Generator Transfer Switch Sizing Calculator
Size a generator transfer switch by total load amps and circuit count.
Get amp rating, manual vs automatic options, and code requirements for home backup power.
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:
- 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
- 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:
| Generator | Recommended Transfer Switch |
|---|---|
| 4-7 kW (portable) | 30A manual |
| 7-12 kW (mid-portable) | 50A or 60A manual |
| 12-20 kW (standby) | 100A automatic |
| 20-30 kW (large standby) | 150-200A ATS |
| 30+ kW (commercial) | 200A+ ATS, 3-phase |
Common circuit selections for partial backup (manual transfer):
| Circuit | Typical Amps |
|---|---|
| Refrigerator | 5-7 A |
| Sump pump | 8-10 A |
| Furnace blower | 5-12 A |
| Microwave | 12-15 A |
| Computer / TV / lights | 5-10 A |
| Window AC | 10-15 A |
| Well pump | 10-20 A |
| Electric range/oven | 30-50 A |
| Electric water heater | 20-30 A |
| Central AC | 20-50 A (usually skipped on portable gen) |
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.