Transfer Switch Sizing Guide: Amps, Circuits & ATS vs Manual
Reading time: ~9 min read
📋 Key Takeaways
- Whole-home installs: the transfer switch amp rating matches the service/panel rating, not the generator — a 200A panel gets a 200A switch even behind a 22kW generator. Generator output only decides whether you need load management.
- Selected-circuit installs: size the switch to generator output — a 22kW air-cooled unit puts out ~92A at 240V, which lands on a 100A-class transfer device; portable-generator interlocks typically run 30A or 50A.
- Generator amps = kW × 1,000 ÷ 240. A 10kW is 42A, a 14kW is 58A, a 22kW is 92A, a 26kW is 108A — the switch must carry at least that on its generator-side rating.
- A service-entrance-rated ATS (with its own disconnect and neutral switching where required) sits between the meter and panel and turns a sub-panel-style install into a clean NEC 230/702-compliant job.
- Manual interlocks and manual transfer switches are legal, cheap, and inspector-friendly — but the homeowner operates them, so they belong on selected circuits, not whole-home standby packages.
The transfer switch is the piece of a standby install that fails inspection when someone sizes it off the generator nameplate instead of the service. The rule is simple once you separate the two questions: the switch rating answers "what am I connected to on the utility side," and the generator kW answers "how much of the house will actually run." The selector below takes generator size, panel amperage, and the whole-home vs. selected-circuits decision, and returns the transfer switch amp rating, the ATS-vs-manual call, and the service-entrance notes that matter for NEC 702 and 230.
Quoting a standby package? Our generators collection covers air- and liquid-cooled units from 169 authorized brands, and electrical supplies stocks the transfer switches, interlock kits, and power-inlet boxes that complete the install.
Transfer Switch Size Selector
Portables run 3–12kW; air-cooled standby 10–26kW; liquid-cooled 25kW+.
Whole-home = ATS ahead of the panel. Selected = subpanel-style switch or interlock.
Generator output: —
Transfer switch rating: —
Recommended type: —
Sizing logic: whole-home switches match the service rating per NEC 702.4/230 with a service-entrance-rated ATS where the switch is the service disconnect; selected-circuit gear is sized to generator output (kW × 1,000 ÷ 240V) rounded up to standard device ratings of 30/50/60/100/150/200A. Informational — the AHJ and the manufacturer listing govern.
Generator Output to Transfer Switch Amps
The first number on any transfer switch quote is the generator's real output in amps. At 240V single-phase, divide watts by volts:
| Generator | Output at 240V | Minimum generator-side switch rating | Typical pairing |
|---|---|---|---|
| 5 kW portable | 21 A | 30 A | 30A inlet box + interlock |
| 7.5 kW portable | 31 A | 50 A | 50A inlet + interlock or 6-circuit manual panel |
| 10 kW | 42 A | 50 A | 10-circuit manual switch or 50A ATS |
| 14 kW standby | 58 A | 60 A | Selected-circuit ATS or 100A whole-home on 100A services |
| 22 kW standby | 92 A | 100 A | 200A service-entrance ATS w/ load mgmt, or 100A selected-circuit ATS |
| 26 kW standby | 108 A | 150 A | 200A service-entrance ATS w/ load mgmt |
| 36–48 kW liquid-cooled | 150–200 A | 200 A | 200A service-entrance ATS, often multiple ATSs on large estates |
| 60 kW+ commercial | 250 A+ | Per NEC 700/701/702 design | Engineered ATS gear, 3-phase, service-rated |
Notice the split: below roughly 12kW the switch is sized to the generator (selected circuits, manual gear). At 14kW and above the economics flip — a 200A service-entrance ATS costs less in labor than moving circuits, and load management handles the gap between 92 generator amps and a 200A panel.
ATS vs. Manual: The Decision Table
| Automatic (ATS) | Manual transfer switch | Breaker interlock | |
|---|---|---|---|
| Operation | Senses outage, starts generator, transfers in ~10 sec | Owner starts generator, flips switch bank | Owner starts generator, slides interlock, flips breakers |
| Generator pairing | Standby units with 2-wire start | Portable or standby | Portable only |
| Coverage | Whole-home (service-rated) or selected-circuit panels | 6–16 selected circuits | Any breaker the interlock feeds — owner picks at outage time |
| Typical installed cost (gear) | $$$ — highest gear cost, lowest callbacks | $$ — mid gear cost | $ — interlock plate + inlet box + breaker |
| NEC hooks | 702.4 capacity, 230 service rules when ahead of panel | 702.4, 702.5 transfer equipment listing | 702.5 — interlock must be listed for the panel model |
| Best for | Standby packages, elderly clients, second homes, medical loads | Budget standby retrofits, workshops | Portable-generator homeowners, rentals, budget jobs |
Two details inspectors actually check: interlock kits must be listed for the specific panel model (universal plates are a red tag in most jurisdictions), and any manual scheme needs the generator breaker in the panel sized to protect the cord and inlet — a 50A inlet behind a 30A breaker is a callback.
Service-Entrance-Rated ATS: NEC 702 Meets NEC 230
When the transfer switch sits between the meter and the main panel, it stops being load-side equipment and becomes service equipment. That triggers NEC 230 requirements on top of Article 702: the ATS must carry the full service rating, include a utility-side disconnect, have service-rated overcurrent protection, and be listed "suitable for use as service equipment" (SUSE). Neutral handling matters too — on a service-rated switch the neutral-ground bond lives at the ATS, so the downstream panel becomes a subpanel with isolated neutral and separate EGC. Get that bonding wrong and you've built a ground-fault path through the generator frame. For separately derived systems or 3-pole switched-neutral designs, follow the transfer switch schematic exactly — this is the one place where "how we've always done it" gets jobs red-tagged.
The practical payoff: a service-rated ATS turns the install into meter → ATS → existing panel, with no circuit moving, no subpanel, and a clean one-line for the inspector. On 400A-class residential services, the standard play is a 400A service-rated ATS (or paired 200A ATSs) ahead of the two 200A panels.
Load Management: How a 22kW Runs a 200A House
NEC 702.4 requires the transfer equipment to either carry the full calculated load or have load management that prevents overload. In practice, a 22kW air-cooled unit (~92A) behind a 200A whole-home ATS uses load-shed modules on the big motor loads: the AC compressor, electric water heater, range, and dryer get priority-ranked, and the ATS drops them if the generator sags. Resist the urge to skip the modules — a stalled 4-ton compressor pulling LRA against a 22kW generator trips the generator breaker, and now the customer has no power and a service call. Paired with efficient appliances and heat pumps, managed whole-home on 22–26kW covers the vast majority of residential jobs; true unmanaged whole-home starts around 36kW liquid-cooled. For hybrid resilience jobs, batteries and storage increasingly share backup duty, and solar panels cut generator runtime on long outages.
Frequently Asked Questions
What size transfer switch do I need for a 22kW generator?
For whole-home coverage: a 200A service-entrance-rated ATS matching your panel rating — the switch must carry the service, and the 22kW generator (about 92A output) runs the house through load-management modules. For selected circuits: a 100A-class transfer device covers the generator's 92A output with margin. Never size the switch at exactly 92A; standard ratings round up to 100A.
Do I size a transfer switch to the generator or the panel?
Both, depending on the coverage design. Whole-home (switch ahead of the panel): size to the panel/service rating — a 200A service needs a 200A switch regardless of generator kW, per NEC 230 and 702.4. Selected circuits (switch feeding only chosen loads): size to generator output in amps (kW × 1,000 ÷ 240), rounded up to standard device ratings of 30/50/60/100/150/200A.
Is a manual transfer switch or interlock legal?
Yes — NEC 702 permits manual transfer equipment, and breaker interlocks are legal when the kit is listed for the specific panel model and installed per its instructions. Manual schemes belong with portable generators and selected circuits; standby generators with automatic start pair with an ATS. Universal or homemade interlock plates fail inspection in most jurisdictions.
What is a service-entrance-rated transfer switch?
A transfer switch listed suitable for use as service equipment (SUSE) that installs between the utility meter and the main panel. It carries the full service rating, includes a utility disconnect and overcurrent protection, and becomes the point where the neutral-ground bond is made — the downstream panel then wires as a subpanel with an isolated neutral. It satisfies NEC 230 service rules and Article 702 in one enclosure.
Can a transfer switch be rated higher than the generator breaker?
Yes, and it usually is. The switch rating must meet or exceed whatever it connects to on each side — the service rating on the utility side for whole-home installs, and at least the generator output on the generator side. A 200A ATS behind a 22kW generator with a 100A generator breaker is correct and standard. The generator breaker protects the generator conductors; the ATS rating carries the service.
Quoting a Standby Generator Package?
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