Neutral and grounding wires are often confused outside of the electrical trade, since both conductors operate with zero voltage. Actually, if you connect the ground wire as a neutral wire by mistake, most devices will operate correctly. However, such a connection is against code, since each conductor has a different functions that are necessary in electrical installations.
The National Electrical Code (NFPA 70 NEC) establishes wiring colors for the neutral and grounding conductors. Standard colors simplify wiring installation and improve safety.
- Neutral wire colors: White or grey
- Grounding wire colors: Green, green-yellow or bare
These insulation colors are only allowed for neutral and grounding conductors, and using them in any of the live conductors is against the code. Since electricians work with the assumption that all wiring of these colors is at zero voltage, using them for live conductors is very dangerous (and against code in the first place).
Make sure your electrical wiring is specified correctly.
Role of the Neutral Conductor in Electric Circuits
To visualize how the neutral conductor works, consider that electrical power is delivered as current across a voltage difference. Although voltage is carried by the live conductor, a neutral conductor is required for two important functions:
- Serving as a zero-voltage reference point.
- Providing a return path for the current supplied through the live conductor.
If only the live conductor is connected to a device, it will not activate because current cannot circulate and there is no voltage difference applied. This is similar to how a hydroelectric turbine requires an outlet to produce motion: if the turbine discharge is blocked, water cannot flow and the turbine cannot rotate, even if pressurized water is supplied at the intake.
When an installation uses three-phase circuits, there may be cases where the neutral conductor is not required.
- For example, a three-phase system with a line voltage of 120V provides an output of 208V between phases, and you can connect a 208V load between two phase conductors without using a neutral wire. Both live conductors carry voltage, but current can flow because they have a different voltage.
- Three-phase loads such as electric motors are often designed to run with three live conductors and no neutral conductor. The same principle applies here - current can flow between live conductors at different voltage.
However, even if some loads don’t use the neutral conductor in a three-phase installation, it is needed for single-phase loads that only use one of the line voltages. In theory, if identical loads are connected to the three phase conductors, their currents cancel out and the neutral conductor carries zero current. However, this is unfeasible in real-life installations, and the neutral conductor carries the current imbalance between the three phases.
Role of the Grounding Conductor in Electric Circuits
Although the grounding conductor has zero voltage just like the neutral wire, it accomplishes a very different function. As implied by its name, this conductor provides a grounded connection for all appliances and equipment.
- Under normal conditions, all current returns through the neutral conductor and the grounding conductor carries no current.
- However, when a line fault occurs, the grounding conductor provides a path for the fault current. Electrical protections can detect this condition, and they disconnect the circuit from the power supply.
Without a grounding wire, appliances and equipment can become energized if a live conductor touches them accidentally. The fault is not disconnected, since electrical protection devices respond to the fault current that flows from the live wire to the grounding wire. In this case, anyone who touches the energized surface suffers an electric shock.
Since a ground fault can affect any circuit, the grounding conductor is required even when there is no neutral conductor. For example, if a motor uses three live conductors and no neutral conductor, it must be grounded nevertheless because any of the live wires can suffer a fault.
Sizing Neutral and Grounding Conductors Correctly
Live conductors are sized based on the current they carry, and the same applies for neutral conductors in single-phase circuits, since they carry the same current as the live wire. Different rules apply for three-phase circuits: a common practice is using the same wire size as the three phase conductors, but some applications require a larger wire size for the neutral conductor.
Grounding conductors for branch circuits are sized based on the capacity of the overcurrent protection device. On the other hand, grounding conductors for the main service entrance are sized based on the capacity of service conductors. The NEC provides tables for both cases.
By working with qualified electrical engineers from the start of the project, you can rest assured that all components are specified according to the NEC and local codes. This not only provides safety, but also a quick project approval with local authorities. Electrical design engineers can also suggest energy efficiency measures to save on power bills.