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Generator Backfeed Safety Checker - Transfer Switch Requirements & Code Compliance

Verify safe generator connection, load capacity, and NEC-compliant transfer switch sizing

Free generator backfeed safety checker for homeowners, shop owners, and electricians. Backfeeding a generator through a dryer outlet or range plug without a transfer switch is illegal, violates the NEC, and kills people. This is not a scare tactic - line workers are electrocuted every year by generators backfeeding onto "dead" utility lines through panels without transfer switches. This tool helps you set up generator backup power the right way. Enter your generator size (watts or amps), panel capacity, and the circuits you want to power during an outage. The calculator determines whether your generator can handle the selected loads, sizes the appropriate manual or automatic transfer switch, verifies that critical loads are balanced across phases, and checks for code compliance issues. It flags dangerous conditions like generators too large for the panel bus rating, missing main bonding jumper conflicts with separately derived systems, and neutral-ground bonding issues that trip GFCI breakers during generator operation. Results include a wiring diagram reference, required disconnect ratings, and a clear pass/fail on NEC compliance.

Pro Tip: The most common generator complaint is "it trips the GFCI breakers when I run on generator power." This happens because most portable generators bond neutral to ground at the generator frame, and your panel also bonds neutral to ground at the main bonding jumper. Two ground-neutral bonds create a parallel path that trips GFCI breakers. The fix depends on your transfer switch type: an automatic transfer switch with a switched neutral eliminates the conflict. A manual transfer switch without a switched neutral may require a floating neutral generator or removing the generator's internal bonding jumper.

Check if your generator can handle specific equipment

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Generator Backfeed Safety Checker
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How It Works

  1. Enter Generator Specifications

    Input generator rated (continuous) watts and surge watts. Use the rated watts for sizing, not the surge or peak number. A "10,000 watt" generator with 8,000 rated watts can only sustain 8,000 watts continuously.

  2. Select Circuits to Power

    Choose which circuits you need during an outage: well pump, refrigerator, furnace blower, sump pump, lights, etc. The calculator totals the running watts and flags any motor loads with high starting surge requirements.

  3. Enter Panel Information

    Specify your panel's bus rating (100A, 150A, or 200A), voltage (120/240V), and whether you have a main breaker. This determines transfer switch sizing and identifies any bus rating conflicts.

  4. Check Safety and Code Compliance

    Review the pass/fail results for load capacity, transfer switch sizing, neutral bonding, GFCI compatibility, and NEC compliance. Red flags identify dangerous conditions that must be corrected.

  5. Get Equipment Specifications

    See the required transfer switch type, amp rating, and number of circuits. Get a reference to the correct wiring configuration for your specific setup.

Built For

  • Homeowners setting up a portable generator with a manual transfer switch
  • Shop owners sizing a standby generator for welders and compressors
  • Electricians specifying transfer switches for residential generator installations
  • Rural property owners ensuring well pump backup power during ice storms
  • Building inspectors checking generator installations for code compliance

Frequently Asked Questions

No. Backfeeding through any outlet without a transfer switch violates NEC 702.6, which requires a transfer switch or other approved means to prevent interconnection of normal and emergency power sources. In jurisdictions that adopt the NEC, this setup is not compliant. Beyond legality, it is genuinely dangerous - it energizes utility lines that line workers believe are de-energized.
The transfer switch must be rated for the maximum current it will switch, which is typically your panel's main breaker rating for a whole-house switch, or the sum of the backed-up circuits for a partial panel switch. A 200A panel needs a 200A transfer switch for whole-house backup. For a 6-circuit or 10-circuit manual switch, the switch is rated per circuit (typically 30A per circuit) and the generator inlet is sized to the generator's output.
Most portable generators bond neutral to ground at the generator frame (a code requirement for standalone use). Your main panel also has a neutral-ground bond. When connected through a transfer switch without a switched neutral, you have two neutral-ground bonds creating a parallel return path. GFCI breakers detect this imbalance and trip. Solutions: use a transfer switch with a switched neutral, or use a generator with a floating neutral (bonding jumper removed), or install a GFCI-compatible transfer configuration.
Unlikely. A typical 2,000 sq ft home with central AC has a 40,000+ watt peak demand. A large portable generator produces 8,000-12,000 watts. You can power critical circuits: refrigerator (150W running), well pump (1,000W), furnace blower (500W), sump pump (800W), and lights (300W) on about 3,000-4,000 watts of running load with a 6,000+ watt generator. Prioritize what matters and use a transfer switch to feed only those circuits.
Disclaimer: This tool provides guidance based on NEC requirements and standard electrical practice. It is not a substitute for a licensed electrician. Generator installation involves life-safety electrical connections that must be done by a qualified person in compliance with NEC Article 702 and local codes. Improper generator connections cause electrocution, fires, and equipment damage.

Learn More

Shops & Outbuildings

Why Backfeeding Kills Linemen

Connecting a generator to your panel without a transfer switch sends voltage back up the utility line. Linemen working on what they think is a dead line get electrocuted. Here's how backfeed happens and what the NEC requires to prevent it.

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How to Size a Generator for Motor Starting Loads

Why running watts are not enough to size a generator. Motor starting surge, NEC sizing rules, transfer switch requirements, and what happens when you undersize.

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