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Question DetailsAsked on 2/11/2018

is a automatic transfer switch considered a sub pannel

the switch would be mounted below the meter box and supply my main panel when a failure of the utility would occur the switch would automatically go to the generator this is a 150 amp switch to match my panels 150amp the generator would be a 20kw unit would this switch be considered a sub panel

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You don't say why you are asking this question - presumably related to some code requirement.

No - a sub-panel is where power is tapped off the main circuit to an ancillary or secondary use or to a second main panel - like a separate sub-panel or additional main panel for a pool or guest house or workshop for instance. (A sub-panel technically comes off the main breaker panel using one or two of its breaker slots as the main breaker for the sub-panel, while a secondary / additional /auxiliary panel is in parallel with the primary breaker panel, coming directly off the main household feed,, and has its own main breaker). A transfer panel, because it transfers all the power for the house, would be considered a primary panel under the code, whether or not it has breakers in it. Would technically be a "service panel" if only has transfer switch, or be a combined "service and main breaker/fuse panel" if it has breakers/ fuses in it.

Sometimes, even today, for high load applications like you are talking, fuses are used in a transfer switch or main breaker or combined meter/fuse panel because they blow more reliably and quicker than breakers in the event of a high amperage overload, and also almost never fail to blow when significantly overloaded. Believe it or not, breakers (depending on amperage rating and amount of overload) are allowed to take as long as 15 minutes to an hour to trip with not grosslyexcessive overlaods, and to near-instantaneously trip out can require as much as about 10 times the rated amperage to trip ! That is why you can sometimes, unless a "dead short", get prolonged (many seconds to even minutes) of arcing at an individual household short before the breaker trips. I had one outdoor entry light which fried as it turned on (with a photocell) conincidentally just as I was coming up the steps - arced like my 120V wire-feed welder in the metal fixture for a good minute before the wires finally melted through - the 15A circuit breaker nevfer tripped, though it tripped immediately in a subsequent breaker function test with more amperage.

And in some locales when putting in a transfer switch, because you are splitting the main power feed into two sources, you would be required to put a 150A main breaker there to protect the main feed circuits between the meter/transfer switch and the main breaker panel. That is actually a good idea anyway - the electrical code has a major flaw in that it lets you have unprotected wiring from the meter to the main panel in many cases, yet these are common locations for electrical fires to start - from weathering, rain/ice instursion, insect nests, physical damage by homeowners, corrosion, slacking off of clamps on aluminum wiring, etc.

The code should require a main breaker protecting all the incoming wiring instead of allowing unportected wire leading to the main breaker. Ideally it would be on the incoming side of the meter panel to protect against shorts there too, but most utilities don't allow that because they want the meter to be the "end" of their circuit. Also, having one on the inbound side of the meter means to do any work on the main breaker panel, the incoming service would have to be disconnected at the transformer or service pedestal. But then, ideally there should also be fuses on the incoming line (Service drop) from the utility at the transformer or service pedestal too, to protect against faults in the incoming service drop and its connections at the house.

I once had a power-pole mounted transformer-fed service drop failure which ended up arcing through and dropping a live line initially on the roof (where it set wood shingles on fire), and then after I threw a timber at it to knock it off to stop the fire source, it fell right by the house, an arced continuously until the utility showed up to disconnect the power at the transformer. This happened because (which I had remedied) there was no fuse or breaker as the transformer, at a savings to them of about $10 per house for two fuses ($100-200 each for pole-mounted breakers put in at time of transformer installation). At another house I had a service drop buried alongside the house arcing out intermittently for a couple of months (and undetectable during those brown-downs and brown-outs because they never lasted long enough for the power company to detect them), which eventually they agreed to dig up and found several spots where large rocks in the unclassified trench backfill had penetrated the insulation and allowed water to cause arcing, whcih then stopped once the arcing heated the soil around the fault enough to vaporize the moisture causing the shorting - so it would only arc for a few to maybe 10 seconds at a time, then be fine for hours to days.

Note, in probably almost all or all locales, the generator is required to have a main breaker on it too, to prevent overload hazards in the line from it to the transfer switch or excessive household load - and should have it anyway to get UL listing, and to protect the generator itself against a dead short or gross overloading downline from it. Most come with them mounted on it, but some larger units (including in your size range) require it be separately installed in a service panel by the generator.

Couple of suggestions - in the event of a generator electrical problem or fire, nice (and required in some areas) to have a remote fuel shutoff panic button some feet from the generator, and also to tie into that automatic shutdown switches for generator overheat and low oil level.

And of course, if unit does not come with it, automatic fuel shutoff switch and loud audible battery-powered alarm in case of fire at the unit - especially since with automatic startup you may not be home or may be asleep when it kicks on. That is one of the reasons I do not like automatic startup generators for homes - because that is commonly when they have a fire or exhibit a fuel leak, so I prefer a battery-powered power-outage alarm or two in the house, with manual generator startup and transfer switch operation.

One thing on the transfer switch - if your main breaker is 150A, but the generator is rated 80-90A (10KW on each side of the 220), pay attention to correct breaker amperages. Also, your generator should have an overload shutdown, because with 150A main panel (assuming you mean the normal phraseology of 150A double breakers, with 150A on each side of the 220, so about 18KW per side or 36KW total household load capacity) but with only 20KW generator capacity, it will only be able to handle about half the total household load before it starts bogging down and browning out - which also overheats the generator. Some larger generators have a low-voltage shutdown, but many do not, so may need to add that as an accessory too. I have seen fancier transfer switches whihc have this function built-in - transferring the switch to an OFF (neither line load nor generator connected) position.

Oh - one other thing on automatic transfer switches - some detect the power loss, start the generator, then when it is up to speed and is voltage is normal do the power transfer to the generator. Many of the cheaper ones do the transfer immediately upon power less, then start the generator. With that kind the generator start is under full load (which can be a real problem if generator is slow-starting or it is cold out so it can't start against generator load) and is hard on the generator and engine regardless to start under load. Some generators automatically cut out the power as its built-in load center during starting and shutdown, but many - including most household sized ones, do not.

Oh - another "one more thing" - consider for your climate if you need fuel recirculation and heating to keep the fuel from gelling and separating if this is a diesel unit. Also if you need engine block heater regardless of fuel type - both to reduce starting wear and tear and promote a fast start, but also to keep the lubricating oil warm. Personally, I do not like the oil dipstick type heaters - if they oerheat they can scorch the oil, and I have also heard of several cases (home backup generators and cabin generators are common in my area) where they shorted out and cause a fire in the crackcase.

And of course if liquid cooled engine, proper antifreeze for your locale - and at least every month (diesel) or two (gas) test run to circulate it and the oil so they do not segregate and separate.

One other thought - depending on your meter you may need or want a power-on indicator on that side if it does not have indicator/LED lights on the meter, to show (or maybe alarm you) that the public power is back on. Some transfer switches cut in automatically when the public power goes out, then transfer back when it comes back on and shuts down the generator, others are not that smart and only kick the generator on but the transfer back and generator shutdown has to be done manually - so of course if there is no way to know if the public power is on you don't know when to shut the generator off.

As you can see, there is a lot ot this game - so choosing a highly experienced generator installation company can be as important or more important than choosing the generator itself, and including the electrical mods and hookup can commonly cost as much as half to as much as the generator itself.

Answered 9 months ago by LCD

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