Generator Interlock Kit vs Manual Transfer Switch (and the Power Inlet Box Both Need)

Why you need one of them

The alternative is backfeeding — a "suicide cord" from the generator into a dryer or wall outlet — and Generac's own page on the subject is worth quoting: utility workers "may assume the lines are de-energized. Backfed power from a portable generator can seriously injure or kill them." It also "violates the National Electrical Code (NEC)" and, when the grid comes back, puts utility power into your running generator. Generac's stated solution is a manual transfer switch, which ensures "the home is either connected to utility power or generator power, but never both at the same time."

The NEC says the same thing in code language. Article 702.5 (2020) — via Mike Holt's summary — requires transfer equipment for any optional standby connection, "listed, designed, and installed so as to prevent the inadvertent interconnection of all sources of supply in any operation of the transfer equipment." The 2023 edition widened "transfer equipment" to "interconnection or transfer equipment." Both devices on this page exist to satisfy that sentence.

How each one works

Interlock kit. Your electrician installs a two-pole breaker (typically 30 A or 50 A) in the top position of your panel, wires it to an inlet box outside, and bolts a slotted plate to the panel cover. The plate can slide to expose the main breaker's ON position or the generator breaker's ON position, never both. To use it: main OFF, slide, generator breaker ON, then turn on the branch circuits you can afford. Everything in the house is reachable; nothing is protected from you turning on too much except the generator's own breaker.

Manual transfer switch. A second small panel next to the main one. Selected branch circuits are moved from the main panel into the switch, each on a three-position toggle: LINE, OFF, GEN. The generator feeds the switch through the inlet; flip circuits to GEN one at a time. Reliance describes its Pro/Tran 2 as "double-throw, break-before-make" — each circuit is disconnected from utility before it touches the generator — and the wattmeters "help balance generator load." Circuits that weren't moved into the switch don't exist during an outage.

Generac's characterization of interlocks — less flexible, more manually intensive — is a transfer-switch maker's view and should be read as one. In practice the interlock is the more flexible of the two on circuit choice and the less convenient on operation, and the acceptance question is the real difference.

The listing question, honestly

This is where the two devices diverge, and where forum arguments come from. A Reliance Pro/Tran 2 is a transfer switch listed to UL 1008, the transfer-switch standard; no inspector questions it. An interlock kit is a panel accessory. A kit from your panel's manufacturer — Square D's QOCGK2C for QO panels, HOMCGK2C or HOMCRBGK1C for Homeline (the series on your panel label decides which), Eaton's BRMIKCSR / BRMIKBR / CHMIKCSR, Siemens ECSBPK01 / ECSBPK02 — is covered under the panel's own UL 67 listing; Eaton's sheet states "UL67 Listed – for use with Eaton loadcenters" and "meets NEC Article 702." Aftermarket kits (interlockkit.com, GenInterlock and others) are tested by an independent lab "to UL 67" and marketed as MET-listed — the vendors' claim, and not the same thing as the panel maker's listing covering the combination.

Whether that satisfies 702.5's "shall be listed" is an authority-having-jurisdiction call. On the Mike Holt forums, a moderator reads NEC 110.3(B) as permitting only the panel maker's kit, another reports that local inspectors "look for" aftermarket kits and reject them, and a third reports Virginia inspectors accepting them. The same thread expects 2026 NEC language to name interlock kits in Article 702 explicitly. The practical rule: buy the kit your panel manufacturer lists for your panel series if one exists; if it doesn't, call the building department and ask before you buy anything.

The inlet box, the plug and the cord

Both systems end at a flanged inlet on the outside wall. Reliance's PB30 instructions cover the standard 30 A version: "30 Amps, 120/240 VAC," mating with "NEMA type L14-30" plugs, wired with "at least three color-coded wires (AWG #10 minimum, AWG #8 maximum)," and "installation of this power inlet box and related wiring must be done by a qualified electrician in compliance with all applicable electrical codes." Generac's own 30 A aluminum inlet is model 6343 (dealer listings); its cord sheet lists L14-30 cords at 10, 25 and 50 feet (6327 / 6328 / 6329, 10/4 wire) and 50 A cords with a 14-50 plug at the inlet end and a CS6364 twist-lock at the generator (6389 / 6390 series, 6/4 wire).

The arithmetic decides which size. L14-30: 30 A × 250 V = 7,500 W — Reliance rates its 30 A kits "for generators up to 7,500 running watts." 14-50 / CS6365: 50 A × 250 V = 12,500 W. A 5,500–7,500 W portable is a 30 A system; a 9,500–12,000 W unit needs 50 A hardware (Generac HomeLink 9854/9855, a 50 A interlock breaker) or it will trip the inlet breaker on every big motor start. The wattage calculator does the sum for your loads.

Two more things the inlet needs. Length: CPSC says generators run "at least 20 feet from homes and buildings, with exhaust directed away from windows, doors and vents" — so the 10-foot cord in most kits is too short for a compliant setup; plan on 25 feet. The sign: NEC 702.7(C), as Placer County's checklist spells it out, requires a label at the inlet reading either "FOR CONNECTION OF A SEPARATELY DERIVED (BONDED NEUTRAL) SYSTEM ONLY" or "FOR CONNECTION OF A NONSEPARATELY DERIVED (FLOATING NEUTRAL) SYSTEM," plus a sign at the service equipment noting the standby source. Which one depends on your generator's neutral, and it matters: portables of 15 kW and under built since 2015 have GFCI-protected receptacles (NEC 445.20), and a bonded-neutral generator feeding a house through a non-separately-derived connection can trip its own GFCI. Generac's note that HomeLink is "compatible with any portable generator with GFCI protected outlets" is the same issue from the other side, and the neutral question belongs on the electrician's checklist. You may read that "outdoor inlets must be GFCI under the 2020 code"; a flanged inlet isn't a receptacle, and that claim isn't supported.

Verified part numbers

You'll still find Generac's 6295 (30 A, 10–16 circuit) recommended online; it's discontinued and absent from generac.com. The current equivalents are the HomeLink 6852/6853 at 30 A and 9854/9855 at 50 A. Whether the 6853 includes a cord is stated differently on Generac's product page and its brochure — check the kit contents before ordering.

Permits, inspection, and who does the work

Both are panel work. Two municipal documents make the point plainly: North Port, Florida — "you must first apply for a permit … and the work must be inspected by the City upon completion," and the transfer equipment "must be installed to prevent the inadvertent interconnection of the normal and the optional system supply." Placer County, California — a building permit application, UL-listed transfer equipment and inlets, and the two signs above. North Port adds the useful boundary: a generator that only powers things through its own receptacles "DO[ES] NOT REQUIRE PERMITS OR INSPECTIONS." The permit itself runs $50–350 in the cost guides.

Reliance's wording is "qualified electrician"; Generac's HomeLink page says "installation by an electrician recommended" and the switch is "pre-wired for the electrician, reducing the amount you pay for installation." This site's rule is simpler: anything on the line side of a main breaker is a licensed electrician's job. The interlock in particular looks like a fifteen-minute install and involves pulling the panel cover on a live 200 A service.

Which one to choose

Repair or replace?

Not the right question for hardware that has no moving parts to speak of, but one honest note: an older Reliance Pro/Tran (31406CRK) without meters works exactly as well as the Pro/Tran 2 — don't replace one that's installed and inspected. And a generator-ready panel (Siemens and others sell 200 A load centers with a factory interlock) is worth the small premium if you're replacing a panel anyway.

When to call a pro

Related pages

What happens when the generator runs and the house stays dark — including the portable-generator cases: runs but no power. How much generator the chosen circuits need: wattage calculator. Why a standby unit shuts down when the house asks too much: Generac 2100 Overload.