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I have a small grid-connected solar PV system. If it is connected to my main load center via a two-pole breaker, how can I safely add a generator inlet to this system ?

The usual method is to connect a generator is via a NEMA 14-30 jack, which is connected to the load center via a two-pole breaker. An interlock prevents this two-pole breaker from being turned on until the main breaker is first turned off; this prevents backfeeding the grid from the generator and endangering line workers. (Some jurisdictions do not allow a simple interlock, but rather a separate transfer switch, for which case my dilemma would not exist).

If the grid-tied solar inverter (GTI) is connected via another breaker, then, since there is no mechanism I know of which can interlock the main breaker with TWO branch-circuit breakers, it would be possible to have both the generator and the GTI connected to the load center simultaneously. This seems problematic. But, is this actually safe, because the GTI's U1741 capability might prevent it from being activated, since it wouldn't detect the generator's output as being up to grid spec) ?

This is all moot if one connects the GTI to the line side of the main breakers (using insulation-displacement connectors such as those made by Buchanan). But even though I've done this, it seems un-wholesome, given that the wiring between the panel and the GTI (not to mention the inverter itself) is not protected by ANY over-current protection device (OCPD), notwithstanding what exists at the power-company's transformer.

Perhaps the only good solution is a transfer switch, or an essential-loads sub-panel, or one of the "generator ready" load centers, which is effectively an essential-loads sub-panel built into the same enclosure.

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No need for an interlock. There's no safety issue here.

The reason for the utility/generator interlock is to prevent back-feeding the grid and killing linemen, as you note. Solar grid-tie inverters don't need that because they comply with UL 1741 (or better!)

But that's not all UL 1741 does. It also makes grid-tie inverters play nice with generators. The code requires the grid-tie inverter to look for a number of signs of grid presence: including stable voltage, extremely precise AC frequency like a grid would have, and the ability to absorb unlimited amounts of current with no affect on AC frequency and minimal effect on voltage.

The chance of a generator delivering that precise an AC frequency is close to nil. And the moment the solar tries to power up and pick up the house's loads, this will unload the generator, causing the engine to "race" before the governor can change the fuel rack to adjust. The UL 1741 inverter will detect this frequency change and go "Nope, this is not the grid" and drop out.

Ordinary inverter generators aren't precise enough either. If you want an inverter that is, you have to get a special type called a grid-forming inverter. It is specifically designed to make power clean enough to "wake up" a UL 1741 grid-tie inverter and get it to come online and generate. But it has one essential trick.

A little-known fact is much of the time, your house uses less power than the solar panels make. (that's true for any solar array worth installing). Houses have 24KW or 48KW services to deal with peak demand, but houses average about 1 KW. So when they use less, they use a lot less. For the grid-forming inverter to work during grid-out scenarios, it must cope with the solar panels vastly outproducing demand.

It does that with a battery bank. If the battery is not full, the grid-forming inverter absorbs all surplus solar production into the battery pack. That keeps voltage within spec, which keeps the UL 1741 solar inverter thinking it's on the grid. Once the battery is full, it stops absorbing the surplus solar, and the solar inverter realizes it is not on the grid. If you turn on a heavy load, the battery is now no longer full, so the grid-forming inverter again tricks the UL 1741 solar into coming online.

You need to turn off breakers anyway. Turn this too.

Here's the thing about using a generator interlock to power a whole panel. Your generator can run some loads, but it can't carry every load in the house at once. So you will need to switch off some branch circuit breakers when you throw the interlock over to "generator". (especially for non-controllable loads like water heaters).

While you're shutting off other breakers, it's no trouble to shut off the solar breaker too, if it were a problem. But I don't expect it to be.



But yes, a triple interlock is possible. It just depends on the fit of the interlocks. Some interlocks, such as the ECSBPK02, CHML and QO2DTI, interlock two adjacent 2-pole breakers. The ECSBPK01 interlock works on two opposite breakers. You would simply need to look at them and figure out how they can overlap. I am confident you can stack the "adjacent" interlock type, so that 1-2 are interlocked as are 2-3. Thus if 2 is on, 1 and 3 must be off.

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  • The main issue with stacking interlocks like the CHML or QO2DTI is getting the nominal limits on retainer positioning for the backfeed breaker to play nice with where the solar breaker usually goes Commented Dec 25, 2021 at 3:44
  • @ThreePhaseEel but does a solar breaker even need a tie down? UL 1741 should shut it down the moment it pops out. Commented Dec 25, 2021 at 3:51
  • The solar breaker doesn't, but the generator breaker does... Commented Dec 25, 2021 at 10:52
  • Yes, it would be pretty natural to turn off the solar breaker as you turn off most of the others (except the ones you hope for the generator to be able to handle). But I guess I'm still not 100% convinced that no harm can come from having the generator and GTI breakers both turned on. Commented Dec 25, 2021 at 19:19
  • I'll look at stacking interlocks if I decide to connect my GTI to a breaker (rather than line-side taps) when I replace my main panel. However, not doable just with ones that interlock adjacent or opposite breakers; I'd need one that interlocks the main breaker too. Commented Dec 25, 2021 at 19:21
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My understanding is that you let the solar system determine what it wants to do, or you change it to work with a battery/inverter storage solution that allows grid-tie AND grid backup. At least some of those do have means to allow a generator in to assist when needed.

If the solar system considers the generator to be "grid quality," it steps in and assists. The generator output drops, and if the total load is less than the solar output, the voltage is limited at maximum and either the solar disconnects itself due to overvoltage (and perhaps a reassessment of the quality of the "grid"), or it reduces output as well.

You cannot "use the generator input to connect solar" because then the solar would be interlocked from ever connecting to the grid.

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  • Duh, on your last paragraph. What a dope - I will delete from OP. Commented Dec 20, 2021 at 17:38
  • So you're confident that no damage will be done if both GTI and generator breakers are "on" (and main breaker is "off"), even if total load drops below solar output ? Commented Dec 21, 2021 at 18:59

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