Generator Portable vs Interlock vs Manual Transfer Decision Matrix

Why this matters

When a homeowner has a portable generator and wants to power circuits inside the house, there are exactly three code-legal ways to do it: a mechanical interlock kit on the main panel, a manual transfer switch (a small subpanel of selected circuits), or a single inlet feeding a generator subpanel. The fourth option people reach for - a male-to-male "suicide cord" into a dryer outlet - kills firefighters and linemen by backfeeding the utility and is never acceptable. Picking the right legal method depends on the panel, the circuits the customer actually needs, and local code adoption. This matrix lets you recommend the correct one on the first visit instead of selling a manual transfer switch to someone who really needs a whole-panel interlock, or vice versa.

The recommendation also has to survive the permit and inspection process. All three legal methods are inspectable, listed installations; the inspector will look for a listed interlock kit matched to the panel, a back-fed breaker with the required hold-down, a properly sized and weatherproof inlet, and correct neutral handling. Getting the method right up front avoids a failed inspection and a return trip to re-do the connection method.

The options

Interlock kit. A listed sliding plate mounts on the panel deadfront so the main breaker and a back-fed generator breaker can never be on at the same time. A power inlet (typically an L14-30 or larger) feeds that generator breaker. The customer can power any circuit in the panel, one panel-load at a time, manually managing total draw. Requires a listed kit specific to that panel make and model.

Manual transfer switch. A factory-built switch with a handful of double-throw circuits (often 6 to 10) that flip individual loads between utility and generator. Each chosen circuit is pre-wired; the customer flips switches and reads the built-in watt meters. Self-contained and unambiguous, but the powered circuits are fixed at install time.

Single inlet plus generator subpanel. A power inlet feeds a small subpanel that holds the chosen critical loads, isolated from the main with an interlock or a service-disconnect-rated transfer device. Cleaner than back-feeding when the main panel has no listed interlock available.

All three break the connection to the utility so the portable cannot backfeed the grid. That isolation is the entire point. The differences are in how many circuits are reachable, how much the customer has to think during an outage, and what the panel will physically accept.

When A/B/C wins

Interlock wins when the panel has a listed interlock kit available, the customer wants access to all circuits, and they are comfortable load-managing manually (turn off the AC and electric range before adding the well pump). It is the most flexible and usually the least material. It needs a properly sized back-fed breaker, a hold-down kit on that breaker per the kit listing, and an inlet sized to the generator.

Manual transfer switch wins when the customer wants foolproof operation, a fixed set of critical loads, and clear per-circuit metering. It is ideal for someone who will not reliably manage load by hand. The tradeoff is that only the pre-wired circuits are covered; adding a circuit later means re-wiring the switch.

Single inlet plus subpanel wins when no listed interlock exists for that panel, when the main panel is full, or when the customer wants a permanent clean home for backup loads separate from the crowded main. It also suits a future upgrade to a standby unit because the subpanel and inlet can often be reused.

The deciding constraints are usually: does a listed interlock exist for this exact panel, how many circuits matter, and will the customer manage load manually. A "suicide cord" is never on this matrix - it is illegal and lethal.

Field decision flow

1. Confirm the generator is a portable feeding the house, not a permanently installed standby (a standby gets an automatic transfer switch, a different conversation).
2. Identify the panel make and model and check whether a listed interlock kit exists for it. If yes and the customer wants whole-panel flexibility, recommend the interlock.
3. If no listed interlock exists, the panel is full, or the customer wants a permanent dedicated backup area, recommend a single inlet plus generator subpanel.
4. If the customer prioritizes foolproof, fixed, metered operation over flexibility, recommend a manual transfer switch sized to the chosen loads.
5. Size the inlet and back-fed breaker to the generator's rated output and the cord set. Verify the generator's neutral-bonding configuration matches the install (a bonded-neutral portable on a system with its own neutral-ground bond can create a parallel neutral path; follow the OEM and the transfer-method listing).
6. Confirm the homeowner understands the operating sequence for whichever method is chosen: with an interlock they must shed large loads before starting heavy draws and manage total wattage by hand; with a manual transfer switch they flip only the pre-wired circuits and read the built-in meters; with a subpanel they run only the loads landed in that panel. Walk the sequence with them at commissioning so the method they paid for actually works during the outage.

References

• NFPA 70 (NEC) Article 702, Optional Standby Systems - transfer-equipment and interconnection requirements for portable and permanently installed optional standby.
• NFPA 70 (NEC) 702.4 and 702.5, capacity and transfer-equipment provisions; 250 series for grounding and neutral-bonding of separately derived vs non-separately-derived systems.
• UL 67 (panelboards) and the interlock kit's UL listing - interlock kits must be listed for the specific panel.
• OSHA and utility safety bulletins on generator backfeed hazards to line workers.
• Generator OEM owner's manual neutral-bonding and grounding guidance for portable units.