Generator Transfer Switch Sizing Calculator

Generator Transfer Switch Sizing

A transfer switch isolates your home from the utility grid when running on generator — preventing back-feed (a SAFETY ISSUE that can kill utility workers) and protecting your generator from utility surges.

The basic rule: Transfer switch amp rating ≥ Total load AND ≥ Generator output capacity

Switches are rated by amps. Common sizes:

• 30A: small / single circuit
• 50A: RV-style, partial home
• 60-100A: most home generators
• 125-200A: whole-house with large generator
• 200A+: premium service entrance ATS

Two switch types:

1. Manual transfer switch (MTS): flip a lever when grid is out
   • Pros: cheaper ($300-1,500), simpler, no maintenance
   • Cons: must be home to switch, manual every time
2. Automatic transfer switch (ATS): detects grid loss, transfers automatically
   • Pros: works while away, instant transfer, monitor lights
   • Cons: more expensive ($800-3,500), requires generator with auto-start

Sizing by generator output:

Common circuit selections for partial backup (manual transfer):

Manual transfer switch (typical):

• 6-10 circuits selectable
• Each circuit can transfer to gen power individually
• Common sizes: 50A 6-circuit, 60A 10-circuit, 100A 16-circuit
• Brands: Reliance, Generac, Eaton, Square D

Whole-house ATS:

• Replaces or sits next to main panel
• Transfers entire load (limited by generator)
• Auto-detects grid outage, transfers in 10-30 seconds
• Brands: Generac, Kohler, Cummins, Eaton

Code considerations (NEC 702):

• Transfer switch is REQUIRED to prevent back-feed
• Must be listed (UL listed): DIY transfer switches often fail inspection
• Inlet box (e.g., 30A 4-prong twistlock) typically required outdoors
• Generator earthing varies by setup (separately derived vs not: consult electrician)
• Permit and inspection required in most jurisdictions

The 50/30/20 rule for portable generators: For a portable 30A generator transfer panel:

• 50% of capacity for steady loads (refrigerator, lighting, blower)
• 30% for occasional loads (microwave, well pump cycling)
• 20% headroom for surge starts

Soft start / hard start:

• Resistive loads (heaters, lights) draw rated amps from start
• Inductive loads (motors, AC, well pumps) draw 2-7× starting current for 1-2 seconds
• Generator must handle starting surge: sometimes more than running surge

The “interlock kit” alternative: A mechanical interlock on a circuit breaker panel achieves the same transfer effect at 1/3 the cost. Code-compliant in most jurisdictions but not all — verify locally.