What an interlock kit actually does
An interlock is a physical mechanical lockout — a small metal plate that slides between your main breaker and a dedicated generator-input breaker in your panel. When the main is flipped on, the generator breaker is physically blocked. When the main is flipped off (grid is down), you can slide the generator breaker on. It's impossible to have both on at once.
That's what makes it code-compliant: the NEC requires a positive means of preventing backfeeding the grid. A manual transfer switch does this; an interlock kit does this; running an extension cord through a window does neither.
Why this matters: Without a mechanical interlock, a portable generator wired to your panel could back-feed the utility lines during an outage. That can kill a lineman working to restore power, trip the generator when grid comes back, or destroy your equipment. Carriers will refuse to re-energize a house they suspect has uncontrolled backfeed.
Panels we install interlock kits on
Interlocks are panel-brand specific — a Square D kit doesn't fit a Siemens panel. We stock kits for the common brands:
Square D (QO, Homeline)
Most common in Upstate SC. Kits available for QO + Homeline, standard and HOM variants.
Eaton (BR, CH)
Newer builds often use Eaton. BR and CH series both have compatible interlocks.
Siemens
Common in mid-century and early-2000s homes. Kits available for PL + PN-series panels.
Murray
Siemens-compatible in most cases. We'll confirm the kit fits during the quote visit.
Leviton
Newer panel brand. Interlock kits available. Less common in Upstate SC but we've done them.
Others / older panels
Federal Pacific, Zinsco, Pushmatic — we do NOT install interlocks on these. Safety risk. Panel replacement recommended first.
The install itself
- Quote visit (15–20 min) — we photograph your panel, identify the brand and available slots, confirm the inlet location. Price is locked before any work begins.
- Permit pull — we submit under SC License 2942 for your city or county. Usually approved in 1–7 business days depending on jurisdiction.
- Install day (3–5 hours on-site) — inlet box mounted, conduit run to panel, generator breaker installed with the interlock plate, labels applied per code.
- Inspection — the city/county inspector signs off. You don't need to be home. We stand for the inspection.
- Walkthrough — we show you exactly how to power up safely: main off, generator running, interlock slides, generator breaker on, pick your circuits.
What's included in the $1,197–$1,497
- Interlock kit matched to your panel brand
- Generator breaker (30A or 50A) installed in the panel
- Inlet box mounted exterior (NEMA L14-30P or CS6375 for 30A; NEMA 14-50P or CS6364 for 50A)
- All wiring, conduit, weatherproofing
- Permit pull + permit fee
- Inspection stand-up
- Labor (one day, no return trips)
- Walkthrough + written "how to power up" card left on your panel
Common questions
Can I install an interlock myself? SC code requires licensed electrician work for panel modifications. The interlock kit itself is available at hardware stores, but pulling the permit and passing inspection requires a licensed contractor. DIY installs that fail inspection cost more to fix than having us do it right the first time.
How long does an interlock last? The mechanical plate is good for the life of the panel. Cycling it on/off doesn't wear it out in any practical sense.
What if I move? The interlock stays with the house — it's a code-compliant home upgrade. Appraisers + home inspectors generally view it favorably.