have grown over the past decade. Critical business functions are becoming more reliant on continuously supplied electrical power and technological advances have only heightened this. To address technician safety and industry demand, there have been modifications and changes made to the National Electrical Code (NEC) regulations. As a result, it can be tricky to navigate the changing regulations that ensure that the circuits supplying critical loads are safe, stable, and efficient. This article will address the cost effective, NEC compliant options for branch circuit wiring for a variety of power distribution systems.
What is a multiwire branch circuit?
A multiwire branch circuit is a branch circuit with a shared neutral. This means there are two or more ungrounded (hot) phase or system conductors with a voltage between them and a shared neutral. Multiwire branch circuits are used in offices, hospitals, hotels, high-rise residential buildings, or similar facilities for a variety of power distribution systems including lighting, devices, and receptacles.
What are the requirements for multiwire branch circuits?
The National Electrical Code (NEC) mandates the simultaneous disconnect of multiwire branch circuits that supply electricity to more than one device on the same circuit. To be more specific, NEC® 210.4(B) Disconnecting Means states, “Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.” To put it simply, the “simultaneous disconnection rule” means that all circuit conductors, on a multiwire branch circuit, regardless of whether they are being worked, must be disconnected when work is performed on them. As a consequence, there is the inconvenience of disrupted service in unaffected areas supplied by associated branch circuits.
The purpose of simultaneous disconnects
Multiwire branch circuits can be dangerous when all the ungrounded circuit conductors are not de-energized and the equipment supplied from a multiwire circuit is being worked on. Electricians run the risk of electrocution and injury while working on the equipment supplied by a multiwire branch circuit. The circuit can still be energized while work is being conducted or the proper protocol might not have been followed to disconnect the system. Simultaneous disconnects reduce the risk and takes the guess work out of ensuring safe conditions for maintenance.
Over the past decade, however, the electrical industry has been moving away from multiwire branch circuits to avoid the indiscriminate disconnection of power. It is simply not feasible for certain business operations to have an outage necessary for safe maintenance. For instance, consider the case of disconnecting power within a centralized location in a hospital and the power to lights at patients’ beds goes out across the facility. Service cannot be interrupted for circuits supplying critical loads.
As an alternative, the use of individual branch circuits with dedicated neutrals for each ungrounded circuit conductor ensures that each circuit can be independently disconnected. A failure in one circuit will not adversely affect another
How?
An option is to install a neutral for each ungrounded branch circuit conductor. Once this is done, the branch circuit is no longer “multiwire” and simultaneous disconnection is not required. By replacing a home run cable, a neutral per phase Type MC cable eliminates shared neutrals and allows electrical systems to continue to operate while repair or maintenance is performed on one circuit without affecting other branch circuits within the cable. A dedicated neutral for each phase conductor allows electricians to have greater flexibility in their work because only the affected single-phase breaker will trip. Additional benefits include:
Improved Safety
Safety is at the forefront of the NEC’s requirements on simultaneous disconnects. The risk of injury and shock is high for electricians servicing equipment when neutral wires may still be energized even after the proper lockout/tagout procedures have been implemented or when they mistakenly think both phase and neutral are shut off. While there are various modifications that electricians can make to avoid the hazards of shared neutrals, including training and warning signs on electric panels, neutral per phase cables will substantially reduce this risk.
Easy to Use
Because only the affected single-phase breaker will trip, neutral per phase Type MC cables not only decrease the time it takes electricians to identify and trace a faulty circuit, but also reduce the length of unnecessary outages. The benefit to this is two-fold. Building operators can have confidence that their facilities will continue to function with reduced risk of outages while maintenance is being performed, and electricians increase their efficiency and reduce labor charges.
Reduced Cost
Neutral per phase cables offer both installation time savings and maintenance time savings. Instead of running two separate circuits, technicians are now required to only run one multiple circuit cable—and terminate, fit, and secure one cable versus two.
Certain manufacturers offer neutral per phase cables made out of aluminum armor. Aluminum armor offers its own advantages by being lighter weight than steel, the material more commonly found in cables. Technicians can more easily move and position the lighter weight aluminum cables, increasing their efficiency on the job. Aluminum is also inherently RoHS compliant. RoHS, the Restriction of Hazardous Substances, also known as Directive 2002/95/EC, originated in the European Union and restricts the use of specific hazardous materials found in electrical and electronic products.
Reduced Harmonic Distortions
Neutral per phase Type MC cables are also ideal for applications where harmonic distortion is a concern due to shared neutrals. Prime examples are computers, variable frequency drives, electrical test equipment, and office equipment. Branch circuit wiring to computers and other electronic equipment may present additive harmonic currents from nonlinear switching loads.
When a current builds up on the neutral, the neutral can overheat. The additional current will cause the problem to be compounded when multiple circuits are connected to the shared neutral and buildup will occur. This buildup damages the conductor, which will have adverse effects on equipment, degrade the quality of the power, and ultimately require replacement.
Neutral per phase cables can mitigate this harmonic distortion by reducing currents in the neutral conductor.
