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I have a Generac Guardian 22kw hardwired in via an RTS switch. When the power goes out (and it does so fairly often) the generator kicks on automatically and there is barely a flicker in the lights.

However I am all about redundancy. I also have a 15kw "portable" generations that I would like to be able to use as a backup to my backup should the 22kw go down during a blizzard or something. I have several 50amp welding and RV plugs. Could I wire up an inlet box and provide power that way? As long as the RTS switch is flipped and keeping it shut off from the main power line that should be fine right? Am I missing anything? Thanks!

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  • Is your transfer switch a whole-house/service entrance switch, or is it feeding a subpanel of its own? Nov 15, 2020 at 4:26
  • It is "whole house." I own a ranch. From the pole where the service comes in (nothing is buried it's all above ground) I have a line going to the house's panel, another line going to the panel in the shop, another line going to the panel in the barn, and another going another outbuilding. There is no "main panel" for the property but everything is connected on the pole. When the 22kw gen kicks on it provides power to the house and all the outbuildings.
    – Jafruech
    Nov 16, 2020 at 5:41
  • I take it your transfer switch is at the pole? Nov 16, 2020 at 12:43
  • That is correct
    – Jafruech
    Nov 16, 2020 at 13:56
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    The 15kw portable is a DuroMax XP15000EH, but we also have a couple others so plug and play would be beneficial as well. If things were ever to go down in the world it would be nice to do a rotation and give them a break and be able to rotate through oil changes and such. We have a significant amount of propane and fuel storage to keep them running quite a while if it is ever necessary. Last year's "bomb cyclone" blizzard really opened our eyes with power out for 2 weeks and the need for redundancy, oil changes, etc
    – Jafruech
    Nov 17, 2020 at 2:49

3 Answers 3

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The trouble with back-feeding portable generator power through any of those welding or RV plugs while the transfer switch is set to generator position is the same problem with relying on turning off the main breaker before doing the same: it's manual, not interlocked, and sooner or later somebody is going to get it wrong.

Instead, daisy-chain another transfer switch. It could be another RTS series switch if you're so inclined, but the transfer switch could also be as simple and inexpensive as a main-breaker type panel with a generator interlock kit for a branch breaker.

I gather you'll want to keep the automatic switch-over for the 22 kW generator working and add a manual alternative in case the 22 kW breaks down or you just decide not to use it. To do that, add a 150A or 200A main-breaker panel. Wire the main breaker to the 22 kW generator. Add a branch breaker in the 100A-150A range and wire that to the "generator input" connection of the existing transfer switch. Finally, add a 70-100A branch breaker in the designated "generator backfeed" location of the panel and wire this to an inlet socket.

With the above arrangement there are two operating modes. Either the main breaker is on, allowing 22 kW generator to connect to RTS input, or else the generator interlock is activated so that the inlet for the portable generator is connected to the RTS input.

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  • I have been using the manual interlude code kits for a while as a back up to a backup this is probably the best and least code compliant plan.
    – Ed Beal
    Nov 15, 2020 at 16:53
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What Greg Hill says.

Your idea is a "checklist" method, i.e. step 1 switch off main breaker step 2 switch on gen inlet breaker (more steps than that obviously). To work, your idea absolutely depends on humans following the checklist precisely, when they are a) in the dark, b) in cold/rain/wind, c) in stress/panic, d) they tried following the checklist and it did not work. Especially in situation d), no force in the universe is going to keep people on-script.

NEC is written by the NFPA, which gathers fire and accident data from every fire chief in America. What the data has proven, without a doubt, is that checklists do not work and have a long history of creating hazardous situations.

The Marines can tell you why: in a crisis, people do not rise to the occasion, they sink to the level of their training. And training like that is a perishable skill: when they need it in the military, they train it weekly. You have to admit, it's pretty unrealistic to remember a checklist procedure you created 4 years ago and haven't used since.

So NFPA determined that checklists are hopeless and only an interlock will do. They're not terribly expensive. For "across-from" i.e. backfed panels, they are as little as $27. (plus $12 for the generator breaker obviously). For main-breaker-separate panels they are often in the $60 neighborhood, UL-approved from the manufacturer. Having an interlock downstream of an ATS is certainly no problem, as Greg Hill discusses.

The person working with that system has confidence that the hardware was installed competently and to Code, so when they're in a tizzy "trying anything", they won't mess with that.

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Nesting transfer switches is OK...

The good news is that nothing prohibits you from having a "hierarchy" of transfer switches, so the existing "whole house" service entrance rated ATS can stay on the pole since you are indicating that you can manually operate the switch to the generator position.

Given that, what I'd do is get a NEMA 3R interlocked-breaker transfer panel with a 100 or 125A utility breaker and a 60A generator breaker; a Reliance Controls TCA1006DR will do the trick, although there may be other products out there that provide this featureset. You'll want to run a 1.5" liquidtight between this switch and the existing ATS on the pole; inside this conduit, you'll be running 2 1AWG Al hots that extend the existing generator feeder from the ATS enclosure (where it's disconnected from the generator terminals on the switch) to the new switch's "utility" terminals, 2 1AWG Al hots that connect the new switch's "load" terminals to the "generator" hots on the ATS, a 1AWG Al neutral that connects the neutral bar on the new switch to a spare neutral lug on the existing ATS, and an 8AWG copper ground (bare or insulated) that connects the grounding bar on the new switch to a spare grounding lug on the existing ATS. Don't forget to pull the bonding screw out of the new transfer panel!

...but you need to mind your neutrals

The issue, though, with what we've done so far is that while you have the hots safely isolated under all circumstances, permanently installed residential standby generators and automatic transfer switches are all configured for a floating neutral, where the main panel supplies the neutral-ground bond regardless of what position the transfer switch is in. However, most portable generators provide their own neutral-ground bond so that they can safely be used on a jobsite. Normally, this would mean that you'd need a transfer switch with a switched neutral pole on it, but we'd have to have both switches switch the neutral in your situation, which is impractical.

However, there is a way out of this using naught but an appropriate transformer, an inlet kit or box, and a bit of cleverness. In your case, a 15kVA, 240/480VAC to 120/240VAC, single phase, encapsulated transformer in a NEMA 3R enclosure will do the trick quite happily, as all we need this transformer to do is isolate the portable genset from the rest of the system so that the offending neutral-ground bond can't cause us any trouble.

This transformer gets wired up with its H1 and H3 terminals connected to one of the inlet's hot legs, its H2 and H4 terminals connected to the other hot leg on the inlet, its X1 terminal connected to one generator hot terminal on the new switch, its X4 terminal connected to the other generator hot terminal on the new switch, and its X2 and X3 terminals connected each other and to the neutral bar on the new switch. This all can be done with 6AWG copper wire, using another length of liquidtight conduit (3/4" is fine) to connect the transformer to the manual transfer switch, and the inlet box to the transformer for that matter. The inlet ground connection gets connected to the transformer ground lug, and the transformer ground lug gets connected to the ground bar on the new transfer switch for that matter, with some 10AWG or 8AWG copper wire.

What this gets us is a setup where the generator supplies a neutral/ground bond of its own, but the transformer isolates that bond from the rest of the electrical system, letting it play off in the corner all by its lonesome while everyone else congregates around the neutral-ground bond in the existing service entrance ATS. (That's why nothing gets wired to the inlet's neutral connection -- we don't need or care about it in this case!)

Sidebar: Why not put the manual transfer switch on the load end of the ATS instead?

If you're wondering why I didn't put the new manual transfer switch "downstream" of the ATS, routing the ATS' load connections to the new transfer switch, it's because that'd require a manual transfer setup that can handle the full utility load current. Considering we're splashing out on a transformer to work around the N-G bonding problem, adding more cost to the system that we don't technically need is undesirable.

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