When half of your home’s 120V outlets are dead and your 240V appliances (like well pumps or AC units) fail to start during a power outage, you are experiencing a dropped hot leg. In a North American split-phase system, power is distributed via two 120V conductors (L1 and L2). The loss of a single leg indicates a hardware failure in the “chain of power” between the generator’s alternator and your home’s bus bars. This is typically caused by a failed conductor in the power cord, a tripped single pole on a transfer switch, or an internal generator fault.
Fast-Fix: The 45-Second Solution
This condition is technically unsafe for all 240V equipment. While 120V devices on the “live” leg may work, 240V motors will attempt to “single-phase,” drawing excessive current through the remaining live winding, leading to rapid thermal failure. You must immediately shut down the generator or open the 240V double-pole breakers to prevent motor burnout.
Immediate Safety Status
- Kill the 240V Breakers: Manually flip the double-pole breakers (HVAC, Dryer, Water Heater) to the “OFF” position.
- Inspect the Inlet: Do not touch the generator plug if you smell ozone or burning plastic; use an insulated tool to disconnect if necessary.
- Neutral Integrity: Ensure the neutral connection is still solid. A lost leg with a loose neutral can cause voltage to “float,” potentially sending 200V+ into 120V electronics.
- No “Jumpering”: Never attempt to bridge the L1 and L2 slots in an outlet to restore power; this bypasses all overcurrent protection.
Symptom Branching: Low vs. High Risk
- If 120V lights are steady on half the circuits → Low Risk: Likely a broken wire in the L14-30/L14-50 cord or a failed transfer switch pole.
- If 120V lights flicker or dim when the fridge kicks in → High Risk: The system is attempting to pull power through the windings of a 240V appliance (backfeeding internally).
- If the generator is screaming/stalling → High Risk: Massive phase imbalance is causing a magnetic drag on the alternator, risking permanent stator damage.
System Analysis (The “Why”)
Standard residential power is split-phase. The utility (or generator) provides two hot legs and a neutral.
The voltage is defined as:
VL1 to N=120V
VL2 to N=120V
VL1 to L2=240V
Your breaker panel alternates bus bars. If L2 is lost at the generator or the transfer switch, every second breaker in the column loses power. Because 240V appliances require the potential difference between L1 and L2, they receive 0V (or “phantom” voltage) across their terminals and fail.
The Most Likely Culprit
- 70% Probability: Faulty Power Cord/Inlet. Constant plugging/unplugging of the L14-30 twist-lock cord often breaks one of the two hot conductors at the terminal.
- 20% Probability: Transfer Switch Contact Failure. One side of the double-pole relay in an ATS or one pole of a manual switch has carbon buildup or mechanical fatigue.
- 10% Probability: Generator Breaker or Winding. One of the two poles on the generator’s onboard circuit breaker has tripped or failed internally.
The Cost of Delay: 1hr → 24hr
- 1 Hour: Nuisance loss of lights; potential food spoilage in unpowered refrigerators.
- 4 Hours: High risk of “single-phasing” damage to well pumps or sump pumps if they are left in the “on” position without a full 240V supply.
- 24 Hours: Environmental Forensics alert: Sustained phase imbalance can cause the generator’s rotor to overheat due to asymmetrical magnetic flux, leading to a total “gen-end” failure (1,000+repair).
Diagnostic Differentiators
- Generator vs. Cord: Unplug the cord from the generator. Use a multimeter to test the L1−N and L2−N pins directly on the generator. If both show ≈120V, the generator is fine; the cord is the failure point.
- Cord vs. Panel: Plug the cord into the generator but leave it disconnected from the house. Test the female end of the cord. If you only see 120V on one side, the cord is dead.
- Internal Link: If the generator shows no output at all, refer to Output Loss: Generator Running but House Not Getting Power.
The “Right Now” Protocol
- Shut down the generator.
- Disconnect the power cord and inspect the prongs for “pitting” or blackening.
- Flip all 240V breakers to OFF. 4. Check the generator’s onboard breakers; reset both poles even if they don’t look tripped.
- Restart and test voltage at the generator outlet before reconnecting to the house.
Red Flag Stop Triggers
WARNING: If you detect a “hot” or “metallic” smell coming from your breaker panel, or if you see 120V lights getting brighter than usual, you have a Neutral-to-Ground fault or a Floating Neutral. Shut down all power immediately. This can cause 240V to surge through 120V circuits.
The Professional Inspection Path
An electrical systems engineer will use:
- True RMS Multimeter: To verify phase-to-phase voltage (240V) vs. phase-to-neutral (120V).
- Load Bank Testing: To see if a leg “drops” only when under load (indicating a high-resistance connection).
- Thermal Imaging: To find the specific lug in the transfer switch that is overheating due to a loose connection.
Estimated Repair & Replacement Cost
- Minor (Reset/Loose Screw): $150 – $250.
- Moderate (New L14-30 Power Cord): $100 – $300.
- Systemic (Transfer Switch or Generator Stator Repair): $600 – $2,000+.
Symptom Escalators
- If the house power is flickering but both legs are present: THD Analytics: Why Lights Flicker on Generator Power.
- If the issue is specifically with one side of the panel only: Busbar Issues: One Side of Breaker Panel Not Working on Generator.
Final Circuit Check
Operating on a single leg of a split-phase system is an emergency state for your electrical infrastructure. While it may seem like a “half-fix” to keep the lights on, the risk of destroying expensive 240V motors and causing a generator winding failure is extreme. Diagnose the cord and inlet first; 70% of these faults are solved by replacing a $150 power cable.