two portable generators on one service

[rlitman]

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.

Nobody is talking about syncing the generators. With two separate transfer switches this is not an issue.

But my take is that it is highly unlikely that ALL of the loads will be on simultaneously. I still think that one generator should be plenty for all of that stuff, and if the electric hot water can be lived without, I'm almost positive that one will be fine.

 

[VHF]

There has been quite a bit of confusion on this thread. Let's try to clear up a few things!

Transfer switches come in a variety of configurations. For this discussion, there are 3 relevant configurations (each of which could be either manual or automatic):

1. Stand-alone transfer switch -- essentially a big DPDT switch in its own enclosure which is placed ahead of a main panel or subpanel and switches between utility and generator power for everything in that panel.

2. Transfer switch with its own subpanel -- this combines a subpanel with a single large DPDT switch. Popular with Generac generators. Often the transfer switch is mounted next to the main panel and circuits to be protected are relocated from the main panel to the transfer switch's built-in subpanel.

3. Individual circuit switches -- this type of transfer switch has an individual SPDT or DPDT switch for each circuit so that for each circuit you can select utility/off/generator. In this case, each circuit is individualy switched after the breaker in the panel from which is supplied. Popular with smaller portable generators to provide flexible manual load management. GenTran/Reliance makes this configuration of transfer switch.

Now, about hooking two generators to one panel. The thing you cannot do is energize the bus bars in a panel from multiple sources at the same time (utility or generator.)

However, you can (and this was already mentioned) have two GenTran/Reliance-style transfers switches (configuration 3) each fed from a different generator protecting various circuits that are all fed from a single main panel. Because you are switching the circuits individually after the circuit breaker, generator power is never being placed on the main panel bus--it either has utility power or is dead (when the utility power is off.) An individual circuit is either fed utility power from its circuit breaker or generator power from its respective generator.

Having all the neutrals tied together and all the grounds tied together is not a problem--in fact it is the correct way to do it for a typical residential installation (switched neutrals would come into play in a "separatley derived system" which is sometimes applicable in a commerical setting.)

For what the OP is trying to acomplish, the most practical approach might be a Configuration 3 (Reliance) transfer switch fed by the first generator to allow selected individual circuits in the house to be manually switched between utility and generator, and a manual 100A Configuration 1 transfer switch (big DPDT switch) installed in the feeder to the apartment and connected to the second generator inlet. This transfer switch can be installed in the house near the main panel (intercepting the feeder from the 100A breaker that feeds the apartment), facilitating locating both generator inlets in the same general area.

Load management for for the apartment would need to be done by turning off some breakers in the apartment (such as the heat pump or water heater) as needed to avoid overloading the generator.

 

[theoldwizard1]

The other issue I have seen with the Reliance (or other brand) individual circuit transfer switch is that they typically only allow one 240V load and that is typically limited to 20 or 30 amps.

Again, there are many types of these devices.

 

[theoldwizard1]

Having all the neutrals tied together and all the grounds tied together is not a problem--in fact it is the correct way to do it for a typical residential installation (switched neutrals would come into play in a "separatley derived system" which is sometimes applicable in a commerical setting.)

No offense VHF, but are you a certified master electrician ? IMHO, 2 generators sounds like "separatley derived system" to me.

Bottom line, it has to be approved by the local building inspectors.

 

[Rick1550]

Just to keep thing straight, the one transfer switch I have is a 50 amp Reliance- 10 circuit w/ 6- 110's and 2- 220's... Model # 51410C (from Home Depot) Haven't bought the second one yet because I'm thinking if I can hook up both, I will buy a Reliance 100 amp double pole manual transfer switch Model # TCA1006D for the sub panel.

 

[VHF]

No offense VHF, but are you a certified master electrician ? IMHO, 2 generators sounds like "separatley derived system" to me.

Nope, just a hobby for me, and everything I say should be taken as opinion only! I do read a lot, and researched this issue extensivley when installing my own Generac 10kw generator. However, I stand ready to be corrected at any time by Aceman, mrb, and the other pros on the site!

"Separatley derived system" is a term defined in the NEC, and only applies when the neutrals are switched--not for a typical home stand-by generator.

Bottom line, it has to be approved by the local building inspectors.

That's true, and sometimes there is a problem with inspectors not understanding what you want to do even when it is permitted by code! Anything non-typical can be outside their comfort zone. However, legally they can't fail you because they don't like it--they need to cite a code section!

 

[rlitman]

There has been quite a bit of confusion on this thread. Let's try to clear up a few things!

Transfer switches come in a variety of configurations. For this discussion, there are 3 relevant configurations (each of which could be either manual or automatic):

1. Stand-alone transfer switch -- essentially a big DPDT switch in its own enclosure which is placed ahead of a main panel or subpanel and switches between utility and generator power for everything in that panel.

2. Transfer switch with its own subpanel -- this combines a subpanel with a single large DPDT switch. . . .
Now, about hooking two generators to one panel. The thing you cannot do is energize the bus bars in a panel from multiple sources at the same time (utility or generator.)
. . .
Load management for for the apartment would need to be done by turning off some breakers in the apartment (such as the heat pump or water heater) as needed to avoid overloading the generator.

Ok, here's another idea.
You hook up one standalone transfer switch to the power supplying the main panel, and a second standalone transfer switch to the circuit supplying the subpanel.

With the 2nd transfer switch set to "line" and gen2 off, you can turn the first switch to "gen" and when you run gen1, the entire house runs on gen1.
If/when you want the added power of gen2, you turn up gen2, and transfer the other switch. The benefit of this, is you can choose whether or not to use the second generator, and can save lots of propane.

If you use properly sized inlet disconnect plugs for the two generators, if one generator fails, you can disconnect and swap in the other one, and still power up everything.

 

[Rick1550]

I will be hooking up the two gens running on LPG, in about 3-4 weeks, as follows:
Gen 1 to the main panel through the 50 amp-10-circuit Reliance transfer switch I have, and I will hookup all and see if the 8000 watt gen. can handle it by juggling, if need be. If it can't handle it, I will then add the Gen 2 to the sub panel through a 100 amp manual transfer switch. I will write back and let the forum know how it works after test running and how it handled it all. Thanks so much to all for the input!

 

[macdenewf]

Gents.

I’d like to ask another question on this post. I actually have a 13kw nat gas generator. I’m in Houston and have this connected to my home for emergency power in the event of hurricane (as power has been known to go for over a week..in the sweltering heat after some previous hurricanes)

I have it wired with a proper generator inlet box, the box is wired directly to my main panel and there is a mechanical interlock on it so the mains and generator cannot be powered at the same time. On that note…try finding a generator cable with moulded 60 amp connection on one side with a 50m connection on the other ( 50amp gen inlet box was biggest I could find at a reasonable price ) ..so I made a jumper. For anyone reading..there is absolutely NO reason not to install a backup generator safely. That mechanical interlock was less than a hundred dollars!

Im asking about the proper way to parallel two generators….NOT connect up to two separate transfer switches. And again..i’m only asking as I have a genuine interest and many friends that want a reasonable cost on install. My 13 kw generator was about 3500 (real Honda engine, American made copper wound, brushless alternator) . It seems that some of options out there now for 7, 8 & 9 KW generators are very reasonably priced. Costco has a dual fuel ( gasoline 8450w and propane 6750w) on sale for 550 bucks…and it has a 50 nema connection on it…where as most that size have a 30amp connection. For 1100 dollars I can get the same overall wattage as my big 13k unit. So when thinking about paralleling generators – both of them would be identical in engine size and electrical output.

I have an electrical background…but been years since I’ve done any real engineering calculations so I’m hoping someone knows of real practical ways of connecting 2 generators and if there is any equipment that might be available to make it safe.

There are many posts about using parallel generators but they all point to the latest inverter types that essentially rectify AC to DC then invert the DC back to AC. Many of the gen manufacturers have already preconfigured the system to parallel as it’s fairly simple to sync an inverter. But those are small and even when paired up – youre still only getting 7 or 8 KW. I’m looking to see how to get twice that…..as the ppl I chat with really want the AC to be available….and not by making significant modifications to their electrical and running dual transfer switches.

As I understand it – it’s mainly the phase and loading that are the main concerns – is that correct? I’ve Read many posts on manually trying to sync the generator but that doesn’t look practical for the average home owner. So I guess my question is….is there anything out there (specifically made for) synchronizing 2 portable generators?

2nd part. If you can manage to get them in sync……if the electrical circuit is parallel to both generators…I have heard that the load tends to force the generators to stay in synchronization….is that true?

Anyway. I know there will be tons of comments on achieving this through splitting the electrical loads and having separate panels…etc..and others that will ask why you need so much power ( think 2 AC units) ….…but I’m just seeing of anyone here has some good working knowledge on actually running 2 portable generators in parallel without blowing anything up! Seems to be a lot of knowledge on this site. Hopefully there is something new on the market that might help solve this.





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[theoldwizard1]

So I guess my question is….is there anything out there (specifically made for) synchronizing 2 portable generators?

I have never seen such a device. Small (<3500W) inverter generator can be paralleled for two reasons. First, they are inverter generators. The voltage coming off of the actual coils is converted to DC and then to a very stable 60 Hz A/C output. Second, there is additional circuitry that that allows the second generator to use the the first generator as frequency "source".

Bottom line, can it be done, yes ! But it will cost a LOT OF MONEY ! Both generators outputs will have to be converted to DC, combined and then converted back to AC.

 

[TractorJeff]

Well?
Yes and No!
First thing to consider is that the engines have mechanical governors that maybe have a 5% Droop in them. When the 2 engines are tied together mechanically or electrically, they will PROBABLY only play nice together for a short while before each starts pushing back making the throttles open and close making the speed go up and down (Hunting). This will affect Frequency output.
Second thing to consider is that these generators are not Voltage Regulated. They have a circuit board that excites them to put out a nominal value (240). Unloaded it maybe 242v at 60.5hz or it maybe 248v at 61.5hz. So voltage wise they will circulate current back and forth between each other causing them to run hot.
Tying number 1 and number 2 together means that if you could get them synced, then they will be constantly pushing voltage back and forth between each other which the governors will see as load causing them to hunt.
As more building load is added then they will "Droop" in both speed and voltage again probably not equally due to the mechanical aspects of the engine and the non-adjustable voltage regulation which will lead to hunting.
Modern "real" gensets have voltage regulators which sense voltage drop as load is added and adjust for it. They also have governors that will only droop a 1/2% in rpm to stay locked solid at 60hz. (59.9 to 60.1 is about the max I typically encounter)
If you were experimenting out in the garage, then yes you could parallel them probably successfully for a short period of time and only be able to get to a certain load point before they would start kicking each other off line.

My most recent project involved two 2 megawatt diesel 4800v gensets paralleled with each other, then paralleled to the Utility for Detroit Edison to use in Michigan.

 

[Ole Slewfoot]

I think if you phase the generators the same, then weld or geardrive the crankshafts together, it might work. A single VR from a much bigger unit might also eliminate some issues.

 

[TractorJeff]

In reply to Ole Slewfoot's comment, mechanically tying the two units together, you need to remove one governor assembly from one engine. Then mechanically connect the remaining governor to said unit in effect building a 2 cylinder engine with 2 carb's. Else both governors will fight with each other eventually breaking the mechanical connection.

 

[sberry]

Then there is the fuel cost. Most of the time that the water heater isn't on there will only be light intermittent loads. Switch water heater out to LP would save money.

 

[txvwnut]

Look up generator load sharing. It’s done all the time with same make and model and size of generators. I’m not up on the electrical side of it as I only do the engine side of power generation.

 

[slow]

I believe the Honda EU7000 will parallel with a second unit, but this is a $8-9K setup for 2 generators.

Honestly, i would be looking into a battery bank with a parallel capable inverter for the surge capacity (motor starts and such) and a regular 5-8k generator will power most house loads. Recharge the battery bank with excess capacity from the one generator, or a second generator that only runs the charge the battery bank.

 

[LifeLongWNYer]

I used to do this in the service but the generators MUST be capable and built for it. The need to be set up to automatically synchronize themselves. Think about what AC current is. The voltage goes from+120 volts, through zero volts, to -120 volts, and back 60 times each second. When paralleling generators, they both need to be on the same part of the cycle at the same time. Generators capable of feeding the same load do this automatically. If they can't "autosync", they can't be paralleled.


.

 

[dogdog]

I think it would be just easier to get a whole house generator for this purpose.... they run on LPG too.

 

[Bert_]

I used to do this in the service but the generators MUST be capable and built for it. The need to be set up to automatically synchronize themselves. Think about what AC current is. The voltage goes from+120 volts, through zero volts, to -120 volts, and back 60 times each second. When paralleling generators, they both need to be on the same part of the cycle at the same time. Generators capable of feeding the same load do this automatically. If they can't "autosync", they can't be paralleled.


.

Kind of off topic but I bet you could parallel a couple regular generators. Use a meter between the outputs and wait until it reads zero, then throw the switch to connect them together. Once connected they will stay in sync. No idea on how well they would share the load.

 

[LifeLongWNYer]

That sounds easy Bert, but it won't work. It has nothing to do with the load, as far as current draw, rather it relates to the frequency of the current.

As the load fluctuates, the speed of the generators changes, and the cycles/sec ( frequency ) it puts out is related to the speed. Find any generator which has a frequency meter, and watch it for a while as the load changes. You will see it vary a few cycles, then you will hear the engine speed change, and the cycles per second will return to 60. Even when the load doesn't change, you will see the CPS move up or down a cycle/second, or so.

So the deal is, if one generator is on the uphill side of the sine wave, and the other one is on the downhill side, they will fight each other.

I never actually saw two, or more, generators toss their cookies when a synchronizer died, but heard stories of it, and they say the results are spectacular.



.

 

[TractorJeff]

I explained it all in post 51, but as no one likes to read anything more than the newest post.
Well, enough said!
Next week I need to go look at a damaged switchgear cabinet that tried to sync and out of phase to the Utility. It was 2 meg, witness statements are a "Flash of Light and a Shockwave".
If this is the case, then it will be up to the Customer to pull their building out of service to have the switchgear repaired and cleaned. That will be a long expensive day as the utility will need to come to kill the power after they do an orderly shut down in their plant.

 

[slow]

I'm not sure if you can, but I would like to see pictures of the carnage if you can post it.

 

[finn]

There is a fundamental lack of understanding of ac current and waveform shape, phasing,and frequency among some of the posters here.

Must have missed, or slept through, a few days of high school physics class.

 

[Bert_]

That sounds easy Bert, but it won't work. It has nothing to do with the load, as far as current draw, rather it relates to the frequency of the current.

As the load fluctuates, the speed of the generators changes, and the cycles/sec ( frequency ) it puts out is related to the speed. Find any generator which has a frequency meter, and watch it for a while as the load changes. You will see it vary a few cycles, then you will hear the engine speed change, and the cycles per second will return to 60. Even when the load doesn't change, you will see the CPS move up or down a cycle/second, or so.

So the deal is, if one generator is on the uphill side of the sine wave, and the other one is on the downhill side, they will fight each other.

I never actually saw two, or more, generators toss their cookies when a synchronizer died, but heard stories of it, and they say the results are spectacular.



.

I get that.

I am thinking that since the generators won't be running exactly the same speed, at some point they would be in phase with each other. A meter between the two would read zero at that point. Then you could throw a switch or whatever and connect the two. If you just threw the switch willy nilly and the generators were out of phase at the time then I'm sure there would be some impressive sparks flying since it is a phase to phase short.

Once they are connected electrically I would think that they would be locked together and stay in phase. Can someone confirm that?

I am in no way saying this is a good idea or practical, but I do think it could work to some extent.

*Been doing some reading on the subject.
I seems like it could work, some even talk of using a couple light bulbs in place of a meter. When the bulbs go out you throw the switch, same concept I was thinking of really.
The biggest issue is load sharing. One generator will hog the load or one might even just motor along acting like an electric motor unless the governors are well matched.

 

[mm08822]

Biggest problems with 2 randomly sized consumer-grade gens is the response time to intermittent loads, output voltage, and the steady-state frequency at which each run.

A single gen by itself doesn’t matter if it is running at 59 hz vs 61 hz. Add a second random gen to the mix and any running frequency difference between them will be a problem. There will always be a circulating current between the 2. This will add heat to the windings and slowly reduce the life of 1 or both gens. Even if both gens can synch, it will only be temporary with the frequency not being exact.

As for intermittent loads, both gens will recover from the added load at different rates. During this time, the generated voltages between the 2 will be different and again another reason for useless and damaging circulating currents.

The best way for a homeowner budget to deal with this variable demand is to have 2 xfer switches – 1 for main panel and another for the sub panel. Each gen can handle its load independently this way.
If there is only a single main panel, then buy a gen to handle the worst-case load expected and forget all this potentially dangerous connectivity.

 

[finn]

The frequency of the sign wave put out is a function of the speed of the little gas engine that runs the generator.

In the US, our power is a sine wave at 60 hz, so most non inverter generations run at either 1800 or 3600 rpm, both of which are divisible by 60.

Problem is, these little gas or diesel generators have mechanical governors, which have, by design, hysteresis inherent to the design, so they never maintain exactly the target design speed, especially When the load fluctuates, for example, when the freezer kicks in.

Assuming you could start with the multiple generators in phase, they would drop out of phase shortly because the engine speeds, although they may be close, won’t exactly match, especially when loads fluctuate.

 

[TractorJeff]

Yes Bert, 2 meters is how I validate "In Synch" voltages which also backs up what the 2 "Synch Lights" and Synch Scope are showing!
Still goes back to my earlier post 51 which is re-enforced by the replies in posts 65 and 66.

 

[Stuart in MN]

My most recent project involved two 2 megawatt diesel 4800v gensets paralleled with each other, then paralleled to the Utility for Detroit Edison to use in Michigan.

Here's one of my boss's projects from a few years back - five Cat 3516s, 12.5 megawatts in total.

 

[TractorJeff]

Stuart, any particular reason for painting them black?
We are doing a ton of micro-grid's right now which are black from the manufacturer.
Its hard to see the oil leaks on black!
Yellow, Orange, Gray and Tan are easy colors to see the leaks!
Biggest set I ever did was 16 C175's at 3 Megawatts each for a Server site!

 

[Stuart in MN]

Stuart, any particular reason for painting them black?

It was the customer's request. I think they just thought it would look cool.

 

[T2T2T2]

Did you ever determine if this worked OK? I’m trying to do nearly the exact same thing (4-circuit transfer switch on the main panel, and a 4-circuit transfer switch on a sub-panel that services a very small guest house). Both very low electrical loads. Either want to use 2 small portable inverter generators (one on the main, and one on the sub panel), or a single portable generator with 2 cords, one to each transfer switch. Neutral and ground are bonded in the main panel, and the sub-panel neutral and ground are separate (but are connected back to the main panel). The generators both have floating grounds.

In any event, not much info out there (and seems to be some conflicting info) so just thought I’d follow up with you and see if it had worked, or if you fried anything. Thanks.

 

[AA/FC]

delete

 

[Mike65]

When my wife & I got our emergency backup generator this year we contacted a local licensed electrician & had him do the hookup & he answered all the questions we had, & removed the bonded ground from our generator. He recommended a Gener-link transfer switch that is hooked up behind the meter on the outside of the house that the cable from the generator hooks to. Then I just have to turn on the breakers in the service panel for whatever I want powered by the generator.

GenerLink, Backup Generator | Global Power Products

 

[mike93lx]

When my wife & I got our emergency backup generator this year we contacted a local licensed electrician & had him do the hookup & he answered all the questions we had, & removed the bonded ground from our generator. He recommended a Gener-link transfer switch that is hooked up behind the meter on the outside of the house that the cable from the generator hooks to. Then I just have to turn on the breakers in the service panel for whatever I want powered by the generator.

GenerLink, Backup Generator | Global Power Products

Do you have Dominion? I've been curious if they would allow these

 

[Mike65]

Do you have Dominion? I've been curious if they would allow these

No, we have Appalachian Power. Call your power company & ask them if they allow the use of a Generlink transfer switch. Someone from Appalachain power came to our house & installed it.

 

[mike93lx]

No, we have Appalachian Power. Call your power company & ask them if they allow the use of a Generlink transfer switch. Someone from Appalachain power came to our house & installed it.

Thanks. We lose power so infrequently that an interlock is plenty, but I've been curious

 

[wyliesdiesels]

this thread was started 11yrs ago and last reply was 4.5yrs ago...

 

[TractorJeff]

Yes, it's an old thread but at least a SEARCH was done to try and find answers to a series of questions on Back Up Power.
As an update to big power installs: Before COVID retired me, I have been up to 4800Hp on 13,200Vac a couple of times! One batch was a series of units that we did to power a 3rd world country!

 

[Walkers]

I hate it when dead threads get sucked up from the bilge!