Tech Tip 12
Your Neutral Wire May Be Working Overtime
CONDITIONS FOR A PROBLEM
- 120/208 or 277/480 volt system
- Loads are connected line-neutral
- Loads require 3rd harmonic current
- Single phase rectifiers
- Personal computers
- Some electronic ballasts
- 3rd harmonic current adds in the neutral
- For full neutrals, keep distortion below 30%
- Use double-size neutral
- Use separate neutral for each phase wire
- Specify low harmonic equipment
Overloaded neutral wires are most common in office areas since these locations usually have 120/208 wye distribution with many computer loads connected at 120 volts. Reduced neutrals make the problem even worse. A typical scenario is addition of or converting to equipment that requires high harmonic current.
On a wye power system, if all phase currents are equal, it is common to assume the neutral wire is carrying little or no current. However, with the recent proliferation of single phase power supplies, this may no longer be true. Neutral current can sometimes be higher than phase currents. This is very important in facility wiring systems because neutral wires have no overload protection.
CANCELLATION -- OLD LOAD CURRENT
To understand cancellation of return current in the neutral, consider Figure 1. At each point in time, the neutral current is the sum of the phase currents. For the old load current, summing up the three phase currents at each point in time gives zero current in the neutral. For example, adding the Old Load Current values for each phase current at the vertical dotted line, we find the neutral current at that point in time is zero amps.
ADDITION -- PERSONAL COMPUTER LOAD CURRENT
The waveform for personal computer and other 120 volt electronic loads is very different. Figure 1 shows a typical waveform for a personal computer load. Adding the phase currents at each point in time, we find that the phase currents do not cancel to zero in the neutral. The third harmonic current in the personal computer load current adds in the neutral. For this load current, the neutral current is higher than the phase currents.
Figure 1: Third Harmonic in Neutral
CONDITIONS TO BE CONCERNED ABOUT
The following three conditions must be present before becoming concerned about overloading neutrals due to harmonic loads. 1. The system is wye-connected 2. Most loads are connected line-neutral 3. Loads require 3rd harmonic current
Be sure to use true rms meters when measuring current, especially neutral current.
Since there are usually no overcurrent devices to protect the neutral, we must rely on good engineering design to prevent neutrals from becoming overloaded. A common design practice is to use double-sized neutrals or a separate neutral for each phase conductor when supplying single phase electronic loads from a wye system. Using distribution panels with double-rated neutral bus bars is also recommended.
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.