Interlock Kit vs Transfer Switch (Pros, Cost & When to Choose)
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Best for: homeowners choosing between a listed generator interlock kit and a manual or automatic transfer switch for portable or standby backup.
Not ideal for: utility interconnection protection, commercial selective coordination, or DIY backfeeding with a suicide cord—those are out of scope and unsafe.
Quick answer: interlock kit or transfer switch? #
| Need | Prefer | Why |
|---|---|---|
| Portable generator, few circuits, someone is home | Listed interlock kit (or small MTS) | Lowest cost; prevents main + gen breakers both ON |
| Unattended home / standby generator / whole panel | Automatic transfer switch (ATS) | Starts gen and transfers without you present |
| Selected circuits, want labeled load center | Manual transfer switch (6–10 circuit) | Clearer circuit control than interlock alone |
Rule of thumb: Interlocks solve safe portable hookup when a person is home. ATS solves unattended standby. Neither replaces a UPS for IT blinks during crank/transfer.
Screen amps with the Transfer Switch Size Calculator, then size the generator with Generator Size. Timing/UPS bridge: ATS transfer time. Hub: Power calculator · UPS calculator.
What is the difference (interlock vs transfer switch)? #
| Dimension | Interlock kit | Manual transfer switch | Automatic transfer switch |
|---|---|---|---|
| What it is | Mechanical block on main + generator breakers | Separate switch or subpanel for selected circuits | Contactor/controller that senses utility loss |
| Who operates | You (after gen start) | You | Controller / gen |
| Typical cost band | Lowest | Mid | Highest |
| Whole-home? | Often yes if AHJ allows + inlet sized | Usually selected circuits | Common for standby whole-panel |
| UPS role | Still need UPS for PC/NAS while you set up | Same | UPS bridges crank → ATS close |
Example 1: 7.5 kW portable on a 200 A house #
Inputs: 7.5 kW running, 240 V, PF ≈ 0.8 → ≈ 39 A → screen 50 A inlet class (or 30 A if you intentionally limit loads).
Typical path: Listed interlock + 30–50 A inlet for fridge, furnace controls, sump, a few lights—not whole HVAC. Verify with Transfer Switch Size and Generator Size.
Shop class (sponsored): Generator interlock kit · Manual transfer switch 30 A
Example 2: 22 kW standby, unattended #
Inputs: Whole-panel standby on a 200 A service → screen 200 A ATS paired with the generator listing—not a suitcase interlock.
Typical path: OEM-matched ATS + document crank/warm-up for UPS bridge in ATS transfer time and Generator + UPS.
Shop class (sponsored): Automatic transfer switch 200 A
Which is better for cost, legality, and install? #
| Question | Short answer |
|---|---|
| Which is cheaper? | Interlock ≪ MTS ≪ ATS (equipment + labor) |
| Is an interlock legal? | Only if listed for your panel and AHJ allows—ask the electrician before buying |
| Install difficulty | Interlock: panel-local; ATS: controller wiring + commissioning |
| Can I leave town? | Interlock/MTS: no. ATS: yes (with maintained standby gen) |
5 common mistakes (and how to avoid them) #
- Suicide cord / open-neutral backfeed — Never energize a panel without a listed transfer method.
- Buying 200 A ATS for “headroom” on 100 A service — Feeder/service ampacity still limits you; see the sizing calculator notes.
- Skipping UPS for IT — Transfer can take seconds to minutes; use UPS Runtime for PCs/NAS.
- Ignoring continuous watts on the generator — Size gen first with Generator Size.
- Assuming interlock = automatic — You still start the gen and flip breakers.
FAQ #
Which is better—interlock kit or transfer switch?
Interlock for attended portable backup; ATS for unattended standby; MTS when you want labeled selected circuits. Screen amps in the Transfer Switch Size Calculator.
Is a generator interlock kit legal?
Often yes when the kit is listed for your panel and the AHJ approves—never assume. Hire a licensed electrician.
Do I still need a UPS with an ATS?
Yes for IT blinks during crank and ATS delay. See ATS transfer time.
What size transfer switch for a 10 kW generator?
At 240 V 1φ PF 0.8, I ≈ 52 A → continuous screen ≈ 65 A → often 100 A class depending on loads. Run the numbers in Transfer Switch Size.
Interlock vs manual transfer switch—what’s the install cost difference?
Interlocks are usually the lowest hardware + labor path; a multi-circuit MTS adds a subpanel/switch enclosure and more branch rewiring.
Next step: size amps, then pick hardware #
- Screen the amp frame with Transfer Switch Size Calculator.
- Confirm generator watts with Generator Size.
- If UPS-protected loads matter, plan bridge minutes via ATS transfer time and Generator + UPS.
- Return to the Power calculator hub or UPS calculator hub for the full backup workflow.