# Why are Balanced Loads needed for a 240V Generator?



## Airstreamer67 (Oct 3, 2020)

Does anybody know why it is advised that the loads on the two 120V circuits of a 240V genset be balanced, or the genset will overheat or otherwise somehow be damaged?

For instance, let's say a 240V generator has a total capacity of 6000 watts at 240 volts, with 3000 watts of that capacity available from one 120V circuit and 3000 watts from the other 120V circuit.

If that generator is operated with, say, a 2900-watt load on Circuit A and a 200-watt load on Circuit B, will it cause more damage to that generator than would the same 2900-watt load on a straight 120V generator with a capacity of 3000 watts that has only one circuit for the load?

With a presumed answer of "yes" because the 240V generator needs balanced loads, the question becomes: why?

Just wondering.


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## jeb (Feb 16, 2021)

Airstreamer67 said:


> Does anybody know why it is advised that the loads on the two 120V circuits of a 240V genset be balanced, or the genset will overheat or otherwise somehow be damaged?
> 
> For instance, let's say a 240V generator has a total capacity of 6000 watts at 240 volts, with 3000 watts of that capacity available from one 120V circuit and 3000 watts from the other 120V circuit.
> 
> ...



Because the advertised wattage is based on the combined power output of both phases - phase A and phase B (more accurately L1 & L2).

So, if you have a 10,000 watt rated generator...you CAN NOT pull 10,000watts off a single phase...you can only pull/draw 5,000watts. So therefore if you want to utilize the full capacity of the generator, you will need to balance the loads on each phase so you can pull that advertised power.

-J


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## iowagold (Jan 22, 2020)

well if you had an eu7000is generator you can pull off the rated power at 120 vac.

240 single phase generators and grid power is best when the loads are equal on L1 and L2.
power generation 101 stuff.


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## jeb (Feb 16, 2021)

iowagold said:


> well if you had an eu7000is generator you can pull off the rated power at 120 vac.
> 
> 240 single phase generators and grid power is best when the loads are equal on L1 and L2.
> power generation 101 stuff.


Hey Paul, that is a good question...is he talking inverter generator or conventional? That would potentially change things.


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## Airstreamer67 (Oct 3, 2020)

I'm referring to a conventional generator in post #1. The question is: why do the two circuits of a 240V generator have to be balanced? How does it harm the generator if it's not balanced; ie, what is the the "generation 101 stuff" as contrasted to a 120V generator. Are the two windings in a 240V generator less capable than the one winding in a 120V generator?

And that is interesting that a 240V inverter generator can carry its full load from one 120V circuit. I assume the 120V plugs could be an obstacle to getting advantage of the full production?


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## iowagold (Jan 22, 2020)

I will pm direct airstream


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## billt (Dec 11, 2020)

Airstreamer67 said:


> I'm referring to a conventional generator in post #1. The question is: *why do the two circuits of a 240V generator have to be balanced? How does it harm the generator if it's not balanced;* ie, what is the the "generation 101 stuff" as contrasted to a 120V generator. Are the two windings in a 240V generator less capable than the one winding in a 120V generator?
> 
> And that is interesting that a 240V inverter generator can carry its full load from one 120V circuit. I assume the 120V plugs could be an obstacle to getting advantage of the full production?


This is an excellent question. And one that is very rarely properly explained.... Usually with opinion, and not with fact. I'm interested in this as well. Let's make this simple, and use a photo. What would be the safest and simplest way to tap into the full 9,500 watts from this gen set panel, (non inverter), without overheating or damaging anything in the process? (Assuming one has all the necessary plugs, cords, and adapters required to do so).


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## Airstreamer67 (Oct 3, 2020)

Well above my pay-grade.


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## billt (Dec 11, 2020)

I would think, (and correct me if I'm wrong), that you can safely draw the full 9,500 watts all from the 50 amp receptacle. I base this on the fact that if you are running this unit, (or any other for that matter), off a transfer switch into your main breaker box, that is the receptacle you would tap into. So it stands to reason the unit is capable of delivering all the power it is capable of making through it. Yes or no?

The other outlets pretty much speak for themselves power wise. The output of the 4, 120 Volt outlets, or the 30 amp twist lock outlet, are all self governing by the maximum output each one displays. And common sense would dictate if you were drawing the full output via the 50 amp plug into the transfer switch, you obviously shouldn't be trying to pull more power out of the other outlets while doing so.


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## iowagold (Jan 22, 2020)

what is the load voltage is the big question...
if you are at 125 vac... no but you can balance the load and get close.

if you are at 250 vac load then yes you can get a balanced up to the rating on the generator out.

running mixed appliances voltages is tricky on a good balance...
just remember you want to leave 40% or more as head room so you are not working the generator head to death!
so on rated power use only 60% on an avr style unit..
it is that heat soak thing on the windings!
you can get hot spots and loose the whole winding to melt down on the insulation...


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## jeb (Feb 16, 2021)

Was that 9500 watts continuous or max power rated? If max, as Paul stated you don't want to run it at max for very long, you'll overheat the windings.

For your application of using a transfer switch (assuming you mean feeding panel with an interlock), feeding both L1 and L2, yes the 50Amp plug is the way to go. 










Because the conventional genset is effectively a center tapped transformer, the sine wave is split into phases. If your house has 3 wire 120v circuits which are supposed to be split on phases for the hot side...otherwise you over load the neutral for those circuits.

If you have to run 240v appliances you have no other option anyway.

If 9500 watts is your maximum, then you have 9500/2= 4750 watts per phase.

4750/120v = 39.6Amps per phase. The 50Amp breaker is double pole...a pole for each side...so under the 39.6Amps of max current on that phase and what the genset can do on that phase.


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## Airstreamer67 (Oct 3, 2020)

Has the original question in Post #1 been answered?

Quote: "Does anybody know why it is advised that the loads on the two 120V circuits of a 240V genset be balanced, or the genset will overheat or otherwise somehow be damaged?"

It's the "balanced" part I'm interested in. I understand the advisable maximum of 60% loading factor per circuit. But why is it harmful to have 60% on Circuit A and, say, 5% on Circuit B, which would be an "out of balance" situation and thus presumably harmful? Whereas, that same load on a straight 120V generator would presumably not be harmful, all things else being equal in terms of generator rating vs the load carried.


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## jeb (Feb 16, 2021)

Balanced is not the harmful issue *as long as* you are *under* the rated '*continuous wattage' *for that leg/phase. So if the continuous wattage rating is 9500watts, then you only have 9500/2 to work with. So you have 4750 watts (again I'm assuming this is continuous rating; if its max then you need to adjust down accordingly). 
If you want to run 4750 watts on phase A, and 10watts on phase B you can do that. But you are not efficiently using what the generator is creating. The engine has to turn the genset...and you already have forced the engine (with the governor) to dump more fuel because you put the genset under load. You are leaving energy on the table (or flushing down the toilet). So for efficiency sake, you try to balance the phases (L1 & L2) so that you are using your power more efficiently *and* so you can utilize the *full wattage of the generator.*

If you don't balance the loads, you are never going to utilize the full capacity or capabilities of your genset. Again we are talking conventional generators here...not inverters which have some other parallel 120v options...


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## Airstreamer67 (Oct 3, 2020)

Thanks for the reply! 

If you are correct that there is no danger factor in unbalanced loads, I wonder why it is very commonly stipulated that it can cause generator problems.

I do understand that one would not be able to get the full rating of the generator if the loads aren't balanced on the two 120V circuits, since each circuit can produce only half the load. That factor is not in question.

I actually don't have a problem with balancing my loads, within reason. My electrician has divided the two phases quite well in my subpanel for use with a home generator. It is just curiosity that has driven my question.

Thanks for the patience!


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## exmar (Jun 29, 2015)

Excellent question. IMHO, no, it won't harm the generator to have uneven loads on the two sides of the 120/240V. You will discover fast enough that you can't pull the rated wattage out of one side when it's breaker trips. This will lead you to redistribute loads more evenly if you want to use the rated capacity.


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## Airstreamer67 (Oct 3, 2020)

This is a comment from another forum. There are warnings out there about unbalanced loads; how valid they are is the question:

Quote: "Without getting into physics.. basically if you have things on leg 1 pulling a total of 40 amps and leg 2 is only pulling 5 you have an unbalanced load. The probem, and damage, occurs because of something called negative sequence magnetic flux. Both leg 1 and 2 share a neutral wire. And because of physics and heat, you can damage your generator. No generator will have a perfectly balanced load between phases, but getting it close is important for extending the life of the electronics in your generator. This is some complex sh*t, people get doctorates just to figure it out."


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## jeb (Feb 16, 2021)

Interesting concept...that we usually see discussed regarding power distribution from utility companies...where the turbine generators are 18KV and such. Would be interesting to see if this is significant enough on the smaller size portable generators. 
I wonder if you could measure with a infrared themo the temperature different between a full load on a single leg verse a balanced load? I have read that THD and noise does increase with unbalanced loads...so that is definitely a negative, especially with a 'construction' grade genset that already has high THD.


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## Old man here (Nov 5, 2020)

jeb said:


> Interesting concept...that we usually see discussed regarding power distribution from utility companies...where the turbine generators are 18KV and such. Would be interesting to see if this is significant enough on the smaller size portable generators.
> I wonder if you could measure with a infrared themo the temperature different between a full load on a single leg verse a balanced load? I have read that THD and noise does increase with unbalanced loads...so that is definitely a negative, especially with a 'construction' grade genset that already has high THD.


I don't know if the THD and noise does increase with unbalanced loads, but I can test that theory next week, I have a Fluke 345 Power Quality Clamp Meter arriving next week that measures THD. This is an expensive tool designed for this exact purpose. It will be interesting to see the results of unbalanced loads. 
I do know that you should keep the loads as balanced as you can. It is not possible to do 100% of the time.


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## tabora (Sep 6, 2018)

jeb said:


> I wonder if you could measure with a infrared themo the temperature different between a full load on a single leg verse a balanced load?


Just use an amp-clamp to get it in the ballpark. I have a pair of these mounted on the main panel to monitor the two legs.


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## jeb (Feb 16, 2021)

tabora said:


> Just use an amp-clamp to get it in the ballpark. I have a pair of these mounted on the main panel to monitor the two legs.


I was actually referring to measuring heat generation because of the unbalanced load and "negative sequence magnetic flux". In the past, the only papers I've read on this related to power distribution from utility companies...where the turbine generators are 18KV not our relatively small gensets. 

It would be nice to know if this _really_ is a concern for most portable sub 12KW gensets?


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## iowagold (Jan 22, 2020)

the unbalance can cause bearing issues...
so if you are at rated at L1 and lets say 100 watts on L2 there will be an unbalance on the rotating part of the generator creating vibration al noise..
this can be seen with vibration sensor tools as bearing chatter...
so yes un balanced loads can be a harmonic mechanical real issue!


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## Airstreamer67 (Oct 3, 2020)

Now that is interesting, Iowagold.

The issue of unbalanced loads and the effects on the home generator has been an interest of mine. I don't think it's anything to worry about within reason. Nobody can balance the various 120V loads precisely, and I doubt it makes a significant difference to the longevity of a generator unless there is a really big disparity over a longer period of time.

I'm not paranoid about the topic, but it's not hard to get paranoid on the Internet. I've seen a suggestion that one should add an otherwise-unneeded load to the "lighter" one of a generator's 120V circuits to balance out the loads. I believe it was a guy named Monk who make that suggestion. 

Of course, in some situations, it might be a good idea :-?


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## iowagold (Jan 22, 2020)

give or take a few hundred watts is balanced enough...
it is going to depend on how large of gen set you are using as well as the loads.

on the small L1 L2 gens keep it balanced as close as you can.
so if you have a fridge place it on L1 and furnace place it on L2 kinda thing.

if you watch it with live meters you can make sure you are good to go.

plan your generator system! and then plan some more..


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## jeb (Feb 16, 2021)

This is really an interesting topic, because many times we (or I ) don't think of _all_ the reasons for balancing the loads. We know it is wise for efficiency and pulling full capacity of genset. But long term durability of various components is an aspect too.

Regarding vibration...would be cool if we could measure vibration increases related to unbalanced loads? Such as is this a linear increase with increase of unbalanced loads? Is it only measurable at a specific load/threshold? I imagine that the bearings used in the specific genset would also be a factor of measurable vibration increases due to unbalanced loads.

Anyone on the forum try measuring this to quantify this? Would be interesting data...


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## Airstreamer67 (Oct 3, 2020)

It would be interesting data, but I suspect the vibration would be very slight and difficult to discern without some very expensive and sensitive equipment. 

On the other hand, it wouldn't take much extra vibration at 3600RPM to introduce wear and tear on bearings and their races: 3600RPM x 60 minutes per hour x 24 hours per day = 5,184,000 revolutions of the rotor each day. Any tiny bit of extra stress from an unbalanced rotating mass could really add up. 

I'm feeling vulnerable right now.


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## jeb (Feb 16, 2021)

I'm sure the designers & engineers at Generac, Honda, etc have all tested this. Would be interesting if they have that data and or have publically shared it? Well, the latter I guess that is a stretch


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## iowagold (Jan 22, 2020)

jeb said:


> I'm sure the designers & engineers at Generac, Honda, etc have all tested this. Would be interesting if they have that data and or have publically shared it? Well, the latter I guess that is a stretch


vib data is not for public.
yea not you average shop has $100,000.00 usd in vib tech gear...
we do for predictive maintenance as well as use for balancing rotational parts.

yup things go bump in the night!
lol!
and we have the meters and tools to measure them!

that howl noise is bad, or any vibration noise...
it all adds up over time as wear or stress...
wire fatigue ... etc...


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## jeb (Feb 16, 2021)

I wonder if a person could use the sensors on your phone to try to measure this and use an app such as this one ‎The Best Vibration Meter

Granted its not calibrated so accuracy would be questionable...but if we are just trying to measure the delta or difference of vibration, it might work. Maybe tape phone securely to generator body?


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## iowagold (Jan 22, 2020)

nope, phones are not set up for this spectrum.
and are not setup for the proper conduction.
you want a proper vibe tool.
pm me for a list of proper vibe tools.


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## jeb (Feb 16, 2021)

iowagold said:


> nope


Nope as in it won't work? Or haven't tried it? 😃


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## Old man here (Nov 5, 2020)

jeb said:


> Nope as in it won't work? Or haven't tried it? 😃


I say try it, doesn't cost anything and the accelerometers built in to the phones are very sensitive.


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## Bpizzino (Mar 13, 2021)

I would like to know about severing a power company neutral when backfeeding a 4 wire generator. Especially as it pertains to when power is restored while backfeeding. Does the 2 neutrals cause a significant problem? Also, if the power company neutral is shorted out while down, will this hurt the generator?


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## iowagold (Jan 22, 2020)

Bpizzino said:


> I would like to know about severing a power company neutral when backfeeding a 4 wire generator. Especially as it pertains to when power is restored while backfeeding. Does the 2 neutrals cause a significant problem? Also, if the power company neutral is shorted out while down, will this hurt the generator?


first things first
welcome to the forum.
please start another topic on your subject.

next and most important:
:BACK FEEDING IS ILLEGAL!:
never ever back feed any grid system!
big fines and people can get dead on a back feed!

now if you are talking about a generator inlet with an interlock.
yes those are a proper way to get a generator on your grid system.
or an ats (automatic transfer switch)
those also are a p[roper way to connect to a grid system.

sorry for the rant!
stay safe out there!


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## Airstreamer67 (Oct 3, 2020)

In a previous life I worked for an electric utility company. 

I remember many years ago at one of our power plants an in-service generating unit suffered the loss of a turbine blade. Those spinning turbines and the attached electric generator rotor are all precisely balanced, and the vibrations created by losing just one blade out of the hundreds on the long turbine shaft caused the spinning mass to begin vibrating, and before they could get the thing stopped it resulted in a lot of mechanical damage, including a fire. 

After repairs, the engineers used a lot of time and equipment to balance the turbine to exacting specifications. It doesn't take much of an imbalance for centrifugal force to cause problems for a spinning mass at 3600RPM.


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## iowagold (Jan 22, 2020)

vibe tech and tribology is a good field!
yea been there done that! lol
and still do that!
engineering solutions on my back ground.
power , communications, and more.

a bit of vibration is a BIG thing over time...
and adds up to a real mess in a hurry!
a couple of g's can escalate to 100g's in a few min of run time....
yea for the young guys in the forum vibe tech is a well paying job! $250/ hour and up as a SR predictive maintenance guy.
and starting is $75/ hour for the guys just out of school.
get your stamps (training) for balancing and for oil analysis and you can get the top pay!
classes are expensive, and the math! yup the math is wow!

these days with all the wind turbines and water turbines online a good vibe guy is back in demand!
as well as for aerospace.
they use it in our modern cars and trucks as knock sensors...
I use the tech on fleet gear and trucks...
it helps predict issues for sure.


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## billt (Dec 11, 2020)

With this being of such importance, (balancing the load to achieve max power safely, and without excessive vibration), one would think the operating manuals of most generators would have an entire section devoted to it, explaining the importance of the matter, along with how to safely accomplish it. They don't.

Most people, (especially today), who buy these generators are not electricians. They simply want to make power with the unit. Just as they want to blow snow, whack weeds, or blow off a sidewalk. However, this is not so simple. The power panel could easily be labeled so as to allow for easy load splitting between both sides. But again they're not. Is there a simple formula that can be applied to achieve this, regardless of the unit's panel layout, that you don't have to be an Electrical Engineer to understand?


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## Airstreamer67 (Oct 3, 2020)

I suspect it takes a gross out-of-balance load situation and some time to make a significant difference with home generators. 

As earlier stated, perhaps a near 100% load on one pole and 5% on the other. 

Most homeowners aren't that out-of-balance with their load management. If a homeowner is that off-center, heshe would quickly discover that they are getting just half of the generating capacity they thought they had. That should encourage better load-balancing management pretty quickly.


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## billt (Dec 11, 2020)

Another thing that can help with this, that I'm sure many don't know about, is the use of power splitting plugs, such as this one by Westinghouse. They are red and green color coded to help split the load evenly. It's not rocket science. But it will help in keeping things balanced. They're available in a 50 amp version as well.









Westinghouse 30 Amp 120-Volt L14-30P to (4X) 5-20R Generator Plug Adapter 302116A - The Home Depot


The Westinghouse L14-30P to 5-20R Adapter converts a 4-prong 30 Amp locking receptacle into a four 5-20R 20A standard household outlets. Designed to provide a convenient way to alter the outlet you need,



www.homedepot.com


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