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Why does the requirement for using a "transfer switch" with a backup generator depend on its power rating?

In a residential setting, a transfer switch decouples or isolates all or some house circuits form the utility service in the generator is operational. This can be manual or automatic. Decoupling prevents shorting and back-feed.

I would have thought that in a residential backup setting, the only criterion for a transfer switch is whether the appliances or circuits serviced are dual fed (utility and generator, e.g. through house wiring - so switch required) or single fed (generator alone, no utility and no switch, e.g. by extension cord).

I am asking this from a power engineering and circuit perspective, but I'd also appreciate any thoughts on the reasoning behind code requirements.

Perhaps I am reading this wrong, or the writing is wrong, but it seems to suggest that the power rating matters, as well as an assessment of whether it's "a good idea".

What Generators Need A Transfer Switch?

Standby generators for homes and businesses almost always need one. Since they are always waiting for when the power goes down, it is important to have this extra piece of equipment to keep the power flowing without downtime.

However, portable generators don’t strictly need a transfer switch, but it is usually a good idea. The biggest benefit of having a transfer switch in a residential setting is that you gain the ability to power things up via your circuit breaker panel instead of having to use extension cords. This includes hardwired devices, such as your dishwasher, hot water heater, air conditioning, and ceiling fans. All you have to do is plug the portable generator into the transfer switch and you’re good to go! Is A Transfer Switch Required?

If your generator is above 5,000 watts, you will always need a transfer switch for safety reasons and ease of use. This is important to remember, as the level of power being produced requires the use of a regulator to help keep surges and backfeeding the grid from happening.

Ref: https://www.wpowerproducts.com/news/why-do-generators-have-transfer-switch/

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    \$\begingroup\$ I think the "portable generators" they speak of, used without a transfer switch, involve you unplugging some appliances from your wall receptacles and plugging them into the generator (via extension cord) instead. Power from the generator never enters your house wiring, so no transfer switch is needed. \$\endgroup\$ Commented Jan 12, 2023 at 18:57
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    \$\begingroup\$ "what does that have to do with power rating (over/under 5000W)" 5000W/240V = 20A, resulting in 12AWG (North America) wire. Finding extension cords larger than that, though possible, is uncommon. Power levels like this are better hardwired with a transfer switch, in my fully un-qualified opinion. \$\endgroup\$ Commented Jan 12, 2023 at 19:13
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    \$\begingroup\$ @P2000: You don't wire in a generator without a transfer switch. Full stop, doesn't depend on power rating. The only place power rating comes in is "can you reasonably unplug your loads from wall sockets and plug them into outlets on the generator?" which for large number of small appliances would be very annoying, while large loads such as central HVAC may be permanently wired and not capable of being unplugged/replugged. \$\endgroup\$ Commented Jan 12, 2023 at 20:22
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    \$\begingroup\$ The article is written for a technically unsophisticated audience who want a simple rule to decide if they need a transfer switch. As far as I can see the article does not and does not claim to derive its authority from the electrical code. If your generator can be connected to your breaker box, then code requires a transfer switch as far as I know. I don't think there is any other legal way to do it. \$\endgroup\$ Commented Jan 12, 2023 at 20:35
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    \$\begingroup\$ @mkeith I have that impression too, and you only need a transfer switch if at least one of the following is true: a) you want automatic transfer and/or b) you want to feed into the existing wiring serviced by utility. The power rating has little to do with it, although feeding into house wiring implies a high rating. \$\endgroup\$ Commented Jan 12, 2023 at 21:31

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If the generator is used to provide emergency power to something that is normally connected to "grid power", you DO need a transfer switch to ensure that the generator won't try to power the grid, regardless of the generator power rating.

If the generator is providing power to some load that is not normally connected to the grid, then a transfer switch is not required, regardless of the generator power rating.

On a construction site, you might have a large generator to power an electric crane that is not connected to any other source of power - no transfer switch is required as there is nothing to transfer the load to.

My yacht club has an outstation in an isolated area where commercial power is not available. We have generators there to power the site, but no transfer switches as there is no alternate power source to transfer to. I think the largest generator there is 20 KW.

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  • \$\begingroup\$ agreed, wiring matters, but what does the power rating have to do with it? "If your generator is above 5,000 watts, you will always need a transfer switch for safety reasons and ease of use" (I edited the question with link for quote) \$\endgroup\$ Commented Jan 12, 2023 at 19:13
  • \$\begingroup\$ I think there are practical considerations. With a small generator, running a couple of extension cords to the few things it can power is no problem. With a larger generator, making temporary connections to all the things you want it to power is a lot more work, so a transfer switch is a more convenient solution. The generator size where a transfer switch is preferred would depend on the user's wants. \$\endgroup\$ Commented Jan 12, 2023 at 19:58
  • \$\begingroup\$ from the discussions here it really seems it comes down to the user's wants, as you write, not the power rating, and the text is just unsophisticated writing. \$\endgroup\$ Commented Jan 13, 2023 at 16:05

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