Re-use existing 10-2 wiring run for generator interlock, AND feed both legs of panel?

Question

Following on this question, I am in a similar situation, but with a few different details:

• I currently have 125A utility electrical service to my house, feeding an outdoor Square D Homeline panel. The house was re-wired completely in ~2011, and there are no MWBCs that I am aware of.

• I also have a 3750 running watt portable backup generator, with TT-30 receptacle.

• My goal is to add a generator interlock to my panel, enabling me to feed 120V to both sides of the panel with a 30A double pole breaker and a generator inlet box.

• Due to some nuances of my property lines, a carport, and the location of the panel, locating the generator close-ish to the panel while still allowing for good spacing from the house is surprisingly challenging. In a perfect world, I would install a new run of 10/3 through the crawlspace, ending at an appropriate location for a remote generator inlet.

Here's where things get interesting:

There happens to be a an already-installed run of 10/2 romex directly terminating at an old A/C disconnect box (the A/C was relocated ~10y ago). This old A/C box would make for a perfect generator inlet location.

I'm hoping I can repurpose this existing 10/2 line for my generator inlet rather than pulling a fresh run of 10/3. It would be substantially more convenient to re-use this line vs. embarking on a new wiring job in the crawlspace.

I'm wondering if the following is feasible, code compliant, and safe:

1. Replace the old, unused A/C disconnect box with a 3-prong generator inlet box (e.g. a Reliance PB-31, with an L5-30 connector)
2. Wire the generator inlet to the existing 10/2 line running to the panel
3. Inside the panel, jump the the hot conductor of the existing 10/2 line into both poles of a 30A double-pole breaker by way of a wire nut (hot from the 10/2 IN, two short 10GA wires OUT to the two poles of the breaker)
4. Install the appropriate, OEM, Square D interlock kit on the 30A generator breaker, ensuring that backfeeding is not possible.
5. Connect the generator to the inlet by way of a TT30 extension cord and an adapter to match the connector on the inlet
6. When the backup generator is in use, keep all actual 240V circuits turned off. Only run the minimum required set of 120V circuits

I'm looking for some feedback on #3 above. My reference point for this idea is that I have seen many generator extension cords that adapt a 3-prong connector (like a TT-30) to a 4-prong (like an L15-30) by jumping the X and Y poles at the 4-prong plug. So...can this same jump be safely accomplished inside the panel itself?

Answer 1

Tricky.

With generators we see biblical amounts of jackassery. Not here. This asker has done a first class job on proper design, there's just a fairly arcane safety issue at play.

Since your generator is only capable of 120V, running a new cable is a pointless endeavor that will buy you nothing until you get a larger generator.

Hopefully never, given how quickly battery tech is improving. A battery system can run loads much larger than the generator, using the generator only to refill.

There must not be any multi-wire branch circuits (MWBCs)

These are an ugly little monkey-wrench to your plan. MWBCs are circuits which have two 120V "hot" wires and they share 1 neutral wire. This is only safe when the "hot" wires are opposite phases/poles: think of it as one of them is +120V, neutral is 0V, and the other hot is -120V. (accurate half the time lol; the rest of the time flipped). The neutral carries only difference current: if one hot is 15A and the other is 13A, neutral carries 2A.

Under your scheme the neutrals MWBCs can be overloaded, since they are being fed by the same phase of power. So 15A+13A = 28 amps on the 15A neutral wire!!! WHOOPS!!! That is why it would be a Code violation.

The answer is to search the panel for MWBCs. These are cases where a circuit consists of 2 hot (live) wires and 1 neutral. If the panel is wired with cables, the "red flag" is a red wire in the cable - that being the second "hot". If your wiring is in individual wires in conduit, it's more of a bug hunt - though if done properly, every hot(s)+neutral grouping must be identified, so it's a quick search.

While you're in there, fix any MWBC defects. Current Code requires that all MWBCs be on a breaker with a common handle throw. This can be a 2-pole breaker, or two singles with a UL-listed handle-tie. This protects the maintainer who is trying to turn off the circuit to work on it; turning off only half an MWBC means you get nailed by the other half. Handle-ties are $3 (hard to find though) and 2-pole breakers are $12 normally. They won't be AFCI or GFCI. Handle-tying the MWBC also forces it to be correctly phased, so takes "neutral overload" off the table on utility power.

However these MWBCs still don't work with your plan.

Pick a leg.

Unfortunately, the presence of a MWBC turns your plan into a "nope". At that point your only option is to "choose 1 leg" and power only that leg. You would need to rearrange your breakers so your critical loads are on the same leg. All the more important that you identify and handle-tie MWBCs; because careless rearranging in an MWBC panel can create the problem that all this fuss is about!

Can the MWBCs be eliminated without rewiring?

Sure. You can take both their hot wires and place them on the same 1 lug of the same breaker. (some breakers accept 2 wires per lug; for others, pigtail). At that point, in our example 13A+15A case, that cannot happen because the single 15A breaker will trip. Thus the neutral is protected (at cost of not being able to run as much stuff at once, but chances are that wouldn't come up very often).

However since this amounts to combining circuits, it's not allowed for kitchen, bathroom, laundry and garage receptacle circuits, which have particular rules about how many circuits and what they serve, and combining a MWBC would probably violate those. But if you choose to violate Code, and are picking your poison, this is the least bad.