Thread: Generator Transfer Panel Wiring

Hi All, I plan on doing a sub-panel install that's got an interlocked set of double pole breakers suitable for a generator transfer switch (details below) and I have a question having specifically to do with the 'proper' way to feed a double pole breaker from a 120v generator. I have no need for a louder, more expensive, heavier, gas guzzling 5+kW generator supplying 240V since I only need to run my furnace fan, fridge and some lights during an outage. All I really need to know is whether I can feed 1 bus with the 120v from the generator and still be code compliant. In other words am I 'required' to feed both breakers? I know I can't jumper the two together or something. I have no intention or need to run any 240V appliances from this panel and only need half the slots to service what I need so they would basically be staggered to tap off the one bus that's powered from the generator. And yes I know this introduces load imbalance in the sub-panel when I'm on utility power, but I've calculated that I'd still be less than 80% loaded on just one hot leg if everything in the box was on. Plus I would balance the remaining circuits in the main panel as best I could. Is there any reason this wouldn't be legit, or do I have to feed a breaker I don't need just because it's there?

Installation:
30A 120V Generator /w L5-30 twist-lock recepticle to L5-30 Outdoor power inlet box.
10/3 wire (not using 1 hot wire) from inlet box to a small 2-breaker main lug load center housing an inline 30A GFCI breaker before entrance to sub-panel.
60A double pole feeder from main panel to 70A interlocked breaker (4/3 Wire).
Neutral is not switched and is isolated from ground in the sub-panel. Generator wired for floating neutral.

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A word of caution if you are going to run your furnace on a portable gas generator be careful with anything electronic such as circuit boards or transformers in the furnace. If your 30 amp generator is not regulated to provide 60 Hz, you could blow your circuit boards and or transformer. I know because this happened to me with my oil fired furnace.

I have actually tested the output of the generator on my multimeter as I've added loads and found that it does put out a pretty clean 60hz with voltage between 125-130 volts. It drops only for a moment as loads are added until the governor can bring the revs back up and never seems to fluctuate more than 5% at that instant (57-63Hz). As far as voltage stabilization it has an electromagnetic rotor so the voltage regulator can respond to voltage drops very quickly. I couldn't read a voltage drop of even one volt. Still though, I definitely follow your line of thinking since a lot of electronics like to be within +-0.5Hz. Too bad they don't make a 1000+ watt UPS for cheap. I suppose if you were nutty you could wire a $700 big one meant for a corporate server room inline with the circuit to condition the supply but for as infrequently as we lose power I think I'd take my chances.

Motor loads can induce twice or three times the start up load (amps) until they come up to speed.

I have since bought a very expensive electronically regulated 12K watt PTO driven generator at 50 amp and is designed for farm use with extra windings so it can take motor load start ups very efficiently.

In my opinion I would utilize the 240 option from the generator to the power panel. And an isolation switch to take the back up power off the grid.

Cheers,

Well, so long as it's just a matter of preference, I'm okay with having 120v on one leg and the other be dead during an outage. I just want to make sure I won't fail my inspection because there's a specific NEC code requirement mandating that both breakers be supplied with current.

Charles,
Question; are you on a well or municipal supply? If you are on a well you may want to make certain your pump is not wired for 240V.

Also I found that it was better to put my furnace circulating fan on first and get it up to speed before turning up the thermostat and the oil burner pump/blower coming on, fwiw.

Cheers,

Originally Posted by Charles Balconi

Well, so long as it's just a matter of preference, I'm okay with having 120v on one leg and the other be dead during an outage. I just want to make sure I won't fail my inspection because there's a specific NEC code requirement mandating that both breakers be supplied with current.

You would fail it, yes.

You would not be allowed to energize the distribution panel from that generator.

You would, however, be allowed to have two panels, one with the emergency circuits in it and the main distribution panel with all other circuits in it.

The transfer switch would then be wired up such that the main distribution panel fed the transfer switch 'power side' (utility power side) to the panel below the transfer switch which included the items you talked about. You would only energize one leg of that panel. The generator would then be wired to the 'non-power side' of the transfer switch through a disconnect (which is always left in the 'off' position). When the utility power is lost, the transfer switch falls away from the main distribution feed, removing power from the main distribution panel AND the circuits in the emergency panel. You would then start your emergency generator, and after the generator is up and running with a short warm-up period, you switch a disconnect between the generator and the transfer switch 'on'. You *DO NOT* want to start the generator while already under load, you want to start the generator, let it warm up some, then apply the load.

Now, the only circuits which are energized are those in the emergency panel.

When the utility power comes back on, the transfer switch (if automatic) will automatically pull back in and transfer power from the generator to utility power.

It is then up to you to shut the generator down.

Your transfer switch could be manual instead of automatic, which would not matter as your generator is manual start and not automatic.

The one I had on our house in South Florida had an automatic transfer switch and the generator came on and shut off automatically, with a time delay coming on and shutting off to allow for hiccups in the utility power supply. I believe the time delay was 18 seconds or so on ours.

Ok. Thanks, for this is very helpful and informative. If I'm following your explanation correctly what would fail is having both hots from the main panel feed connected and only having the one from the generator. In other words from both the feeder from the main panel and the generator only a single hot is supplied to the transfer switch (interlocked breakers) and the other half always remains dead because it's unpowered. (Which i'm perfectly happy to do since I only need 6 of the 12 tabs and never need 240 on the emergency panel) The disconnect you mentioned that should be between the generator and the transfer panel would be the inline 30A GFCI breaker placed before the emergency panel entrance. (I'm almost positive this would be required any ways since the generator doesn't have GFCI protection internally, just overload protection). I understand it's also good practice to have all the emergency breakers shut off before even flipping the disconnect breaker on so you can start up the heavy loads individually and give the genset time to stabilize before powering the next load and so on..

Originally Posted by Charles Balconi

The disconnect you mentioned that should be between the generator and the transfer panel would be the inline 30A GFCI breaker placed before the emergency panel entrance.

I understand it's also good practice to have all the emergency breakers shut off before even flipping the disconnect breaker on so you can start up the heavy loads individually and give the genset time to stabilize before powering the next load and so on..

That's what the disconnect is for.

Those branch circuit breakers will already all be 'on' as they were 'on' for use with utility power.

The disconnect between the generator and the transfer switch allows you to crank the generator and get it running and warmed up before operating the disconnect to 'on', which then powers through the transfer switch to the emergency panel, where the breakers are already 'on', to those circuits, and only those circuits.

Trying to power half of a panel which has other circuits in it as not going to fly anywhere I can think of, and you are at risk of back-powering the utility lines that way too ... which is not allowed and is not safe for the utility workers.

You would feed you regular panel as you do now.

Then install a breaker to feed the emergency panel through the transfer switch. You would need a properly sized feeder (hot, neutral, and ground). *IF* the manufacturer of the emergency panel is the same as your regular panel, you could then simply remove the breakers from the regular panel and install them in the emergency panel.

From the emergency panel you would need to run those circuit back over to, but not in, the regular panel, running the circuits in the regular panel out to a junction box where the circuit conductors would tie together with the circuit conductors from the emergency panel. Making sure that all sizes, ampacities, ratings, neutral/hot combinations, were kept together and matching. You *DO NOT* want to inadvertently use a hot from one circuit with the neutral from another circuit - keep everything neat, tied together, and labeled ... so you know what is what.

Your electrical inspector will stand there and say to himself 'WTF?' at first, then it should become readily apparent what you did - as long as you keep everything labeled. Also remember that you DO NOT want to bundle too many of your conductors together for greater than 24", otherwise derating will be required.

When I installed mine down in South Florida, I ran multiple conduits so I would not need to derate the conductors.

The first photo is of the emergency panel and automatic transfer switch with the conduits, the second photo is of the conduits feeding into the bottom of the existing panel, the third photo was of the drywall repairs part way finished.