# 50a Receptacle Generators w/ low THD?



## Kentinada

I just had my house rewired for a generator cutover panel that will be fed by a portable generator using a 50a power line. I bought a Firman generator from Costco rated at 8500w with a 50a receptacle. When we hooked it up today, every circuit that was running from the generator with lights on it had the lights (LED) flickering. I suspected noisy/dirty power from the generator so we turned it off and went back to line power. When I did some more research, I discovered that line power has a Total Harmonic Distortion (THD) of 5% or less. This Firman generator spec sheet said the THD was 25% at full load. I can’t live with constantly flickering lights while on generator power but probably even bigger is the concern i’d Have for my appliances and furnace running on such dirty power. Can anyone share positive experiences using a portable generator rated at 8k-9k watts with a 50a plug and low THD? Brand recommendations?


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## RedOctobyr

Welcome to the forum! Hopefully others with more expertise will chime in. But personally, I wouldn't have expected the imperfect THD to result in flickering lights. Was the generator putting out the proper voltage and frequency (60 Hz)? My "normal" (contractor-style) generator was a Generac, nothing special, and I don't recall ever having issues with flickering lights (I had a lot of fluorescents, and I think some LEDs at the time). 

I would expect the THD to have more of an impact on sensitive devices. But the LEDs need to take the AC and convert it down to DC, perhaps the dirty power was somehow causing trouble with that process. 

What's your budget? And how much power do you need? There aren't as many choices once you get up to the 50A-output range. The Hondas have an excellent reputation for quality, noise, and electrical output, but they're priced accordingly. This one is 6500W max, 5500W continuous, and lists for $2,200: 
https://powerequipment.honda.com/generators/models/eg6500

I'm impressed that the Firman actually lists a THD spec, that's not something that I'm used to seeing. I wouldn't count on finding it for most generators. 

The cleanest electrical output will be from a good inverter-style generator. Those create DC, and use a pure sine-wave inverter to make clean AC from that. They will have a much lower THD, and they can change their RPM based on the electrical load (since the output power frequency is not tied to the engine speed). This makes them quieter (they can just idle when powering a smaller load), and also much more fuel-efficient. Which is cheaper to run, but can also be a big help when fuel is tough to get. 

This 3300W Firman inverter shows 3% THD, for comparison: 
https://www.firmanpowerequipment.com/shop/inverter-w03083/

Their downsides are cost, and output. They're considerably more expensive per-watt, and there are not many that output 240V, which is what you'd want, for tying it into your house. 

This Briggs is an impressive value, for inverter generators. I don't know how good it actually is, but it's 5000W continuous, 6500W starting watts, for $999. And it outputs 240V. 
https://www.homedepot.com/p/Briggs-...h-Briggs-and-Stratton-Engine-030675/301871650

This Champion inverter is even cheaper, at $699, and 5000W/6250W, still providing 240V output: 
https://www.homedepot.com/p/Champio...Generator-with-301-cc-Engine-100519/308252910

That type might be your best bet for low THD, and still a decent power output. But I don't know what kind of loads you need to run. I suspect you will have a hard time finding a very-low THD, and ~8000W 240V output, in the $800 range of what might be a similar unit to your Firman: 
https://www.homedepot.com/p/Firman-...VAR-GCh020gQ5EAQYAiABEgK3kvD_BwE&gclsrc=aw.ds


I am using an inverter generator now (Honda EU2000i), and really like it. It's only 2000W max output, but it just purrs quietly, typically runs my loads for 6-7 hours per gallon, and provides very-clean output, which is nice for peace-of-mind with appliances, etc. I've added a second one in parallel for 4000W-max. A single larger unit would make more sense for most applications, though (I originally bought mine for a different, portable-use purpose).


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## Kentinada

@RedOctobyr, thanks for the input. I'm learning (once again) that you get what you pay for. I need/want a good generator that has a 50a outlet since that is how I had my house wired that will provide as clean a power sine wave as possible without spending multiple thousands of dollars. That said, if I need to spend $2k to get a good generator then that is what I'll have to do. A 25% THD is just not good enough. I saw this Champion generator on Tractor Supply Co today for $1079. It gets 4.9 out of 5 stars and has a 50a plug providing 9200w of power. It states the THD is 15% which is a big improvement knowing line power can be up to 5%. 

https://www.tractorsupply.com/tsc/product/champion-power-equipment-9200w-portable-generator-with-electric-start?cm_vc=-10005

I just don't know if that will be good enough or not. At Costco its easy to return stuff. Not sure about places like Tractor Supply Co. 

I'll look at the other links you provided too. Just didn't have time to go through them all yet.


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## RedOctobyr

Just to mention it, having a 50A-input for the house doesn't require you do use a generator with a 50A-output plug. A 30A-output generator could go through an adapter cable to change to work with a 50A-input. I'm not trying to tell you to go with a smaller generator, but just wanted to mention it. I wouldn't let a plug-mismatch be a complete obsctacle. 

Definitely check return policies. Generators are (understandably) things that can be difficult to return, since they don't want everyone buying them during a big storm/outage, then returning them 3 days later once the lights are back on. So even if you don't have any bad intentions, you sure don't want to find out that TSC won't take something back, if it doesn't fit your needs. 

What kind of loads do you need to run? Any pumps, like for a well, or a sewer pump? Big motors (especially compressors, like for a big AC unit) take a lot of startup watts. 

With a sewer grinder pump (but no well), I think the max load I was able to draw in our house was around 4500W, when I got as many things running at once as possible. Fridge, furnace, sewer pump, lights, etc etc. So I realized that even my 5500W generator was plenty of capacity (for us; every scenario is different is of course). We've since eliminated the sewer pump, which removed any big 240V loads. My first generator was 10,000W (a deal I couldn't pass up). I then realized it was vastly more capacity than I needed, which really didn't provide any benefit. So I changed to the 5500W. And I've now kinda swung the needle the other way, going to a 2000W inverter. 

Having a sense of your requirements helps balance things between having enough capacity, and way-overkill, which is extra cost, noise, and fuel-consumption. My apologies if this was all clearly calculated already, and you definitely need 7800W or whatever


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## Kentinada

I really appreciate the comments. So I had a setup running in my house using a 30a/5000w Porter Cable generator. It ran my furnace, well pump, 2 refrigerators, and some lights. It worked fine and we could limp along. Then I had this "great idea" that I wanted to add 5 more circuits that would make it a little easier to live in the house when the power went out. I added 2 20a circuits in the kitchen and 3 15a circuits that provided more lights around the house. I figured this was the next best thing to a whole-house generator. But I also figured I'd need a bigger generator. I had to have an electrician swap out my cutover panel because it was small and full so I could add the new circuits. I had him replace the 30a feed cable with 50a so I could run a bigger portable generator and be sure I had enough power to run everything. And so the sad story goes. 

And I just saw Home Depot has that the same Champion generator for about $50 less than Tractor Supply but I'm just very gun shy now. Wish I'd left well enough alone.


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## RedOctobyr

Before doing anything drastic, I might double-check that the Firman is working correctly. Checking output voltage and frequency, at least. Does the Firman display those? 

A multimeter will show AC voltage. Frequency is tougher to measure. A Kill-A-Watt meter (a handy tool) will show it. Or a $10 eBay tachometer would show how fast the engine is spinning, and you can calculate it from there (3600 RPM = 60 Hz).


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## Kentinada

The Firman generator has no displays. Where would you measure the AC voltage? From one of the 110 plugs on the generator itself? Or from a plug in the house that is fed by the generator? I have a Kill-A-Watt meter. Again, I assume I'd have to plug it into an outlet being supplied by the generator but does it need a load placed on it, i.e. something plugged into it?


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## RedOctobyr

If you have a Kill-A-Watt, perfect. Just plug it into the generator and see what it shows for voltage, and frequency. Voltage should be between 100V and 120V, I believe (110-120V is common). Frequency should be right around 60 Hz (my Generac would have you set it to 61-63 Hz, I think, with no load). 

It shouldn't matter whether you plug the K-A-W directly into a 110V outlet on the generator, or a powered outlet in the house. If the generator is not powering a load (if no current is flowing through the wiring), then you'll measure the same voltage at the generator, and at a powered outlet in the house. The frequency will always measure the same, no matter where you're measuring. 

If the generator has an auto-idle function, turn that off for the testing. You want the generator running at full speed. Alternately, if it has auto-idle but it can't be disabled (mine had a switch for that), then add a small load, say something like 100-200W. Enough to make it speed up from idle. You shouldn't need to have anything plugged into the K-A-W, mine will display info as long as it's plugged into a powered outlet.


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## tabora

The Briggs and Stratton 30679 has 8k continuous and 10k peak with a THD of 3-6% for $1059 delivered:

https://www.ebay.com/p/Briggs-Strat...e-Generator/563716255?iid=222890126811&chn=ps


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## RedOctobyr

Impressive price, and THD specs!


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## Kentinada

*Need a 50a outlet*



tabora said:


> The Briggs and Stratton 30679 has 8k continuous and 10k peak with a THD of 3-6% for $1059 delivered:
> 
> https://www.ebay.com/p/Briggs-Strat...e-Generator/563716255?iid=222890126811&chn=ps


Thanks but I need a generator with a 50a outlet. Looking at Northern Tool generators now.


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## Kentinada

@RedOctobyr, thanks for all the info and suggestions. I ended up taking the Firman generator back to Costco. I read the reviews posted on Costco's site (which I should have done first) and another user had EXACTLY the same issue with flickering lights. He went through Firman's support and got no help and took his back. So I just went ahead and took mine back too. I just wish Costco had a generator with better specs. I'm now looking at Northern Tool for an option.


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## tabora

Kentinada said:


> Thanks but I need a generator with a 50a outlet. Looking at Northern Tool generators now.


As @RedOctobyr said in post 4, you don't need a 50A outlet on an 8000W generator. Your old 5000W generator was only capable of 20.8A through the 30A outlet. An 8000W generator only outputs 33.3A continuous and will be fine with a 30A outlet.


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## Charlie Farrow

*Another thought*

You will also benefit potentially by choosing a larger machine. The THD can be reduced by going to a larger alternator, so a larger genrator will therefore improve the performance in this regard. It will also help with load acceptance if that is an issue - the TVD and the frequency drop as the load is applied.


Fairly hard going, but Stamford AVK (the alternator division of Cummins) go in depth here:


https://stamford-avk.com/sites/stamfordavk/files/AGN026_B.pdf




You might consider going to a specially made product, where they mismatch the engine and alternator fom the standard sizes for this purpose, but of course the cost is huge and not worthwhile at this size.


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## HarryN

One of the challenges with achieving "really good" THD numbers is that very few gen heads under 25 kW / 3 phase are designed to do it. I say very few because I have looked and could not find any in the past.

3-6% THD is not considered "good" - it is just not extremely terrible.

The US grid is ~ < 1% THD AFAIK and has pretty good stability. Interestingly Russia has an even more stable grid due to their heavy use of very large generators. It is actually quite amazingly stable. ( I haven't been there, just read about this stuff)

A good quality inverter can be 1% THD but a 50 amp x 240 VAC inverter system is a lot more than $1K. More like $1 / watt.


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## PuckSR

RedOctobyr said:


> The cleanest electrical output will be from a good inverter-style generator. Those create DC, and use a pure sine-wave inverter to make clean AC from that.


This isn't correct. inverters, even pure-sine wave inverters, are inferior to good rotating generators.
A generator is a simple device to make. You simply take an electric motor and rotate it with a mechanical device. 

However, a lot of cheap generators don't use brushless motors. As a result, their output wave is VERY non-sinusoidal. 

*What is THD?=Total Harmonic Distortion*
THD is an approximation of the non-sinusoidal components of the waveform. Basically, you use some math(fourier transform) and see how much power is from a 60Hz sine wave and how much is from other waves. All waves, no matter how complex, can be expressed as the sum of a bunch of different frequency sine waves.(Aside: this is how digital music works)

The higher the voltage THD, the less of a true sine wave. Even top-of-the-line industrial inverters(found in large 1MW UPS) have about ~1-2% voltage THD. Creating a sine wave is hard, unless you are spinning a motor.
High-end generators will create a perfect sine wave if connected to a purely resistive load(like a space heater). However, most devices in your house are NOT resistive loads. This means that the load creates worse THD. How bad? It depends on the source impedance of your generator. If it is a UPS, it doesn't really depend on anything. Modern inverters will simply shift their output to fix the distortion, similar to noise cancelling headphones

*What does this all mean?*
A 25kW 3-phase diesel generator is going to have the best waveform output for your house. However, it is going to be the least fuel-efficient option and will encounter issues because it will be <25% loaded most of the time. Plus, you probably have 240V instead of 208V stuff in your house.
An inverter generator is a nice option, but they suffer from some interesting problems with overload. Some models don't handle it well at all. Additionally, you have to be careful because your inverter might be a square wave output(100% THD).

Rating the THD (voltage or current?) of a generator doesn't really tell you much. In the industrial world, we don't rate them that way. Residential units are using the rating to tell you that they used an actual generator and not a random chunk of wire, like a lot of cheapo chinese generators. For practical purposes, anything <10% is probably an indication that they used quality parts. The rating gets confusing because most residential units are going to react poorly to current THD(THD generated by the load) and produce poor voltage THD because of the load. The rating is not qualifying that issue.



HarryN said:


> he US grid is ~ < 1% THD AFAIK and has pretty good stability. Interestingly Russia has an even more stable grid due to their heavy use of very large generators. It is actually quite amazingly stable. ( I haven't been there, just read about this stuff)


Eh. I dont know where you got this idea. The THD of any transformer is going to depend more on the load than the quality of the grid. 
IEEE 519 is the general consensus rule for any facility and if you exceed those limits the utility will get pissed. They allow 8% voltage THD


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## speedy2019

I bought a cheap £250 2800watt generator a year ago and with a halogen bulb I get flickering but no flickering with led or fluorescent lights


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## HarryN

Sorry. I should have said that if your goals are relatively small amounts of power (sub 10 kW) and you want fairly low THD, then a a GOOD off the shelf consumer inverter is better than a good off the shelf consumer generator. This is especially true for the 1 - 5 kW class. I consider low THD to be ~ 1 - 2% or better.

Achieving low THD with the better quality US made inverters is fairly easy. It is rare to see with most of the imports but a few can.


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## jksolo

tabora said:


> As @RedOctobyr said in post 4, you don't need a 50A outlet on an 8000W generator. Your old 5000W generator was only capable of 20.8A through the 30A outlet. An 8000W generator only outputs 33.3A continuous and will be fine with a 30A outlet.


Yes, Northern tool sells their North Star brand, with a honda engine, and less than 6% THD. Do not bother, buy, or use anything that is not 6% or less. That is the accepted rule of thumb. Greater that that can destroy anything that has sensitive electronics, ie. computers, TV's, audio equipment, modern dishwashers, heating systems, anything with a circuit board. Damage may happen right now, or may happen over time. The only product descriptions that address this are the ones that have the low THD. The others don't even mention this, promoting the idea that you can just carry on with "all normal activities". Criminal fraud in my mind.


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## Bulldogger

jksolo said:


> Yes, Northern tool sells their North Star brand, with a honda engine, and less than 6% THD. Do not bother, buy, or use anything that is not 6% or less. That is the accepted rule of thumb. Greater that that can destroy anything that has sensitive electronics, ie. computers, TV's, audio equipment, modern dishwashers, heating systems, anything with a circuit board. Damage may happen right now, or may happen over time. The only product descriptions that address this are the ones that have the low THD. The others don't even mention this, promoting the idea that you can just carry on with "all normal activities". Criminal fraud in my mind.


 Yes and the cheaper Powerhorse stuff has low distortion as well. No Honda engine on those however.


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## ajnuzzi

westinghouse wgen12000df. Relatively reasonably priced for what you're getting and has 5% thd. I can tell you it is night and day running this generator over the cheaper ones with the higher thd. When you hit it with a load it barely hiccups, no flickering, hz stays dead on 60


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## Macko

If your only requirement is a 50a plug and low THD, you can change one on a Honda eu7000 and is rock solid. 

Other options are inverters that will provide good THD is something like this that is coming out soon. This has thd < 3% and a 50a plug.


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## Columbotrek

Kentinada said:


> I just had my house rewired for a generator cutover panel that will be fed by a portable generator using a 50a power line. I bought a Firman generator from Costco rated at 8500w with a 50a receptacle. When we hooked it up today, every circuit that was running from the generator with lights on it had the lights (LED) flickering. I suspected noisy/dirty power from the generator so we turned it off and went back to line power. When I did some more research, I discovered that line power has a Total Harmonic Distortion (THD) of 5% or less. This Firman generator spec sheet said the THD was 25% at full load. I can’t live with constantly flickering lights while on generator power but probably even bigger is the concern i’d Have for my appliances and furnace running on such dirty power. Can anyone share positive experiences using a portable generator rated at 8k-9k watts with a 50a plug and low THD? Brand recommendations?


Sorry to hear of your issue. Hard to say exactly what it is. I use a Westinghouse 9500DF <23% THD. All my LEDs, and my supposedly sensitive electronics work fine with it. Nearly every thing made in the last 20 years uses switching power supplies in which the first thing they do is make DC then convert it back into whatever their device requires. No one is using the old transformer, rectifier, capacitor, regulator supplies any more. You did not say how bad the flickering is. And do you use dimmers with your LEDs? I had a problem with my generator and my Smart Dimmer LEDS until I adjusted the generators governor to make sure the frequency output did not exceed 60 Hz during normal operation. Cleaned it all right up.


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## Columbotrek

Columbotrek said:


> Sorry to hear of your issue. Hard to say exactly what it is. I use a Westinghouse 9500DF <23% THD. All my LEDs, and my supposedly sensitive electronics work fine with it. Nearly every thing made in the last 20 years uses switching power supplies in which the first thing they do is make DC then convert it back into whatever their device requires. No one is using the old transformer, rectifier, capacitor, regulator supplies any more. You did not say how bad the flickering is. And do you use dimmers with your LEDs? I had a problem with my generator and my Smart Dimmer LEDS until I adjusted the generators governor to make sure the frequency output did not exceed 60 Hz during normal operation. Cleaned it all right up.


Seems the smart dimmers use the voltage zero crossing time for something and they use up every micro-second. I suspect that if the frequency is faster than 60HZ, they run out of time and it messes with their internal control system. Out of the box my Wgen was between 63.5 and 61. When I run my Gen at 60.0 or less, the flickering vanishes. I was not able to adjust exactly at 60 because the governor is not that good. It will go up and down a couple of 10ths Hz. So I usually accept 59.0 to 60.0 as a target range. The westinghouse has a multi display which is real handy for setting Hz, But my computers smart ups also shows line frequency and there are plug in devices I have read about which do the same thing.


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