Neutral and grounding wires are often confused outside of the electrical trade, since both conductors have zero voltage. Actually, if you connect the grounding wire as a neutral by mistake, most devices will operate correctly. However, such a connection is against code, since each conductor has a different function in an electrical installation.
The National Electrical Code (NFPA 70 NEC) establishes insulation colors for the neutral and grounding wires. Standard colors simplify an electrical installation, while making it safer.
- Neutral wire colors: White or grey
- Grounding wire colors: Green, green-yellow or bare
These insulation colors are only allowed for neutral and ground conductors, and using them for any of the live voltage phases is against the code. Electricians work with the assumption that wiring of these colors is at zero voltage, and using white or green insulation for a live conductor would be a deadly trap (and against code in the first place).
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Role of the Neutral Conductor in Electric Circuits
To visualize how the neutral conductor works, consider that electric power is delivered as current across a voltage difference. Voltage is carried by the live conductor, but a neutral conductor is also necessary for two important functions:
- Serving as a zero voltage reference point.
- Completing the circuit, providing a return path for the current supplied by the live conductor.
If only the live conductor is connected to an electrical device, it will not activate because current cannot circulate, regardless of the voltage 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.
When an installation uses three-phase power, there may be cases in which the neutral conductor is not required.
- A three-phase system with a line voltage of 120V provides 208V between phases, and you can connect a 208V load between two phases without using a neutral wire. Both live conductors carry voltage, but current can flow because they have different voltages.
- Three-phase loads like 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 voltages.
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, when identical loads are connected to the three phases, 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
The grounding conductor has zero voltage just like the neutral conductor, but it accomplishes a 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 has no current.
- When a line fault occurs, the grounding conductor provides a return path for the fault current. Electrical protection devices can detect this condition, and they immediately disconnect the circuit from the power supply.
Without a grounding connection, appliances and equipment become energized if a live wire touches them accidentally. The fault is not disconnected, since protection devices can only respond when there is a fault current in a grounding wire. In this case, anyone who touches the energized surface will suffer an electric shock.
Since a ground fault can affect any circuit, the grounding conductor is required even when there is no neutral wire. For example, if a motor uses three live conductors and no neutral, grounding is still required because any of the live wires can cause a fault.
Sizing Neutral and Grounding Conductors Correctly
Live conductors are sized for the current they are expected to carry, and the same applies for neutral conductors in single-phase circuits (they carry the same current as the live wire). However, different rules apply for three-phase circuits: a common practice is using the same wire size as the 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, using tables provided in the NEC.
- 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 engineers can also suggest energy efficiency measures to save on power bills.