Transfer switch and interlock kits

Why transfer equipment is non-negotiable

NEC 702.5 requires interconnection or transfer equipment that is listed and prevents inadvertent interconnection of all power sources. Optional standby systems with fixed or portable generators need this, the narrow exception for supervised temporary portable connections does not cover homeowner backfeeding.

Violation modes from backfeeding through a dryer outlet:

1. Power flows past the meter onto the distribution system where crews assume conductors are dead
2. Exposed male plug prongs are energized, shock hazard
3. Utility return onto an out-of-phase generator is a destructive fault inside the service panel

The compliant interlock path costs $400–$900 installed , not a reason to improvise.

Automatic transfer switch (standby path)

Monitors utility, signals generator start, transfers load after voltage and frequency stabilize, retransfers after utility stabilizes, shuts generator down. Residential units are open transition, with a brief power break during transfer (~10 seconds total including generator start).

Service-rated vs non-service-rated

• Service-rated (service-entrance): contains utility service disconnect; mounts between meter and main panel; becomes the service disconnect; neutral-to-ground bonding moves into the switch. Generac RXSW line is service-entrance rated.
• Non-service-rated: no main breaker; mounts downstream; feeds essential-circuits subpanel. Generac RXSC line is not service-entrance rated.

Whole-house vs partial-house is largely an ATS decision, not just generator kW.

Smart ATS features worth asking about

• Load management for up to four HVAC circuits (Generac 200A smart ATS)
• Integration with Smart Management Modules for larger loads
• Utility approval for service-entrance models in the metering path
• Withstand rating (Generac 200A: 22,000A at 240V)

Manual transfer switch (portable middle path)

A 6–10 circuit manual switch runs $1,000–$2,200 installed plus inlet box and permit. You choose which circuits receive power, which is useful when central AC is off the table. Still requires going outside to start the portable and flip the switch every outage.

Interlock kit (budget portable path)

Mechanical interlock on the main panel: main breaker and generator breaker cannot both be on. Paired with an exterior L14-30 or L14-50 inlet and cord to the portable. Total compliant hookup often $1,400–$3,700 installed including permit, plus the generator purchase.

Sized correctly (7,500–12,000 running watts), an interlock can carry furnace blower, refrigerator, well pump, lights, and one 240V load. Starting central AC while everything else runs is usually off the table.

Grounding and neutral: the field failure mode

Whether the generator is a separately derived system depends on the transfer switch, switched neutral vs solid neutral. Two simultaneous neutral-to-ground bonds produce current on the grounding path. Fix: remove factory generator bond (with field marking per 445.11) or use switched-neutral transfer equipment.

Service-rated ATS installs make the main panel a subpanel, isolate the neutral bar. Non-service-rated: bond stays at main panel; never add a second. This is where good installers earn their fee.

Retail ATS pricing context

Component reference (retail, not installed): Generac 200A smart ATS $699–$809; 200A ATS + 20-circuit load center $1,299; 100A + 16-circuit $589. ATS installation labor commonly $600–$2,500 depending on panel complexity, another reason install exceeds equipment cost.

Bottom line

Standby generators ship with or specify an automatic transfer switch. Portable owners need a listed interlock or manual switch, full stop. We did the installer research for the install side; the electrical code requirements are the same whether you hire from our list or elsewhere.