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Total Harmonic Distortion of Portable Generators


Hello Everyone,

I just registered on this forum looking to obtain some insight on the total harmonic distortion (thd) metric for portable generators. I currently own a new Westinghouse WH7500E Generator which is 7500 continuous watts and 9k peak watts. However, it has come to my attention that it has a 9% thd. As such, I've become a bit paranoid that it will do damage to the electronics in my home and fail to adequately operate the variable speed blower on my furnace.

My panel is wired with a 30amp inlet and an interlock kit.

My question is how a manufacturer can state a specified thd figure when it is pretty much all based on the rpms which are not electronically governed like standby or inverter generators.

I was considering buying a Briggs and Stratton 8k running/10k peak model because they state a thd of 3-6%.

Isnt thd largely dependent on different loads initiating and the rpm engine at that time? I don't understand how Briggs can claim a lower thd number than the Westinghouse model. Am I overthinking thing entire thing?

Lastly, whether I keep the Westinghouse generator or move on to a Briggs, will the installation of an aftermarket natural gas kit have a material impact on the frequency or thd of these generators? I know there's an adjustment for the air/fuel ratio but I don't want to install it if it will negate the manufacturer's stated specs for thd.

Thanks in advance for the information!


In Memory Of
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Most new electronics won't be affected by 9% THD. Almost all now use switching supplies that first rectify incoming line voltage and turn it into DC. This is then switched into a small transformer and then rectified again to the final lower voltage. Feedback circuit controls high side switching to regulate output.


Thanks for the response. Do newer furnaces typically have switching power supplies as well? I'll be installing an 80% 2 stage, variable speed York furnace in a few months.

In your opinion, does it make sense to switch to a generator with 3-6% thd instead? In a power out yin only concerned with running lights, garage door openers, well pump, some led lights, furnace, and maybe a TV with router/modem.


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the generator specifications I looked up show bonded neutral
this is not to code for an interlocking breaker transfer.

furnace controls are a bit tricky they are usually built to run on the mains
and anything else is trial and error. ask your HVAC contractor

the more traditional generator excitation, like the power company uses,
has lower noise than the cap regulated cheap ones. YMMV

as to the furnace, a lot depends on how the 2 stage firing is accomplished

electronic or 2 speed wound motors
how well the electronic speed controls are made
tell the contractor you want very traditional furnace controls if possible.
usually the 80% ones are OK, when you get to 90% things are way more complex.
again ask the HVAC contractor I ran a pair of york 80% with no problem
originally on a 5KW Coleman
but those furnace models are 25 years old.

Ive seen flame controls that wont work with the hot and neutral reversed.

worst case an isolation transformer for the furnace might kill the noise
I have never had a problem [yet] but I have not tested my new [Goodman brand] furnace on the generator!
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In your opinion, how should I ground the generator with an interlock switch? In your mind, does it make sense to sell the 9% thd generator for thr Briggs generator that states 3-6%? Thanks!

Jack Hottel

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You have encountered one of the paradoxes of using generators. OSHA requires that portable generators have their neutral bonded to generator frame for use on job sites. The NEC requires that in a residence the neutral to ground bond be connected in the service disconnect panel and that there be no others. Furnaces using flame rectification require a neutral to ground bond to operate. There are other requirements, see attachment.
Before I purchased either a generator or a furnace I would make sure that the respective contractors would guarantee that the units proposed would work together, and that operation would be demonstrated before they were paid. Good Luck!



I apologize, but some of this is simply over my head. I know the generator ground is bonded to the frame but will this not work with an interlock kit? I'm not sure if it is also grounded inside the panel. Thanks a lot!


In your opinion, how should I ground the generator with an interlock switch? In your mind, does it make sense to sell the 9% thd generator for thr Briggs generator that states 3-6%? Thanks!
In my opinion, you follow code, which requires an unbonded 4 wire system. Then you get it inspected, so if there are any issues in future, you are covered.

I am friends with a heating contractor. During every storm, he gets extremely busy with no heat calls. He usually finds:
1. the flame control system will not work with generator setup.
2. the control board has failed.

He is a licensed plumber, not an electrician, so does not diagnose electrical issues.

The boards on high efficiency systems usually have to come from manufacturer, they are not stocked locally. He says after a storm, often talking about weeks for board delivery. People cannot wait that long, so he replaces furnace. $$$$!

My suggestion would be to find out if there are any models local plumbing supply houses stock the electronic parts for, and buy one of those.

Try to get assurances that the unit you choose will work on a standby generator (unlikely any manufacturer will give any guarantee, and most void warranty if unit is powered from a portable generator)

I like oldstuff

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It's a great topic regarding furnace control boards on these new furnaces.
I had a Bryant 80% er put in last year to avoid the cost and complexity of the higher efficiency models.

Chatting with the installer he responded that I did good. He went on that the 90+ units have a $1200 control board. And,,, if it goes out it usually takes out the same board in the central air unit and he dreads telling people that the bill is $2700. They're soo sensitive, a lightning hit blocks away can kill them.

I asked about generators and being a sharp guy he said they don't like the choppy waveform of the newer inverter based units. Said they'll usually just not start but he's seen some smoke a device or two on the boards. So in my case with my 70's style generator with no electronics I'll cross my fingers and flip the switch.

*As to my nifty new $7000 furnace and AC, it's noisy as hell and only required two visits in the last year. The blower motor cooked itself then a fuse on the board popped. What do I expect. An American assembled machine with every component made in China. My old unit was 37 years old and never sneezed.

Don't buy Bryant, Trane, Carrier and many others as they are all the same machine painted different colors and are all Chinese.


I intend on installing a 2 stage, variable speed 80% York furnace. But to the best of my knowledge, from an electrical perspective, it's identical to a high efficiency furnace, just without the secondary heat exchanger (someone correct me if I'm wrong).

Do you guys have any sort of advice of what type of generator I should buy. Like I said, I own a brand new Westinghouse wh7500e that has great reviews a few no reports of equipment failure but has a 9% thd and brush altenator. It would cost me an additional $500 to go to a Briggs and Stratton with a thd of less than 6% so I don't know if it's a worthwhile purchase. I just want to minimize my risk of damaging something.


Last Subscription Date
oh fun

you may be able to remove the bond at the generator or not
a good background in electronics and the generator circuitry is necessary.
[some generators have flashing circuits that make this difficult]

yes the ground and neutral go to the same place in the main panel
the neutral caries the neutral load, the difference in 120V loads on the 240 legs
the earth ground keeps the generator frame at ground potential.
with the two also connected wrongly [for this use] at the generator
the ground also carries some of the neutral load
the voltage drop on the mis-combined neutral and ground wires
raises the generator frame above ground potential.
and on a dark and stormy night it might be just a slight tingle or fatal
you may be new to the forum, we prefer to see you become an old friend.
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Mikey NY

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I own many generators and rarely need one to run my home. I am still a little perplexed with this new craze to have a whole house generator. How long is you power outage going to be. Mine are usually a few hours and rarely a few days. put in a wood stove instead. The only reason I could justify a whole house generator is if I had a basement that floods. And that could be run with a portable. Or maybe if you have elderly folks in the house that may need oxygen etc. I would never connect any of my electronic to ANY generator, properly connected or not. Electricity seems pretty simple, hot leg, neutral and ground. Nothing to it. NOT so, neutral bonding, grounding etc :DeadHorse are very serious issues. Whatever you do, absolutely have it inspected by the nys electrical inspector. Don't take some local electricians word. file for the permit it may save your life.


I have no problem keeping tvs off etc but my chief concern is my furnace. I don't want a 9% thd generator to cause damage to the furnace or cause it to not operate. The house is relatively large so wood burning stove wouldn't suffice. Does it make sense to switch to a 6% thd generator or is it just a waste of money?


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most furnaces don't give a hoot

but most furnaces were in order of operation
24V transformer
gas-valve [and built in pilot safety]
stack switch and blower motor,

now days to get 80% efficient at a minimum
24V transformer
relay and draft motor / damper
bellows switches
glow coil ignition and flame sensor control box
stack switch and blower motor,

more electronics coax every BTU into the house, 80% and up
leaving the furnace exhaust cool enough for plastic pipe

how the furnace gets along with the generator
depends more on the furnace than the generator.

inverter generators are not the answer
I refurbished a 1978 ONAN 6.5 KW NH for my daughters residence
even on wheels it is barely portable
NOTHING electronic save a battery charging circuit diode.
with a little TLC it will churn out the KWs
long after pacific rim screamer junk is recycled.

you will find here proponents of 70s American 1800 RPM generators
because they will happily chug along for weeks on end
while, in the same interval, the pacific rim junkerator has
launched internal parts out the aluminum casting

internet ratings not withstanding.
you can be sure if it is Westinghouse no longer applies
even [the current] ONAN put their name on and had to recall some cheap junk.
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Last Subscription Date
I suspect that 9% thd vs 6% thd is mostly marketing.

I also suspect that that 9% figure comes because the type of generator set (capacitor regulated) doesn't put out a perfect sine wave. It puts out a wave that is close to sine but distorted by 9%. (Thus the 9% Total Harmonic Distortion figure). The voltage is still 120 and the frequency is 60, just the exact waveform is distorted from a "perfect" sine wave. As far as 95% of the stuff in your house, it won't notice the difference. The stuff that does notice shouldn't be in critical applications, because power isn't perfect!

I personally wouldn't worry about it (I have a westinghouse 7500 set too). If it really bothers you, I'd be considering finding a used Onan JB set to replace it rather than anything from a box store. They would run 7.5Kw all day long for a year and run at 1800 rpm. This thread discusses onan waveforms https://www.smokstak.com/forum/showthread.php?t=120202 and AC THD. It links a few others too.

Also it would be _very_ interesting to research just exactly what the standard figures are for distortion on the sine wave that comes into your house from the power company after it's been through umpteen transformers, by how many big motors and industrial complexes with welders and other nasty power messer uppers, power factor corrected with capacitors, etc. Not to mention computer power supplies, cfc bulbs, led power supplies in your own home .

Can anyone speak to the THD% from the power company and how bad or good that gets??

This paper suggests:

Application Class THD (%)
Sensitive Applications .
• Airports/Hospitals 3%
• Telecommunication Facilities . 3%
General Applications
• Office Buildings/Schools 5%
Dedicated Systems
• Factories 10%
Figure 5. IEEE 519 standards for total harmonic voltage distortion.

David (still learning) Meed

also might want to glance at this thread

especially post 15 showing a jb waveform on a scope against ac power waveform


Interesting, I won't waste money going from a 9% non-inverter generator to a 6% non-inverter generator. I had briefly considered the installation of an 11k natural gas standby but it's hard to swallow a $6k price tag for the very occasional power outage.

In your opinion, does it make sense to convert the Westinghouse wh7500e to natural gas? I know it results in a 10-20% reduction in power.


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Imagine the "distortion" from a sine wave that is the output of a Modulated Square Wave inverter! They erroneously call it a modified sine wave, but that is pure false advertising. There is nothing sine wave about it except that it is AC. Yet, lots of stuff will run just fine with it...