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Interlock or Transfer Switch?

Posted by ohmmm (My Page) on
Sun, May 29, 11 at 14:44

Recent power outage here after tornados in the area. Total time out was about 10 hours starting around 5pm. Used candles and flashlights for the evening. (There was much grumbling from the females, that there was no tv/internet. The grumbling got much louder when the water pressure went to zero.)

1. House is all electric, forced air heat, stove, 80 gal water heater etc. No sump pump since it is a slab foundation.

2. Main panels are in garage right next to the two car garage door. Two 200A Square D panels.

3. Location is central Oklahoma. Talking with the neighbors, it seems that power being down for more than a day is a relatively rare occurrance.

So, with that being said it would be nice to have power to:

1. some general lighting up (8cf bulbs - 144w total)

2. the side by side fridge/freezer(800w)

3. the well pump(1500w)

4. kitchen vent hood and an portable electric kitchen grill(2000w)

5. microwave oven(1200w)

6. tv and cable box(300w) (Though I don't know if the cable tv service goes down with the electric. I assume it does, since I think they have to use repeaters to get the signal out farther distances. In that case, tv/internet access will not be possible)

7. 2 home office computers(assuming internet access is still up)(700w)

8. In winter, a 1500w 120v plug in radiator type heater for the bedroom.

Obviously the water heater and central a/c can't be run on a portable generator.

There would be enough residual heat in the water heater to take a quick shower if need be, provided the well pressure tank was at full pressure. Turn the shower off while you lathered up and then back on for a quick rinse off. Even if you have a portable generator powering the well pump, that electric water heater (which typically cannot be connected to portable sized genset) is going to be filling with cold well water. So the residual heat in the water heater will drop rapidly unless you shut off the incoming cold water supply to it. If you remember to do that, then multiple people could get showered using the above quick method.

That's a big issue for those on well water, when the electric is out, your well pump is out. That means that you only have water pressure as long as the well pressure storage tank still has working pressure. Once it drops below 40psi, the pressure switch is supposed to trigger the well pump to come on and refill the pressure tank back to 60psi. That won't happen if the electric is out. Which means, soon, there will be no water pressure at all in the entire house after flushing toilets, washing hands, etc. And if you were approaching the 40 psi cut in mark just when the power outage hit, you have even less useable water pressure now.

Also if you have battery powered timers on some portable outdoor sprinklers, better shut them off quick or your pressure tank psi will drop to zero FAST. Not a good thing.

So back to the interlock/transfer switch question.

Since not all the circuits desired during a power outage are on one panel here, not sure an interlock would be the best route to go. How would that have to be hooked up across two 200A panels?

I am thinking a transfer switch would be better. I could probably get away with a 20 ft cord from the generator to the transfer switch and just run the cord under the garage door. Bit less of a hassle than punching through brick to mount a plug box. Though I still have my core bit from the shed electric project.

I know the prewired Reliance transfer switches you can get a Depot and Lowes probably won't have long enough wires to reach across the two panels and hook into the circuits on the panel nearest the garage door. I would have to add wire to get that to work.

Thoughts or suggestions as to brand/model of transfer switch?

The generator size I was thinking of would be around 7k-8k watts, gasoline powered. Bigger is probably better for more flexibility. Assuming also that ALL generator powered stuff is not going to be on at the same time.

Follow-Up Postings:

RE: Interlock or Transfer Switch?

First, you should decide if you are going to do this the "best" way or the "cheapest" way.

One of the best ways is a big honkin' manual transfer switch with a name like Square D, Ronk or ASCO on it. Your dual 200A panels might mean you have a 400A service, switches in that size are not inexpensive. But such a switch may last the life of the house.

A cheap way is a couple of those Reliance switches and some extension cord for your generator. Or a pair of interlocked breakers in each panel and so on.

You wrote "Obviously the water heater and central a/c can't be run on a portable generator..." I agree that you can't run those items with the size generator you will probably buy. But "portable" generators are readily available that will run your house and 5 or six more like it- however, you don't buy them at the big box store, and you tow them with a truck.

RE: Interlock or Transfer Switch?

If your WH is around 4.5kw, an 8kw generator will certainly drive it.

In the philosophy of the Reliance box, you could add a subpanel (like the shed) that has the circuits in it you want to use during the emergency. Then use a transfer switch between the subpanel and the main panel that also connects to your generator outlet. The main panel could feed with a 60a breaker (or whatever you need) to the subpanel.

Get a gas grill to cook on.

RE: Interlock or Transfer Switch?

With a 8000W generator, you're only going to move some of the circuits. You can splice the one you want to move inside the panel it is currently located in, over to the transfer switch with the same sized wire. You need to feed the transfer switch with a breaker and wiring of adequate size (it will tell you but 40A and #8 CU would probably fill the bill.

RE: Interlock or Transfer Switch?

Have you done a cost analysis on getting rid of the grumbling females vs. a generator and accessories. ;-)

I've told this story before but it bears repeating here. A friend of a friend had a big, whole house generator with auto start, auto transfer. One cold day, just before Christmas, the power goes out. Of course, the family barely noticed the outage. The hours go by and night falls when there is a knock at the door. It's the neighbor. "Look John, we know you have that big generator and are nice and warm over here but could you do me just one small favor? "Sure," says John. "Turn off the #$%@! Christmas lights."

There's a generator for every need and is only limited by your budget.

RE: Interlock or Transfer Switch?

Reliance makes this type of panel as well:

Basically combining the sub panel with a generator plug and an interlock. Rated at 7,500w, 100amps, 30amp generator input

Any glaring issues with using that?

RE: Interlock or Transfer Switch?

It would seem to be a lot simpler if I could just use an interlock on each panel. But how to feed both panels with one generator?

Found this response on another site:

"you could install an interlock kit in each panel and give each panel its own inlet. Then you have the option of using two generators, or one generator with a Y cable. (you wouldnt want the Y cable connected when on utility power due to the bridging neutral situation)"

RE: Interlock or Transfer Switch?

"...a lot simpler if I could just use an interlock on each panel. But how to feed both panels with one generator?..."

Maybe buy a generator with two suitable outlets? When you get up around 10kw many portable sets have both a L14-30R and 14-50R receptacle.

RE: Interlock or Transfer Switch?

Hi Mr. Ohmmmm,

I know your pain. We lost power for 5 days after a tree fell in a storm. And there was much grunbling, and I installed a generator set. Having paid over $US 800 for a crached heat exchanger in the HVAC due to freezing. Luckly no burst pipes.

I think a good-ish solution is a petrol generator with a suitable transfer switch. The best genset I know is a Honda EU6500iS in this class. This has an inverter and although wheelable is enclosed, so you it is better protected against rain etc. Usually the genset comes with a recomended transfer switch which is appropriate for the genset capacity. Honda has a transfer switch as an acessory.

Petrol gensets are the cheapest, however you will need to store lots of petrol 24 hours = 20 gallons, and you will need to wheel out the genset when it may be cold and damp.

A more expensive solution is a propane powered genset. These typically come with their own transfer switches because they have auto start and auto switch over.

After my power outage, I installed a Onan RS 20000 from Costco. A bit of googling will probablly bring up my old posts.

Genset solutions can get expensive and you have to think carefully how practical it is for you to wheel about equipment, possibly in bad weather, and keep petrol around. Note petrol will last only a year if you use a fuel stabilizer. propane will last almost indefinately.

Anyhow good luck with your project.

Warmest regards, Mike.

RE: Interlock or Transfer Switch?

Yes, still pondering and doing research online. There is a natural gas line out front, the builder just chose to cheap out and not run a line to the house. We have a gas fireplace, but no gas supply. Duhhh

Portable units certainly can be used for other things as need be. But, I can't say I have ever really needed one for a project. Usually a long extension cord will do the trick.

Considering the real estate market and this particular town, spending $5000 for a permanent generator setup right now would probably add about $100 value to the property. So basically I am leaning toward cheap.

Another issue is weather. If the power is out here, it is usually weather related. Which means rain here, lots of rain. And most smaller generators are not designed to be running out in the downpour. So the power is out and it is pouring rain. Now where do you put the generator? Can't put it in the garage and run it. The nearest covered area is the back porch, about 60ft around the outside of the house from the main panels. Hmmm Build a doghouse for the generator?

RE: Interlock or Transfer Switch?

A lot depends on how much you want to spend.

To me, dealing with an outage means we maintain essentials, and not luxuries. That means we keep the gas furnace, water pump, freezer, and refrigerator running, and provide a little CF light in each main room. That's it.

OK, it doesn't take that much to keep the computer modem and network up, and a laptop doesn't use much energy, so that too. Maybe a small television set. But that's it. I mean it. :)

I don't expect to have all the comforts of home when the power is off. If we can comfortably read our books, take showers, and so on - that's enough for us. We can cook regardless, thanks to the gas stove (just light with a match), and the water heater is gas too.

Enough power to run an electric space heater - no way would I pay for that kind of extra genset capacity.

We have a gas furnace and a woodstove with backup-powered fan, what else do we really need? I say let the whiny family members whine.

Running an electric heater is redundant anyway, if your backup power is a genset. You have an engine making waste heat. Why not just duct in the heat from its radiator if you want some extra?

As for fuel, I've seen natural gas fueled gensets that sat for years nearly unmaintained, and started right up when they were needed. The downside is that in a really dire situation nat gas service may be interrupted.

But if it's that bad, you're also going to have trouble getting more gasoline or Diesel fuel when you run low. And large capacity onsite liquid fuel storage is a hassle, for where to put it, and keeping it fresh and safe. If you absolutely have to store fuel onsite, at least LPG won't go stale or evaporate.

You could also go all out and install a grid-intertie PV system with backup battery and (optionally) genset. Then you can (eventually) pay for your backup system through the reduction in your electric bill from the energy your PV generates year-round.

RE: Interlock or Transfer Switch?


One word of warning when using petrol gensets is the risk of Carbon Monoxide poisoning. This sadlly turns out to happen quite alot. So running near a window, porch, garage is a definate no-no.

You need to decide where to run your genset. It needs to have good ventlitation away from your house, and needs to by dry, and needs to be secure (generator theft can be rampant during an outage).

Failing having the ideal spot, I would wait untill the worst of the weather is past. That may be hunkering down for 4 hours or so. Then once the weather is clear, crank up the generator for showers etc. Then shut down again over night.

Each time you will have to wheel the generator out of the garage, and make sure no one steals it, or chain it to something.

I know it sounds like a hassle, but hopefuly this will only happen once in a while. So you are willing to put up with the hassle for the cheaper priced solution.

I would say overall it can be quite a bit of work, in that you need to keep and maintain a genset, fuel, etc. But when you need it, it sure is nice to have :)

Best, Mike.

RE: Interlock or Transfer Switch?

I suggest you do the electrical part of this first, since you're pretty good at it.

What I meant about 'Reliance philosophy' was that they require you to do the wiring inside the panel (instead of pulling the circuits out to the subpanel). You could do this with your own subpanel and wire the circuits you want to each of your main panels. You would power the panel from a single 60a (or whatever) breaker from the nearest main and put the transfer switch in between. The other side of the TS would go to a socket for the genset. You could decide the size and location of the genset later on.

RE: Interlock or Transfer Switch?

Basically I want to spend the least amount possible. The generator and cord can be included with the house sale as a bonus if need be.

I have taken photos of the main panels with circles around the proposed generator circuits. I may swap out a 20a master bedroom, for the receptacle circuit the microwave is on(counter top unit).

Comes out to seven 20A circuits and
two 30A circuits(well pump, aerobic septic)

Now the aerobic septic has an air pump that runs continously. But that does not draw much at all. The tank pump kicks on about 2 hours after sunset and pumps out the chlorine treated tank to sprinkler heads. Based on our use here, it only pumps out once per evening.

The reliance transfer switch unit I was looking at is here:

The one review questioned the quality of the internals and the wire gauge used on it.





RE: Interlock or Transfer Switch?

With this box, you don't have 1 transfer switch, you have 9. And that's fine.

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