two portable generators on one service

[Rick1550]

Hi!
I am new to this forum, and have a question or two on generators, hopefully someone can help me with. I have two 8000/10000 watt tri-fuel generators I am using LPG with, due to having a 500 gal. tank I will be connecting both generators to. (I realize one large whole house generator would have been better), but I acquired two portables for a few reasons. That aside, my questions are, is there a problem wiring two generators to the same 200 amp service panel? I also have, or will have, two 50 amp transfer switches, inlet boxes and cords to do the proper hook-ups. Is there any reason you can't do this, if they are wired properly? And the second question is...I have a 100 amp sub-panel fed from the main box on a breaker, I would like to hook one of the two generators to. The sub panel is quite a distance,(at the other end of the house feeding an apartment over the garage) from the main panel I want to hook up to. I need to keep the generators near the main panel and 500 gal. tank for the obvious reasons, instead of one at the main and one at the sub panel. Is there a way to do what I have suggested? Thanks, and hope someone can help!

 

[Gary S]

Time for some serious reading.
http://en.wikipedia.org/wiki/Synchronization_(alternating_current)

 

[Speedy Petey]

Is there a reason you need more than an 8000 watt generator?
That's more than enough to run the essentials plus quite a bit more in the average home.

 

[Rick1550]

I need to supply two refrigerators, a freezer, hot water boiler for heat & hot water, well pump, two sump pumps, a few lights, TV, and a microwave.... and off the sub panel feeding the apartment over the garage, a 1500 watt electric heater, a refrigerator, a few lights, TV, and a electric hot water heater. Without doing the precise math, I thought that might be a stretch for one 8000 watt generator. I have two, and hoped I could use to both to supply all these things. But after reading the link supplied, I get the synchronization issue. Is there no way to make this work, short of a large whole house generator, since I already have two portables? Is there a way to isolate the neutrals and ground from each transfer switch, perhaps with a whole other box, so they will not be somehow, tied together? Thanks for answering!

 

[pattenp]

With two transfer switches you can feed the main with one gen and the sub with the other gen. Right off hand I don't see an issue where you need to isolate the ground and neutral between the two.

Edit: On portable gens the ground and neutral is usually bonded. When using a portable gen to supply power to the house system the neutral/ground bonding in the gen should be removed.

 

[mopar4wd]

As mentioned above you can pretty easily power the main house with one gen and the sub panel with another. If you need to split up the main house you are going to need some more tricks (possibly a sub panel with certain loads for one genset.) You can accomplish this by placing the transfer switches between the main panels and the subpanels. This is done frequently in the marine world where multiple gensets are common.

 

[CNGsaves]

Might be cheaper to run buried yellow polyethlene plastic pipe for propane over to the subpanel area to power the smaller generator there. Also, might be cheaper heat anyway from propane than electric for apartment? That way apartment subpanel could have it's own manual interlock so that genset would provide power to just apartment (ie prevent any backfeed risk).

Larger genset would be located over near main panel and propane tank to minimize that hookup cost. Bigger genset would be here and if possible it COULD provide power to house and apartment, if only minimal load (thus only need the one generator). ATS (automatic transfer switch) would connect big genset to main electric panel. Maybe the 10Kw would handle both house and apartment. If not, the separate genset for apartment would operate independently through the subpanel.

 

[Rick1550]

Guess I should have written that differently...both gens are identical, they are 8000 run/10000 surge. So unless I'm misunderstanding, I can wire one gen w/ transfer switch to the necessary circuits in the main panel and then the second gen's transfer switch to the 100 amp breaker in the main panel that feeds the sub panel for the apartment? Or must I put the second gen all the way down at the other end of the house, wired into the actual sub panel itself? Also, if both are wired into the main, that means each neutral and ground wire from the transfer switches will be on the same neutral/ground bar in the main panel. That doesn't create a problem? Sorry if the questions sound ignorant, but this generator wiring is new to me, and that's exactly why I came on this forum...I want to make sure I'm doing this right.

 

[pattenp]

The transfer switch for the sub can be in the main panel switching the 100A breaker to the sub. I don't know of a problem with the neutral/ground being the same for the two transfer switches. Even if the second transfer switch was at the sub the ground and neutral are bonded back at the main panel.

 

[Sureshot]

Is this an emergency back up or a full time gig? If only for emergencies you do not need everything running at once.
eg:the well pump and microwave.
If you need the furnace and heater going you can put your fridge items in a tote and set them outside or in a shed etc. A deepfreeze will stay cold a long while if you keep the lid shut. You can then cycle the power to it between other loads if the time gets extended.
Is the well pump 110 or 220v?
Is the boiler electric?
What voltage is the electric hot water heater in the apartment?
A tank of hot water will supply a few warm showers even after losing power.
You don't give a location so not sure what your temps and needs are.

Forgive me if you have these things figured out but the info given raises lots of questions.

 

[Rick1550]

It is for an emergency only, not full time. I live in a rural area of southeastern Pa., and we lost power a number of times last year. Some for a couple up to eight hrs. and twice for about a week each time. I do understand I don't have to run everything at once and could cycle between things, but since I have two gens, I thought I might as well hook up both and eliminate having to juggle. The well pump is 220v, and the heater is (110v) oil fired/hot water (boiler) with baseboards and has a coil for domestic hot water. The only things I need to run with the second gen on the sub panel is a few lights, refrigerator, 1500 watt portable elect. heater, (winter only) TV, and possibly a 220v hot water heater. (I could probably do without the h/w heater if need be) but I thought a 10000 watt(surge) gen by itself on the sub panel, could handle it. My main concern was if I could hook up 2 gens to the same service. Any other suggestions? The help is greatly appreciated!

 

[Speedy Petey]

From what I am seeing, without the water heater an 8kW should be more than enough. I see nothing that stands out as a huge draw. People start listing everything they want to run and get all nervous that it is so much. Electronics are very little draw, IMO they are almost not worth mentioning with larger gensets. Even a few lights will never even add much to the draw in the scope of things.

I'm not trying to say you should not do this, but I am asking WHY bother. Do you realize how much gas an 8kW genset uses??

If you do, two transfer panels/switches is the only way to go.

 

[rct]

Although the more expensive whole house generator is an option, you could consider double checking your loads if one can sustain the essentials, giving you a back-up genset. If you want more than 8Kw and since you are already set up for a portable generator, you could consider a larger portable genset as well. I think winco makes a 12000 trifuel portable and if I recall generac was making portables up to 17000. Finally, you may be getting to the range where a diesel such as a 15Kw military surplus trailer gen makes sense.

 

[Sureshot]

The money saved on one transfer setup and the extra wire/install fees to go to the far panel + the recoup of cash by selling one genset would go towards buying a larger one and you would still have a small portable for your other uses.

Those pumps will draw on startup. That would be my only concern. I would leave the sump pumps on and everything else but the well pump and electric heat items. They could be cycled as needed. That is if you are in a situation where they are active. If not just run everything and learn the limits. In short I would try one 8k and see if you can live with it. I suspect you can.

 

[Rick1550]

I already have the two-8000 watt portables that will run on LPG, so purchasing a whole house or another larger portable is not what I wanted to do. From the posts I have read so far, it seems as though the one alone, might supply enough if I do not use the hot water heater in the sub panel, and perhaps if I do a bit of juggling, if required. I'm thinking I might try wiring just one gen first to check how it handles it all. I have a 10 circuit transfer switch (two-220's & six-110's ) which I will wire...one- 220 to my well pump and the other 220 to the 100 amp breaker that feeds the apartment sub panel. The sub panel then will run a 1500 watt room heater (if it's in winter), a TV, a light or two, and a refrigerator. The six remaining 110's in the main panel will feed my sump pumps (if needed), TV, 2- refrigerators, a freezer, a few lights, a microwave and a hot water boiler for heat (if winter) & hot water. (Sure seems like a lot of draw for one 8000 watt gen., even if I juggle?) Maybe not, if I juggle wisely. Does this seem feasible, or should I just wire both in right away, since I have them, each with it's own transfer switch, and be done with it? Any other thoughts? And thanks for all the help, so far!

 

[Rick1550]

Thanks Sureshot! (Just read your post after I sent my reply)

 

[mopar4wd]

I think your best bet would be to have a transfer setup for your main panel as described with one genset. The subpanel could than have an inline transfer switch. I assume you current transfer switch is like this one http://www.northerntool.com/shop/tools/product_200317085_200317085
what I would suggest is to use this for the main panel and install one of these
http://www.northerntool.com/shop/tools/product_200196624_200196624
for the sub panel
I'm no expert but I think the neutral being combined would be OK but somone else maybe able to answer that better.

 

[tdkkart]

Uh???
Nobody's mentioned the fact that you not only have to have the rpm of the generators exactly the same, you also have to have the frequency phase sync'd between the 2 generators.
Not doing this right can cause some serious damage to everything, including the generators.

 

[Sureshot]

Have you considered 12v sump pumps?
My next place will have them and wiring for basic 12v lights for backup.
Reduces your load and adds a safety factor.

 

[Milton Shaw]

I would use two transfer switches and make sure there are no interconnective circuits between them, or you wil have problems with sync. The water heaters are your big load and they might exceed the output of your units. You might have to switch heating elements in one of the water heaters so that each heater pulls less watts but still will run off the generator. Resistance heat is a power hog always so make sure you add them up correctly and split them between the two generators . If you use smaller heaters in the water heater your temp will not change just the recovery times. I don't know if anyone has tried this but a heat exchanger between the exhaust from the generators to the input of the water heaters would provide a lot of heat quicker than any normal water heater would. Should be as simple as water tank, pump and boiler (heat exchanger) and some safety valves. I have gas water heat so water heat is not a problem for me when running on backup generator. Propane water heater would also fix problem also.

 

[the1nonlyjl]

Is this an emergency back up or a full time gig? If only for emergencies you do not need everything running at once.
eg:the well pump and microwave.
If you need the furnace and heater going you can put your fridge items in a tote and set them outside or in a shed etc. A deepfreeze will stay cold a long while if you keep the lid shut. You can then cycle the power to it between other loads if the time gets extended.
Is the well pump 110 or 220v?
Is the boiler electric?
What voltage is the electric hot water heater in the apartment?
A tank of hot water will supply a few warm showers even after losing power.
You don't give a location so not sure what your temps and needs are.

Forgive me if you have these things figured out but the info given raises lots of questions.

Exactly. I think its pointless to have this thread posted here. Again one generator with 8,000 watts is plenty. What the heck with this person's thinkings? Does he think he run the hospital in his home?

 

[theoldwizard1]

With two transfer switches you can feed the main with one gen and the sub with the other gen. Right off hand I don't see an issue where you need to isolate the ground and neutral between the two.

Edit: On portable gens the ground and neutral is usually bonded. When using a portable gen to supply power to the house system the neutral/ground bonding in the gen should be removed.

This is the ONLY SAFE solution. There is no way you can wire 2 generators to feed the main.

The big issue is making certain you can not back feed from the sub panel to the main.

Look into the Generac Nexus Smart Switch. Use a 200A for the main and a 100A for the sub.


And hire an electrician who is experienced in installing generators and ATS.

 

[theoldwizard1]

Exactly. I think its pointless to have this thread posted here. Again one generator with 8,000 watts is plenty. What the heck with this person's thinkings? Does he think he run the hospital in his home?

He already bought 2 of them, he wants to use them.

I'm guessing the apartment may be a rental or for an older relative who does not have the capability to deal with anything.

 

[Rick1550]

That is the case. I already have the two gens and my elderly father lives in the apartment. Since I'm getting mixed responses on hooking both into the same panel due to a syncro issue, I will try just hooking up the one. Some say you can hook up two, some say you can't. And as far as the "pathfinderdude" response.....I thought a forum was to exchange ideas and help someone who may not have all the answers, but for some reason you think otherwise. Much easier to end the info and try to belittle someone, in your opinion. Thanks for that. Aside from that, thanks to all others for your much appreciated input. Any more input on this syncro issue, would be appreciated, not only by me, but by others who may be contemplating the same thing.

 

[pattenp]

That is the case. I already have the two gens and my elderly father lives in the apartment. Since I'm getting mixed responses on hooking both into the same panel due to a syncro issue, I will try just hooking up the one. Some say you can hook up two, some say you can't. And as far as the "pathfinderdude" response.....I thought a forum was to exchange ideas and help someone who may not have all the answers, but for some reason you think otherwise. Much easier to end the info and try to belittle someone, in your opinion. Thanks for that. Aside from that, thanks to all others for your much appreciated input. Any more input on this syncro issue, would be appreciated, not only by me, but by others who may be contemplating the same thing.

Don't worry about him. There's one in every crowd.

Anyway, I think the running two gens in parallel issue is when they are supplying the same load, hooked together on one transfer switch. I can understand the conflicting info you are getting. Doing the one gen, one transfer switch, is for sure the no issues way.

 

[Speedy Petey]

That is the case. I already have the two gens and my elderly father lives in the apartment. Since I'm getting mixed responses on hooking both into the same panel due to a syncro issue, I will try just hooking up the one.

There is no mixed response. You CANNOT hook two portable gensets to the SAME panel. Not unless they are designed for it, like the little EU units.

 

[Rick1550]

Speedy Petey wrote:
"There is no mixed response. You CANNOT hook two portable gensets to the SAME panel. Not unless they are designed for it, like the little EU units."

Forgive me then, because I must be misunderstanding. Some were saying you can hookup two gens with separate transfer switches by putting one on through a transfer switch to supply the circuits in the main and the second gen. through another transfer switch connected to the 100 amp breaker (in the main panel) that supplies the sub panel in the garage. If I'm understanding you correctly, you're saying since they are in the same panel this will not work?

 

[Speedy Petey]

Speedy Petey wrote:
"There is no mixed response. You CANNOT hook two portable gensets to the SAME panel. Not unless they are designed for it, like the little EU units."

Forgive me then, because I must be misunderstanding. Some were saying you can hookup two gens with separate transfer switches by putting one on through a transfer switch to supply the circuits in the main and the second gen. through another transfer switch connected to the 100 amp breaker (in the main panel) that supplies the sub panel in the garage. If I'm understanding you correctly, you're saying since they are in the same panel this will not work?

NO, I am saying you cannot have two gens feeding the same panel through the same transfer switch.
If you have a transfer for each gen you can do it. For instance you can have two Gen-Tran type panels off one main panel. Or two main breaker interlocks.

 

[Stuart in MN]

If the transfer switches are configured properly, the generator feeding the subpanel will be separated electrically from the generator feeding the main panel, which will be okay. Not knowing the particulars of your specific generators, it is unlikely that they can be simply connected together.

 

[tdkkart]

Simply....

2 generators, 2 panels, You can hook up one generator to each panel as long as the panels are COMPLETELY disconnected from each other.

 

[CNGsaves]

Giant Elephant in The Room question remains . . . . why do you have 500 gallon propane tank when you have:
a) Electric boiler / hot water in main house
b) Electric 1500 watt heater, hot water, etc in apartment??

Both of those buildings likely should be powered by the propane . . . right?

Simple answer is one genset next to main house with ATS (auto transfer switch) which MIGHT power everything (especially if main heat/HW was being covered with propane rather than electric). Then bury yellow plastic pipe for propane line over to the apartment to power the 2nd genset over near that subpanel (could have simple interlock switch for that generator that would only be needed if genset at main house couldn't handle everything) - - thus apartment would be isolated from any chance of backfeed and survive by itself on the 2nd genset.

 

[Rick1550]

Let me clear this up....the 500 gal. propane tank is there to supply a pool heater, a fire pit and BBQ grill. That is all the tank supplies. My plan is to tee off the main of it, and add two more valves and regulators to run the two LPG gens. My house has an oil-fired hot water baseboard system with a built in coil for domestic hot water. The apartment has a heat pump/AC unit which the gen. probably cant handle, so we would use a 1500 watt portable heater if the power went out. He already runs it now most of the time in winter, because he seems to like it better than the heat pump blowing hot air through the vents in the apartment. The apartment also has a 220v hot water heater which we could probably do without in a emergency. he also would only need power for TV, refrigerator, and a few lights.

Post from CNGsaves:

"Simple answer is one genset next to main house with ATS (auto transfer switch) which MIGHT power everything (especially if main heat/HW was being covered with propane rather than electric). Then bury yellow plastic pipe for propane line over to the apartment to power the 2nd genset over near that subpanel (could have simple interlock switch for that generator that would only be needed if genset at main house couldn't handle everything) - - thus apartment would be isolated from any chance of backfeed and survive by itself on the 2nd genset."

I already have a reliance 10 circuit transfer switch (not auto)to wire to the main. I guess you're saying I could put the 2nd gen for the apartment over by the sub panel itself and wire it in with a transfer switch, but then what is the difference whether it is at the sub panel wired in or wired in to the 100 amp breaker in the main service panel that feeds the sub panel? Either way the sub panel is still not isolated due to the fact the neutral and ground still goes back to the main panel from the sub panel. Could I still get the possible backfeed you're referring to? Or doesn't the neutral and ground have anything to do with that?

 

[VHF]

I already have a reliance 10 circuit transfer switch (not auto)to wire to the main. I guess you're saying I could put the 2nd gen for the apartment over by the sub panel itself and wire it in with a transfer switch, but then what is the difference whether it is at the sub panel wired in or wired in to the 100 amp breaker in the main service panel that feeds the sub panel? Either way the sub panel is still not isolated due to the fact the neutral and ground still goes back to the main panel from the sub panel. Could I still get the possible backfeed you're referring to? Or doesn't the neutral and ground have anything to do with that?

You can put the transfer switch for the second generator in either physical location, as long as the only thing it can feed is the subpanel.

Neutral and ground would not be switched anywhere. All neutrals would be tied together (including both generators), all grounds would be tied together (including both generators.) You must use a 4-wire cord from the generators to the inlets, and there must be a 4-wire feed to the subpanel. Neutral and ground would only be bonded at your main panel. Neutral-ground bonding must be lifted at the generators--some generators have a switch for this, some have a pigtail to the frame that must be disconnected. (Make sure to reconnect if you use the generator stand-alone.)

 

[JerryC]

Forgive me for jumping in... I'm just learning this stuff. I'm confused. I thought a transfer switch would send power from a generator through itself through the onboard breakers and not through the panel.

If that is the case then why cant you have two transfer switches at the same panel using power from two generators?

What am I missing?

In his case could he do an interlock to backfeed the main panel so he could power the sub panel with one generator and use a transfer switch with the other generator for the other circuits?

 

[theoldwizard1]

I already have a reliance 10 circuit transfer switch (not auto)to wire to the main.

IMHO, this is the wrong equipment for your application !

I know this stuff is over your head, but it is also difficult to explain without taking the time to draw extensive diagrams.

I guess you're saying I could put the 2nd gen for the apartment over by the sub panel itself and wire it in with a transfer switch, ...

Let's start here.

An Automatic Transfer Switch (ATS) is very much like the old fashioned "knife switch".

There are 2 input terminals, one on the left side and one on the right (the feed from the main and the feed from the generator). The output (the circuits in the sub panel) is the center connection (2 blades are required for 240V).

The "automatic" part is the mechanism that physically moves the "knife" blades. Some type of "logic" (computer or person) must determine that there is no power coming from the main and that the generator is hooked up and making power to flip the switch from one feed to the other.

So you see the ATS is "upstream" of the sub-panel and it is impossible to "backfeed" into the main.


Things are not much different for the main panel, with a couple of exceptions. Permanent "whole house" generators are sized to be capable of making enough power for the whole house UNLESS there is a mechanism to to turn off some of the loads. This is called "load shedding".

With that reliance 10 circuit transfer, you have to plan ahead of time what are your most important 10 circuits. With this setup, the other circuit will get no power from the generator.

With a load shedding ATS, it is all automatic (except made the initial "cutover"). Again the ATS is placed ahead of the main panel (see previous picture and discussion). But this time, high load appliances (boiler, well pump, electric stove, electric dryer, etc) are removed from the main panel and wired directly into the ATS. The computer in the ATS determines if there is enough power to run everything. If not, it starts turn off the high load items.

This works because, most of the time you are not using a large amount of power. If you boiler is your highest priority, do NOT put it on a load shedding circuit. Just remember, when the boiler turns on something else may need to be turned off.

In your case, the first thing I would have the main ATS shed would be the sub panel. This way you can start the generator for the main and it WILL feed the sub panel as long as the load is not too high. Once the second generator is started, the second ATS will disconnect the load from the main.


Now, because you do not have a fully automatic "system", you will need to be the "logic" and physically flip the switch on each transfer switch after the generators are plugged in and started up.


You still need to hire an electrician who is experienced at wiring this up and fully understands your needs.

 

[theoldwizard1]

Forgive me for jumping in... I'm just learning this stuff. I'm confused. I thought a transfer switch would send power from a generator through itself through the onboard breakers and not through the panel.

Many (manual) transfer switches work that way. They have their own breakers and when you plug your generator in and flip the switch on the transfer panel for that circuit, power from then generator goes to the load instead of coming from the power company.

These transfer switches typically handle 6-10 circuits

If that is the case then why cant you have two transfer switches at the same panel using power from two generators?

On the surface you could have one generator/transfer panel handling one set of 10 circuits and the other generator/transfer panel handling a different set of 10 circuits.

What am I missing?

The 2 transfer panels and generators would be sharing neutral and ground. I think this could be a BIG issue because the generator are not perfectly synchronized.

In his case could he do an interlock to backfeed the main panel ...

Interlocks are not used with these types of transfer switches.

 

[Rick1550]

So....I'm back to what I said midway through the posts...No matter if I put the 2nd gen and transfer switch on the sub panel itself or on the breaker in the main that feeds it, it CAN NOT be done because they share the same neutral and ground? (Since there is no way to hook up two gens on the same house unless they are in sync, I am back to one gen, and try to juggle critical circuit needs.)....Thanks to all for the input...it was much appreciated!

 

[Rookie2]

My 2 cents: you have have two 4000watt 110volt legs from the generator, you will need to balance the load to use every watt of power.

I would use one 200 amp manual transfer switch on the whole shebang and turn off or cycle your breakers as needed.(pumps etc). one generator only.

BTW, that 500gal tank may not produce enough gas vapor at 0 deg F . may need to run it by the supplier.

 

[theoldwizard1]

So....I'm back to what I said midway through the posts...No matter if I put the 2nd gen and transfer switch on the sub panel itself or on the breaker in the main that feeds it, it CAN NOT be done because they share the same neutral and ground? (Since there is no way to hook up two gens on the same house unless they are in sync, I am back to one gen, and try to juggle critical circuit needs.)....Thanks to all for the input...it was much appreciated!

NEVER SAY NEVER

Seeing as you are using portable generators that means you are willing to do some work.

What you need is a 200A 3 pole double throw "safety switch" for the main box and a 100A 3 pole double throw safety switch for the subpanel. The switch is and ON-OFF-ON switch. This would be installed between the main breaker box and the feed from the meter and between the main breaker box and the subpanel. ON in one direction would feed power from the power company (or main box) and ON in the other direction would feed power from one of your two generator.

A safety switch is a giant sized version of the little knife switch I showed a picture of before inside of a large metal box. You need 3 pole because you are switching 2 hots and neutral so that the generator neutrals are never connected. Because (in theory) there is never any current flowing through the ground circuit they can all be combined.

These things are ridiculously expensive (over $1000 retail) but frequently show up on eBay of CL for a couple of hundred bucks.


Again, I am NOT a licensed electrician. Hire a knowledgeable, experienced electrician. This needs to also be reviewed and approved by your local building inspector.

 

[aandpdan]

So....I'm back to what I said midway through the posts...No matter if I put the 2nd gen and transfer switch on the sub panel itself or on the breaker in the main that feeds it, it CAN NOT be done because they share the same neutral and ground? (Since there is no way to hook up two gens on the same house unless they are in sync, I am back to one gen, and try to juggle critical circuit needs.)....Thanks to all for the input...it was much appreciated!

Rick,

It DOES NOT MATTER that you are sharing the neutral and grounds. Think about what would happen if you tested your transfer panel and the utility power was on - NOTHING. Without a complete circuit there will be no current between the generators.

You could even put BOTH transfer panels on the main panel, it won't matter as long as you supply them from a different source. They do not feed onto the bus - and will NOT backfeed.

You just can't hook both generators together - hots AND neutrals/ground.

Also, you'll do fine with a BURIED 500 gallon tank. The temperature won't get low enough to cause you any vaporization problems.