Generator Transfer Switch Question

CompactTractorFan · Jul 8, 2012

With all the talk about generators recently, I was think back to this question I had back when were were looking into purchasing a generator:

Let's say you have a nice big 15kw generator, like this one: Generac 5734 Portable Generator - GP15000E - 15,000 Watt Electric Start Portable Generator). You want to hook up a 50 amp manual transfer switch like this one: Reliance Controls Q510C - 50-Amp (120/240V 10-Circuit) Transfer Switch w/ Interchangeable Breakers (the 50 amp/12,500 watt unit is the biggest they make in this style)

The switch is rated for 50 amps/12,500 watts. The generator is rated for 63 Amps @ 240 Volts/15,000 running watts. It has one 50 amp (240v x 50 amps = 12,000 watts) outlet among other smaller ones. Assume you have the proper 50 amp cord going from the generator to the transfer switch inlet. Would it be safe and legal to use the higher powered generator with the smaller transfer switch???

The wattage coming from the largest generator outlet should be less than the 12,500 watt max input for the transfer switch, right?

schmism · Jul 8, 2012

youll never pull more current through the breaker that protects the transfer pannel, than what the breaker is rated for.

Think of it this way... the "grid" that powers your house is a generator (which the power plant is) capable of putting out 300 MW or 300,000KW. but the wireing and breaker that feds your house is only capable of 200 amps. certantly much less than the 300MW that the generator can put out.

in short as long as the cord and recepticals are matched to the transfer pannel, you could power it with a full size power plant and it wouldnt matter to the plug/receptical/transferpannel

JHTFarmer · Jul 8, 2012

Shouldn't the transfer switch be rated above the amperage required for the generator or that of the line supply to the home, whichever is greater?

i.e., if your are powering your home with a little 15kw generator but have say 200 amp service to the home don't building codes require installation of a transfer switch rated above 200 amps?

Larry Caldwell · Jul 8, 2012

Just install a whole house transfer switch and load balance using your breaker panel. The smaller transfer switches require a lot of rewiring, and if you someday add a load not covered by the transfer switch you are out of luck. Generlink has a meter base transfer switch that doesn't require any rewiring at all, it just mounts under your watt meter. You can use any size generator you want, so if you replace your generator someday with a larger one, you will be all set.

techman · Jul 9, 2012

I have found if you don't need an automatic transfer switch, then the best solution is: Generator InterLock Kit

You just need to add a breaker and this kit and you can power your entire panel. Safe and easy.

here is my panel:

paul

Stimw · Jul 9, 2012

techman said:
I have found if you don't need an automatic transfer switch, then the best solution is: Generator InterLock Kit

You just need to add a breaker and this kit and you can power your entire panel. Safe and easy.

here is my panel:

View attachment 272649

paul

That's a pretty slick way to handle the transfer but they sure are proud of it at $149.00 !!!

techman · Jul 9, 2012

Stimw said:
That's a pretty slick way to handle the transfer but they sure are proud of it at $149.00 !!!

Well the picture is my own version, based on those available for purchase. Of course it is best to show the commercial product that is UL approved. For many people it is not worth the time to do a DIY. The price is a bit high, but much lower than the other transfer switches discussed.

Paul

Highbeam · Jul 9, 2012

I dislike those small generator subpanels. They only allow a few circuits to be transfered. Much better is the interlock kit. I replaced my whole house panel with a generator panel that has a version of the interlock built into the cover. Great idea.

It is nice to have all circuits powered up during an outage. I can walk into the bathroom and flip the switch on and there is light. Just like normal. Bad part is that you don't know when the line power is back on.

techman · Jul 9, 2012

Highbeam said:
Bad part is that you don't know when the line power is back on.

I solved that problem with a small pilot light on the panel (looking at my panel's picture). It is a neon light (1/25 watt) mounted on an insulator with a spring contact to touch the main lug on the main breaker. Small fuse for protection in wired-in. The light will always show utility power no matter the position of the main breaker. Only thing is I need to occasionally go to the basement to check the indicator light.

paul