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What Happens if the Neutral is Lost in the Main or Subpanel?

Electrical Technology
0 6 minutes read

What Will Happen When the Neutral is Broken in the Main Panel or Subpanel?

The neutral wire provides a return path for current back to the power source, completing the electric circuit. It is insulated and carries current under normal conditions back to the transformer. According to NEC standards, the color used for neutral wires is white, while IEC and BS7671 standards use light blue. In today’s post, we will discuss the hazardous effects of a lost neutral in the main disconnect, main panel, or remote subpanel.

What Happens if the Neutral is Lost in the Main or Subpanel

If the neutral connection is lost or broken in the main panel, several critical issues can arise:

  1. Unbalanced Voltage Across Circuits: Without a neutral, the 240V supply remains active, but 120V circuits can become unstable. This instability can lead to unbalanced voltage, where one side of the circuit may receive a higher voltage than the other. Appliances and devices connected to these circuits can experience overvoltage or undervoltage, potentially damaging sensitive electronics or causing them to malfunction.
  2. Flickering or Dimming Lights: In some cases, lights may flicker, dim, or brighten unexpectedly due to fluctuating voltages across circuits. This is a common sign of a broken neutral and can be hazardous if left unaddressed.
  3. Increased Risk of Electrical Fire: If the voltage fluctuates excessively, it can cause wires, connectors, or devices to overheat. This overheating can lead to insulation breakdown and, in extreme cases, electrical fires.
  4. Electric Shock Hazard: Without a stable neutral, certain metal parts connected to the electrical system may become energized, posing an electric shock risk.

If you suspect a broken neutral, it’s essential to turn off the power at the main panel and contact a licensed electrician immediately. A broken neutral is a serious issue that requires professional assessment and repair.

Hazardous Consequences of a Missing Neutral in the Panel

Case 1 – Absence of Neutral in the Main Panel

Suppose the neutral is lost in the service equipment (main panel) or service disconnect. In a ground fault condition, current flows back from the load to the neutral bar in the remote distribution panel (subpanel), which is connected to the neutral terminal busbar in the main panel. Since the neutral and ground terminal busbars must be bonded in the main panel or first service disconnect, current now flows back to the supply source (i.e., transformer) through the equipment grounding conductor and ground rod, as it is the only available return path.

Click image or open in new tab to enlarge

Absence of Neutral - Metal Parts are Energized with 72V

If the load resistance is 20Ω and the grounding path to the transformer has 30Ω of resistance, the returning current will be 2.4 amps, forming a series circuit:

  • I = V ÷ R
  • I = 120V ÷ (20Ω + 30Ω)
  • I = 120V ÷ 50Ω
  • I = 2.4 Amp

This 2.4-amp current may not trip a standard 15- or 20-amp breaker. To trip the breaker, a GFCI breaker is needed because it senses unbalanced current in the neutral. If it detects that the current in the hot and neutral wires is not the same, it will immediately trip and disconnect the circuit to ensure personal safety.

Since the supply voltage is 120V, assume the resistances of the load and ground path to the transformer are 20Ω and 30Ω, respectively. Therefore, the output voltage in the circuit is:

  • VOut = (VIn × R2) ÷ (R1 + R2)
  • VOut = (120V × 30Ω) ÷ (20Ω + 30Ω)
  • VOut = (120V × 30Ω) ÷ (20Ω + 30Ω)
  • VOut = (3,600) ÷ (50Ω)
  • VOut = 72V

The circuit is still working but the light is dim with flickering. This way, all metal parts in the above circuit are energized with 72V.

If there is no subpanel used, the circuit will look like this:

Standard Breaker Poses Shock Hazards During Fault - Lost Neutral

 

Case 2 – Absence of Neutral in the Main Panel & Phase-to-Ground Fault

Now, consider the same scenario—absence of neutral in the main panel—but with an additional fault where the hot wire touches a metallic part of the device, such as a bulb holder.

In this ground fault condition, the returning current takes an incorrect path back to the power source. As shown, the fault current flows through the ground wire, as there is no neutral connection, making the grounding conductor the only available path.

Click image or open in new tab to enlarge

Lost Neutral and Ground Fault - Metal Parts are Electrified with 120V

With almost zero load resistance in a short circuit, 4 to 10 amps of current will flow, assuming a 30Ω resistance in the grounding path:

  • I = V ÷ R
  • I = 120V ÷ 30Ω
  • I = 4 Amp

This 4-amp current is insufficient to trip a standard 15- to 20-amp breaker typically used for household appliances in main and subpanels. Therefore, a GFCI breaker is required to address this issue.

In this scenario, the load is off, but all metallic parts, including metal raceways and equipment enclosures, are energized with 120V because they are bonded to the ground terminal.

The case will be similar without a subpanel used in the premises.

Risks of Standard Breakers During Broken Neutral & Ground Fault

Case 3 – Absence of Neutral in the Subpanel

Now, consider another scenario where the neutral is lost in the subpanel while it remains available in the main panel.

This is not a hazardous case because the load is off due to the lack of a neutral connection, similar to other load appliances. However, keep in mind that the neutral wire carries current up to the neutral bar in the subpanel. Touching it may cause electric shock, which is why a GFCI breaker is necessary for added protection.

Click image or open in new tab to enlarge

Broken Neutral in Subpanel - Circuit is Incomplete

This explains why the neutral and ground are separated in the subpanel and only bonded in the main panel. If accidental bonding between neutral and ground were to occur, the same issue as described in Case 1 would arise, as the fault current would find a path back to ground.

Good to Know:

  • All metal parts, including metal raceways, panelboard cabinet, frames, equipment, should be properly grounded i.e. connected to the ground rod via the grounding conductor. NEC – 408.40.
  • The Ground wire should not carry any current back to the earth during normal conditions.
  • The main panel needs a dedicated neutral busbar terminal connected to the main neutral busbar located in the main panel.
  • The grounding electrode conductor shall be connected to the grounded (neutral) service conductor in the main panel or first service disconnect. NEC – 250.24(A).
  • Grounding conductors (Ground Wires) and grounded conductors (Neutral Wires) are not to be connected together anywhere on the load side of the main service disconnect, 250.24.
  • In a subpanel, only the grounding conductor is permitted to connect back to the the grounding terminal bar in the main panel. NEC – 408.40.
  • Bonding the neutral and ground in the subpanel may pose serious electric shock and fire risks for handymen or anyone working on the appliance. Ensure that they are separated in subpanel and only bonded in the main panel. If you are unsure, contact a licensed electrician to ensure the job is done correctly.

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Electrical Technology
0 6 minutes read

Electrical Technology

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