Is a 50 amp transfer switch a good match for a 17,500-watt generator?

Question

I'm looking to buy the Guardian Ultrasource 17,500 watt generator as a back-up generator for my home. It comes with either a 50 or 60 amp transfer switch (see below). My electrician said that a 50 amp switch is not powerful enough to transfer all 17,500 watts from the generator to the house. He said I would need something closer to 100 amps, and that it would cost about $400 or so.

I don't know much about electricity, but I figure that the manufacturer of a 17,500 watt generator probably wouldn't include a 50 or 60 amp switch, unless their engineers had determined they were a good match for each other.

One last note: The Guardian PDF spec sheet says a "60A Manual Transfer Switch" is included, but the Guardian website says a "50-amp high power outlet" is included.

http://www.guardiangenerators.com/PublicPDFs/UltraSourceSpec.pdf

http://www.guardiangenerators.com/Products/Ultrasource/Ultrasource.aspx


So who's right? My electrician, or the folks at Guardian generators?

Answer 1

Lots of information here, and most of it I agree with. While 17500 watts will require a current of 73 amps at 240 volts, this is in excess of the maximum rating of the highest rated generator outlet which is 50 amps. If you are using this outlet, a 60 amp transfer panel will be adequate for your use.

Ask your electrician about the generator neutral connection and whether the transfer panel switches neutral. In many cases, even from the same manufacturer, the two are not supplied in a compatible configuration and should be modified.

Read more about connection and other safety issues on my web page.
http://members.rennlist.org/warren/generator.html

Answer 2

This is an interesting issue. It is perhaps similar to some of the other things you see now days, such as a 6 Peak HP air compressor that could run on a 15A 120v circuit. Well at 745 watts per horsepower, the "Peak" inrush in a 100% efficient motor would 4470 Watts, while a 15 amp circuit is good for at most V*I=W 120*15=1800w or 2.4 HP, so you know this compressor is running at most at 2 HP continuously even if it has a 90% efficient motor. So with their advertised 6 Peak HP they are leading you to believe that the motor is 3 times the size that it actually is!

You may want to read the small print in the instructions and see just how much connected wattage is allowed with this unit. You can then use the Voltage * Current= wattage formula to determine just exactly how many amps this generator will supply on a continuous basis.

My guess is that the 50 or 60A transfer switch is rated for the maximum continuous load, which would suggest that the continuous load is not 17500 watts.

One other thing however. If this generator is generating 230V power but splitting it to two 120V busses through a two pole breaker, that would only be 76 amps per pole, so perhaps they are jacking up the advertised watts only a little instead of a lot.

Answer 3

tropicalt's wattage formula is correct. As he/she said, a 60 amp is not really enough, let alone a 50. A 50 amp outlet is high power only in the sense that most outlets are less than that. The term "high power" is pure marketing hype. Is the 50 amp outlet the only power out? If so, it sure is less than the maximum.

Are you going to expect this generator to run a fully powered electral house? If you have an A/C and electric oven and electric stove on at the same time, you need a lot of power. Keep that in mind. It is seldom worth the extra expense to get even more power than this unit, you will just need to conserve.

Only you can balance the costs of an extremely high amperage switch with the fact that you might use this once every three years. Then again, you may live in a semi-wilderness area where power routinely goes out for days at a time. If so, having a switch that can handle the full output of the generator would make even more sense than the puny 60 amp one.

Answer 4

Your electrician is right. Watts = amps x volts and therefore at 17,500 watts you are pulling almost 73 amps at (I'm assuming 240 VAC). This is probably a maximum output of the generator.

What is the existing service at your house? What devices will be on the generator or will it be the whole house? Have you thought of a contactor type transfer switch rather than manual?

The 50 amp plug is fine for the application though.

Answer 5

50 Amp Transfer Switch

Answer 6

I've been an electrician for 20 years, the 60 amp switch will not work.Also the generator will not run the entire house,even if you get a 100 amp switch. For future reference watts divided by volts = amps.17,500 divided by 220 is 80 amps.So on a 100 amp switch you may be able to run a few lights at 15 amps per circuit, your refrigerator at 20 amps and your entertainment center at 20 amps ,and that's about it.

Answer 7

The generator's spec sheet shows 50A at 240 volts as the highest power single outlet available. This is 12,000 watts (amps times volts) - not the full output of the generator, but the highest power circuit in common usage with a standard plug. To get the full generator output, you need to be drawing power from the other outlets as well - the 120/240 volt and 12 volt circuits listed. If you want to be able to switch all of the available power from the generator to your house, you need to switch another 5500 watts - say the 240/30 amp circuit, then put a circuit breaker on it at 20 amps so you don't overload the generator with everything working.
The factory only includes a transfer switch for the heaviest circuit.

Answer 8

Wow, thats a pretty big portable generator when you said 17,500 watts I was assuming you had one of those automatic whole house generators.

Personally I got a 15,000 watt natural gas automatic generator, here is a picture of the 100 AMP transfer switch...

http://www.guardiangenerators.com/Products/Residential/QuietSource/QuietSource16kW.aspx

http://www.dishretailer.com/house/103CANON/IMG_0323.JPG
http://www.dishretailer.com/house/103CANON/IMG_0324.JPG

I think the reason why the manufacture included only a 50 amp switch is because of the size of the cable and the connector used to connect the transfer switch to the generator.

Im almost willing to bet if you take a look at the locking plug, its only rated for 50 or 60 amps!

Answer 9

Lots of good math and theory in these answers...but...Code says that a continuous duty circuit {which this would be} can only be loaded to 80% of it's capacity. Use the sixty disconnect, It is actually designed to handle 72 amps, but is rated continuous duty at 60amps. I hope that somewhere in this circuit there's a 60 amp breaker. Also, don't know what part of the country you live but in Arkansas a 100 amp disconnect costs me about $90 bucks.

Answer 10

egads...I say mostly all wrong here!
(30 plus year master electrician and inspector currently....)
first off your electrician....needs his head examined...or you didnt hear him properly since amperage isnt "PUT OUT" its used...thus in this case its not like water where it will burst your lines it simply is used as needed...and if it goes OVER the rated breakers or devices rating it simply trips and shut down...
also a 17.5 k generator rating is the PEAK rating it has...and cannot run at that peak for a long period of time anyways!
I would guess the average running rating of it would be aprox 15k of power tops to be used...(15000 watts = 62-68 amps depending on voltage max at any duration to be used...) thus the generator company also is incorrect in its transfer switch because it should be at least 70 amps..
However they erred on the safe side and limited you to a 60 amp switch to ensure you dont overload the generator...and you shouldnt use more than 80% of the power at any one time anyways...to be safe per code. so while they could have used a 70 amp switch... going to a 60 wasnt neccesarily wrong on their part..it simply tells me possibly the max normal running output is actually lower than even the 15k i estimated...