50a Receptacle Generators w/ low THD? - Power Equipment Forum : Power Equipment Forums
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post #1 of 14 (permalink) Old 04-06-2019, 08:12 PM Thread Starter
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50a Receptacle Generators w/ low THD?

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 canít live with constantly flickering lights while on generator power but probably even bigger is the concern iíd 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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post #2 of 14 (permalink) Old 04-07-2019, 09:55 AM
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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/gen.../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...verter-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-S...0675/301871650

This Champion inverter is even cheaper, at $699, and 5000W/6250W, still providing 240V output:
https://www.homedepot.com/p/Champion...0519/308252910

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-1...E&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).

Last edited by RedOctobyr; 04-07-2019 at 10:01 AM.
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post #3 of 14 (permalink) Old 04-07-2019, 10:10 AM Thread Starter
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@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/pr...t?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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post #4 of 14 (permalink) Old 04-07-2019, 10:39 AM
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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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post #5 of 14 (permalink) Old 04-07-2019, 10:48 AM Thread Starter
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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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post #6 of 14 (permalink) Old 04-07-2019, 11:50 AM
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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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post #7 of 14 (permalink) Old 04-07-2019, 01:40 PM Thread Starter
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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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post #8 of 14 (permalink) Old 04-07-2019, 03:18 PM
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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.

Last edited by RedOctobyr; 04-07-2019 at 03:30 PM.
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post #9 of 14 (permalink) Old 04-07-2019, 03:19 PM
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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-Stratt...0126811&chn=ps

Last edited by tabora; 04-07-2019 at 11:01 PM.
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post #10 of 14 (permalink) Old 04-07-2019, 08:35 PM
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Impressive price, and THD specs!
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