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I would like to have a standby generator installed for my house. Based on my own assessment of our electrical needs, along with my desire to have a slower-running generator to reduce noise, and my desire to get an adequate generator at the best price, I've made a preliminary choice of the Generac 22kW (RG022). This is the smallest propane-fueled generator in the Generac line that runs at 1800 RPM.

We have been working with a local dealer, who sells both Generac and Kohler products. My initial conclusions have been stymied by some assertions that the dealer has made, but which they have refused to provide documentation for:

  1. They claim that "code" requires that either we install a generator capable of supplying the entire 400 amps that the home's panel could be configured for (the current service is, if I recall correctly, 320A and this is more a by-product of having the service split into two panels, because we have a detached garage with bonus room above, so there is one panel for each building), or we can instead hire an electrician to install a power monitor for a month and then size the generator based on the results of that monitoring.
  2. They claim that, in spite of the statements by the manufacturer (e.g. "Can Be Installed Just 18" From a Structure - Capability to be installed 18 inches away from a structure, ideal for areas with tight lot lines. Generac’s is the only manufacturer of standby generators to have 18” offset on most of its residential lineup."), we will be required "by code" to locate the generator a minimum of five feet from the building.

I keep putting "code" in quotes because that is literally the only information this dealer has provided to me, even after I ask for a more precise explanation, i.e. for them to provide me with an actual reference to the actual code that imposes these requirements. All they ever say is "that's what code requires".

My question is: are these actual legal requirements for the generator installation? If so, where are these legal requirements documented, and by whom are they imposed?

I guess a follow-up question would be: if these are in fact not legal requirements, what are the ramifications of just selecting and siting the generator according to my own preferences and expectations, instead of following the "requirements" as stated by the dealer?

For example, suppose I've miscalculated at some point, or maybe additional equipment is installed later on — what would happen if more load was put on the generator than its rated to provide? Would there simply be a sag in the voltage on the service from the generator? Or would it actually damage the generator and/or the electrical system in some way?

The house is in Washington State, which I presume affects to at least some degree what codes are applicable.

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    The muffler will be the primary factor to control noise, not RPM. Most of my neighbors have whole house Generacs ( nat gas) and they are noisy . When the power goes out , the whole neighborhood hums. I have a 2000 W Honda portable ( fridge, TV, lights, room AC) and I can't hear it above the neighbors' machines. I never considered any code. – blacksmith37 Sep 9 at 21:47
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    What loads do you actually want to power from the generator? There are several very good reasons that commercial/institutional setups do not use whole-building transfer (and they also largely apply to residential applications as well); instead, they provide the utility source to the transfer switch as a feeder from the main panel/switchgear and then have a separate set (or sets, in a place like a hospital) of standby and/or emergency busses that are fed from the ATS and thus power only the designed standby and/or emergency loads. – ThreePhaseEel Sep 9 at 23:50
  • @ThreePhaseEel: The most important loads would be our well pump, septic system, propane-heat furnace, water heater, and EV charging. The water heater and EV charger are the highest-draw elements. I would have loved to have a split system, but after wrestling with the electricians to try to work out a plan for that which accommodated circuits in the main house and detached garage without making for overly-complicated circuit breaker setup (mainly, to ensure breakers for circuits were in the same building where the circuit was), I gave up and settled on whole-house. – Peter Duniho Sep 9 at 23:54
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    @PeterDuniho -- well, septic, and furnace all make sense. Is replacing the WH an option, and can I ask why the EV charger is so critical? Also, this sort of problem is something I've seen before...there's really no need to go whole house here, and that's probably part of your problem. Furthermore, is the "bonus room" above the garage simply some sort of workroom, or is it intended to be dwelling space? – ThreePhaseEel Sep 10 at 0:25
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    @ThreePhaseEel: ... Detached building water heater: one hot tap, electric instant point-of-use, it's totally fine if that doesn't work during a power outage. Utility is Puget Sound Energy. The "bonus room" is living space, but it's well-insulated, and has only leisure activities; it can mostly be unpowered in an outage without any problem, but it's the building where our Internet connection comes in, so I'd want power for that. – Peter Duniho Sep 10 at 0:31
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  1. They claim that "code" requires that either we install a generator capable of supplying the entire 400 amps that the home's panel could be configured for (the current service is, if I recall correctly, 320A and this is more a by-product of having the service split into two panels, because we have a detached garage with bonus room above, so there is one panel for each building), or we can instead hire an electrician to install a power monitor for a month and then size the generator based on the results of that monitoring.

No

Sounds like an effort to "upsell". You can have as small or large a generator as you want. Quite often a generator will be sized for critical loads (e.g., refrigerator, air handler for furnace) + a couple of convenience circuits. Or "everything but the air conditioning and the electric dryer". Or anything in between. Or "everything".

It is entirely up to you. Just understand that if you put in a generator that can't handle "everything" and then try to use it for "everything", you will run into trouble. Which might take the form of "generator turns itself off because it is overloaded" - and then you have nothing running at all.

  1. They claim that, in spite of the statements by the manufacturer (e.g. "Can Be Installed Just 18" From a Structure - Capability to be installed 18 inches away from a structure, ideal for areas with tight lot lines. Generac's is the only manufacturer of standby generators to have 18" offset on most of its residential lineup."), we will be required "by code" to locate the generator a minimum of five feet from the building.

Maybe

There may be some truth to that. A state, county or local building code may place additional requirements that override the manufacturer's minimum requirements. Or they may be citing old requirements that have been superseded by Generac's 18" capability and they just don't know it. Or they may be just making it up - e.g., perhaps to try and sell you on a separate pad for the generator instead of being able to use concrete already in place next to your house.

  • Are there requirements on the size of the transfer switch? – JACK Sep 9 at 20:46
  • @JACK I don't know, but a transfer switch smaller than your generator would mean you couldn't use full capacity and a switch larger than your generator really wouldn't hurt anything. – manassehkatz-Reinstate Monica Sep 9 at 21:09
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While I appreciate the answers posted so far, the more comprehensive of the two did not include any references. Granted, it's hard to point to a reference that says something isn't required. But, I was hoping for some kind of citation. So, I went looking for one.

I believe I've finally found the applicable requirements, in the National Electrical Code (NFPA 70).

Article 702, "Optional Standby Systems" has this to say:

702.4 Capacity and Rating

(B) System Capacity. The calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method.
(1) Manual Transfer Equipment. Where manual transfer equipment is used, an optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system. [emphasis mine]
(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
    (a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
    (b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

I read (B)(1) to mean that the user gets to pick the load, and thus the size of the generator. However, if an automatic transfer switch is used, then (B)(2) applies, and unless the system also automatically manages the load, the generator is required to be able to supply 100% of the actual load being transferred by the switch (calculated according to Article 220, as stipulated in the main (B) paragraph).

The referenced Article 220 is a fairly complicated document. It has two different parts that address calculation of service loads (which is what I presume applies here, since the generator is acting as the electrical service), Part III and Part IV.

Part III is the more complicated of the two, and generally involves a fine-grained approach, making assumptions about lighting loads based on building square footage, but otherwise calculating the actual loads on each branch-circuit, appliances, etc. Part IV is an "optional" method, approved as an alternative to Part III. It too is complicated, but in a coarser-grained way (e.g. it allows each 20A circuit to be assumed to carry 1500 VA).

Part IV is also where we find the alternative means of determining the service load, by measuring the actual demand:

220.87 Determining Existing Loads. The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under all of the following conditions:
(1) The maximum demand data is available for a 1-year period.

Exception: If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflected the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.

(2) The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.
(3) The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.

Now, the language here all seems to relate to adding load to some service, either installing new service (i.e. where the original load is 0), or adding to existing service. But, the electrician who installed the recording ammeter on my system tells me that they use the same 125% margin for the generator sizing, as well as take into account assumed heating loads if the measurement is done during the cooling season.

Phew!

TL;DR: it seems that, while not precisely accurate — at no point does the code say you have to size the generator to the existing service capacity, i.e. 320 A in my case — it is true that when an automatic transfer switch is used, some load calculation is required, and the generator size must be sufficient to meet the demand of that load.

As for the other question, about the placement of the generator, all I could find on that was another NFPA reference, this time in NFPA 37, Chapter 4:

4.1.4 Engines Located Outdoors.
4.1.4.1 Engines and, if provided, their weatherproof housings that are installed outdoors shall be loacted at least 1.5 m (5 ft) from openings in walls and at least 1.5 m (5 ft) from structures having combustible walls except as provided in 4.1.4.1.1 or 4.1.4.1.2.
4.1.4.1.1 A clearance less than 1.5 m (5 ft) shall be permitted where all portions of structures that are closer than 1.5 m (5 ft) from the engine enclosure have a fire resistance rating of at least 1 hour.
4.1.4.1.2* A clearance less than 1.5 m (5 ft) shall be permitted where it has been demonstrated through methods acceptable to the authority having jurisdiction that a fire within the enclosure will not ignite combustible structures.

I presume that Generac's claim of 18" from structures is based on some demonstration of their generator enclosures not igniting combustible structures. Why the local dealer claims that this exception applies only to the generators with air-cooled engines and not those with liquid-cooled engines, I don't know. I would guess that if I wanted to push the issue, I could point to the manufacturer's advertising and technical documents.

It does seem possible that Generac is, as they claim, the only manufacturer which has gone to the trouble of demonstrating that their enclosures are capable of containing a fire. Thus, other generators would in fact be required to be 5' from the building.

Both of these sections of the regulations are, of course, not technically my local regulations. But they are part of electrical and fire code adopted throughout the US. So it seems reasonable to assume that these codes do apply in my case.


Finally, as an aside I wondered what the consequence would be of having a generator insufficiently sized for the actual load. I did not get any concrete information on that question, but from what I've been able to glean, it seems that output voltage would start to drop, until such point as there is insufficient voltage for the alternator's field coil, causing the alternator output to drop to zero, and the generator to shut off. I haven't seen anything that indicates any damage would be done to the generator. You'd just lose power again until you went and manually got the generator running again (after, of course, turning enough of the load off that the generator could handle what remained).

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With a dealer being THAT uncooperative, my 1st recommendation is give someone else your business. Sounds like they are trying to scam you.

I would trust the manufacturer spec if you cannot get a real answer from your local building code officials.

  • Yeah, that dealer has been dismissed, on the general principle that they refused to provide actual customer service (i.e. by answering reasonable questions). But, I want to make sure I really understand the rules before I start talking to a new one. – Peter Duniho Sep 13 at 18:38

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