This article was posted before 01/01/2011 and is most likely outdated.

Neutral-to-Ground Voltage (11-15-99)

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Neutral-to-Ground Connections Power Quality Issues

Author's Comment: Code references in this article are based on both the 1996 and 1999 National Electrical Code. The first reference is to the 1996 NEC and the italic reference is to the 1999 Section number. The Code Section numbers in the graphics, however, are based on the 1999 NEC.

In the past few issues I explained the National Electrical Code requirements as well as the dangers of improper neutral-to-ground connections. In this issue, we will review the power quality problems caused by improper neutral-to-ground connections, particularly electromagnetic interference to sensitive electronic equipment and elevated equipment case ground voltage.

VOLTAGE BETWEEN METAL PARTS TO EARTH

Proper Neutral-to-Ground Connection

When a neutral-to-ground connection is properly made in accordance with the NEC, the voltage between any metal part of the electrical system to the earth will be zero volts.

Improper Neutral-to-Ground Connection

The National Electrical Code requires a neutral-to-ground connection to be made at service equipment only and there shall not be any neutral-to-ground connection on the load side of service equipment [250-23(a), 250-24(a)(5)], except as permitted in Section 250-61 [250-142].

Author's Comment: Because of confusion on proper grounding and bonding, many electricians install the main bonding jumper that is supplied with the panelboard on the load side of service equipment making an improper neutral-to-ground connection (I personally did it many times).

Electromagnetic Interference

When a neutral-to-ground connection is made at the load side of service equipment in violation of the NEC, the feeder neutral current will divide and return on both the feeder's metal raceway as well as the feeder's neutral conductor (parallel path). This improper neutral-to-ground connection permits neutral current to return on the metal parts of electrical equipment (i.e. metal raceways).

When neutral current (or any ac current) travels on the metal parts of electrical equipment, the electromagnetic field generated from the flow of alternating current is not able to be canceled. This uncanceled electromagnetic field can negatively impact sensitive electronic devices.

IMPORTANT: Improper neutral-to-ground connections create parallel paths for neutral currents on the metal parts of the electrical system, including any metal shielding of low-voltage and limited-energy cables!

Author's Comment: There is the unproven health issue about the effects of electromagnetic fields on humans.

Elevated Ground Voltage

When a neutral-to-ground connection is made at the load side of service equipment in violation of the NEC, the voltage difference between the equipment ground and the earth will rise to equal the voltage drop of the neutral conductor at that location in the electrical system. The elevated ground voltage can be calculated by the following formula:

E(Voltage Drop) = I(Current) x R(Resistance)

Author's Comment: Today's office buildings contain large quantities of single-phase nonlinear loads such as personal computers and laser printers. These loads, when on a Wye 4-wire 3-phase system, produce odd triplen harmonic currents that add (instead of cancel) on the neutral conductor causing the neutral conductor to carry elevated neutral current. In addition, when high amperage loads such as laser printers or copiers cycle on, they cause rapid increases of neutral current which elevates the voltage on the grounding conductor.

Feeder. If a neutral-to-ground connection were made at the panelboard in violation of the NEC, the feeder neutral current would divide and return on both the feeder metal raceway and the feeder neutral conductor. Under these conditions, the voltage difference between any part of the electrical system to the earth will have risen to 1.25 volts (the voltage drop of the feeder neutral).

Author's Comment: The voltage drop of the feeder neutral conductor was 3 volts in Figure 1-1, but the parallel paths for the neutral current reduces the feeder voltage drop to 1.25 volts (for the purpose of example) in Figure 1-3.

Branch Circuit. If a neutral-to-ground connection were made at the receptacle in violation of the NEC, the branch circuit neutral current would divide and return on both the branch circuit metal raceway and the branch circuit neutral conductor. Under this condition, the voltage difference between any part of the electrical system to the earth would have risen to some value more than 3 volts but less than 4.5 volts.

VOLTAGE BETWEEN NEUTRAL AND GROUND

Proper Installation.

In a proper neutral-to-ground installation, the voltage between the neutral conductor and any metal part of the electrical system will be equal to the voltage drop of the neutral conductor in accordance with the following:

  1. At service equipment, the voltage difference between the neutral conductor and the service equipment case will be 0 volts.

  2. At panelboards, the voltage difference between the neutral conductor and the equipment grounding conductor (panelboard case) will be equal to the voltage drop of the feeder neutral conductor, which is 3 volts.

  3. At branch circuits, the voltage difference between the neutral conductor and the equipment ground (ground contacts of the receptacle) will be equal to the voltage drop of the feeder and branch circuit neutral conductors, which is 4.5 volts (3 volts feeder and 1.5 volts branch circuit).

Author's Comment: Computer and copier manufacturers insist (for warrantee purposes), that the voltage between the neutral and the ground contacts of the receptacle should not exceed 1 to 3 volts, and some manufacturers actually specify that the voltage must not exceed .5 volt! This is practically impossible to achieve in the real world without violating the NEC and I wonder if these manufacturers have any clue as to why they contain this requirement in their specifications. Maybe this subject should be explored in depth in the future.

Improper Installation

Feeder. If a neutral-to-ground connection were made at the panelboard in violation of the NEC, the voltage difference between the panelboard case ground to the neutral conductor would be 0 volts. Under this condition, the voltage difference at the receptacle contacts would only be equal to the voltage drop of the branch circuit conductors.

Branch Circuit. If a neutral-to-ground connection were made at the receptacle in violation of the NEC, the voltage difference between the grounding contacts of the receptacle to the neutral conductor would be 0 volts.

Author's Comment: At service equipment, the voltage between the neutral-to-ground will always be 0 volts.